Search

Zooplankton phenological shifts may affect energy transfer through pelagic food web and up to fisheries, but few studies have compared zooplankton phenology across a wide latitudinal range of water temperatures. We examined the phenological variations of zooplankton at four sites along a latitudinal gradient from the Bay of Biscay [Bilbao and Urdaibai (BU)], the English Channel (Plymouth; L4), and the North Sea [Stonehaven (SH)] from 1999 to 2013. Zooplankton taxa showed interannual phenological variations that were opposite in direction between the southernmost BU and the northernmost SH sites. The East Atlantic pattern was the climate teleconnection best related to zooplankton phenological variations. Among local variables , salinity at BU was best related to phenological changes at BU as opposed to those at SH. Locally, chlorophyll a was most relevant at SH and temperature at L4. While we did see some imprints of temperature in causing expected directions of phenology shifts, i.e. towards earlier appearance of spring taxa and later appearance of autumn taxa in warm years, these patterns were by no means clear-cut. The dominant role of temperature in driving phenological variation suggested by some studies seems to be obscured by biotic and climatic controls acting differently along our environmental gradient.

The colonization success of non-indigenous species (NIS) of copepods that arrived in the estuaries of Bilbao and Urdaibai (Bay of Biscay) and other expanding species (OES) of copepods that arrived in the former estuary during the 1998–2015 period, as well as the impact of such species on the density and seasonal patterns of congeneric species, total copepods and total zooplankton have been analysed. The estuary of Bilbao was successfully colonized by the NIS Acartia tonsa and Oithona davisae (since 2003) and by the NIS Pseudodiaptomus marinus and the OES Calanipeda aquaedulcis (since 2010), coinciding with the progressive improvement of environmental conditions in the rehabilitation process of this estuary. The three NIS species also arrived in the estuary of Urdaibai during the same time period, but only A. tonsa became abundant, likely due to inherent natural constraints for the other two species. A. tonsa in both estuaries, and O. davisae and C. aquaedulcis in the estuary of Bilbao became influential in the density, composition and dynamics of total copepods and zooplankton in water-masses of 33 or lower salinity. The highest impact was found in the inner estuary of Bilbao, where total copepods and zooplankton reached densities comparable to those of the outer estuary and copepod maxima moved from spring to summer due to the contribution of the colonizing copepod species. The congeneric species most affected by the NIS were the neritic species Acartia clausi and Oithona nana in the estuary of Bilbao, and the brackish species Acartia bifilosa in the estuary of Urdaibai.

Seasonal and interannual variations of total mesozooplankton abundance and community variability were assessed for the period 1998–2005 at 3 salinity sites (35, 33 and 30) of the estuaries of Bilbao and Urdaibai (southeast Bay of Biscay). Spatial differences in mesozooplankton seasonality were recognized, both within and between estuaries, related to differences between sites in hydrodynamic features and anthropogenic nutrient enrichment that drive phytoplankton biomass seasonal cycles. The within estuary seasonal differences in mesozooplankton community were mainly shown through seaward time-advances in the seasonal peak from summer to spring along the salinity gradient, linked to differences in phytoplankton availability during the summer, in turn, related to nutrient availability. These differences were most marked in the estuary of Urdaibai, where zooplankton seasonal pattern at 35 salinity (high tidal flushing) resembled that of shelf waters, while at 35 of the estuary of Bilbao zooplankton showed an estuarine seasonal pattern due to the influence of the estuarine plume. Cirripede larvae contributed most to the mesozooplankton seasonal variability, except at the outer estuary of Bilbao, where cladocerans and fish eggs and larvae were the major contributors, and the inner estuary of Urdaibai, where gastropod larvae contributed most. Total mesozooplankton increased at 30 salinity of the estuary of Bilbao and 35 salinity of the estuary of Urdaibai. Interannual variability of mesozooplankton at the lowest salinity of the estuary of Bilbao was mainly accounted for by copepods due to the introduction of non-indigenous species during estuarine rehabilitation from intense pollution. However, bivalve larvae and gastropod larvae showed the highest contributions at 35 salinity of the estuary of Urdaibai. At the rest of sites, the opposite interannual trends of polychaete larvae and hydromedusae generally made the highest contribution.

