long-term changes in a brackish lagoon, lady's island lake, south-east ireland

Long-Term Changes in a Brackish Lagoon, Lady's Island Lake, South-East IrelandAuthor(s): Brenda HealySource: Biology and Environment: Proceedings of the Royal Irish Academy, Vol. 97B, No. 1(Jul., 1997), pp. 33-51Published by: Royal Irish AcademyStable URL: http://www.jstor.org/stable/20499983 .

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LONG-TERM CHANGES IN A BRACKISH

LAGOON, LADY'S ISLAND LAKE,

SOUTH-EAST IRELAND

Brenda Healy

ABSTRACT

Lady's Island Lake is a natural, percolating, coastal lagoon, isolated from the sea but subject to wide fluctuations in water-level and salinity as a result of more or less regular breaching of the dune barrier. Observations over seventeen years revealed changes in the salinity regime every two to four years, with poly-euhaline phases alternating with oligo-mesohaline phases. The dominant

macrophytes were Ruppia cirrhosa or Potamogeton pectinatus, depending on the salinity. The fauna was rich by comparison with lagoons in Britain. Continuous shifts in species dominance linked to gradual changes in salinity were punctuated by sudden, more extensive changes resulting from tidal exchange, which sometimes caused mass mortalities of the resident brackish fauna. Popula tions recovered quickly, however, and are believed to have recolonised from peripheral refuges.

There was no evidence of extinction of resident species and colonisation from neighbouring brackish waters was not discernible. The observed time-lag in the response of the community to changing conditions, and differences in the responses of the species, mlay explain the lack of correlation between species presence and environmental factors in many lagoonal studies. Snapshot studies for conservation purposes may misclassify brackish lagoons during a state of

transition.

B. Healy, Department of Zoology, University College, Belfield, Dublin 4, Ireland.

Received 21 December 1995. Read 22 May 1997.

Published 31 July 1997.

INTRODUCTION

Lady's Island Lake is a natural, brackish, per colating lagoon, situated on the south coast of County Wexford (52011'N, 6?25'W; grid ref. T 1006), about 3km west of Camsore Point. Early but notoriously inaccurate maps show an open inlet at this point, but a complete barrier is known to have existed from the late seventeenth century, although there may have been periods since when the lagoon was open to the sea and tidal. Analysis of sediments indicates that a freshwater lake, which formed 4500-5000 years ago, existed until about 1700, when salt water first entered (R.W.G.

Carter, pers. comm.). As there is no outlet for water except by seepage through the baffler, the level rises in winter, flooding surrounding land and backing up into sewers. In most years, therefore, flooding is relieved in spring by cutting a channel through the dune baffler to let out excess water.

The opening is allowed to seal naturally, and the lagoon may become re-isolated within two weeks or it may remain open to the sea for up to six

months. The procedure has been camred out since at least the seventeenth century (Murphy 1956), and the lagoon in its present state owes its distinc tive character to this simple form of management.

Lagoons, in the strict sense, are defined as natural physiographic features comprised of shal

low open bodies of saline or brackish water, iso lated or semi-isolated from the adjacent sea by a barrier of sand or shingle, but which nevertheless receive salts from the sea as a result of natural causes (Barnes 1987; 1989a; Bamber et al. 1992). The characteristics of lagoons are shallow water (mean depth generally 2m or less), often undergo ing significant seasonal fluctuations in volume, salinity which varies widely, both spatially and temporally, and a species-poor community of brackish water plants and animals, many of which are almost entirely confined to non-tidal waters but are capable of high rates of productivity. La goons are relatively short-lived features in geologi cal terms and are subject to continual change as a result of evolution of the barffer and succession of the vegetation. They are among the most severely affected of coastal habitats by changes in sea-level, storm surges which damage bafflers, and modifica tions of the coastline which alter long-shore cur rents or the supply of sediment. Almost every

where, lagoons have been modified, exploited or even obliterated by human activities, and it is doubtful whether any truly natural ones remain in

westem Europe.

Lagoons have been designated as a high pnror

ity for conservation in the 1992 EU Habitats Directive (92/43/EEC), chiefly because, as geo morphological features, they are under threat.

BIOLOGY AND) ENVIRONMENT: PIWCEEDIINcS CF THE ROYAL IRISH ACADEMY, VOL. 973, No. 1, 33-51 (1997). 0 ROYAL IRISH ACADEMY 33



BIOLOGY AND ENVIRONMENT

Lady's Island Lake is recognised as an area of scientific interest more specifically for its wintering

waterfowl and breeding terns) for two species of charophytes, Lamprothlamnium papulosum (Wallr.) J.

Groves and Chara canescens Desv. and Lois., listed in the Red Data Book (Stewart and Church 1992) as 'vulnerable' in Ireland, and for the rare cotton

weed Otanthus maritimus Hoffm. and Link. on the dunes at the south shore. Its two islands are pro tected as a Refuge for Fauna under the 1976

Wildlife Act, and the lagoon has been designated as a Special Protection Area under the EU Birds

Directive (79/409/EEC). The highest priority for conservation has been the internationally important breeding colony of roseate terns (Sterna dougallii

Montagu) which, together with three other tern species, all of which are listed in Annex 1 of the

EU Birds Directive, nest on islands in the lagoon. The obligation to protect these terns has brought conservationists into conflict with the local com

munity over the practice of the annual breaching of the barrier, because when the opening fails to close early the water-level may continue to fall until the channels separating the islands from the

mainland dry up sufficiently to allow access of predators, which raid the nests. Reproductive fail ures in the 1980s are believed by some to be linked to long penrods of tidal exchange and low water level, conditions which became more frequent during this period than in the previous decade. The reasons for the delayed closure of the breach in recent times are unclear, but it seems likely that removal of gravel from the barrier near Carusore Point, which continued spasmodically until 1994, is at least partly responsible. The rate of extraction increased from 1970 and the amount removed far exceeded supply, with the result that the dune cliff at Carnsore Point retreated Sm between 1978 and 1980 (Carter and Orford 1980).

This paper summarises seventeen years of ob servations on salinity, water-level, fauna and flora. The data are uneven, having been acquired for a range of purposes. Differences in methods, sam pling effort and frequency of observations have

meant that few of the observed changes in the flora and fauna are fully quantifiable. However, general patterns of change have provided an insight into the dynamic nature of the system and the causes of fluctuations in conditions and biota.

METHODS

Observations were made between 1975 and 1991. In the summer of 1975, the Department of Fisheries conducted a survey of the lagoon to determine its potential for aquaculture. Emphasis was on fish and invertebrates, but an unpublished report (M. Crowley and M. Wallace) includes

measurements of salinity, water-level and height of

34

the dune barrier. From June 1976 to December 1978 the lagoon was visited monthly by the author to make faunal collections and to record changes in salinity and water-levels. A more detailed study,

which included granulometry of the sediments, nutrient measurements, and density estimates of pelagic and benthic invertebrates, as well as mea surements of E. coli contamination and reports of fish catches, was carried out in 1976-7 by Bates (1977). The ecology and population dynamics of two crustacean species were investigated in more detail in 1979-80: Palaemonetes varians (P. Fox, unpublished) and Lekanesphaera hookeri (Norton and Healy 1984).

Qualitative sampling in later years was mainly concerned with monitoring the presence of com

mon pelagic, benthic and sessile species, but quan titative and semi-quantitative sampling of pelagic and benthic fauna was carried out at three shore sites on two occasions in 1991 (Galvin 1992).

Throughout most of the study period, only con spicuous and easily identified species occumng in the littoral zone were recorded, and insects and polychaetes in particular have been neglected. Only in 1976-7 and 1991 was complete identifi cation of collections attempted. Failure to record a species during a given period, therefore, does not

mean that it was not present.

Most observations were made in shallow water near the shore at four sites: in the south-east, at Lady's Island, in Bunargate Pool and, less fre quently, at the north end. Details of methods are given in Healy et al. 1982 and Galvin 1992.

