DISTRIBUTION AND ANNUAL VARIATIONS OF

LOXOCONCHA ELLIPTICA IN THE GERNIKA

ESTUARY (BAY OF BISCAY)

ANA PASCUAL Unk.,ersidad dd Pals Vasco, Opt~ K~raligrafla, aaodin~mic.a y Paleontologta, Aptdo.644, E-48080 Bilbao.

PIERRE CARBONEL Universttd Bordaam I, Ddpartement de Gdologie at Ocdanographie, Avenua des FaculMs, F-33405 Talence Cdd~.

ABSTRACt

The high number of individuals of the ostracode Loxoconcha elliptica appear in the Gernika estuary (Bay of Biscay), has made it possible to carry out a study on the said species. In accordance with its annual distribution, it has been possible to differentiate two zones in the estuary : That of the head with abundant especimens, and the mouth which is lacking in them. Also, a greater number of individuals were observed on the right bank than on the left one. Therefore, the presence of this ostracode indicates the separation of the estuarine and marine environments. When analyzing the relation existing between the aforesaid organism and the physico-chemical factors of the environment, as well as its life cycle, it can be proved that Loxoconcha elliptica is able to supply information about the prevailing environmenlIal conditions and characteristics (substratum, currents, oxygen etc) and contribute information to paleoecological studies (changes of the water masses through time).

DISTRIBUTION ET VARIATIONS ANNUELLES DE LOXOCONCHA ELLIPTICA DANS LA Ilia DE GERNIKA (GOLFE DE GASCOGNE)

RI~UM~

Le grand nombre d'individns de l'ostracode Loxoconcha elliptica vivant dans l'estuaire de Gernika (Golfe de Biscaye) a permis de r6aliser une 6tude de cette espEce. En accord avec sa distribution annuelle, nous avons pu diff6rencier deux zones dams cet estuaire : ceIIe de la partie interne avec beaucoup d'individus et ceIle de l'embouchure) tr~s pauvre. Norm avons observ6 un plus grand nombre d'individus sur la rive droite que sur la gauche. La pr6sence de cet ostracode marque la s6paration entre les domaines estuarien et marin. L'analyse des relations entre les facteurs physico-chimiques du milieu et ces organimes ainsi que leur cycle de vie montre que Loxoconcha elliptica peut fournir des informations pr6cises sur les conditions du milieu (substrat, courants, oxyg6ne dissous, etc.). Ces informa- tions peuvent 6tre utilis6es ~t la caract6risation de milieux anciens (variations dans le temps de la circulation des eaux).

D1STRIBUCION Y VARIACIONES ANUALES DE LOXOCONCHA ELLIPTICA EN EL ESTUARIO DE GERNIKA (GOLFO DE VIZCAYA).

RESUMEN

E1 elevado nftmero de individuos del ostrficodo Loxoconcha elliptica presente en el estuario de Gernika (Golfo de Vizcaya), ha permitido la realizaci6n de un estudio sobre dicha especie. De acuerdo a su distribuci6n anual, se han podido difereneiar dos zonas en dicho estuario : la de cabecera con abundantes ejemplares, y la desembocadura carente de eUos. Se observa adem~, un mayor nftmero de individuos en la margen derecha queen la izquierda. La presencia de este ostr~icodo marca por tanto, la separaci6n de los medios estuarinos y marinos. A1 analizar la relaci6n existente entre dicho organismo y los factores fisico-q-~micos del medio, asf como su cido de vida, se comprueba que Loxoconcha elliptica es capaz de sumini~trar informaci6n sobre las condiciones y caracterlsficas ambientales (substrato, corrientes, oxigeno, etc) reinantes, pudiendo asf mismo aportar informaci6n en estudios paleoecol6gicos, como en el caso de variaciones en el tiempo de la circulaci6n de masas de agua.

KEY-WORDS : OSrRACODS POPULATION, ESI'UARINE ENVIRONMENT, HOLOCENE, BAY OF BISCAY, SPAIN.

MOTS-CLt~S : OSTRACODES POPULATION, ENVIRONNEMENT ES'TUARIEN, HOLOCI~NE, GOLF~ DE GASCOGNE, ESPAGNE.

PALABRAS ,CLAVE : OSI'RACODOS, DINAMICA POBLACIONAL, MEDIO AMBIENTE ESTUARINO, HOLOCENO, GOLFO DE VIZCAYA, ESPA/qA.