This contribution summarizes the scope and results of a selection of the studies presented to the XV International Symposium of Oceanography of the Bay of Biscay (ISOBAY 15), that took place in Bilbao (Basque Country, Spain) from 22nd to 24th June 2016. Firstly, a brief introduction to the status of the Bay of Biscay, its presence in scientific literature over the last two decades, and the aims of the present edition of ISOBAY are presented. A second part shows the main findings of the works submitted for publication in this special issue. They represent new steps in the knowledge of oceanographic, geological and biogeochemical processes, biodiversity, ecosystems´ structure and functioning, and human impact in the Bay of Biscay. On the other hand, they also contribute to the overall knowledge of marine systems.

The effect of climate variability on estuarine water environments was assessed in two systems of the Bay of Biscay, the estuaries of Bilbao and Urdaibai, with contrasting morphology, hydrodynamic features and anthropogenic influence. To that purpose, the main time scales of variability in relevant environmental factors were established along spatial salinity gradients and, using a combination of multivariate and regression analyses, the contribution of each factor to the total variability, as well as the influence of climate factors in the seasonal and inter-annual estuarine environment variations were assessed. The major seasonal modes of variability in the water environments of both estuaries were accounted for by water temperature together with salinity stratification and/or chlorophyll a. This seasonal variability was associated with climate variability, as shown by the relationship with air temperature and river flow. The major inter-annual modes of variability were also accounted for by water temperature, along with dissolved oxygen and/or chlorophyll a in Bilbao, and chlorophyll a and/or transparency in Urdaibai. These were also associated with climate variability, in this case summer air temperatures. Water quality variables, such as dissolved oxygen and transparency, were found to be sensitive to reveal the effect of long term anthropogenic activities.

Estuarine and marine ecosystems are subject to different sources of stress including changes in environmental physicochemical variables (nutrients, oxygen availability, temperature, salinity, pH) or exposure to a large cocktail of environmental pollutants. In the estuary of Bilbao, despite the improvement in environmental quality due to replacement of most polluting industries and the progressive implementation of an integrated sewage treatment plan, chronic pollution by metals and hydrocarbons still remains, together with eutrophication and pollution by emerging contaminants. The Unit of Formation and Research “Protection of Ecosystem Health” was created in 2012 through the strategic aggregation of three consolidated research groups: Cell Biology in Environmental Toxicology (CBET), Phytoplankton of estuaries and coastal areas (FITEAC) and Research on marine and estuarine pelagic environment and planktonic communities (MarEsPlank). The three groups have a long tradition in studying the health of marine and estuarine ecosystems from different perspectives, focusing in the Bay of Biscay and especially in the estuary of Bilbao. In this work we review data on the health status of the Bilbao estuary using early warning tools at the cell and tissue levels, phytoplankton assemblages and zooplankton ecology. In spite of the seasonal and interannual variability recorded for some parameters, a general recovery trend was observed for the health status in the estuary of Bilbao, with sporadic critical events such as the Prestige oil spill.

Variations in mesozooplankton abundance and community structure in response to water quality improvement and to variations in hydro-climatic conditions were studied in the inner estuary of Bilbao from 1998 to 2011. A process of recolonization was observed with a marked increase in copepod abundance and a smaller increase in the abundances of appendicularians, meroplanktonic bivalves and gastropods, at the expense of the decrease in groups of gelatinous predators. Within the copepods, an initial phase in this recolonization occurred through an increase in neritic copepods. However, in a second phase, the non-indigenous species Acartia tonsa and Oithona davisae increased and became dominant. In the last 2 years, a third non-indigenous species Pseudodiaptomus marinus was observed in lower densities, and Calanipeda aquaedulcis abundance increased becoming co-dominant with A. tonsa. All non-indigenous copepod species had summer/autumn peaks and were characteristic of brackish conditions, which suggests that unsaturated ecological niches in brackish waters were important for the settlement of non-indigenous species. Pollution abatement allowed for zooplankton recolonization, but variations in community structure, both at the broad taxonomic group and at the copepod species level, were greatly influenced by hydro-climatic factors, temperature being the single best explanatory variable of zooplankton variations.