RESULTS

PHYSlOGRAPHY OF THE LAGOON AND BARRIER

The lagoon is 3.7km long (north-south) with a maximum width of 1.8km (east-west), and cov ers a maximum area in winter of about 450ha (Fig. 1). The maximum depth in winter is 5m, but in summer the depth is generally no more than 1 1.5m. There are two islands: Sgarbheen, which becomes submerged in late winter, and Inish, which is never su-bmerged. The former 'Lady's Island' is now joined to the mainland by a cause

way. Sediments range from mud at the northern end and in sheltered bays to coarse sand and gravel at the southern end. The bed is stony in places, e.g. at the southern end of the peninsula at Lady's Island and on the south shore, and there are numerous rocks in the south-east. The dune bar rier is composed of stratified coarse to fine gravels

with grain sizes of up to - 44, topped with small aeolian sand-dunes, interrupted by relict washover fans (Carter and Orford 1980; Ruz 1989). The height of the barrier reaches 8-1Om IOD (Irish Ordnance Datum-= Chart Datumn - 0.2m) but is



LONG-TERM CHANGES IN A BRACKISH LAGOON

S Xr~~~~~~~~~~~~~~~~~~~~~~f . .X

*Lady's Island

I .-WE)CFORD < Raven Point )

* fish Is.

Bunargate

Roessara Harbour /* 4

1~ <_r %

t _~~~~~~~~~~~~~ . _ SS

F - I L a a f b g n s ..

%~~

5 km 1 km

Fig. 1-Lady's Island Lake and location of neighbouring lagoon-like systems.

lower at the point where the cut is made. The mlinimlluml width of the barrier in winter, from the edge of the lagooin to high-water mark oIn the shore, is a little over 300m (measured in March 1989). The meani spring tidal range of the open sea in the area is 3.6mii.

WATER-LEVEL

Fluctuations in water-level are seasonal, rising from Septemlber-October by a little over 2m to reach a maximiiumi in .spring, when the barrier is usually breached (Fig. 2). In years when breaching is not carried out, the level stabilises at around

5.6mn 01) and starts to fall in early summer, when evaporation and seepage through the barrier ex ceed inputs from streanms, drainage and run-off from surrounding land, and direct rainfall.

By the end of January water has usually cov ered most of Sgarbheen, part of the pilgrimage path which borders the Lady's Island peninsula, and the stony beach on Inish where common and Arctic terns nest. From about D)eceimiber, B3unar gate l'ool in the south-west (Fig. 1), which is isolated in summer, becomiies joined to the lagoon, allowing mixing of the waters and their fauna. A

water-level range of 3.02m was recorded between 1976 and 1981 (Fig. 2). As the average depth of the lagoon in summer is only an estimated 2Iml, the

winter rise represents a nmajor change in volume and results in considerable dilution of salts.

Breachinig of the barrier is usually carried out in March-April. Following completion of the cut, the water flows out for several days until the lagoon level falls below high-tide mark. The la goon then becomes tidal until the opening seals as a result of onshore and long-shore transport of beach sedinment. A tidal range of 80ccm was Im1ea suLred in the lagoon in July 1983. D)uring tidal periods, the water falls well below levels which would be reached as a result of loss by seepage and evaporation, and large expanses of the substratum are exposed. The bar which forms across the sealed opening continues to build up gradually but finally stabilises. lDuring the period 1975-8 and in March 1989 the final level of the bar was 6.8m 01),

which is nearly 3m above extreme high water of spring tides, although waves may wash over during stormls. The water-level in the lagoon usually re

mains above mean tide level, even in summiler, so the amount of landward percolation is probably always smiiall.

35



BIOLOGY ANI) ENVIRONMENT

35

30

3925

Z' 20

=15

"W 10

5

1975 1976 1977 1978 1979 1980

35

t2

1975 1976 1977 19784 1979 1980

20S

10

5

. ~~~~~~~9-Y-_T |,-r-r1 r a-Y . -r .r*rw?y'9 w{a~-y1-g-7 U- rrrrrrr WrrnV i

1981 1982 1983 1984 1985 98

35

03O

-25

420

25

203

197o98 98 99 19

36




SALINITY

Seawater at approximately 34%o enters the lagoon by seepage through the barrier, by salt spray being washed into the lagoon from the barrier, by occasional overwash of the entrance bar and, most importantly, by tidal flow following breaching. Salinities of between 2%o and 40%o have been recorded in the lagoon as a whole. At most times

when the lagoon was isolated the salinity was 0.5-2.0%o higher at the southern end than at Lady's Island and lower in the extreme north, where a small stream enters. Bunargate Pool also receives some fresh water, and during its periods of isolation, which can last for up to nine months, the salinity falls below that of the lagoon. Concentra tions of 2-1 9%o were recorded here between 1977 and 1991, but an exceptional level of 26%o was reached in September 1990. All other point sources of fresh water have no significant effect, and the main body of the lagoon has a more or less uniform salinity. Vertical stratification has been observed during calm weather in summer.

Seasonal fluctuations in salinity corresponded to changes in water-level (Fig. 2). In years when the breach sealed early before any significant tidal exchange could take place (e.g. in 1977, 1978 and 1979), the annual range was from 6-8%o in winter to 12-16%o in summer. Occasional higher sum

mer readings near the southern end (Healy et al. 1982) may be due to landward seepage of seawater during spring tides under favourable conditions, i.e. when lagoon levels are low and tides high, or after storms when seawater, perhaps concentrated by evaporation, drains from the barffer. Some high salinities at the southern end in autumn-spring

may have been due to overwash of the inlet bar, e.g. in November 1977, December 1978 (R.W.G.

Carter, pers. comm.) and October 1981; there was no evidence of washover at other places.

In years when there was a major influx of ti dal seawater (e.g. 1980, 1981, 1983), the salinity throughout most of the lagoon rose from < lO%o before breaching to >25%o after two weeks, and in six of the seventeen years for which measure

ments were available salinities in summer reached over 30%o. Vanrations from year to year (Fig. 2) were chiefly caused by differences in the propor tion of lagoon water which was replaced by sea

water during periods of tidal flow. This depended on the length of time the inlet remained open, which in turn depended on tides and weather, and the degree of mixing brought about by winds. In calm weather, tidal water flows north through the centre of the lagoon and ebbs along the same route, so that regions on either side are little affected. However, even when the opening closes quickly, the salinity throughout the lagoon can rise rapidly, especially if there are strong cross-winds during tidal flow (e.g. in 1980).

The wide range of salinities measured means that the lagoon's inhabitants can expenence condi tions corresponding to almost all categonres of

mixohaline (brackish) waters: oligohaline (0.5 5.0%o), mesohaline (5-1 8%o), polyhaline (18 30%o) and euhaline (30-40%o) (Venice System 1958). During the period of these observations, there were changes in the salinity regime every two to four years, with high-salinity poly-euhaline phases in 1975-7, 1980-4 and 1988-90 alternat ing with low-salinity oligo-mesohaline phases in 1977-9 and 1985-7 (Fig. 2). The reasons for this pattern of cyclical change are not known. During high-salinity phases there were wide seasonal fluc tuations in salinity, while in low-salinity phases differences between sununer and winter were rela tively small.

WATER TEMPERATURE

The minimum and maximum day tempera tures of nearshore lagoon water in 1976-8 were 10C and 26?C respectively, compared with 6?C

and 17C in the open sea. Shallow water in the littoral zone sometimes reached over 30?C. Ice formed occasionally in winter but was of short duration.

NUTRIENTS

The only measurements of dissolved nutrients are for 1977 (Bates 1977) and 1991 (Galvin 1992). These were years when macrophytes were abun dant and no phytoplankton blooms were observed. An annual cycle of nutrient uptake and release can be envisaged, with submerged macrophytes re

moving nutrients from the water during the grow ing phase and releasing them gradually in autumn and winter when deciduous stems accumulated in banks on the shoreline. Additional nutrients from agricultural run-off and domestic effluents in creased during the study period (unpublished re ports from various sources), and phytoplankton blooms (diatoms, filamentous chlorophytes or cyanophytes, depending on the season), the first of

which was observed in 1979, have become more frequent.