Manuscrit ddpos6 le 17.06.1991 Manuscrit accept6 d6finitivement le 10.09.1991

Geobios, 1992~ 25, fasc. 4 |

• 495-503 t)

496

I N T R O D U C T I O N

In the course of a general study on benthic foraminifera and ostracodes in the Gemika estuary, the presence of a large number of individuals of the species Loxoconcha elliptica with an unequal distribution was detected. This led us to carry out a more specialized work in order to know the population dynamics of that species along the estuary, the objective being to use this information in paleoenvironment studies.

This work consists of 3 parts : In the first the studied microfauna is briefly cited. Then, the distribution of Loxoconcha elliptica is shown and, finally, an ecological significance for the obtained data is proposed.

The Gemika estuary is situated in the Bay of Biscay, Lat. 43 ° 20', Long 3°W, is 12 km long with a maximum width of 1 km and an average depth of 3.5 m at high tide.

Two kinds of substrata can be distinguished : the head formed by marshland colonized by Phragmites, Suaeda, Spartina, Salicornia, etc, with fine silt-clay sediment, and the sandy mouth which constitutes the banks in the middle of the estuary and the peripheral beaches.

This estuary has been the subject of several works about its hydrography (Gobiemo Vasco 1986), plankton (Orive, Santiago & Villate 1984, and Orive 1988) and benthic microfauna (Pascual 1990).

In accordance with its physico-chemical characteristics, the estuary is divided into two zones: The first being the head with important fluctuations of salinity (1-32 %o), large amounts of nutrients and organic material, and the other b6hag the mouth where the salinity becomes stable (32-34 %o) and the nutrients decrease.

The pH presents basic values, while the averse concentration of dissolved oxygen is situated at 61cm/1. The temperature varies throughout the year betw6en 11 and 23 °C, and the alkMinity between 0'13 and 0'28 gr/1.

Samples were taken along the Gernika estuary during autumn 1985, and winter, spring, summer 1986. The sediment was collected by means of Boltovskoy's scraper and was later placed in a solution of methanol and Rose Bengal.

13 samples were taken from the head of the estuary (Murueta) to the mouth (Fig. 1) in each season. Furthermore, during the summer and in order to iden- tify the upstream zone the species could reach, 8 more samples were taken including the channelled section of Gernika (G15 and G16) and the abandoned meanders of Arteaga (G14). Another was obtained in the distal part of the estuary (G71) next to the island of Izaro.

MUNDAKA

BAY OF BISCAY

o

G "! 1

PEDERNAL:

A3CPE DE

BUSTURIA

BILBAO GERNIKA

Figure i - Location of the sampling in the Gernika estuary, with. the different subst ra tum types. Substrat et localisation des dchantglams dam l'estuaire de Garnika.

An approximate total of 500 specimens was studied. The male, female and juvenile forms were separated in order to establish the percentages existing between them in each season, and to be able to identify the population dynamics.

BENTHIC MICROFAUNA

26 species of ostracodes were identified, of which 7 did not present living individuals. Of the 60 benthic foraminifera found, 11 had exclusively dead specimens. The biggest part of this microfauna is typical of lukewarm and warm waters, as the estuary is situated between the East- Atlantic and Lusitanic regions.

497

The head of the estuary presents an asemblage formed by the species : Loxoconcha elliptica and Leptocythere castanea, in the case of the ostracodes, and Ammonia beccani tepida, Elphidiurn articulatum and Haynesina depressula in the case of the foraminifera.

In the mouth Aurila convexa and Urocythereis oblonga (phytal-coasl~al ostracodes) and Obicides lobatulus, Quinqueloculina seminulum and Elphidium crispum (foraminifera).

L O X O C O N C H A E L L I P T I C A (PI. 1, PI. 2)

Brady 1868, define it as a salt (euryhaline) species of the river mouth, and sometimes marine riving in bays below the influencx: of fresh water coming from the rivers.

Elofson 1941 and Wagner 1957 describe it as of salt waters, mainly among plants and at a slight depth. Yas~ini 1969 defines it as shallow (0-4 m) and adapted to salinity fi'om 6-20 %0. Carbonel 1980 cites the said species as having a phytal euryhaline character. It is found especially in mesohaline waters, and more rarely in ollgobali~es (case have been found of forms living in salinity of 0.5 %0 and in sabkhes with values greater than 40 %0).