A two-layer box model based on salinity and freshwater inflow data was developed and used to estimate net water circulation, contributions of gravitational circulation exchange and tide-driven exchange, and turnover times for the estuary of Bilbao, a small estuary of the Basque coast (Bay of Biscay). Average monthly estimations for the 2001–2010 period were made and related to river discharge and saltwater inflow. Seasonal variations of surface-layer outflows were strongly related to the river discharge regime, even in the lower estuary (inner Abra harbour). Bottom-layer salt-water inflow from the outer Abra was the main driver of bottom landward flow, vertical advection and surface-layer outflow in the inner Abra, but not in the channelized zone that extends from the inner limit of the Abra harbour to the tidal limit. Gravitational circulation exchange dominated in the entire estuary over the annual cycle. Tide-driven exchange proportionately increased in summer and showed the highest contribution (42%) in the lower estuary in August. Flushing and residence times increased in summer in relation with the decrease of freshwater discharge, although in the innermost zone of the estuary they were also high in winter due to the retention of freshwater at the inner estuary under extremely high discharge conditions. Flushing and residence time maxima of 21.5 and 28.6 days respectively were obtained for the entire studied zone in August. It is of note that turnover times differed largely between the upper (flushing time of 0.4–2.4 days) and bottom (flushing time of 2–10 days) layers in the channelized zone. Results supported intuitive conclusions drawn in previous studies about the spatio-temporal dynamics of dissolved oxygen, chlorophyll and zooplankton populations in the estuary of Bilbao, in relation to the effect of water circulation and turnover.

Dissolved oxygen dynamics in estuarine and coastal environments are complex and highly variable, which highlight the need to compile information from many different types of estuaries. Small estuaries where euhaline habitats dominate are particularly ill represented in the literature. As a contribution to fill this gap, a study on dissolved oxygen dynamics was conducted in the small estuary of Urdaibai (inner Bay of Biscay). Spatial and temporal variations in the percentage saturation of dissolved oxygen (DO-saturation) along the salinity gradient of the estuary and the role of hydro-climatic factors and eutrophication as drivers of those variations at seasonal and inter-annual time scales were analyzed (period 1998–2008). DO-saturation showed an inner to outer estuary increasing gradient. DO-saturation below the salinity gradient layer was either significantly higher than or not significantly different from that in the salinity gradient layer. DO-saturation showed summer minima, but hypoxia was rarely observed. At the outermost estuary, seasonal variations of DO were small and it is hypothesized that DO dynamics were governed mainly by tidal exchange and turbulence. In the intermediate and inner estuary, seasonal variations of DO-saturation were best explained by river discharge, and to a lesser extent by chlorophyll a and temperature, each factor gaining relevance in different periods of the year. In intermediate and inner zones, river discharge exerted a positive effect on DO-saturation, likely via an increase in the renewal rate of DO. At the inter-annual time scale, unlike at the seasonal scale, temperature did not show a significant negative relationship with DO-saturation.

Background: The introduction of NIS to estuaries and coastal embayment is of great concern. Commercial ships’ ballast water discharge and the northwards progression of species due to the ongoing climate change arise as the main factors explaining the rising occurrence of NIS species in Northern Atlantic waters. In this regard, regular monitoring of the plankton communities is paramount as to be able to respond properly to this potential issue. Results: While monitoring the invasive copepod Acartia tonsa populations in the estuaries of Bilbao and Urdaibai (Basque country, Spain), we report here the Asian copepod Pseudodiaptomus marinus for the first time in the Iberian Peninsula waters. Individuals from both species were collected from July to October, 2013 for DNA sequencing of mitochondrial cytochrome c oxidase subunit I gene (MT-CO1). Phylogenetic analysis of MT-CO1 confirmed P. marinus identity. Conclusions: Phylogeographic distribution of A. tonsa haplotypes in Europe along with the Bilbao port traffic patterns suggested a secondary invasion from an European source to Basque estuaries. The successful establishment of the A. tonsa population and the appearance of Pseudodiaptomus marinus confirm the need for regular plankton monitoring of estuarine and port waters. This applies also to nearby systems as these populations could represent a source of future dispersal. DA’s work was supported by the ZabaldUz Program (PhD fellowship). SGIker technical and human support (UPV/EHU) is gratefully acknowledged. Thanks to T. Matellanes (Bilbao Port Authority) for providing maritime traffic data. Special thanks to the anonymous reviewers that greatly improved the different manuscript versions. The sampling program received funds from the University of the Basque Country (UFI11/37) and the Basque Government (GIC10/168).