CHANGES IN FLORA

The wide fluctuations in water-level mean that frnging beds of emergent reeds and sedges are poorly developed and mostly limited to the north ern end and a few sheltered bays. Submerged

macrophytes grew luxuriantly or were nearly ab sent, depending on the salinity. During extended oligo-mesohaline periods, Potamogeton pectinatus L. and Ruppia cirrhosa (Petagna) Grande, in varying proportions, formed dense beds in shallow water, together with species of Cladophora and Entero

morpha, and Chaetomorpha sp. in deep water. Pota

37



BOLOGY ANI) ENVIRONMENT

1976 1977 1978 1979 1980 1981

Clupea harengus _

Pleuronectes platessa _

Platichthys flesus

Crangon crangon -_

Arenicola manna _

Pomatoschistus microps

Praunus flexuosus

Hydrobia ventrosa

Cerastoderma glaucum

Pa/aemonetes vanans

Anguilla anguilla -

Hediste diversicolor _

Gasterosteus aculeatus =

Gammarus zaddachi __

Lekanesphaera hooked

Neomysis integer

Limnephilus affinis u

Sigara stagnalis

Ischnura elegans

Idotea chelipes

Potamopyrgus antipodarum

Ruppia cirrhosa _

Potamogeton pectinatus __

Chaetomorpha sp. |-_

40

Fig. 3-Changes in abundance of the more important species, 1976-81 (semi-quantitative or subjective estimates

only).

inogetoti pectitiatus increased and R. cirrliosa declined during extended periods of low salinity (<1 6%o), while in periods when the salinity remained > 16%' (Fig. 3) R. cirrlw(sa completely replaced P. pectinatus, Ruppia inaritihnus L. appeared, Cliaeto iii(rplia disappeared, Enteroniorphia was seasonally abundant near the shore, and the charophytes Laniprothamniiumn papulosunil and Clhara caniescenls could be found in sandy or stony shallows. Most of these species disappeared within a few months

38

during extended periods of tidal flow when salinity reached 3()%o or nnore, although somile Ruppia rhizomes survived to regenerate the following spring. They were replaced by Chot)rda filum (L.) Stack., scattered plants of Polysiplioitia, Cera(i(nii and other unidentified Ithodophyceae, and when high salinities persisted Fucius sp. becanme estab lished in stony areas. Macrophyte beds were absent or poorly developed fron nmid- 1981 until 1986 while summner salinities were high, but had recov



LONG-TERM CHANGES IN A BKACKISH LAGOON

ered by 1987. The flora corresponds to the Ruppi etum cirrhosae Iversen, 1934 association, with the sub-assoctation potametosum pectinati Verhoeven, 1980, which has an upper salinity limit of 27%o, characterising the low-salinity phases, and the sub association chaetomorphetosum lini Verhoeven, 1980,

which has an upper salinity limit of 20-36%o,

characterising high-salinity phases.

Floating fucoids and laminarians were often common in tidal water and tended to collect in

depressions on the lagoon bed, where they de cayed causing deoxygenation of the water and blackening of the sediments. Decomposing Ruppia or Potamogeton and algae could also be present at such times, and the combined effects of nutrients released from decaying vegetation, increased sewage discharge related to seasonal tourism and pilgrimages, and the absence of an aquatic

macroflora to take up excess nutrients sometimes resulted in unpleasant smells associated with anaer obic decomposition and black mud.

AQUATIC MACROFAUNA

Between 1975 and 1991, 97 macrofaunal spe cies (> lmm in shortest diameter) were identified and a further eleven unidentified taxa recognised (Table 1). Also identified, but not included here,

were some smaller taxa: eight Enchytraeidae, eleven Rotifera, two Copepoda and two Ostra coda. Among the species recorded, twelve are lagoonal specialists, more or less confined to en closed, non-tidal brackish waters (Verhoeven 1980;

Barnes 1989b; Davidson et al. 1991; Bamber et al. 1992), sixteen have a more general distribution in brackish waters, including salt-marshes and estuar ine habitats, 43 are euryhaline marine species with

various capacities for tolerating reduced salinity, and 29 are commonly found in fresh water (Table 1). The lagoonal and brackish water species were generally the most abundant.

CHANGES IN FAUNA

Temporal changes in faunal composition and abundance were, as for macrophytes, linked to shifts in the salinity regime and the extent of tidal incursions. Changes in the relative abundance of the dominant species (semi-quantitative and sub jective estimates only) during the period 1976-81 (Fig. 3) reveal two suites of species which replaced each other in oligo-mesohaline and poly-euhaline phases. A high proportion of the species associated with oligo-mesohaline conditions were insects, many of which can also live in fresh water (Hemiptera, Trichoptera, Odonata, Coleoptera,

Chironomidae); others are found in many low medium-salinity lagoons and lagoon-like systems

(Cordylophora caspia, Neomysis integer, Gammarus zaddachi, Idotea chelipes, Lekanesphaera hookeri,

Conopeum seurati) or are freshwater species which tolerate some salt (Corixa panzeri, Notonecta viri dis, Potamopyrgus antipodarum, Pungitius pungitius).

Meso-euhaline phases were marked by a decrease in low-salinity species and the presence of com

mon species of seashores and estuaries (Arenicola marina, Praunusflexuosus, Crangon crangon, Carcinus maenas, Idotea baltica, Potamoschistus microps, Pla tichthys flesus, Pleuronectes platessa) and of non-tidal

brackish waters with frequent influxes of seawater (Cerastoderma glaucum, Hydrobia ventrosa). Only three or four species appeared to tolerate both ends of the salinity range for extended periods: Hediste diversicolor, Palaemonetes vaoians, Anguilla anguilia and, possibly, GCsterosteus aculeatus. The latter spe cies occurs on the open coast at Camsore Point (Healy and McGrath 1982) and is presumed to

be present in local fresh waters, but it is not known whether the freshwater, brackish water and marine populations belong to the same breeding population.

The way in which some siminlar species re placed each other during a change of salinity regime is shown by changes in the relative num bers in qualitative collections. Neomysis itnteger,

which was rare in 1976, completely replaced Praunius flexuosus between March and May 1977, following a fall in salinity from 24%o in September 1976 to 9%o in March 1977. The two species frequently coexisted, often in different proportions in different parts of the lagoon. In August 1981, for example, 30% of mysids were P. flexuosus at the southern end but only 2% at the northern end.

However, proportions did not always reflect salin ity because the populations do not respond imme diately to salinity fluctuations (Fig. 4). Neomysis integer appears to be more tolerant of high salinity than Praunus flexuosus is of low salinity. Hydrobi ids, in contrast to mysids, replaced each other more gradually. Hydrobia vetntrosa, a polyhaline species, persisted throughout 1977-8 in salinities of < 16%o and coexisted with the more abundant Poota mopyrgus antipodarum, albeit in low densities, into the 1 980s. However, following the catastrophic

mortalities of the rnid-1980s (see below), H. ven trosa was slow to return and numbers remained low until 1991. In most years it was possible to find both species but again proportions differed in dif ferent parts of the lagoon. The distribution of the two species in March 1980 was typical of meso haline phases, with H. ventrosa comprising 90% of hydrobiids at the southern shore and 60% at Lady's Island, while only P. antipodarum was present at the

northern end.

