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Intertidal zone of DelawareInlet, Nelson, New ZealandD. J. Stanton a , B. B. Bohlool a b & CillaBeasley aa Cawthron Institute , P.O. Box 175, Nelson,New Zealandb Department of Microbiology , University ofHawaii , Honolulu, U.S.A.Published online: 30 Mar 2010.

To cite this article: D. J. Stanton , B. B. Bohlool & Cilla Beasley (1977)Intertidal zone of Delaware Inlet, Nelson, New Zealand, New ZealandJournal of Marine and Freshwater Research, 11:3, 577-587, DOI:10.1080/00288330.1977.9515696

To link to this article: http://dx.doi.org/10.1080/00288330.1977.9515696

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N.Z. Journal of Marine and Freshwater Research 11 (3) : 577—87

INTERTIDAL ZONE OF DELAWARE INLET,NELSON, NEW ZEALAND

D. J. STANTON, B. B. BOHLOOL* and CILLA BEASLEY

Cawthron Institute, P.O. Box 175, Nelson, New Zealand

ABSTRACT

A study was made of Delaware Inlet (41° 10'S, 173° 26' E), Nelson, NewZealand, during February—April 1976. The catchment contains sparse animaland human populations, and supplies unpolluted influent waters.

Over 90% of the inlet was intertidal, with surfaces of predominantly sandinterspersed with mud, gravel, cobbles, and shell. Less than 10% of the sedimentswere colonised by macroscopic vegetation, principally Juncus spp. with Salicorniaaustralis, Zostera muelleri, Viva lactuca and Enteromorpha spp. Two microscopicorganisms (Euglena obtusa and Oscillatoria ornata) were studied. Dense aggrega-tions of molluscs, particularly Amphibola crenata (mud snail) and Chionestutchburyi (cockle) were present in specific areas.

Salinity of the water fluctuated widely from <4‰ at the river mouth to35.0‰ in the main channel at high tide. Nitrogen levels (N02-N, NO3-N, NH4-N,Kjeldahl-N) were determined on influent and waters of the inlet. For the mainchannel, levels of NO3-N, NH4-N and Kjeldahl-N tended to be substantiallyhigher around low water than at high tide; respective maxima and minima were0.016 and 0.001 g.m-3, 0.050 and 0.001 g.m-3, and 0.35 and 0.10 g.m-3. For waterfrom river and streamlets, average levels of nitrogen components were similarto those for the main channel at low tide.

INTRODUCTION

The shoreline of Tasman Bay is indented by numerous tidal inlets.In the southeast is Nelson Haven on which ecological studies were madeby Davies (1931). He applied the name Wakapuaka to the northernend of this haven and this conforms with the current local designation.To the northeast of Nelson Haven and 10 km distant is Pepin Island,with Cable Bay to the south and Delaware Bay to the east. BehindPepin Island, the Wakapuaka River enters a shallow tidal inlet emptyinginto Delaware Bay. On recent maps this inlet is not named, althoughsome early maps record it as "Wakapuaka mud flat". To avoid con-fusion, the inlet behind Delaware Bay is provisionally designated theDelaware Inlet (41° 10' S, 173° 26' E).

Following recent eutrophication studies of Waimea Inlet and NelsonHaven by Updegraff et al. (1977), Delaware Inlet suggests many interest-ing features for comparison. The contrasts are in the sparsely populated

* Present address: Department of Microbiology, University of Hawaii, Honolulu,U.S.A.

Received 6 September 1976; revision received 20 January 1977.

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catchment (83 km2), the relatively unpolluted freshwater river, theabsence of significant polluting influents, the dewatered condition at lowtide, and the constricted outlet channel containing well-mixed waters onboth ebbing and flooding tides. The situation is very favourable forinvestigating the origins, concentration, and ultimate destiny of nutrientcomponents of inlet waters.

The data for this survey were recorded from February to May 1976.The aim was to provide a base for further studies of the inlet bysurveying the texture, flora, and fauna of its intertidal flats, and byassaying the salinity and nitrogen levels of its waters.

DESCRIPTION OF THE AREA

Geologically, the entire watershed of the Wakapuaka River andDelaware Inlet is within a sequence of volcanic and sedimentary Permianrocks (Beck 1964). The topography and complex, igneous rocks ofPepin Island were described by Lauder (1964).

Cable Bay was named following the installation there of the trans-Tasman telegraph cable in 1876, when the district was as sparselypopulated as it still is.