Metabarcoding is becoming an increasingly valuable alternative approach to biodiversity assessment, due to the combination of extreme sensitivity and potential for the highest taxonomic resolution in a cost- and time-effective methodology. To evaluate the capacity of metabarcoding for estuarine plankton monitoring, a comparison between the results obtained with this approach were compared with those based on traditional taxonomic analysis (microscopy). Database incompleteness, one of the main limitations of metabarcoding, was somewhat overcome by the addition of DNA sequences for local species, which increased the taxonomic assignment success from 23.7 to 50.5 %. When the communities were studied along with environmental variables, similar spatial and temporal trends of taxonomic diversity were observed for metabarcoding and microscopic studies of zooplankton, but not for phytoplankton. This is most likely attributable to the lack of representative sequences for phytoplankton species in current databases. In addition, there was high correspondence in community composition when comparing abundances estimated from metabarcoding and microscopy, suggesting semiquantitative potential for metabarcoding. Furthermore, metabarcoding allowed the detection and identification of two non-indigenous species (NIS) found in the study area at abundances hardly detectable by microscopy. Overall, our results indicate that metabarcoding is a powerful approach with excellent possibilities for use in plankton monitoring, early detection of NIS and plankton biodiversity shifts.

Seasonal, interannual and spatial variabilities were analysed for the > 100 μm and > 200 μm zooplankton assemblages in the estuary of Bilbao throughout 1997–2000, and related to hydroclimatic and water-quality factors. Seasonal and interannual patterns of abundance differed between assemblages, and only the abundance of the > 100 μm zooplankton was correlated with temperature. The large-sized zooplankton decreased more drastically than the small-sized zooplankton with decreasing salinity, and showed highest correlations with dissolved oxygen and water transparency. Seasonal changes were the major variability mode of zooplankton composition in both assemblages. Interannual changes associated to phytoplankton biomass and temperature were more evident in the small-sized zooplankton, whilst spatial differences related to oxygen depletion were more evident in the large-sized zooplankton. This indicates that small zooplankton was more sensitive to climate-related factors, while large zooplankton was more sensitive to water quality. Small copepods, cirriped larvae, appendicularians and Noctiluca were the main taxa responsible for the interannual variations in the > 100 μm assemblage. In the > 200 μm assemblage, large copepods were the main zooplankton responsible for the spatial variability related to oxygen depletion. Cnidarians and cladocerans responded mainly to decreases in salinity, and polychaete larvae were found to be unaffected by the decrease in dissolved oxygen and salinity. Results suggest that size-related differential responses of zooplankton should be considered when zooplankton monitoring is performed to assess the effect of climate forcing and pollution in coastal and estuarine environments.

The effect of environmental factors (river discharge, water temperature and dissolved oxygen saturation) on the abundance and distribution of Acartia populations and the interactions between their congenerics was evaluated by means of transfer function (TF) models in the estuary of Bilbao during the period 1998–2005. The recorded species were Acartia clausi, Acartia tonsa, Acartia margalefi and Acartia discaudata. Acartia clausi dominated in the entire euhaline region of this estuary until 2003 when it was displaced from the inner part by A. tonsa. This invasive species (A. tonsa) was found for the first time in 2001 and colonized successfully the inner (salinity 30) and intermediate (salinity 33) waters of this estuary since 2003. The TF models revealed an immediate and negative effect of A. tonsa on A. clausi in the intermediate salinity (33) waters, where these species showed the highest spatial overlap. The results indicate that environmental changes from 2003 influenced the abundance of Acartia species, being unfavourable for A. clausi. The decrease of A. clausi abundance in low salinity waters was related to a significant decrease of dissolved oxygen saturation levels, whereas the increase of temperature was linked to a significant increase of A. tonsa. Acartia margalefi and A. discaudata were scarce over the entire period, but they were found to be valuable indicators of hydrological changes, which were associated to climate factors. These two latter species increased in abundance and expanded their seasonal distribution, and in the case of A. margalefi also its spatial distribution, in 2002, coinciding with the period in the time-series when autumn–winter rainfall and summer temperatures were lowest, and dissolved oxygen saturation levels were highest.

This study is a part of the zooplankton monitoring program carried out in the euhaline region of the estuaries of Bilbao and Urdaibai (Basque coast, Bay of Biscay), and analyses between-estuaries differences in zooplankton spatial and temporal patterns in relation to environmental conditions between July 1999 and May 2001. Environmental variables measured were water temperature, dissolved oxygen saturation (DOS), Secchi disk depth (SDD) and chlorophyll a. Relationships between zooplankton community and environmental variables were analysed using canonical correspondence analysis; between-estuaries differences in environmental conditions and distribution of zooplankton taxa in relation to salinity were tested using Mann― Whitney U-test. Spatial differentiation of the zooplankton community was higher in the estuary of Bilbao, with the relative abundance of most of the taxa decreasing more pronouncedly towards the upstream estuary than in the Urdaibai related to significantly lower values of DOS and SDD, reflecting the higher degree of pollution, in the Bilbao estuary. However, the successful establishment of the Acartia discaudata and A. margalefi populations, and the first records of another Acartia species, Calanipeda aquaedulcis and Eurytemora affinis in the Bilbao estuary, along with the increasing similarity between zooplankton assemblages of the Bilbao and Urdaibai estuaries in relation to the period 1997―1999, represent a new step in the recovery of the zooplankton community in the estuary of Bilbao responding to the improvement of water quality.