Periods of gradual change, such as that moni tored between 1976 and 1978, were punctuated by sudden changes brought about by extended tidal periods which sometimes caused widespread mor talities e.g. in 1985. As the water-level fell follow

39




ing breaching, the benthic fauna of shallow water was exposed and individuals of Cerastoderma glau cum, Mya areniaria, Arenicola marina, and Carcinus maenas could be seen dying at the surface, espe cially during warm weather. It appears likely that pelagic macrofaunal populations would have been severely depleted by being washed out to sea, the effects being greatest when there was good mixing of sea and lagoon water or a long tidal period. Surviving benthic and pelagic individuals were

subjected to a rapid rise in salinity, and salt con centrations often increased further owing to evap oration after the inlet closed, only falling to polyhaline levels in late autumn or winter. Sur vivors were also vulnerable to predation by marine fish and crustaceans which had entered with the tides. In years when low water-level and high salinity persisted for several months, previously common species often disappeared completely from the main body of the lagoon. Bunargate Pool

Table 1 -Macrofaunal species recorded in Lady's Island Lake, 1975-91. Ecological categories: MI,

marine species entering occasionally in tidal water, surviving for short periods only;

M2, marine species becoming established for a few months, usually disappearing

when the salinity falls in winter; M3, marine species surviving lower salinity in

winter, persisting for 1-3 years after entry; M4, marine-estuarine species surviving in

polyhaline phases but not reproducing successfully, declining and finally disappearing

in extended oligo-mesohaline phases; M5, marine-estuarine species capable of repro

ducing in poly-euhaline phases but declining and disappearing in oligo-euhaline

phases; B, brackish water species also occurring in tidal waters; Li, lagoonal special

ists favoured by higher salinities; L2, lagoonal species tolerating a wide salinity range;

L3, lagoonal species favoured by lower salinities; F, freshwater species occurring

occasionally in the lagoon, mainly in Bunargate Pool; A, accidentals washed into

the lagoon from the surrounding land. Abundance categories (maximum observed):

a = abundant, c = common, f= frequent, o = occasional, r = rare.

Ecological Favoured Abundance Region

category salinity

Cnidaria

Aurelia aurita (L.) M1 High o South

Cordylophora caspia (Pallas) B Med.-high c Mid-south

Rhizostoma octopus (L.) Ml High r South

Annelida

Amphicteis gunneri (M. Sars) M3? Med. o ?

Phyllodoce maculata (L.) M3? Med. f Al > 5%o

Arenicola marina (L.) M4 Med.-high c All > 5%o

Capitella capitata agg. M3 High o South

Hediste diversicolor (Miiller) M5 Low-high a All

Polydora ligni Webster B High? c All > 5%o

Pomatoceros triqueter (L.) M2 High o South, rocks

Nais elinguis (Muller) B Med. -low a Al

Tubffex costatus Clap. B Low-high f All?

Cirripedia

Balanus improvisus Darwin B High a South, rocks

Semibalanus balanoides (L.) M2 High o South, rocks

Mysidacea

Mesopodopsis slabberi (v. Ben.) B Med. r North

Neomysis integer (Leach) B Low-med. a All

Praunus flexuosus (Muiller) M5 Med.-high a All?

Isopoda

Asellus aquaticus L. F Low r Bunargate

Idotea baltica (Pallas) M4 High f South

40




Table 1 -(continued)


category salinity

Idotea chelipes (Pallas) L2 Low-med. c All

Idotea granulosa Rathke M2 High o South

Lekanesphaera hookeri (Leach) L2 Low-med. a All

Amphipoda Gamnmarus locusta (L.) M2-3 High r South

Gammarus zaddachi Sexton B Low-high a All

Orchestia gammarella (Pallas) A Low-med. o South

Decapoda

Carcinus maenas (L.) M4 Med.-high c All ex. Bunargate

Crangon crangon (L.) M4 Med.-high c All ex. Bunargate

Liocarcinus hiolsatus (Fabr.) M3 Med.-high c Deep water

Palaemon serratus (Pennant) M2 High r South

Palaemonetes varians Leach L2 Low-high a All

Trichoptera

Limnephilus affinis Curtis B Low.-med. f All ex. south

Ylodes reuteri (McLachlan) F Low r Bunargate

Odonata

Ischtnura elegans (v.d. Linden) B Low-med. f All ex. south

Hemiptera

Corixa affinis Leach F Low f Lady's Island

Corixa panzeri (Fieber) F Low f North + Bunargate

Gerris lacustris (L.) F Low o Bunargate

Nepa cinerea (L.) F Low o North + Bunargate

Hesperocorixa linnaei (Fieber) F Low r Bunargate

Notonecta viridis Delcourt B/F Low f All ex. south

Sigara concinna (Fieber) L3 Low f Bunargate

Sigara dorsalis (Leach) F Low f North + Bunargate

Saldidae A Low o Shore

Coleoptera

Agabus conspersus (Marsham) L3 Low r Bunargate

Cercyon ustulatus Preys F Low r Lady's Island

Coelambus confluens (Fabr.) F Low r Bunargate

Coelambus impressipunctatus (Schall.) F Low r Bunargate

Dryops luridus (Erichs.) F Low r Bunargate

Enochrus bicolor (Fabr.) L3 Low r North

Enochrus halophilus (Bedel) LB Low r Bunargate

Helophorus brevipalpis Bedel F Low r Bunargate

Hydroporns planus (Fabr.) F Low r Bunargate

Hydrotus inaequalis (L.) F Low r Bunargate

Laccobius bipunctatus (Fabr.) F Low r Bunargate

Noterus clavicornis (Deg.) F Low r Bunargate

Ochthebius bicolon Germ. F Low r Lady's Island

Ochthebius dilatatus Stephens f LOW r BUnargate

Ochthebius ma1rinus Paykull LB Low r Lady's Island

DiPtera

Chironomus aprilinus Meigen B Low-mned. ?Bunargate Chironomus sp. ? Low ?Bunargate

41



BIOLOGY AN) ENVIRONMENT

Table 1 (continued)


category salinity

Einfeldia sp. ? Low-high a All

Ephydra riparia Fallen B Med.-high c All

Macropopia sp. ? Low ? Bunargate

Procladius sp. ? Low ? Bunargate

Psychodidae ? Low o Bunargate

Larvae indet. 3 spp ? Low ? Bunargate

Acarina

Arrenurus sp. F Low r Bunargate

Thyas v)igilans Lundblad F Low r Bunargate

Thyopis sp. F Low ? Bunargate

Mollusca

Hydrobia ulvae (Pennant) M2 High r South

Lepidochiton cinerea (L.) M2 High r South

Lymnaea pereger (Muller) F Low o Bunargate

Littorina littorea (L.) M3 Low o South, rocks

Mya arenaria L. M4/5 Med.-high c All ex. Bunargate

Mytilus edulis L. M4/5 Med.-high f South

Potamopyrgus antipodarum (Smith) F/B Low a All

Bryozoa

Conopeum seurati (Canu) L2 Low-high c All

Chondrichthyes

Scyliorhinus canicula (L.) M2 High o Deep water

Osteichthyes

Ammodytes tobianus L. M2 High r South

Anguilla anguilla L. M/B Low-high f All

Atherina presbyter Valenc. M2 High ? Deep water?

Crenimugil labrosus (Risso) M2 High ? Deep water

Clupea harengus L. M3 High f Deep water

Cyclopterus lumpus L. M2 High r South

Dicentrarchus labrax (L.) M2 High f Deep water?

Gaidropsarus mediterraneus (L.) M2 High r South

Gasterosteus aculeatus L. F/B/M Low-high c All

Nerophis lumbriciformis Pennant M2 High r South

Platichthys flesus (L.) M4 Med.-high c All

Pleuronectes platessa L. M3 Med.-high f All ex. Bunargate

Pollachius pollachius (L.) M2 High o Deep water

Pollachius virens (L.) M3 High f Deep water

Pomatoschistus microps (Kroyer) M3 Med. -high c All

Pomatoschistus minutus (Pallas) B High o South

Pungitius pungitius (L.) F Low r Lady's Island

Sardina pilchardus (Wahlbaum) M2 High ? ?

