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The impact of the occurrence of the non-indigenous species (NIS) Acartia tonsa, Oithona davisae, and Pseudodiaptomus marinus and of the range-expanding copepods Acartia bifilosa and Calanipeda aquaedulcis on the structure and diversity of zooplankton communities was analyzed using 18 years (1998-2015) time series from the contrasting estuaries of Bilbao and Urdaibai (Basque coast, Bay of Biscay). Changes in the structure of communities were assessed by using multivariate analyses of taxa abundances and changes in diversity by using descriptors of alpha, beta, and gamma diversity. The most evident changes occurred at the upper reaches of the estuary of Bilbao, where an abundant and less diverse brackish community, dominated by the NIS, A. tonsa and O. davisae, succeeded a low abundance and more diverse community of neritic origin. The later establishment of C. aquaedulcis was linked to further changes in the structure of the community and a progressive increase in diversity. The seasonal pattern of diversity at the inner estuary and the beta diversity in the estuary were also significantly affected by the arrival of the NIS and C. aquaedulcis. In contrast, the original low diversity brackish community of the estuary of Urdaibai, clearly dominated by A. bifilosa, was far less affected by the arrival of the same copepod NIS, and A. bifilosa remained as the species best related to the changes in zooplankton structure and diversity in the brackish habitats of the estuary. Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. This work was supported by the University of the Basque Country (UPV/EHU) (UPV/EHU, GIU19/059) and the Basque Government (PIBA2020-1-0028 & IT1723-22) and through a grant to Z. Barroeta to carry out her PhD (UPV/EHUn Ikertzaileak Prestatzeko Kontratazio Deialdia 2016).

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.

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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.

Since 2007, the non-indigenous calanoid copepod Pseudodiaptomus marinus Sato, 1913 has been increasingly recorded in numerous European sites, spreading at an unexpectedly fast pace over a short time-span. This species presents specific biological and behavioural traits which make it of particular interest for ecological and applied research topics. On 29–30 January 2018, 29 scientists from nine European Countries established the EUROBUS (Towards a EURopean OBservatory of the nonindigenous calanoid copepod Pseudodiaptomus marinUS) Working Group (WG). This WG aimed at creating a European network of institutions and researchers working on the various aspects of the biology and ecology of P. marinus, with an open forum where sharing experience and know-how among WG participants. This brought to an updated distribution map of P. marinus in European waters, as well as to the identification of priority research lines and potential joint initiatives under the WGEUROBUS umbrella. This contribution, stemming from the experts participating at the WG, represents the manifesto of the current and future initiatives developed within WGEUROBUS. info:eu-repo/semantics/publishedVersion

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 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.

Feeding is critical for larval survival and it becomes a determining factor in the annual recruitment in species such as European anchovy Engraulis encrasicolus. Additionally, since differences in the plankton community may affect the larval feeding patterns, an interregional comparative study may contribute to understand these patterns concerning larval development and environmental features. The diet, feeding pattern and prey selectivity in relation to the ontogeny of the larval anchovy in the Cantabrian Sea (inner Bay of Biscay) and the Catalan Sea (NW Mediterranean) were compared in this study. Larvae fed preferentially on copepod eggs, nauplii and calanoid postnauplii, selecting them actively. Larval diet was similar before notochord flexion in the two areas. Nonetheless, for larvae of comparable size, prey preferences skewed towards small preys in the Cantabrian Sea, and towards large preys in the Catalan Sea. Differences appeared in the middle-sized larvae (6.5–9 mm) due to prey size and type. Diet differences were attributable (a) to the ability of the Catalan Sea larvae to capture bigger preys at a smaller individual size; and (b) to differences in prey preferences and availability. Therefore, geographic variety in larval populations and food environment features may promote variability in the larval anchovy diet.

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.

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).