The effect of the North Atlantic Oscillation (NAO) indices on climatic conditions and their subsequent influence on water temperature of two Basque estuaries (estuary of Bilbao and estuary of Urdaibai) were assessed by transfer function (TF) models for the period 1997–2006. Results showed that air temperature had an immediate (lag=0) and significant negative response to the NAO, whereas rainfall was not correlated with this climate index. The negative correlation between NAO and air temperature was found to be stronger with the seasonal indices derived from the differences in surface pressure between Iceland and Azores than with that derived from the principal component time-series of the leading eigenvector of the sea-level pressure in an Atlantic sector. The correlations between rainfall and river discharge, and between air temperature and water temperature were positive and highly significant in both estuaries. The response of water temperature to air temperature was immediate in both estuaries, whereas one-quarter lagged responses were also observed in the estuary of Bilbao, which is deeper and more stratified than the estuary of Urdaibai. Our study provides evidence that on the Basque coast the NAO plays an important role in climate variations, which in turn affect estuarine water temperature.

The relationships between chlorophyll a concentration and environmental (climatic and associated hydrographical) factors were investigated in the estuary of Urdaibai (Bay of Biscay) in different salinity zones of the euhaline region, using time-series for the period 1997–2006. Transfer function (TF) models were used on quarterly data (3 month mean values) to establish possible relationships between time-series. In the non-nutrient limited waters with salinities of 30 and 33 PSU, a chain of effects from the North Atlantic Oscillation (NAO) to chlorophyll a was established, where air temperature followed inversely the NAO index, water temperature followed air temperature and chlorophyll a followed water temperature. Each of the steps occurred with a lag of 0 (within the 3 month period); however, the effect from NAO to chlorophyll a showed a lag of 1 (a mean of 3 months delay). Consistent with this result, annual mean chlorophyll a biomass in the 30 and 33 PSU salinity zones showed a significant positive relationship with annual mean water temperature and a significant negative relationship with the 12 month mean NAO index from October of the previous year to September. In the 35 PSU salinity zone, no significant relationship between chlorophyll a and NAO or water temperature was observed. It is suggested that nutrient limitation distorts the effect of temperature on phytoplankton biomass enhancement in the outer estuary (35 PSU salinity zone).

The nutritional environment for fish larvae is analysed for the Basque continental shelf (Bay of Biscay), during the main anchovy-spawning period (spring) over two years. The quantity and quality of Suspended Particulate Matter (seston) is analysed for total particles, microparticles (20–200 μm) and mesoparticles (200–1000 μm). Separately, the characterisation of the

The abundance and hatching success of Acartia bifilosa resting eggs in subtidal sediments of the Urdaibai estuary was analysed in the context of a study on the spatial and temporal dynamics of this species. Two sediment cores of 20 to 30 cm depth were obtained at two sites of the middle zone, where the accumulation of mud and organic particles is favoured. Laboratory incubations were performed to determine the hatching success of eggs and identify newborns. Resting egg abundance was around 107 eggs m -2 , and was higher at the site where the A. bifilosa population maintains an optimal position in the water column. The lower egg abundance in upper layers, as corresponds to winter, was associated with the seasonal dynamics of planktonic populations. On average, the hatching success was >50% at both sites. The lack of significant differences in the hatching success according to sediment depth suggests that eggs remain viable after being buried for a long-time in anoxic conditions. This enables us to infer that the benthic egg bank of A. bifilosa in this estuary is a safe source of recruits into the planktonic population, and plays a crucial role in recuperationand maintenance of the population.