Solea vulgaris Quesnel M2 High r South

Spinachia spinacliia (L.) M2 High o South

Sprattus sprattus (L.) M2 High ?? Symphodes melopes (L.) M2 High r South

Taurulus bubalis (Euphrasen) M2 High o South

42




100 -* e p 80~~~~~~~~

| 60

40

20- * X

10 20 30 40

Salinity (%Q)

Fig. 4-The proportion of Praunus flexuosus in catches of

mysids and the corresponding water salinity. Data from

1976-91.

was not affected by the tidal regime, however, because it quickly became isolated and the salinity rarely exceeded 19%o. Similarly, the marshy region at the extreme northern end, although experienc ing a substantial fall in water-level, was scarcely affected by tidal water and the salinity remained low. Isolated pools, diluted by rainwater, often persisted throughout the summer here and in the marshy region east of the causeway at Lady's Is land. In July 1988 the salinity in the main body of the lagoon was 36-40%o, but was 7%o at the northern end and 4%o in pools at Lady's Island. These low-salinity refuges, like Bunargate Pool, often held brackish water species at high densities

which could later spread into the main body of the lagoon when conditions became tolerable.

The true brackish species reproduced in the lagoon, and some crustaceans produced two or even three broods a year (Healy et al. 1982;

Norton and Healy 1984); thus populations could build up rapidly in successive years. Some species appeared in numbers which could be described as 'blooms', e.g. Neomysis integer, Sigara stagnalis and

chironomid larvae, when salinities and marine predator levels were low. None of the recorded species were entirely dependent on macrophytes, but some were closely associated with them for food and shelter (Lekanesphaera hookeri, Idotea che lipes) or as surfaces for attachment (Cordylophora caspia, Conopeum seurati). The sessile juveniles of Cerastoderma glaucum were abundant on Ruppia in years when the adult population was dense. Most of the marine species probably do not breed suc cessfully in the lagoon, and numbers generally declined with falling salinities in winter and during the following year. Exceptions were Praunusflexu osus, which produced broods in 1976, and possibly

Mytilus edulis and Mya arenaria, juveniles of which were recorded at times when adult-sized individu als were common in the lagoon. Berried Carcinus maenas and Crangon crangon were frequent, but no

recently settled juveniles were observed. The pop ulations of some of the marine species were re newed following each breach when individuals from the sea entered in tidal water (C. maenas, C. crangon, Platichthysflesus); others were infrequent or spasmodic in their appearance (Pomatoceros triqueter, Balanus improvisus, Idotea baltica, Liocarcinus holsatus, Clupea harengus, Pollachius virens).

COLONISATION AND RECOLONISATION

The brackish component of the fauna ap peared to be persistent in spite of wide fluctuations in population density. Only five of the 24 brackish

water species (categories L and B in Table 1) recorded in 1977-8 were not found in 1991 (Cordylophora caspia, Nais elinguis, Ischnura elegans,

Enochrus halophilus and Mesopodopsis slabberi). Of these, two are insects which could easily re colonise, M. slabberi was represented by only one specimen in 1977, and N. elinguis tends to be seasonal and could have been present outside the sampling periods.

No new species which could have originated in other brackish waters were encountered during the observation period. The main enclosed brack ish water bodies in the area which could be con sidered as potential sources of colonists are Tacumshin Lake, a percolation lagoon similar to Lady's Island Lake, situated 1.5km to the west, and the drainage channels of three artificial impound

ments, the North and South Slobs, at 5km and 17km respectively (15km and 23km by sea), and the Ballyteige drainage system, which opens into the Cull Inlet about 20km to the west (Fig. 1).

Large volumes of water are pumped out from all three impoundments into the open sea, and indi viduals of some species might be capable of colonising Lady's Island Lake in tidal water. How ever, the only brackish water species to be taken during regular sampling over two years on shores at Carnsore Point was Neomysis integer, which was sometimes common, with swarms including ovi gerous females (McGrath 1984). Each of these local lagoon-like systems harbours its own charac teristic and persistent set of species, some of which are known to have existed for at least sixteen years. Species not recorded from Lady's Island Lake (or on the Wexford coast, except for Corophium volu tator) include Leptocheirus pilosus Zaddach, Cyathura carinata (Kroyer), Littorina tenebrosa (Montagu) and Zostera marina L., all common in the North Slob; Corophium volutator (Pallas) in both Tacumshin and the North Slob; Allomelita pellucida (Sars) in

Tacumshin; and Hydrobia neglecta Muus at Bally teige (Galvin 1992; personal observations). Cy athura carinata, Corophium volutator and Zostera

marina were present on the North Slob in 1977 and 1991-2; Leptocheirus pilosus and Littorina tene brosa were found in 1991 and 1992.

43




The marine faunal component was more vari able than the brackish fauna. Some new marine species, not previously recorded, often appeared in the lagoon following breaching of the barrier. All are common on the local shores or are known to be present in neighbouring offshore waters. Plank tonic stages were sometimes frequent in water entering the tidal inlet, especially in May andJune. Some marine species were short-lived in the la goon, e.g. jellyfish, while others survived for up to three years. Many appeared spasmodically and there was obviously an element of chance in their colonisation. For example, herrings are a feature of the lagoon well known to local fishermen, but their appearance is erratic. Those caught in 1975 belonged to a single cohort (M. Crowley and M.

Wallace, unpublished report) and would have en tered as one or a few shoals of young fish. Fish caught in 1977 (Bates 1977) were probably sur vivors of the 1975 invasion. Herrings were not reported again until 1982 and are presumed to have entered as one or more shoals. The appear ance of Liocarcinus holsatus and Pollachius virens in large numbers in 1980 indicates the presence of aggregations of these species in the region of the inlet at the appropriate time.

Entry of marine species in overtopping or overwashing waves is most likely to occur after the inlet bar has formed but before it has reached its

maximum height. Storms, especially those from the south-east, which are most likely to produce overwash (Orford and Carter 1982), are more frequent in autumn and winter, but by this time planktonic stages are rare and adults have moved offshore into deeper water. Entry in seepage water is another possibility, but salinity readings indicate that only small amounts of seawater enter in this

way and that seepage is irregular and infrequent, occurring mainly in mid-late summer.

An artificial introduction of mussels occurred between 1975 and 1977. Numbers increased in subsequent years and small individuals were found, and thus they may have reproduced in the lagoon.

Many of the insect species are known to occur in neighbouring brackish or fresh waters. The following were taken from feeder streams at the northern end and/or on the western shore: Corixa panzeri, C. affinis, Sigara stagnalis, S. dorsalis and Limnephilus affinis.

Recolonisation by brackish water species fol lowing the mass mortality observed in 1985 oc curred rapidly during 1986, and by late 1987 there

were lush beds of Ruppia and thriving populations of pelagic fauna. The rapid recolonisation was most likely due to expansion of fragments of original populations surviving in refuges (see above). Some species may even retreat into fresh waters or may survive for short periods out of water. Gammarus zaddachi was frequent in a feeder stream on the

44

western shore; Lekanesphaera hookeri was common under stones well above water-level in 1990 at a time when the population density in the lagoon

was low.

USE OF THE LAGOON BY BIRDS

Lady's Island Lake is an important waterfowl site in winter. An assessment of the lagoon in

Grimmett and Jones 1989 mentions mute swans, Bewick's swans, whooper swans, wigeon, teal, shoveler, scaup, tufted duck, red-breasted mer gansers and coot as being important. The mute swans appeared to feed almost exclusively on Rup pia and Potamogeton and their numbers were low when growth of these plants was poor, for example during periods of high salinity. Species with a

more varied diet are probably also favoured by the low salinities at which small invertebrates tend to be most numerous. Red-breasted mergansers, cor

morants and herons, which feed on larger prey, were sometimes frequent. Waders feed on exposed parts of the substratum in autumn when the water level is low enough.

The two islands support Ireland's largest tern colony. In 1989, more than 1843 pairs nested here, including 1317 pairs of sandwich terns (Sterna sandvicencis Latham), 450 pairs of common terns (S. hirundo L.), a small number of Arctic terns (S. paradisaea Pontoppidam) and 76 pairs of the rare roseate terns (S. dougallii). Breeding success largely depends on the islands remaining isolated in sum

mer because low water-levels allow predators to cross the exposed mud-flats. No terns nested in 1983 when the inlet remained open throughout the summer, and numbers of roseate terns re

mained low until 1989 (Fig. 5). However, the numbers of all species were low in 1982 when the inlet closed within two weeks, and few roseate terns nested in 1987 or 1988, years when the barrier was not breached. Low water-level was thus not the only reason for poor reproduction in roseate terns.