Its distribution, therefore, is very comparable to that of Cyprideis, and it is able to live in waters of 0 to 60 %o and euryhalines (Carbonel & Pinson 1979).

With regard to its geographical distribution, Brady et aL 1874 cite Loxococha elliptica on the coasts of England. Elofson 194.1 indicates this species in the North Sea, Baltic sea and Mediterranean, Kruit 1955 finds indi- viduals in the Rhone delta. Yassinl 1969 describes specimens in the Arcachon basin and the Mediteranean littoral. Carbonel 1980 cites the Gironde estuary, Arcachon and the Bay of Bidasoa.

Note also that this species appears in Africa, in the Tunis lake (Carbonel & Pujos 1982), in Senegal (Carbo- nel & Pinson 1979) and in paleocene paleolakes in the north of the Sahara (Fontes et al. 1985, Gasse et al. 1987).

In accordance with the distribution of Loxoconcha elliptica in the Gernika estuary two bands can be differentiated : Zone 1 extends from the head to the narrowing of the estuary off Axpe de Busturia (G3) and Canala (Gll), as well as the zone parallel to the cliff on the right bank up to the A.rketas landing stage (G10). It is characterized by the appearance of abundant specimens of Loxocon- cha elliptica. Zone 2 corresponds to the mouth where individuals of this species have not been found except in the G-6 sample du~lg the winter (Fig. 2).

\

G6 )GI0 ARKETAS

G 3

AXPE DE BUSTURIA

CANALA

0 3 0 0

Zone i.- L.elliptica t-,-------- L.castanea

Zone 2.- A.convexa, U.oblonga

L.elliptica,L.castanea, and A.convexa, U.oblonga, C.elongata

Figure 2 - Zones in the Gernika estuary according with the different ostracodes assemblages. Zones de l'estuaire de Gernika en foncaon des associations d" ostracodes.

Likewise the scarce presence of this species can be observed on the left bank, although on the right bank there is a high concentration of individuals (Fig. 3).

As for the number of individuals, a maximum number in summer and a minimum in autumn is observed. In winter this number increases but decreases again in the spring (Fig. 4).

In autumn there is only one sample (G10) with living representatives and it registers a large abundance of

<

498

AUTUM WINTER SPRING SUMMER

Figure 3 - The number of living individuals of Loxoconcba ellip- tica in each example. Nombra d'tndtvidus vtvants de Loxoconcha elltptica darts chaque dchantalon.

juvenile .forms. In winter the number of living ones increases with a balance between males and females along all the right bank of the estuary. In spring the number of individuals descends. In summer a great increase in the number of specimens is observed with a boom of juvenile forms.

Upstream of the head there exists a perfect balance between n~ales, females and juveniles. On continuing downstream, the females are found on the left bank and the males on the right

Note also the perfect gradation which exists along the estuary with regards to the number of individuals. At the head, where the salinity is less, the greater number of forms of Loxoconcha elliptica are registered, while as are advances towards the mouth the number decreases, and these forms disappears completelly in zone 2 (Fig. 4).

The Arketas sample (G10) situated geographically in Zone 2 is an exception to this general scheme since a

high number of living specimens appears, dominated by juvenile forms in all the sampling.

E C O L O G I C A L S I G N I F I C A N C E : ITS I N T E R P R E T A T I O N

In the previous section we have shown that Loxoconcha elliptica is located, in the Gernika estuary, exclusively in Zone 1 or the head, with is logical if one bears in mlnd that we are dealing with an phytal - euryhallne ostracode and that the aforesaid zone is situated in the marshes of the said estuary.

The euryhalinity of the species has been proved by annlizfng the results obtained in the summer sampling, where a great number of specimens are observed at the head of the estuary where the salinity deacreases to 1%o.

Previously we have shown the great difference that exists on both banks of the estuary with regard to the

PLATE 1

Fig. 1,2,3,4,8 - Loxoconcha elliptica BRADY, 1868. 1 - ~ Left valve (G-12P). Valve gauche. 2 - ~ Left valve (G-13P). Valve gauche. 3 - ~ Carapace in dorsal view (G-13-P). Carapace : Vue dorsale. 4 - (~ Hinge and muscle scars (G 12-P). Valve gauche : Vue interne, charnidre et empreintes musculaires. 8 - Q Internal view of a left valve (G-12-P). Valve gauche : Vue interne.