Variations in prosome length and width, dry weight and condition factor of female Acartia clausi copepods were studied at three salinities (35, 34 and 33 psu) in the euhaline region of two estuaries (Bilbao and Urdaibai) of the Basque coast, with different level of anthropogenic impact. Effect of the environmental variables upon the morphology of A. clausi females on a small geographic scale is discussed. In general, biometric variables showed no significant differences between the two estuaries, but dry weight and condition factor were significantly higher in the estuary of Urdaibai at 35 and 34 psu, whilst at 33 psu they were higher in Bilbao. Body dimensions decreased significantly with decreasing salinity in both estuaries, however, no similar trends were observed for dry weight and condition factor. Temperature appeared the main variable to account size variations, but once eliminated seasonal effect of the temperature body size was related with oxygen concentration in the estuary of Bilbao and with salinity in Urdaibai. This study reveals that morphological characteristics of A. clausi not are only dependent on the temperature, but also, within a limited geographical zone, on local differences in environmental variables, mainly salinity and oxygen concentration.

The abundance and spatial distribution of copepod species were compared in the euhaline region of the polluted estuary of Bilbao [mean biological oxygen demand (BOD): 49748.05 kg day -1 in 1996] and the unperturbed estuary of Urdaibai. Sampling was performed at fixed salinity sites, where data of temperature, dissolved oxygen saturation, particulate organic matter, Secchi disk depth and chlorophyll were also obtained. Differences between the estuaries and within the estuaries for environmental variables and copepod abundances were tested statistically. The dominant species Acartia clausi and Paracalanus parvus differed significantly in abundance between estuaries, and most of nerilic species diminished more drastically with decreasing salinity in Bilbao by the effect of pollution. The different responses observed among species in relation to pollution and water desalination are discussed. The lack of the brackish-water species in Bilbao indicated that water quality worsening upward the estuary of Bilbao not only limited the penetration of neritic species but also prevented the development of autochthonous estuarine species. At the outer euhaline region of Bilbao, however, pollution was found to be low enough to enhance the development of tolerant neritic species that are favoured under moderate pollution conditions. Results indicate that comparisons of copepod abundances and distributions with salinity in estuaries may he a useful tool to evaluate the health of the different estuarine pelagic habitats.

The fecundity and hatching success of Acartia clausi were analysed at fixed salinity sites (35, 34 and 33 psu) in two nearby estuaries (Bilbao and Urdaibai, Basque coast, Bay of Biscay) from March to June 1997. Field incubations were conducted to estimate egg production rates and hatching success, and the size of eggs and experimental females measured. Water temperature and dissolved oxygen saturation were also determined, as well as seston samples to quantify food abundance and quality. Between-estuary and within-estuary differences were tested statistically, and correlation and regression analyses were used to determine relationships between reproductive and environmental variables. Egg production rates were higher in the organically enriched estuary of Bilbao; this denoting that food supply controls the fertility of A. clausi in these systems. Temporal patterns of egg production differed between estuaries, and were associated with different nutritional factors in each estuary. Within the salinity range analysed, egg production reached higher values at intermediate salinity (≈34 psu) in both estuaries. This was interpreted as the result of the interaction between the positive effect of food increase, and the negative effect of physicochemical conditions with decreasing salinity. Egg size variations mainly occurred temporally in relation to female size, but no clear trade-off between egg size and egg number was observed in any case. A drop in hatching success in Bilbao, mainly in waters of

The abundance and spatial distribution of mesozooplankton were compared in the euhaline region of the polluted Bilbao estuary and the unperturbed Urdaibai estuary. The environmental variables measured were salinity, temperature, dissolved oxygen saturation, chlorophyll a, particulate organic matter and Secchi disk depth. Differences in the environmental variables and zooplankton between the estuaries and within the estuaries were tested statistically. Differences in the patterns of mesozooplankton abundance with salinity in each estuary indicated that they were affected by pollution in the Bilbao. Total mesozooplankton, and several mesozooplankton categories were more abundant in the Bilbao at higher salinities but in the Urdaibai at lower salinities. Therefore, we deduced that, in the Bilbao, zooplankton abundance was enhanced by the moderate pollution at the outer euhaline zone and limited by the poorer conditions of the inner zone. The response of the various zooplankton categories to the pollution is discussed.

