Cah. Bioi. M ar. ( 199 1), 32 : 105- 11 9 Rose off Zooplankton assemblages in the shallow tidal estuary of Mundaka (Bay of Biscay). Fernando Villate Laboratorio de Ecologfa , Facultacl de Ciencias Uni versiclacl del Pafs Yasco, A pcl o. 644, 48080 Bilbao, Spain Abstract Spatial and temporal variability of zooplankton assemblages were studied in the tidal es tuary of Munclaka , from M ay 198 1 to July 1982, in relati on to factors controlling es tuarine hydrodynamics. Salinity range and maintenance of mixed wa ters were related to the tidal amplitude and rain fa ll, which drives ri ver discharge. A t high tide, the study zone appeared mainly as a polyhaline system, in which Acania bi/ilasa was the only perennial component of the es tuarine- marine assemblage , and true es tuarine copepocls (genus Eurytemora) were absent. A mong neritic holopl ankters, Paraca/anus parvus and Acartia c/ausi dominated and no season ally stabl e euryhali­ ne and st"'nohaline assemblages were found. A mong meropl ankton groups, larvae of Polychaete Spionicls was pre­ dominant toward the upper es tu ary, while Biva lve ve ligers had the lowest penetration in the es tuary from the neri­ tic zone. Some nectobenthic organisms assoc iated with muddy benthic habitats, such as Praniza larvae of Paragnatilia formica and ostraco cl Cythericl s, characteri zed the pl ankton community of the upper es tu ary during periods of low abunclanre of es tuarine holoplankton. According to th e hierarchica l clustering of sa mples from zoo­ plankton data, summer-autumn , winter and spring periods were separated as se asonal units, which also di ffered with regard to hydro logical fea tures . The mixed wa ter persistency and the deve lopment of estuarine spec ies were higher in summer, when low freshwa ter inflows and neap tides co incided. By contrast, high-sa linity wat ers and neritic zoopl ankton filled the es tuary in spring by the effect of spring tides co upled with low freshwa ter inflows. Resume : La variabilite spatio-temporelle clu zooplancton a ete etucli ee cl ans l'estu aire cl e Munclaka pendant 14 mois (mai 198 1 - juillet 1982 en foncti on des facteurs qui reg lent l'hyclrocl ynamisme. Les variati ons cle la sa linite et la persistance des masses cl 'ea u saumatres ont ete mises en rapport avec !'amplitude cle la maree et les prec ipita­ ti ons, qui gouvern ent le debit flu vial. A pleine mer, la zone etudiee apparalt comme un milieu surtout polyhalin, cl ans lequel Acania bifilosa es t la seu le espece perennante de !'ensemble es tuarien-marin et les copepocl es vrai ­ ment estuariens (genre Eurytemora) sont absent s. Panni l'holopl ancton neretique Paracahuws parvus et Acartia c/ausi sont les es peces cl ominantes et on n'a pas observe cl 'ensembles euryhalins ou stenohalins temporellement stables . Panni le meroplancton, les larves cl e polychetes spi onicles preclomin ent vers l'interieur de l'es tuaire, tancli s que les ve ligeres cle lamellibranches ont la plus fa ible penetrati on cl ans l'es tu aire. Quelques organismes nectoen­ thiques provenant des foncls vaseux, principa lement des larves prani zes cle Paragnatilia fonnica et des os tracodes, sont les co mposants les plus caracteri stiques de la communaute pl anctonique de l'interieur cle l'es tu aire pendant les periocl es cl e faible abonclance cl e l'holoplancton es tuarien. Suivant le regroupement hierarchique des echantillons se lon les clonnees cle zooplancton, les periocl es ete-automne, hi ver et printemps ont ete separees comme unites sa i­ sonnieres, lesquelles se cliffe rencierent au ss i par les aspec ts hydro log iques. L a persistance des eaux mixohalines et le cleve loppement des especes es tuariennes atteint son max imum en ete, qu ancl le debit fluvial es t faible et pendant les marees de morte-ea u. Par co n11·e , les eaux marines et le zoo plancton neritique envahissent tout l'estuaire au printemps a la faveur des marees de vive-ea u et cle fa ibles apport s flu viaux. I NTRODUCTION Among environmental factors influencing th e compos ition and dynamics of zooplankton communities in estuari es, the vo lume of fresh water inflow, the tidal exchange and th e sali­ nity di stribution pattern pl ay a predominant rol e. Given the wide va ri ety of estuari es with regard to th e above mentioned hydrological characteri stics ; the recognition of different estu ary types and the study of zooplankton in relation to its particular environmental pro- 106 F. VJLLATE perti es seem to be useful to th e transfer of knowledge on zooplankton dynamics between estuaries of the same type. In this sense, zooplankton composition, zonation and