In the early days, forest clearing and extension of farming probablyled to some erosion of hillsides and thus to sediment accretion in Dela-ware Inlet. Today, pastures of the slopes have been invaded by scrub orplanted in exotic forest, and erosion and sedimentation are restrained.

Moderate applications of phosphate and lime to pasture over recentdecades have maintained the fertility for grazing of sheep and cattle.Pastoral run-off would be the principal contributor to any eutrophicationof streams or inlet waters.

Flow measurements by the Nelson Catchment Board for 9-13 Feb-ruary 1976 gave the daily discharge of the Wakapuaka River as85,000 m3. A number of streamlets contribute small quantities of fresh-water to the inlet.

The area of Delaware Inlet at high water is 3.1 km2; the tidal flats havean even, level surface dissected by sparse, shallow channels. The tidalrange varies from 2 m at neaps to 4 m at spring tides. At low water neapsonly a small volume of water remains in the inlet, and for a short periodat low water springs it is restricted to the deepest channels. Today'spattern of channels of the river and tidal drainage differs from thatdepicted on early maps.

At the head of Cable Bay a massive 500-m-long, 20-m-wide boulderbank joins Pepin Island to the mainland. Even at high water this barriereffectively separates the waters of the bay from the inlet.

A spit of cobbles and sand 2.25 km long divides the waters of DelawareBay from the inlet, ingress and egress of all water being through anarrow channel (<200 m wide) at the western end. On the flooding tide,

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• • > ,

FIG. 1—Aerial view of Delaware Inlet, Nelson, New Zealand, October 1972;Wakapuaka River enters at lower left. At upper left the 500-m-long boulderbank joins Pepin Island to mainland and divides Cable Bay from the inlet.Pepin Island (centre) rises to 402 m. On right of Pepin Island is the outletchannel with bar accentuated by turbid water, and the 2.25-km-long spitseparating the inlet from Delaware Bay.

Photo : Aerial Surveys Ltd.

a bar at the seaward end of the channel causes turbulence and mixingof the water, while 500 m up-channel a rocky reef causes equivalentturbulence in the ebbing waters. A trough-like channel between thosetwo points provided a very favourable site for sampling well-mixed waterof the incoming and outgoing tides (Fig. 3, Site 3). The inlet and itsenvirons are shown in a 1972 aerial photograph (Fig. 1).

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FIG. 2—Sketch map of Delaware Inlet, Nelson, New Zealand, showing surface texture of intertidal flats, March 1976. tsDow

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SURVEYS OF THE INTERTIDAL FLATS

TEXTURE

Of the Wentworth grain sizes as grouped by Folk et al. (1970), onlyboulders were absent. Extent and depth of textural surfaces tended toalter as buoyancy of particles was influenced by climatic and tidalphenomena.

Five textural surfaces (mud, sand, gravel, cobbles, and shell) weremapped. (Fig. 2). Large flat expanses of firm sand with few channelspredominated. In the shallow extremities of the inlet to the east and westand off the Wakapuaka rivermouth, mud accumulated to a depth of15 cm over subsurface sand or gravel. Surface gravels occurred nearthe mouth of the river and some streamlets. Cobble and gravel surfacesalso occurred near parts of the main channel and over some adjoiningflats.

Accumulations of dead shell were a textural feature of the banks ofprincipal channels. Where gradients steepened or where water movementincreased, shell, gravel, or cobble surfaces became more prevalent.The major textural features of the inlet were sandflats 55%, mudflats15%, gravel and cobbles 15%, shell beds 5%, and tidal channels 10%.

FLORA

Paucity of vegetative cover was a feature of Delaware Inlet. Figure 3shows the areas, aggregating approximately 30 ha of the five principaltypes of macroscopic vegetation.

Rushes {Juncus spp., principally / . maritimus) occupied the largestarea, particularly when the growth on shore was included. The areaof most prolific growth was around the mouth of the Wakapuaka River.Minor narrow strips were present along the shoreline at the eastern andwestern extremities of the inlet, usually where streamlets entered. Thepresence of rushes promoted the accumulation of fine detritus andsediment.

Salicomia ausiralis was sparse and was often present on the seawardside of Juncus stands, particularly where the surface was gravel or sand,but was seldom associated with mud. Three patches of Salicornia grewon the sandy shoreline around the central section of the inlet.

The eelgrass Zostera muelleri was plentiful in discrete sections of themain channels where change of direction or gradient promoted thedeposition of waterborne sand. In the southeast corner of the inlet asmall patch was growing at a near-shore, shallow-water site.