The reproductive response involving egg number and size was studied for the egg carrying copepod Euterpina acutifrons in two estuaries with different nutritional environment (Bilbao and Mundaka). To characterise the available food for copepods, the concentration of particulate organic matter (POM), the combined concentration (PGL) and proportion of particulate protein, carbohydrate and lipids and the concentration of chlorophyll a were measured in the

The egg production of the copepod Acartia bifilosa was measured and related to environmental variables and food availability in two estuaries located in the same biogeographic region (Bay of Biscay) but showing very strong differences in abiotic and biotic features: the Gironde estuary (France) and the estuary of Mundaka (Spain). The study was conducted during the spring-summer-autumn period of 1994, Food availability was evaluated by analysing the chlorophyll a (Chi a), the particulate organic carbon (POC) and the easily extractable macromolecular compounds such as proteins, carbohydrates and lipids of the seston. The egg production of copepods was estimated from field incubations with natural water, and phytoplankton feeding of adult females was estimated by means of the gut fluorescence method. The nutritional environment of the Gironde was characterised by high amounts of suspended particulate matter (SPM) with low food value, emphasising the mainly detrital origin of the organic matter (OM). In Mundaka, the higher contribution of phytoplankton to the seston led to marked increase, in particulate food value accounting for up to 35% of organic matter. The weight-specific egg production was found to be sharply higher in Mundaka (ranging from 0.2 to 0.63 x 10(-3) day(-1)) than in the Gironde (ranging from 0 to 0.13 x 10(-3) day(-1)), but the seasonal trend of variations was similar, the highest weight-specific egg production rates occurring in early summer and the lowest in autumn in both estuaries. Egg production was not correlated linearly with temperature since maximal egg production occurred at intermediate temperatures. In Mundaka, the egg production showed a significant positive con-elation with the chlorophyll and the Chl/SPM and the POC/SPM ratios. This coupled with higher values of algal food availability (Chi a/SPM: 10 to 1870 mug g(-1)) and gut fluorescence (between 0, 12 and 0,38 ng Chi a Eq ind(-1)) indicate that a herbivorous diet could cover the energy requirements of A. bifilosa and support egg production. In the Gironde, the algal food availability and the gut fluorescence were lower (Chi a/SPM: 10 to 80 mug g(-1) GF: 0.09 and 0.25 ng Chi a Eq ind(-1)), and the egg production showed significant positive correlation with the particulate food value, suggesting that other sources of carbon rather than phytoplankton ere responsible for the observed changes in egg production, Results indicate that the particular seston properties of each system may be responsible for the noticeable differences in A. bifilosa fertility among estuaries. (C) 2002 Elsevier Science B.V All rights reserved.

The grazing activity of nanozooplankton natural assemblages on phytoplankton was analysed in the polyhaline and euhaline zones of the estuary of Mundaka throughout the spring–summer–autumn period. Grazing experiments were conducted in situ using the dilution method. Incubations occurred within a relatively wide range of initial chlorophyll concentration (1–10 μg chl l −1 ), but saturated feeding kinetics were not observed in most experiments. The heterotrophic nanoplankton grazed on average between 43 and 51% (mean values from all dilutions data and the 3 more diluted data, respectively) of the phytoplankton stock in the polyhaline zone, and between 36 and 43% in the euhaline. The grazing impact on primary production averaged around 65% in both zones, but showed higher variability in the euhaline (0–223%) than in the polyhaline (15–117%). Herbivore pressure was found to be higher in polyhaline waters on average, but between-zone differences were not significant. The nanozooplankton herbivory was not clearly related to water temperature, nor to phytoplankton biomass (expresed as chlorophyll concentration), and the coupling between production and consumption was weak, being more unbalanced in the euhaline zone. Nanozooplankton grazing was sometimes found to cause phytoplankton decline, while at other times its impact was negligible. In addition, indirect evidence suggests that nanozooplankton grazing activity may be affected by predation pressure from mesozooplankton in the polyhaline zone. Seasonal patterns of phytoplankton exploitation by heterotrophic nanoplankton differed among saline zones, suggesting that the fate of phytogenic carbon in the estuary of Mundaka may be highly variable both spatially and seasonally.

Seasonal and inter-annual variations of dissolved oxygen (DO) along the estuary of Bilbao were investigated from 1998 to 2008, during its rehabilitation phase from pollution, to determine whether anthropogenic or natural forcings or both govern DO dynamics and hypoxia. Both seasonal and inter-annual variations of DO were best explained by hydro-climatic factors, sewage pollution and phytoplankton dynamics in the inner, intermediate and outer estuary respectively. The most remarkable intra-decadal improvement in DO occurred in the halocline layer of the intermediate estuary, where the factor that best explained these changes was sewage pollution abatement. However, in the estuarine hotspot for hypoxia, i.e. inner estuary bottom waters, no parallel response to sewage pollution abatement was observed and hydro-climatic factors were the main drivers of inter-annual DO variations. Differences in the degree of stratification and flushing accounted for this differential response of DO to anthropogenic and climate-related forcings at both axial and vertical scales.

