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The interannual variations in absolute and relative densities of copepods from the neritic waters of the southeastern Bay of Biscay and their relationship to climate teleconnections and local environmental factors were assessed using time series for the 1998–2015 period. Opposite patterns of variation of the absolute densities of spring Acartia clausi and Centropages typicusspecies vs. summer/autumn species, mainly Oncaea media, but also Ditrichocorycaeus anglicus, Oithona nana, Temora stylifera, and Oithona plumifera were detected. This type of opposite patterns were also observed between the relative densities of the spring A. clausi and summer Paracalanus parvus species. These opposite density patterns were positively correlated to all seasons North Atlantic Oscillation (NAO), spring East Atlantic pattern (EA), summer and autumn water temperature and summer chlorophyll a. They were negatively correlated to summer EA pattern, the winter and spring Atlantic Multidecadal Oscillation (AMO) and water temperature, and the upwelling index of all seasons. In these patterns of copepod variation two regime shifts were detected, one in 2008 towards an increase in the spring species and the other one in 2014–2015 towards an increase of summer species. This latter regime shift coincides in time with the abrupt community shifts predicted in the literature for 2014.

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 realized ecological niches of native zooplankton and non-indigenous species (NIS) were analysed in the contrasting estuaries of Bilbao and Urdaibai from 1998 to 2015 in order to study their responses to the particular features of each estuary and the biotic and abiotic changes along the study period. The marginality and tolerance of zooplankton taxa was estimated for the two estuaries together using the OMI analysis, whilst, by means of the WitOMI analysis, niches were differentiated into spatial subniches to assess the differences between estuaries and into temporal subniches to determine the variations in time within each estuary. The changes in the niche overlap of the main native and NIS species were calculated with the D-metric. Results showed that the combined effect of salinity gradients and seasonal variations, linked to temperature, defined the main spatio-temporal niches of zooplankton taxa in the overall environmental scenario of both estuaries. Thus, those factors separated winter-spring neritic, summer-autumn neritic, warm water affinity brackish and limnetic species. Secondarily, river discharge and hydrological stability accounted for the higher presence of freshwater and some brackish species in the estuary of Bilbao and tychoplanktonic organisms in the estuary of Urdaibai. At the regional scale the marginality and tolerance of zooplankton taxa reflected a combination of spatial and seasonal niche breadth and difference of abundance between estuaries. The number of taxa with niches not clearly explained by the environmental variables studied was higher in the estuary of Urdaibai, whilst the new arriving species occupied realized niches well defined by the environmental variables and overall zooplankton taxa showed a better discrimination from generalist to specialist behaviours in the estuary of Bilbao. This was related to inherent abiotic and biotic features of each estuary, which make the estuary of Urdaibai more refractory to the settlement of NIS species. Moreover, in the estuary of Bilbao, several neritic and brackish species experienced temporal changes in niche breadth and overlap that was attributed both to the environmental improvement related to rehabilitation plans in the system and the impact of the species that arrived during the study period. In the estuary of Urdaibai only small changes attributable to the limited impact of NIS were inferred, since no meaningful changes in environmental conditions were perceived. This work was supported by the University of the Basque Country through research funds to Consolidated Research Groups (GIU16/69) and through a grant to Z. Barroeta. The authors thank the WGEUROBUS of the International Council for the Exploration of the Sea (ICES) for facilitating research on P. marinus.

Zooplankton seasonality and its environmental drivers were studied at four coastal sites within the Northeast Atlantic Shelves Province (Bilbao35 (B35) and Urdaibai35 (U35) in the Bay of Biscay, Plymouth L4 (L4) in the English Channel and Stonehaven (SH) in the North Sea) using time series spanning 1999–2013. Seasonal community patterns were extracted at the level of broad zooplankton groups and copepod and cladoceran genera using redundancy analysis. Temperature was generally the environmental factor that explained most of the taxa seasonal variations at the four sites. However, between-site differences related to latitude and trophic status (i.e. from oligotrophic to mesotrophic) were observed in the seasonality of zooplankton community, mainly in the pattern of taxa that peaked in spring-summer as opposed to late autumn-winter zooplankton, which were linked primarily to differences in the seasonal pattern of phytoplankton. The percentage of taxa variations explained by environmental factors increased with latitude and trophic status likely related to the increase in the co-variation of temperature and chlorophyll a, as well as in the increase in regularity of the seasonal patterns of both temperature and chlorophyll a from south to north, and of chlorophyll a with trophic status. Cladocerans and cirripede larvae at B35 and U35, echinoderm larvae at L4 and decapod larvae at SH made the highest contribution to shape the main mode of seasonal pattern of zooplankton community, which showed a seasonal delay with latitude, as well as with the increase in trophic status.