Sea lettuce Viva lactuca occurred close to the Zostera beds on sandysides of channels. Practically all Ulva was attached to buried livingcockles. Paucity of detached fragments suggested that those were carriedto sea from the channels by the ebbing tides.

Enteromorpha was more sparse than Viva and was also attached toburied, living cockles. Usually the Enteromorpha occurred to landwardof the Viva.

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FIG. 3—Sketch map of Delaware Inlet, Nelson, New Zealand showing areas of vegetation in the mtertidal zone, April 1976. t/2paD

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Two microscopic algal genera were observed and their distributionstudied. For convenience and clarity these are plotted on Figure 4 withthe molluscs. Euglena obtusa was initially observed in early March as agolden film on sand, and its population intensified from March through-out April. Dense populations were plotted at five locations on warm,sandy flats with similar aspects relative to tidal coverage, tidal channelsand water movement. Dense populations of Oscillatoria ornata was asso-ciated with Euglena at two sites. Both organisms were concentrated in theupper 10 mm of moist sand; although Euglena showed strong positivephototaxis, Oscillatoria was strongly negatively phototactic. After ex-posure to bright sunshine for an hour, the sand surface was golden withEuglena, and after an hour of shading this1 was superseded by a greenfilm of Oscillatoria. Detailed studies are in progress on the phototacticresponses and nitrogen fixing potentials of these organisms (B. B. B,unpublished data).

MOLLUSCS

The four dominant genera of molluscs occupying distinct habitats havebeen plotted in Figure 4.

Mud snails Amphibola crenata were present in dense populations overlarge areas of the surface of the inlet. They preferred high tidal flatswith fine textured detrital or muddy surfaces, and avoided sandy surfaces.

Cockles Chione stutchburyi tended to occupy slopes and flats adjacentto the main channels. Mud, sand, and fine gravel all supported verydense populations. As depth of tidal water decreased, or surface ex-posure increased, populations became more sparse. Extensive banks ofdead shell were usually present over and around the living cockles.

Pipis Paphies australe were only present in two small areas in coarsesand and fine gravel and were exposed only for short periods at lowtide. Cockles were also prolific at and around one site.

Mussels Mytilus edulis were present in large numbers on a small areaof cobbles in the main channel on the western side of the rock reef. Thisarea was exposed only for short periods at low tide.

Smaller populations of other molluscs and crabs were not recorded.

WATER PARAMETERS

Factors to be considered in evaluating coastal seawater quality werediscussed by Updegraff et al. (1977). Their methods were applied forthe measurement of salinities and determination of nitrate, nitrite, andKjeldahl-N in the Delaware Inlet waters in February-March 1976.

SALINITY

During this late summer period, seawater temperatures in the inletranged between 16°c and 21°c. At full tide, the waters showed normalseawater salinities of 34.5-35.0%0, with little variation with depth.

Sig—12

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FIG. 4—Sketch map of Delaware Inlet, Nelson, New Zealand showing main populations of molluscs, Euglena, and Oscillatoria,April 1976.

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Hourly measurements over the tidal cycle at Site 3 showed that atspring tide salinities varied from 35.0%o at high water to 33.0%o at lowwater. On a neap tide the corresponding salinities were 34.8-31.8%c.

Salinities measured at low water along the main channel showed thatunder the influence of river inflow there was a progressive decrease to3.8%O at Site 27. At this time, in an adjacent channel receiving less fresh-water (Site 28) the salinity was 24.2%O. After some hours exposure onsand flats, small puddles showed rising salinities, maximum 40%o.Measurements indicated that wide salinity fluctuations could occur atspecific sites at particular periods of the tide.

NITROGEN

While much information is available on the dynamics of the nitrogencycle in soil/plant relationships, the situation in coastal waters is moreobscure. The paucity of intertidal vegetation in Delaware Inlet indicatedthe desirability of information on sources and concentrations of nitrogencomponents of the water.

On two occasions, on 2 March 1976 with a spring tide and on25 March with a neap tide, water samples were taken hourly over tidalcycles from the main channel at Site 3 (Fig. 3). A total of sixty sampleswas collected from the inlet, the channels, tidal puddles, streamlets, andriver for determining NO2-N, NO3-N, NH4-N and Kjeldahl-N.

The range of NO2-N was always low in tidal and river water, (0.001-0.004 g-mf'). Only three samples were higher: two slightly saline samplesof water trickling from foreshore seepage gave 0.005 g.nr8 and0.009 g«.mr3, and saline water from a low tide channel showed 0.005 g.nr3.