Fig. 5,6,7,9,10 - Loxoconcha elliptica BRADY, 1868. 5 - 9 Carapace in dorsal view (G12-P). Carapace : rue dorsale. 6 - Hinge (G12-P). Charni~re. 7 - Larval stage (G10-P). Forrne juvdnile. 9 - ~ Internal view of a right valve (G12-P). Valve droite : rue interne. 10 - ~ Right valve (G12-P). Valve droite.

Scale = 100/am.

Geobios n ° 25, fasc . 4,

PI. 1 A. Pascual & P. Carbonel

i

AUTUM,

!

500

\

C

WINTER

L

Figure 4 - Relation between juvenile and adult (males, females) forms of Loxoconcha elltptica in the Gernika estuary. Adultes (ma- les, femelles) et imma- tures de Lexeeotwha eHiptl4a darts l'estuai- ra de Gamika pandant l'annda.

~d

~? ~J SPRING

tH

L

50]

I00

75

50

25

A

%°"'.° ." -~''"'°"

W Sp Sum A

1 0 0

60

B 20

\ t ~

% / % %,.. J %%

W Sp Sum A

d m m

. . . . . . . . . j

Figure 5 - The life cycle of Loxoconcha elliptica. A = In the Gemika estuary. B = In the Eyre delta (Carbonel 1980). Cycle de Loxoconcba elliptica. A = Darts l'estuaire de Gernika. B = Dans le delta de l'gyre (Carbone11980).

SAMPLES

GI-V G2-V G3-V G4-V G5-V G6-V G7-V G8-V G9-V G10-V G11-V G12-V G13-V G14-V G15-V G16-V G17-V G18-V G19-V G20-V G21-V

SAND SAND SAND SAND S I L T COARSE MEDIUM FINE VERY FINE CLAY (>1 ram) (1 - o ' 2 5 ) (o'25-o'125) (o't 25-0'006) <2mm

0'1 2'6 2'5 2'6 92'2 2'0 3'2 6'6 12'0 76'2

22'3 58'7 1 '2 1 '0 16'8 7'3 87'2 5'3 0'2 ---

42'0 36'0 2'8 1'8 17'4 4'2 84'8 10'4 0'6 ---

15'5 79'0 5'5 . . . . . .

26'0 71'7 2'3 . . . . . . 5'3 87'6 6'5 0'6 --- 3'8 31'0 10'5 7'7 47'0 0'9 40'0 6'6 6'5 46'0 1'2 51'5 14'0 4'6 28'7 3'0 46'0 8'2 4'9 37'9

15'2 70'0 2'0 1 '6 11 '2 86'0 7'9 0'6 0'6 4'9

4'4 46'0 17'8 6'0 25'8 40'8 39'5 3'2 1'2 15'3

3'7 3'2 83'2 9'6 0'3 9'6 45'0 9'0 8'0 28'4

14'4 81'2 4'4 . . . . . . 11 '7 85'6 2'7 . . . . . .

Tableau 1 - Granulometr ies (%) of sediment in the Gernika estuary. Granulom~tries (%) du sddiment dam l'es~aim de Gernika.

disposition of this species. So, on the left bank the number of individuals is less than on the right, and even absent in samples such as G1 which is, in general, very rich in foraminifera and ostracodes (Pascual 1990).

In order to explain this disposition of Loxoconcha elliptica in the Gernika estuary we have to know the vital needs of an ostracode. As described by Carbonel 1980, the individuals require to breathe, feed and protect themselves, in short: Nutrients to live and make their shell (nitrates, phosphates, silicates, etc), oxygen and shelter. These parameters are bound to four factors : contributions by the currents and their intensity, type of substratum, cheminal changes in the water-sediment inter phase and the life cycle of the Ostracode.

As for the water dynamics or, what is the same, the inteusit3f of the currents and their contribution, we can observe that they vary according to the position, be it in the channel or on the edges more or less distant from the same. Also, the speed of the water on the bottom undergoes certain variations when it crosses zones of vegetation, as is the case of the marshes.