seasonal variations are better documented for large estuaries, while less information is available for small tidal estuaries. Along the Basq ue coast there are several estuaries of this type, which are being studied during th e last decade. Nevertheless, few works have been focused on brackish water zoo­ plankton: Yillate & Orive (1981a, 1981b) reported on the spatio-temporal distribution of copepocl and claclocera populations during an annual cycle within the Plencia estuary. D'Eibee & Caste! (1982) dealt with total zooplankton composition and distribution from fresh waters to euhaline waters four times in the year in the Aclour estuary. San Yicente et al. (1988) described midsummer zooplankton composition and distribution in the Orio estuary. In the Munclaka estuary, a preliminary study of zooplankton community was made as part of a multi-disciplinary ecological study on the valley and estuary of Gernika-Munclaka. Data on species composition, distribution and seasonal variations are shown in a recent paper (Villate, in press). In the present contribution, seasonal and spatial zooplankton assemblages in the Munclaka estuary are differentiated by multivariate analysis techniques and related to environmental conditions throughout an annual cycle. The effect of river discharge -stated from rainfall- and tidal rhythms on the zonation and development of popu­ lations will be analysed in an attempt to predict zooplankton dynamics within the es tu~r y. Study area The Munclaka estuary (43° 20 ' N, 3° W) is a mesoticlal system about 13 km long, in which intertidal areas form the larger percentage of th e total estuarine area. Tidal amplitude ranges from around 4 m at spring tides, to around 1 m at neap tides, and the estimated ratio of tidal prism volume and estuarine water volume fluctuates from 1.89 to 0.90 (Villate et al., 1989). It is barely navigable during low tide but at high tide the average depth ranges from 2 to 5 m depending on tidal amplitude. It remains navigable up to the Gernika chan­ nel, a narrow channel through which the study area receives fresh water inputs from Oka stream. The ri ver discharge can be considered low, since mean values of 0.048 m3 s- 1 at low-flow periods and 4.8 m 3 s-1 at hi gh-flow periods have been recorded. According to salinity gradients at hi gh tide, five sampling sites were located between the channel and the coastal line (Fig. 1). Three stations were in the inner salt marshes zone and the other two in the outer estuary. M ATERI AL AND METHODS Sampling was performed from May 1981 to July 1982. Over this period 17 surveys were carried out monthly or at 15 clay intervals. Sampling started one hour before hi gh tide at sta- Z OOPLANKTON IN THE M UN DA KA ESTUARY 107 tion I and fini shed two hours later at station 5. In February and March 1982, at neap tides, station 1 was not used due to the shallow depth of the water column. MUNDAKA N Km 0 2 CD Gernika Channel Fig. I : Maps of the Bay of Biscay and the study area in the estu ary of Mundaka, showing depth cont o urs in mete rs at low tide and the locati on of five sampling s ites. 108 F. YILLATE To determine salinity, water samples were obtained at the surface and near the bottom with a Van Dorn 6 I bottle. Dail y rainfa ll values were provided by the Meteorological Service of Sondika Airport. Zooplankton was coll ected usi ng a mouth-reducing cone net with 35 cm diameter mouth, 1. 2 m long and 1.13 m2 of 250 j.lm aperture filtering mesh. Two horizontal hauls were carried out at each station, one near the water surface and the other near the bottom. When depth was less than 2 m, only one haul was taken. The net was towed for 5 minutes 1 at a speed of 0.4 m·sec- • Samples were preserved immedi ately in 5 % borax buffered for­ maldehyde. The filtered volume of water was estimated from the net mouth area and the haul length. Filtration efficiency was considered to be lOO %. The identification and coun ­ ting of organi sms was done under a stereoscopic microscope. The majority of the holo­ plankton forms were identified to species level. Among the meroplankton, medusae were identified to species, but the other groups were categori zed as class or families. To relate spatial and temporal vari ations of zoopl ankton species and groups to environ­ mental parameters, surface and bottom data were averaged for the water column. Statistical analysis of zooplankton data were performed using the SPAD multivariate analysis package (Lebart & Morineau, 1982). The grouping of samples from zooplankton composition and abundance was made by means of the nearest-neighbor clustering method (Benzecri , 1982).