In the tidal water, the range of NO3-N was generally low (0.001-0.01 g.nr3) . On the two occasions on which Site 3 was sampled over atidal cycle, nitrate was very low around high water, 0.001 g.nr3 and0.003 g«nr3, but gradually rose with the ebb to a maximum about 1 hafter low water, 0.016 g.nr3 and 0.013 g.irr3 (Fig. 5). Water from tidalpuddles tended to be below 0.01 g.nr3. Samples taken from tidal channelsat low water showed 0.01 g.nr3, but levels increased progressively upchannel to a maximum of 0.10 g.nr3. Five samples on different daysfrom various points in the Wakapuaka River showed an average of0.11 g.nr3 (maximum 0.14g-nr3). Samples from some streamlets andshore margin seepages were higher (average 0.80 g.nr3); the highestlevel of 2.0 g.nr3 was recorded from a very small shore-margin seepage.

The range of NH4-N in tidal cycle samples from Site 3 was 0.001-0.050 g.nr3. The range fluctuated more than that for nitrate, and showeda similar tendency for highest levels around low tide (Fig. 5). At lowwater, the range in tidal channels was 0.008-0.060 g.nr3, with an erratictendency to become higher with progression shoreward. The range of 12samples from streams and the river was 0.008-0.046 gonr3. The highestlevels were obtained from two tidal puddles, 0.117 g.nr3 and 0.127 g.nr3.

Kjeldahl-N determinations on unfiltered samples tended to averageabout ten times higher than those for the corresponding soluble

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Tide

FIG. 5—Levels of nitrate, soluble ammonium, and Kjeldahl nitrogen in waterfrom Site 3 in the main channel, collected during a spring (2 March) and aneap (25 March 1976) tidal cycle.

ammonium levels. Kjeldahl-N levels for Site 3 (Fig. 5) also tended tobe highest near low water (range 0.10-0.35 g.nr3). In freshwater fromstreamlets and river the range was 0.15-0.36 g.nr3. Kjeldahl-N, as withNH4-N, tended to be highest in small tidal pools and seepage (0.30-0.67 g.nr3).

Lower levels of nitrogen were found in flood tidal waters comparedwith water from inlet and catchment. In the main channel (Site 3),levels of NO3-N, NH.|-N, and Kjeldahl-N were substantially higheraround low water than at high tide, with respective maxima and minimaof 0.016-0.001 g.nr3, 0.050-0.001 g.nr3, and 0.35-0.10 g.nr3. Overall,nitrogen averages from river and streamlets were of similar order tothese low tide maxima. Water from tidal puddles and shoreline seepagesshowed higher concentrations. Inflowing freshwaters, seepages, andmicrobial degradation of intertidal detritus appeared to be the principalcontributors of nitrogen to the waters of the inlet. This nitrogen appearedto be rapidly assimilated, dispersed, or diluted in the tidal waters.

ACKN OWLEDGM EN TS

Gratitude is expressed to Dr W. J. Wiebe for advising and participating in theearly stages of these studies, and to Dr Elizabeth A. Flint and Dr J. D. Stout foridentifications and information on Euglena and Oscillatoria. We also thankMr A. W. Parrott for information on molluscs, Mr G. Pemberton for measuring

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the flow of the Wakapuaka River, the Chemical and Biological Services sectionof the Cawthron Institute for nitrogen determinations and preparation of diagramsand Aerial Surveys Ltd, Nelson, for the photograph of Delaware Inlet.

LITERATURE CITED

BECK, A. C. 1964: Sheet 14 Marlborough Sounds. "Geological Map of NewZealand 1:250 000". N.Z. Department of Scientific and IndustrialResearch, Wellington.

DAVIES, W. C. 1931: Tidal-flats and salt-marsh studies in Nelson Haven. Part I -The ecology of Nelson Haven, its mudflats and the Wakapuakareclamation. N.Z. Journal of Science and Technology 12: 338-60.

FOLK, R. L., ANDREWS, P. B. & LEWIS, D. W. 1970: Detrital and sedimentary rockclassification and nomenclature for use in New Zealand. N.Z. Journalof Geology and Geophysics 13 (4): 937-68.

LAUDER, W. R. 1964: The geology of Pepin Island and part of the adjacent main-land. N.Z. Journal of Geology and Geophysics 7 (1) : 205-41.

UPDEGRAFF, D. M., STANTON, D. J. & SPENCER, M. I. 1977: Surface waters ofWaimea Inlet and Nelson Haven: a preliminary assessment of quality.N.Z. Journal of Marine and Freshwater Research 11 (3) : 559-75.(this issue).

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