In the Gernika estuary we observe that the G1 sample without any riving specimens remains in the backwater of the channel, while the rest of the samples which do present this Ostracode are situated in zones subject to the influence of the same. That is to say, the current exerts as a limiting factor but only up to a point. It is beneficial for the nutritional contributions it cames, but it can be funest as an originator of the sweeping of the substratum.

As regardes the substratum, we have observed that zone 1, where the species Loxoconcha elliptica is found, has a sediment of fine grain silt-clay with the exception of the channel which presents sand with a medium-size grain. The latter constitutes an unstable deposit which is not very favourable for the colonization of ostracodes and, in fact, individuals have not been found. The stron currents which are registered on the bed makes this sediment lack nutrients.

Outside the channel, on the edges, the granulometries offered as a result a fine substratum, although with slight variations on either side of the estuary (Table 1). So, on the left bank and, more concretely, off the Astilleros de Murueta, the silt-clay part reached more than 90 % of the sample. On the right, on the contrary, and although the free component was dominant, the sand reached up to 40 % of the total of the sediment. If the disposition of Loxoconcha elliptica is observed one can prove that the representation of the said species is poorer on the left bank that on the right.

Loxoconcha elliptica is a periphytal species (Carbonel 1980), that is to say, its way of life and its nutrition take

502

place in the water-sediment interphase, and it is in this superficial film where the cycles of S, N, P, C, are carried out with the help of certain bacteriums. That is why it is necessary to have a detrital sandy part in the sediment since the silt and clay can be too compact and restrain the ascent of molecules like S (Gouleau 1975) which are essential in the life of the aforesaid ostracode.

We believe, therefore, that the fact that a greater num- ber of examples of Loxococha elliptica are found on the right bank of the Gernika estuary is due to its having a mixed silt-clay sediment with a layer of sand. The oppo- site happens near the Astillero de Murueta (G1) on the left side of the estuary where this layer does not exist.

With regard to the life cycle of the Loxoconcha elliptica, Carbonel 1980 studied this species in the E~e delta (Fig. 5) and observed that adults and juveniles were constantly present, with two maximums of the latter in April and October. The sex ratio was balanced in October and, to a lesser degree, in April, registering therefore two generations in the year.

Following the life cycle of Loxoconcha elliptica in the Gernika estuary has contributed the following data (Fig. 5) : Adults and juveniles are present throughout the year, although the males disappear in autmnn. The proportion of juveniles is maximum during antnmn. The sex ratio is balanced in winter and at the end of spring and, the same as in the Eyre estuary, two generations per year were observed.

C O N C L U S I O N S

The presence of living specimens of the ostracode Loxoconcha elliptica in the Gemika estuary have led us to differentiate two zones. The head or zone 1 with the presence of abundant forms and with two defined subzones : the left bank with a smaller number of speci- mens and the right bank with a greater representation of forms. Zone 2 the mouth where individuals are not found.

The study of the water dynamics and ~ e nature of the substratum, provide data about this zonation. A certain current providing oxygen and nutrients is necessary to find this ostracode. The nutrition of Loxoconcha elliptica takes place in the water-sediment interphase, and appears to require a fine clay sediment accompanied by a sandy layer capable of allowing the exchange of molecules such as P or S which are necessary for the life of the ostracode. This fact seems to be the cause of the different disposition of Loxoconcha elliptica on both sides of the channel. The left bank has a silt-clay sediment which is too compact for the above-mentioned nutritional exchange. The fight bank, which has a Silt-clay substratum with sand, is suitable for the life of this species.

503

The different disposition of Loxoconcha elliptica on both sides of the estuary seems to indicate , moreover, the existance of a channel of flow (sea water) and a channel of ebb (continental water).

On studying the population evolution it is proved that juvenile and adult individuals exist throughout the year. The sex-ratio is maximum in winter and at the end of spring, with the percentage of juveniles being superior in autumn, and there are two reproductive phases during the year.

We have observed, in the light of this work, that Loxoconcha elliptica is capable of providing answers to the variations in the environment thanks to the relation that exists between its individuals and the sediment and hydrology. Therefore, it could be used in recons- tructions of old sedimentary environments contributing data on the water masses, type of circulation and even coastal evolution.

R I ~ F I ~ R E N C E S B I B L I O G R A P H I Q U E S

BRADY G.S. 1868 - A monograph of the recent british Ostracoda. Trans. Linn. Soc. London, 26, (2) : 353-495.