Contribution of latitude, distance offshore and environmental factors to variations in zooplankton assemblages across the Northeast Atlantic Shelves Province, from the Bay of Biscay [Bilbao 35 (B35) and Urdaibai 35 (U35)] to the English Channel (Plymouth L4; L4) and the North Sea (Stonehaven; SH), were assessed mainly by redundancy analysis. For coarse zooplankton groups latitude explained the main between-site differences, and meroplankton contributed more than holoplankton. Latitudinal differences were best indicated by contrasting abundances of cirripede larvae and doliolids (most abundant at the lowest latitude sites) and bryozoan and polychaete larvae (most abundant at the highest latitude site). Doliolids were best indicators of temperature-mediated latitudinal differences. The interaction between latitude and distance offshore or salinity and phytoplankton biomass explained smaller percentages of the variability. The main differences in copepod and cladoceran genera reflected the oceanic influence, with highest presence of Corycaeus and Oncaea at L4, likely related to the higher influence of off-shelf water intrusions, and neritic Acartia dominating at SH, U35 and B35. Podon and Evadne, which decreased from south to north, reflected latitude-related differences driven more by salinity than by temperature. Instances where a single species (e.g. Acartia clausi) dominated showed common relationships with temperature, consistent with a common thermal niche. Differences in co-generic species dominance between sites depicted the latitudinal gradient.

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.

Zooplankton abundance series (1999–2013) from the coastal sites of Bilbao 35 (B35), Urdaibai 35 (U35), Plymouth L4 (L4) and Stonehaven (SH), in the Northeast Atlantic were compared to assess differences in the magnitude of seasonal, interannual and residual scales of variability, and in patterns of seasonal and interannual variation in relation to latitudinal location and trophic status. Results showed highest seasonal variability at SH consistent with its northernmost location, highest interannual variability at U35 associated to an atypical event identified in 2012 in the Bay of Biscay, and highest residual variability at U35 and B35 likely related to lower sampling frequency and higher natural and anthropogenic stress. Interannual zooplankton variations were not coherent across sites, suggesting the dominance of local influences over large scale environmental drivers. For most taxa the seasonal pattern showed coherent differences across sites, the northward delay of the annual peak being the most common feature. The between-site seasonal differences in spring–summer zooplankton taxa were related mainly to phytoplankton biomass, in turn, related to differences in latitude or anthropogenic nutrient enrichment. The northward delay in water cooling likely accounted for between-site seasonal differences in taxa that increase in the second half of the year.

The rapid warming of the wolrd’s oceans during the last few decades has affected distributional patterns of marine planktonic communities. Here, we analyse links between sea warming and changes in copepod community composition over the last 3 decades (1980−2012). We used zooplankton time-series data which included 79 species of copepods collected at 3 sites in the eastern North Atlantic (Bay of Biscay and the Kattegat Sea) and the Mediterranean Sea (Gulf of Saronikos). First, using community β-diversity metrics, we analysed temporal patterns of copepod community composition changes over time and its relation to local environmental conditions. Second, to test whether the changes in copepod community composition correspond to community thermal preferences, we used the community temperature index (CTI) and compared CTI interannual changes with local temperature trends. The β-diversity analysis reveals a high temporal turnover in the copepod community composition at the 3 sites (30−45%), with a significant similarity decrease over time (‘decay’) associated with both niche descriptors and demographic stochastic processes. CTI results reveal that both in the Kattegat and Saronikos, where the ocean warming rate was the highest amongst sites, copepod community changes are linked to temperature variability, suggesting that the community is tracking their thermal niche over time. Our findings unveil the fundamental role of abiotic factors structuring copepod biodiversity over time and reveal that the local velocity of ocean warming and the species thermal thresholds are key to rearranging copepod community composition in coastal ecosystems.