The hydrographic structure of Abra Harbour, a tidal embayment located at the seaward end of the highly polluted estuary of Bilbao, is influenced by the inflow of a polluted estuarine plume, the asymmetry of the harbour basin, and the tidal circulation pattern. Multivariate analysis of the spatial variability of the zooplankton between May 1981 and May 1982 showed that significant differences in zooplankton abundance and species composition occurred from the inner-eastern to the outer-western side, related to the horizontal structure of the system. The annual temperature cycle, however, was the major source of temporal variability, and the seasonal stratification in the water column was responsible for the predominance of vertical differences in zooplankton composition during the spring-summer period. Wind-induced turbulence and tides were other sources of variation. With increasing turbulence (rough sea), the spatial gradients in zooplankton composition were not as clear, and with decreasing tidal height the compositional differences in the horizontal dimension tended to be more evident. Spionid larvae accounted for strong local differences within the Harbour. They were usually segregated from other taxa, and mainly associated to the deeper waters characterized by a greater hydrological instability.

The effects of environmental factors on variations of dissolved oxygen (DO) saturation was assessed in 2 estuaries (Bilbao and Urdaibai) in the Bay of Biscay with contrasting hydrography and anthropogenic impact, for the period 1997 to 2006, using generalised additive models (GAMs) and generalised additive mixed models (GAMMs). Spatial and temporal variations in DO saturation and the current severity and extent of hypoxia were higher in the culturally eutrophicated and stratified Bilbao estuary. Seasonal patterns of DO saturation and the hierarchy of factors controlling them differed both between estuaries and between salinity zones. In the inner Bilbao estuary, the main factor explaining DO-saturation variations was temperature, with a dominant effect on oxygen consumption processes. In contrast, in the outer Bilbao estuary, the main factor explaining DO-saturation variations was chlorophyll a, suggesting that the effect of oxygen production through photosynthesis was stronger than that of oxygen consumption. In the inner Urdaibai estuary, both a negative relationship with temperature and a positive relationship with chlorophyll a were detected. The outer Urdaibai estuary showed strong water-column mixing and very high tidal flushing, and ventilation processes were more important than oxygen production or consumption processes. River discharge affected DO saturation only at the inner waters, but when the effects of several variables on DO-saturation variability were tested in an additive model, river discharge was excluded from the models. The effects of temperature-dependent processes on DO-saturation dynamics decreased from the inner to the outer zones of both estuaries., Sí

Daily variations in the plankton community of the Gernika estuary (Bay of Biscay) were studied over three week-long periods in order to evaluate the effects of large increases in river runoff due to intense rain pulses. Both tidal and river inflow changes determined the hydrological zonation within the estuary during the study periods. Chlorophyll a appeared to be related to riverine inputs and resuspension processes rather than to phytoplankton growth dynamics. The spatio-temporal distributions of plankton assemblages were related to hydrological conditions. Thus, a successional progression involving short-term interactions among plankton populations was severely affected by increased river discharge. Freshets removed neritic populations and returned the plankton community to an initial state. As hydrological conditions became more marine, the progression towards a community with metazoan predominance was paralleled with a rise in neritic plankton abundance further up the estuary. The development of estuarine populations, which can reach high densities under stable conditions, seemed to be limited by frequent river runoff disturbances occurring in this estuarine system.

Abundance and biochemical composition of organic materials in sediments of the estuaries of Mundaka and Bilbao (SE Bay of Biscay) were analysed to assess the effect of organic wastes released to these systems. Organic and labile contents were higher in the sediments of Bilbao, denoting organic enrichment with poorly decomposed materials by human dumping. Spatially, organic matter distribution skewed seaward in Bilbao and upward in Mundaka, in agreement with the location of major point sources of sewage. Labile material, proteins and protein:carbohydrate ratio showed upward increases in both estuaries, attributed to the oxygen restrictive conditions to decompose organic materials. Vertical distribution of organic components into sediments reflected the history of human activities. In Bilbao, the significant reduction of organic and labile compounds, and the decrease of protein:carbohydrate values in top layers seem a result of recent remedial actions to reduce urban sewage inputs and improve environmental conditions. Higher contributions of proteins and lipids were associated with anthropogenic materials, and higher contributions of carbohydrates with autochthonous materials. High protein:carbohydrate values indicate poorly decomposed materials of human origin. The close relationship of carbohydrates with chloropigments in Mundaka suggests that phytobenthic populations and derived detritus contributed to a greater extent to the organic fraction in this estuary.