DISCUSSION

Analysis of the long series of observations on Lady's Island Lake shows how important such studies can be for a proper understanding of the character of brackish water commnunities and demonstrates the difficulties involved in attempting to classify brackish waters when only single obser

vation sets, or even two-year studies, are available. A two-year investigation of Lady's Island Lake in 1976-7 might have led to a classification of the inhabitants as a poly-euhaline assemblage corre sponding to a Cerastoderma glaucum-Hydrobia ven trosa community (Muus 1967), while a similar study in 1978-9 would have recognised an oligo

mesohaline community with a high proportion of




Weeks 3 3 4 6 2 22 3 14 0 0 13 0

Sandwich 500o

Common 2000

Roseate 200'

Arctic 100{

1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989

Fig. 5-Fluctuations in the numbers of four species of tern nesting on Inish and Sgarbheen Islands, 1978-89, in relation

to length of tidal periods (data supplied by the Irish Wildbird Conservancy).

insects. Similarly, a plant ecologist could have recognised different sub-associations of the Ruppi etum cirrhosae association (Verhoeven 1980) de pending on the year of observation. A classification based on the means of seawater input could be equally misleading. It is now clear that a previous publication (Healy et al. 1982), which described the lagoon during a period of change from high to low salinity, did not give a complete picture of the extent of the change which the community expe riences. Not all lagoons have conditions which fluctuate as widely as those described here, but all are subject to an amount of unpredictable change, so that snapshot studies, used to compare lagoons for purposes of classification or assessment of con servation value, are liable to show up differences in salinity and living communities that are no more than transitory. A delayed response to changing salinity may explain the failure by some authors to find a relationship between salinity and features such as species-richness and presence or absence of a particular species (e.g. de Kroon et al. 1985;

Barnes 1987). An inventory of brackish water resources would need to take into account the geomorphology and hydrology of the sites, as well as recent history, if the information were to be in any way predictive and the final classification

meaningful.

A high degree of variability is one of the characteristics of lagoons. The range of variation in

salinity and biotic assemblages in Lady's Island Lake is unusually large because management attempts

produce far-reaching changes in conditions, but the time-scale of the variations is probably normal for most enclosed brackish systems, whether natu ral or managed. The dynamic pattern observed was one of continuous shifts in species dominance linked to gradual changes in water quality, punctu ated by occasional more rapid and extensive changes brought about by a combination of cli

matic events and human interference. Events capa ble of causing a major change in conditions in other brackish waters in County Wexford dunrng the period of these observations included the de struction of a section of a dune baffler owing to erosion on the north-east coast, which allowed the sea to break through; failure of the pumping sys tem at Ballyteige, which resulted in flooding of drainage canals onto surrounding land; extensive storm-induced overwash at Tacumshin and Bally teige; and artificial drainage of Tacumshin Lake (the drainage pipe has since become blocked and the lagoon has returned to its natural state) and natural closure of its outlet to the sea. Some events are thus due to management initiatives or failures,

while others are caused by severe weather or

45



BIOLOGY AN1) ENVIRONMENT

changes in sediment transport patterns. They are probably representative of the kind of coastline processes which govern the changing character and survival of lagoon-like systems everywhere.

The number of faunal species recorded in Lady's Island Lake is unusually large for a single lagoon, these systems being generally poor in spe cies. The complete list of 97 macrofaunal species recorded over seventeen years (Table 1), and a further eleven taxa recognised but not identified, includes representatives from all phases in the la goon's recent history, but the list is by no means complete because few of the many marine species

which are assumed to have entered during long tidal periods in the 1980s were identified. It must also be stressed that this has been an extensive rather than an intensive study, so that even impor tant lagoonal species could have been overlooked.

The length of the sampling period only partly explains the apparent cumulative richness of the fauna. In 1977-8 alone, 58 faunal species were recorded (Healy et al. 1982, omitting Enchytraei dae, Rotifera, Copepoda and Ostracoda) compared

with a total of 44 species and a maximum of seventeen in 26 East Anglian lagoons (Barnes 1987), a total and maximum of 47 in 166 English lagoon-like systems, excluding The Fleet (Bamber et al. 1992), and a total of 58 and a maximum of 36 in fourteen Ruppia communities in The Nether lands (Verhoeven 1980). Most sites in these studies

were sampled in two years or more, but the faunal lists may not be directly comparable because sam

pling methods and levels of identification differed. Among the 58 species recorded in Lady's Island Lake are eight belonging to groups not identified in most British studies (Chironomidae, Cole optera), but even when these are omitted the list remains comparatively long. One explanation could be that 1977-8 was a transitional period

when the salinity fell steadily and a number of insect species were present, but which followed a year of extensive tidal exchange when a large number and variety of fish entered; the year may thus have been exceptional. Unfortunately, there

were no similar studies in other years to confirm this. There is some evidence that high species-rich ness is characteristic of Irish lagoons (Galvin 1992) and also those of north-west Scotland (Smith 1987), while English lagoons generally contain an impoverished and specialised fauna.

There is little evidence in the literature that lagoons reach equilibrium richness and they ap pear, in general, to be undersaturated (Barnes 1988). Nevertheless, species-richness can, to some extent, be predicted from two of the determinants of island biogeography theory (MacArthur and

Wilson 1967), namely area and opportunity for colonisation. Lady's Island Lake is a large lagoon by comparison with those investigated in England,

46

where the majority were less than 4ha (Bamber et al. 1992). Its large size allows a range of environ

ments to be juxtaposed and enables viable popula tions of plants and animals with different requirements to occupy different parts of the la goon, e.g. at the northern and southern ends or in

muddy and stony areas; it also allows refuges to exist. Peripheral refuges appear to act as recolonisa tion patches of brackish water species, and there

may be deep-water refuge patches as well. Area was found to influence species number in most types of saline pond in the south of England (Sheader 1986), but in other studies the species area relationship did not hold (Barnes 1987; 1989a;

Bamber et al. 1992), probably because it is habitat heterogeneity rather than area per se which is the causal factor. Over 210 species have been recorded in Britain's largest lagoon, The Fleet, with an area of 450ha, a linear system open to the sea with a salinity gradient which provides a wide 'choice' of environmental conditions (Seaward 1980; Barnes 1989b).

There have been a number of attempts to correlate species-richness in lagoon-like systems

with a range of geographic, physiographic, histori cal and water-quality parameters with only partial success (Heerebout 1970; den Hartog 1974; de

Jonge 1974; Verhoeven 1980; Barnes 1987; 1988; 1989a; Bamber et al. 1992; Galvin 1992). In all these studies, some aspect of salinity was identified as the predominant factor, species-richness being correlated with salinity at time of visit, deviation from full-strength seawater, mean chlorinity and chlorinity fluctuations, and average chlorinity and fluctuations. Other parameters, such as age, size, shape, proximity to other brackish waters, sedi

ment diversity and seawater inflow type, were generally only recognised as contributing factors and their influence only appeared significant

within defined geographic areas or physiographic types. Owing to the qualitative observations it is not possible to compare faunal richness in different years in the present study, but it can be predicted that the number of species would decline during extended low-salinity phases when marine species are eliminated, while during and after long tidal periods, when marine species would be plentiful, oligo-mesohaline species could persist in peripheral refuges and the species-richness of the system as a

whole would therefore remain high. An analysis of variability in Dutch brackish waters over eight years led Heerebout (1970) to the conclusion that the number of marine species decreases and the number of brackish water species increases with decreasing median chlorinity and increased vari ability of chlorinity. This could have been pre dicted, given that high salinities usually mean comparatively easy access for marine species, most of which are less tolerant of fluctuations than brackish water species. In most comparative studies




of lagoons, it has been found that those which have the most species sustain occasional, frequent or regular influxes of seawater (e.g. Sheader 1986; Gravina et al. 1989). Barnes (1989a) found no such relationship for English lagoons, however.