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).

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 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.

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 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.

18 pages, 13 figures, 8 tables, The abundance, composition and mesoscale variability of the microplankton (53-200. μm) and the mesoplankton (0.2-2. mm) fractions in relation to oceanographic factors and phytoplankton biomass were compared off the Catalan coast (NW Mediterranean) during the summer stratification (June) and autumn mixing (November) periods in 2005. This work aims to determine whether the two plankton fractions that more contribute to fish larval diet respond to a common variable environment, and this study constitutes the first attempt to analyse, in parallel, the spatial structure of both fractions in this area. From June to November microplankton abundance increased mainly by the increase of dinoflagellates, tintinnids and radiolarians, and mesoplankton decreased due mainly to the decrease of long-horned dinoflagellates, cladocerans, doliolids and appendicularians. Plankton mesoscale variability in relation to environmental variables showed higher complexity in June, where environmental horizontal and vertical gradients were more marked than in November. In June, the major mode of variability of the microplankton was mainly accounted by the patchy distribution of several tintinnid species dominated by Rhabdonella spiralis associated to the subsurface phytoplankton biomass. The main mode of variability of the mesoplankton was related to the intrusion of the Ebro river plume and the related aggregation of doliolids and cladocerans, dominated by Evadne spinifera. In November, the major variability pattern in both fractions was a combination of inshore-offshore and eastern-western gradients in taxa distributions shaped mainly by the course of the Catalan Current along the shelf-break. Spatial differences in planktonic food pathways in each period are discussed on the basis of literature on plankton feeding habits and types, and on the diet of fish larvae of the main species from the same surveys. © 2014 Elsevier B.V., This research was funded by the CTM2004-03510-C02-01 and 02 ⁄MAR Project. The team of the UPV/EHU also received finnancial support for this work from the Basque Government (GIC10/168) and the University of the Basque Country (UFI11/37)

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.

12 pages, 7 figures, 4 tables, The feeding ecology of the larvae of the two most important small pelagic species in the western Mediterranean, anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus), is investigated. Samples were collected in early summer (anchovy) and autumn (sardine). Composition of the diet and prey selectivity were compared over the course of ontogeny in anchovy (2.7-14.5 mm standard length, SL) and sardine (5.5-15.8 mm SL) larvae. Anchovy larvae begin feeding on prey items >150 m at smaller sizes than sardine larvae, and the diets of both species are based mainly on copepod nauplii and postnauplii. Seasonal differences in the composition of the plankton influenced the contributions of prey types to the diets of the two species, e.g. the cladoceran Evadne spp. in anchovy and the tintinnid Codonellopsis sp. in sardine. Although copepod eggs are generally present in the diets of larval clupeoids, they were not major components of the diets of the species considered here. Despite morphological similarity, selection patterns were different between the species and changed through ontogeny. © 2009 International Council for the Exploration of the Sea, The research was funded by projects CTM2004-03510-C02-01/MAR and CTM2004-03510-C02-02/ MAR. EM acknowledges predoctoral FPI Fellowship support from Spain’s Ministry of Education and Science

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.

The day-to-day variations over a 17-day period in the plankton community of a shallow embayment located in the Basque Country (Bay of Biscay) were studied and related to changing environmental conditions. Fluctuations in river runoff due to several rain pulses determined most of the changes in the chemical properties of the embayment. Other physical forcing functions like wave originated high turbulence in the water column, regulating the quantity and quality of seston and chlorophyll a. As a response to the combined effects of these processes, different patterns of variation were observed in phyto- and zooplankton populations. Skeletonema costatum developed after nutrient inputs into the system and resulted in enhanced primary production levels. Population growth was also controlled by turbulence, while the disappearance of this pulse could be associated with the depletion of silicate. Zooplankton abundance fluctuations can be explained mainly by their motility to avoid the environmental stress imposed by water turbulence and sediment resuspension.

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.