BRADY G.S., CROSSKEY H.W. & ROBERTSON D. 1874 - A monograph of the Post-Tertiary Entomastraca of Scotland, including species from England and Ireland. Pal. Soc. London : 122-232.

CARBONEL P. & PINSON J. 1979 - Les Cyprideis t6moins de l'6voludon des sels en milieu laguno - lacustre sons climat semi-aride. In KRSTIC N. (ed) : Taxo- nomy, biostratigraphy and Distribution of Ostracodes. Serbsko Geolochko Dmchtvo, Beograd, 211-217.

CARBONEL P. 1980.- Les Ostracodes et leur int6r& dans la d6finifion des 6cosyst6mes estuariens et de la plateforme continentale. Essais d'applications ~t des domaines anciens. MOrn. Inst. Gdol. Bassin d'Aqui- taine, 11 : 350 pp.

CARBONEL P. & PUJOS M. 1982 - Les variations architecturales des microfaunes du lac de Tunis : Relations avec l'environnement. Burdeos. Occeano- logica Acta, n ° sp : 79 -85.

ELOFSON O. 1941 - Zur kenntnis der marinen Ostraco- den Schwedens mit besonderer Beriicksichtigung des Skageraks. Zool. Bidrag. Fran. Uppsala : 216-534.

FONTES J-Ch., GASSE F., CALLOT Y., PLAZlAT J.C, CARBONEL P., DUPEUBLE P.A. & KACZMARSKA I. 1985 - Freshwater to marine Like environments from holocene lakes in northern Sahara. Nature, 317 : 608-610.

GASSE F., FONTES J-Ch., PLAZIAT J.C., CARBONEL P., KACZMARSKA I. De DECCKER P., SOULIE-MARSCHE I, CALLET Y. & DEPEUBLE P.A., 1987 - Biological remains, geochemisty and stable isotopes for the reconstruction of environmental and hydrological- changes in the holocene Lakes from north Sahara. Paleogeogr., PaleoclirnatoL, Palaeoecol., 60, (1/2) : 1-46.

GOBIERNO VASCO (DEPARTAMENTO DE AGRICULTURA Y PESCA) 1986 - Estudio oceanogr~tfico de la Ria de Mundaca. Servicio Central de Publicaciones del Gobierno Vasco. Colecci6n Itsaso, 3 : 205 p.

GOULEAU D. 1975 - Les premiers stades de la s6dimen- tation sur les vasi&es littorales atlantiques. R61e de l'6mersion. ThOse Sc. Nat. Nantes, CNRS AO 10992, T. I : 241p., T. II : 123p.

KRUIT C. 1955 - Sediments of the Rhone delta. Grain size and microfaune. Mouton and Cie. La Haye : 141 p.

ORrVE E., SANTIAGO J. & VILLATE F. 1984 - Variabi- lidad de algunos par,ttmetros fisicos y biol6gicos de la Ria de Mundaka. Cuadernos de Area de Ciencias Naturales, Serninado de Estudios Gallegos, 1 : 129- 139.

ONVE E. 1988 - Cambios espaciales del Fitoplancton y variables relacionadas en el estuario de Gernika. Kobie (Serie Ciencias Naturales), 17 : 93-101.

PASCUAL A. 1990 - Utilizaci6n de los Foramlnfferos bent6nicos y Ostr~codos para un mejor conocimiento del medio ambiente en los estuarios vizcainos : Aplicacion a l a s rfas de Gemika y de Bilbao. Universidad del Pals Vasco /E.H.U. Tesis Doctoral. Inddita : 345 p.

WAGNER C.W. 1957 - Sur les Ostracodes du Ouater- naire r6cent des Pays - Bas et leur utilisation dans l 'ttude gtologique des d6p&s holoctnes. S. Graven- hage Facultd des Sciences de l'Universitd de Paris. Mouton. Serie A n ° 707, n ° d'ordre 7, 33 : 158 p.

YASSlNI I. 1969 - Ecologie des Associations d'Ostra- codes du Bassin d'Arcachon et du littoral Atlantique. Aplication gt l'interprttation de quelques populations du Tertiaire aquitain. Bull. Inst. GOol. Bassin Aquitain, 7 : 288 p.