Absolute measures of faunal richness are never attempted and the limitations set by different

workers vary, particularly as regards body size and level of identification. Dipteran larvae are almost never identified, despite the fact that the distribu tion of many chironomid species can be correlated

with salinity (Parma and Krebs 1977). The recog nition of ten dipteran and nine coleopteran taxa in Lady's Island Lake in 1991, including larvae, none of which could be identified to species, suggests that failure to identify these could introduce a serious source of error into species-richness esti

mates. As these groups are mainly found in low salinities, it puts in doubt the generally accepted theory of Remane (1971) that the lowest diversity in brackish water systems is to be found at between 5%o and 7%o (for application of the theory to lagoons see Sheader 1986 and Barnes 1989b). Both

Verhoeven (1980) and Ingolffson (1994), who in cluded insects in their analyses, also failed to find support for Remane's theory. Faunal distribution data for Lady's Island Lake also show that a proper assessment of species-richness requires sampling of the entire system, including areas with extreme conditions like the south shore near the inlet and the mouths of freshwater streams. It could be argued that species which only occur infrequently in these marginal habitats are accidentals and not true members of the lagoon community. On the other hand, these extreme conditions do, on occa sion, become quite widespread, and euryhaline

marine species or oligohaline species could, if the appropriate conditions persisted, also become

widespread. The long tidal periods of the miid 1980s are by no means unusual for Lady's Island Lake.

The relatively large number of species coexist ing in Lady's Island Lake comprises three, more or less distinct, components which differ in their responses to the environmental changes in the lagoon. The resident brackish populations (cate gories B and L in Table 1) undergo continual changes in relative abundance, sometimes disap pearing from areas for several years at a time but never becoming extinct in the lagoon as a whole. This conclusion is based on a comparison between collections made in 1977-8 and 1991, which were remarkably simnilar in spite of at least one mass

mortality (in 1985) during intervening years. The marine component (M1-M5), in contrast, is made up of ephemeral populations whose survival de pends on continuing high salinity. Many can, as adults, live at reduced salinity for quite long peri ods, but all eventually decline to extinction owing

to their inability to reproduce. Marine species which occur in the lagoon on a regular basis are those which are regularly present in inflowing

water and tolerant of salinity fluctuations. The freshwater component (F and F/L) contributes to

the resident lagoon community when conditions allow. The relative importance of the three com ponents varies spatially and is continually changing, the rate of change depending on the extent and frequency of tidal incursions and, to a lesser de gree, on variations in rainfall. The diversity of the

system varies over time for the same reasons.

Salinity has thus been identified as a factor closely correlated with the survival of individuals and the species composition of the living commu nity in Lady's Island Lake. Examination of both distribution data and experimental observations on salinity tolerance in the literature indicates a salin ity range in the order of 5-35%o for all common brackish water species and mlany euryhaline marine species. Most meso-polyhaline species can be found, somewhere, in salinities equivalent to sea

water, yet few occupy such a wide range of salinities within a given water body and their ranges within different systems vary (Barnes 1988).

Evidently there are other factors involved. In shal low water, temperature is likely to be important, and temperatures fluctuate more rapidly and

widely in enclosed shallow waters than in the open sea. Embryos and larvae, which generally have narrower tolerance limits than adults, are especially vulnerable to temperature extremes. The rapid replacement in 1977 of Praunusflexuosus, a com

mon species of the open coast and estuaries, by Neomysis integer, a mostly brackish species, can be explained when their temperature tolerances are compared. Adults of both species can live in salini ties from 0 to 36%o, but P. flexuosus is less tolerant

of low salinity and N. integer is less tolerant of high salinity at high temperatures (Vlasblom and Elgers huizen 1977). Different salinity tolerance ranges for development of eggs and juveniles of the species (Vlasblom and Elgershuizen 1977) could explain the rapid replacement of P. flexuosus by N. integer during a period when the species were breeding, the salinity fell below 9%o and water temperature increased fromn 6C to 17?C. The limiting effects

of temperature on survival at unfavourable salini ties have been shown for a number of other brackish water species occurring in Lady's Island Lake, including Idotea chelipes (Vlasblom et al. 1977), Limnephilhs affinis (Sutcliffe 1960), Sigara stagnalis (Savage 1979) and Cordylophora caspia (Arndt 1989). A combination of high salinity and high temperature, with the additional stress of high BOD produced by high nutrient levels, caused widespread mortality of invertebrates and vegeta tion in Lady's Island Lake on at least one occasion during this study. Ideally, salinity measurements

47



BIOLOGY AND ENVIRONMENT

should be accompanied by records of temperature and estimates of nutrients, BOD and dissolved oxygen if effects on fauna are to be assessed. The role of Ruppia and other macrophytes in absorbing nutrients and increasing dissolved oxygen in warm weather is believed to be important for the persis tence of animal populations in the lagoon. Other parameters which might limit species distributions, and which were not measured in this study, in clude pH (Bamber et al. 1992) and water move

ment. Replacement of one species by another, simi

lar one often, but not always, a sibling species is a feature of systems which experience salinity changes or in which there are salinity gradients (Barnes 1980). Examples in Lady's Island Lake are Potamogeton pectinatus and Ruppia cirrhosa, Neomysis integer and Praunus flexuosus, Idotea chelipes and L baltica, and Potamopyrgus antipodarum and Hydrobia ventrosa. It is assumed that competitive interactions are involved in the replacement process. In the case of Ruppia and Potamogeton, differences in hi bernating organs and growth patterns result in P. pectinatus outcompeting R. cirrhosa at low salinities, while R. cirrhosa ousts P. pectinatus above 1 6%o. Between 7%o and 16%o, the two species can coex ist (Verhoeven 1979). It is much more difficult to demonstrate competition between faunal species, however. The replacement of P. flexuosus by N. integer, for example, can be explained by different responses of the species to salinity and temperature,

without invoking direct competition. Species pairs of mysids, hydrobiids and idoteids have all been shown to coexist in Lady's Island Lake, although it is not known for how long. Recorded differences in the proportions of mysid and hydrobiid species along the north-south gradient corresponded to environmental gradients, of which salinity was the

most obvious. Records of coexistence in this study seemed to coincide with low faunal abundance at the sites sampled. None of the 26 East Anglian lagoons analysed by Barnes (1987) contained more than one mysid species, and only two contained

more than one hydrobiid. Coexistence of these species may not be common in lagoons, therefore, and its occurrence in Lady's Island Lake may mark transitional phases in a widely fluctuating system. Some hydrobiids are commonly sympatric (Muus 1967; Fenchel 1975; Cherrill and James 1985) but

H. ventrosa and P. antipodarum are rarely so.

Differences in the persistence of species appear to be related to both their salinity tolerance ranges and recruitment rates. Species tolerating low salin ity (e.g. Neomysis integer, Potamopyrgus antipodarum, Sigara spp, Palaemonetes varians) were generally

more persistent and recolonised more rapidly fol lowing population crashes than those associated

with high salinity. The low-salinity refuges and the decline at low salinity of predatory shrimps, crabs

48

and fish, which can severely deplete the popula tions of smaller species (Barnes 1994), would be important. In contrast, species favouring high salin ities, such as Cerastoderma glaucum, Pomatoschistus

microps and Hydrobia ventrosa, must survive wider fluctuations in salinity during meso-polyhaline phases, and populations may take several years to recover from high mortality, although they can eventually reach great abundance. Differential fe cundity may further enhance the differences be tween high- and low-salinity phases or between regions of the lagoon. Many of the low-salinity species have high reproductive rates, producing large broods or several generations in a season, sometimes resulting in blooms (e.g. Neomysis in teger, Sigara stagnalis, chironomids). Of the three species pairs mentioned above, Potamogeton pectina tus has a longer growing season and more growth

options than Ruppia cirrhosa (Verhoeven 1979), Potamopyrgus antipodarum can produce nearly five times as many young per year as Hydrobia ventrosa (Lassen and Clark 1979) but has a lower reproduc tive output at high salinities (Siegismund and

Hylleberg 1987), and Neomysis integer starts to breed earlier in the year than Praunusfiexuosus and may produce three broods in a season (personal observation).

Information on colonisation processes and fre quency of extinction of populations is of crucial importance for an understanding of the origins and persistence of communities in enclosed brackish waters; unfortunately it is rarely available. In Lady's Island Lake, opportunities for colonisation are dif ferent for the three components of the system (see earlier). The immigration of brackish water species from outside the system could not be detected during this study, although studies of enclosed brackish waters elsewhere have shown that coloni sation can be surprisingly rapid (de Kroon and de Jonge 1984). Faunal differences between Wexford lagoons, some of which are known to be long standing, suggest either that colonisation rates are generally slow or that potential colonists do not succeed in becoming established because condi tions are unsuitable. Similar persistent differences

between neighbouring lagoons have been reported in Britain (Barnes and Heath 1980; Barnes 1987).

For Lady's Island Lake, the temporary inlet is an obvious route of entry which allows frequent colonisation by marine organisms, often in substan tial numbers. However, the erratic appearance of

many marine species shows that colonisation is, to some extent, a random event, depending not only on an appropriate means of dispersal and the exis tence of a pathway for entry into the lagoon but also on the presence of a species, tolerant of lagoon conditions, in the area at the moment when the pathway is available. The probability of colonisa tion by this route could be expected to vary




between species. Colonisation of land-locked brackish waters, without inlets, is poorly docu

mented. Direct transfer of live animals from the sea into isolated water bodies has not been properly investigated and is often not seriously considered, perhaps because the barrier systems, both natural and artificial, appear to be practically insuperable. This is by no means certain, however, especially for gravel barriers, which may allow greater vol umes of water to percolate than those composed of sand. Little et al. (1989), for example, reported the presence of several species of marine molluscs in the landward face of the shingle barrier of The

Fleet, Dorset. Landward seepage streams issuing as springs from sand or shingle barriers have been reported from localities in Britain (Hunt 1971; Little et al. 1989) and Ireland (Galvin 1992), but apart from some mollusc species in the barrier of The Fleet there are no records of species taken directly from them. Circumstantial evidence for colonisation through a south Wexford barrier is provided by the presence of Amphipholis squamata Delle Chiaje and other marine species in a localised section of a drainage channel fed directly by a seepage stream from Ballyteige Burrow (Galvin 1992). No such streams were observed on the shore of Lady's Island Lake, although they may form temporarily following spring tides or storms

when the water-level in the lagoon is low. Barnes (1991) believed movement of floating green weed

with its associated fauna to be a likely means of dispersal of lagoon fauna.

All of the insect species recorded are capable of flight and would thus be able to colonise the lagoon whenever conditions were appropriate. Some corixids, for example, can migrate for dis tances of up to 80km without wind assistance (Popham 1964). There has been much speculation about the means and opportunities for aerial colonisation of isolated lagoons by non-flying in

sects and other small invertebrates, but very little real evidence. Frequently mentioned mechanisms for the transfer of animals from distant sources include transport on the feet or beaks of birds, transport of small individuals and eggs by the larger aquatic insects such as beetles and corixids, and aerial dispersal by violent winds-not proven, but a likely possibility for aquatic plant seeds and small

molluscs (Rees 1965; Boyden and Russell 1972; Verhoeven 1979). However, common brackish water species have neither resistant phases nor effective dispersal mechanisms. Large numbers of

wintering waterfowl visit all the major brackish waters in south-east Wexford and are known to move from one to another, yet differences be tween the invertebrate faunas of these systems appear to be persistent.

Comparative studies of lagoons and other en closed brackish waters have led to a general con

clusion that comnmiunities are structured principally by the salinity regime but that differences between them owe as much to chance dispersal and extinc tion as to any unique ecological features. A view of lagoons as systems in which the very fast rate of change leads to low persistence of species (Barnes 1991) is not supported by this study, however.

There is certainly a high turnover of marine species in Lady's Island Lake, but their presence is depen dent on an artificially created inlet. The resident species showed themselves to be persistent in spite of population crashes. The rapidity of recolonisa tion following mass mortality and the similarity of the faunal lists for 1977-8 and 1991 are taken as evidence that the source of recolonists during re covery phases lies within the system and that resident populations of brackish water species are not subject to extinction in the true sense. It is suggested that the existence of peripheral refuges provides the opportunity for brackish water species to survive unfavourable periods lasting certainly for a few months and possibly a few years. The pres ence of such fragments of original populations

might easily escape notice during surveys, espe cially in large water bodies, and the apparent disappearance of formerly comnmon species would therefore be interpreted as extinction. Survival in refuges does not preclude the possibility that immi gration from other brackish waters also occurs. The data available do not allow a distinction to be made. The slow reappearance of hydrobiid snails following widespread mortality in the mid-1980s, by comparison with other invertebrates, could have been due to any or several of the following factors: poor survival in refuges, narrower limits of tolerance during the recovery phase, slower migra tion rates within the lagoon, lower reproductive rates, or poor dispersal from other brackish waters. If the Lady's Island Lake community, and others in the vicinity, are indeed persistent, then they can be viewed as relicts of assemblages which had their origins in the past. The special characteristics of two Norfolk lagoons led Hunt (1971) and

Williams (1972) to a similar conclusion.

The lagoon community as a whole, under its present management regime, might be viewed as a system in perpetual disequilibrium (Whittaker 1995), structured by two levels of disturbance: low-level disturbance caused by small seawater influxes and climatic variations which engender gradual changes that are to some extent pre dictable, and high-level disturbance resulting from tidal exchange, the effects of which are, at present, unpredictable, and can be catastrophic. If, how ever, the three main components of the system brackish, marine and freshwater-are considered separately, then a different picture emerges. The resident brackish water component appears, actu

ally, to conform to an equilibrium model, since it

49



BIOLOGY AN1) ENVIRONMENT

quickly returns to its original state following severe disturbance and there is no evidence of species turnover in the true sense. The marine compo nent, in contrast, is a direct consequence of distur bance and is almost entirely dependent on the changed conditions. As these do not persist, marine colonisers are quickly extinguished, not so much by their failure to compete with resident popula tions but more by their inability to adapt to falling salinity. Members of the freshwater component penetrate into the lagoon to varying degrees, ad vancing and retreating along environmental gradi ents as conditions alter. Severe disturbance merely causes populations to contract into freshwater habitats whence they can recolonise freely. If breaching of the dune barrier were to be halted, as favoured by most conservationists, the catastrophic effects would be eliminated. The probable out come would be extinction of the marine faunal component within a few years, with a correspond ing reduction in faunal richness and greater stability of the system as a whole. The lagoon would then evolve naturally into a predominantly freshwater system, with saline conditions, produced by sea

water percolating through the barrier, confined to the southern end. In the absence of significant fluctuations in water-level and salinity, fringing beds of sedges and reeds could be expected to develop, and the lagoon would evolve by natural succession in the manner of a freshwater lake.

ACKNOWLEDGEMENTS

I am grateful to local observers and fishermen, in particular Seian Redmond and the Scallan broth ers. Thanks are also due to the many students who helped with the collection and analysis of samples, notably Dick Bates, Lorraine Feegan and Paul Fox. Jim Hurley gave useful advice and provided some water-level measurements, Mary O'Connor iden tified collections from fresh waters, and Garth Foster identified a collection of beetles. My special thanks go to David McGrath for permission to use his unpublished data on mysids and for his helpful criticisms of the paper. Early studies were part funded by the Electricity Supply Board.

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long-term changes in a brackish lagoon, lady's island lake, south-east ireland

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