13 pages, 7 figures, 1 table, Feeding ecology was analysed for the first time in the larvae of the European hake (Merluccius merluccius) to determine whether their diet and selectivity were constrained by environmental conditions and how these feeding characteristics were related to ontogeny, prey availability and visual capabilities. Larvae collected during both day and night were analysed, and it was found that feeding incidence was high, regardless of the time of day. Examination of the visual system corroborated the hypothesis that hake larvae should be able to cope with a wide range of photic conditions and to forage even at low light intensity. A clear preference for adult calanoid copepods and, especially, for Clausocalanus spp. was observed in all sizes analysed. Prey number increased with larval size, but prey size did not. This finding indicates that hake larvae behave as selective and specialist predators that consume an increasing number of prey rather than larger prey during larval growth, The research was funded by projects CTM2004-03510 C02-01/MAR and CTM2004-03510-C02-02/MAR. E. Morote acknowledges predoctoral FPI Fellowship support from Spain’s Ministry of Education and Science

The growth strategies of pelagic larvae of 3 fish species from the Bay of Biscay belong- ing to 2 different shape groups, the eel-like anchovy Engraulis encrasicolus and sardine Sardina pilchardus, and the tadpole-like horse mackerel Trachurus trachurus, are compared in terms of changes in their biochemical composition and energy allocation. In anchovy and sardine, the relative amount of protein increased and relative amounts of carbohydrates and lipids decreased throughout their growth. The biochemical composition in horse mackerel did not change significantly during growth. Differences between the 2 shape groups could be associated with the various energy alloca- tion patterns related to dissimilar growth strategies. Although the patterns of change in biochemical composition during growth are different in the 2 shape groups, they show a very similar biochemical composition at the notochord flexion stage, indicating convergences in life-history patterns of the dif- ferent species.

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.

Growth and condition of larval Engraulis encrasicolus larvae in the Bay of Biscay were analysed relative to their biochemical composition. Protein, carbohydrate, lipid, DNA and RNA contents of larvae were quantified during larval ontogeny to analyse changes associated with growth. Proteins were the main organic component of E. encrasicolus at every larval stage, followed by lipids and carbohydrates. The increase in relative protein content in terms of protein percentage (P%) and the decrease in lipid relative content (L%) of the total organic matter (OM) with size observed are consistent with the definition of Type 1 larvae. Protein relative content increased until the P% in larvae reached ~71%, whereas L% and carbohydrate relative content (CH%) declined, exhibiting minimum values of ~20, and ~6%, respectively. In general terms, the feeding condition of larvae, as defined by their RNA/DNA ratio, was good. Food did not seem to be a major factor limiting larval growth in the study area throughout late spring and early summer. The RNA/DNA ratio was size-related and increased during larval growth up to an asymptotic optimal value of ~3.6 in postflex- ion larvae in relative protein content, in terms of protein percentage (P%). The 'actual' RNA/DNA ratios found in a given larval population when compared with the 'starving' RNA/DNA ratio, and using an 'absolutely optimal' value as complementary information, could be used to determine the overall status of the population. Parental effect of low egg quality is a possible explanation of delayed ontogenetical development of larvae from July 2000.

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).

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.

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)

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

10 pages, 7 figures, 2 tables., The present study analyses for the first time the diet of larvae of the clupeid Sardinella aurita in the NW Mediterranean. Gut contents of larvae from first feeding (3.9 mm) to early postflexion stages (14.7 mm) were analysed. The type and abundance of ingested prey were related to the plankton composition in the environment in order to determine prey-type selectivity. The feeding incidence observed was among the highest reported for clupeid larvae, with higher values during the day (68.6%) than at night (7.7%), indicating that Sardinella aurita larvae are diurnal predators. The mean number of ingested prey was 3.3 (±2.51). The width of the prey increased from < 30 to 410 μm with the development of the larvae, but the niche breadth did not vary with the size of the larvae. The diet was based on different stages of copepods (nauplii and postnauplii) and cladocerans (mainly Evadne spp.), though the preference for each type of prey varied, with higher preference for copepod nauplii in larvae < 8 mm and a higher preference for Evadne spp. in larvae ≥ 8 mm. The copepod based diet of S. aurita larvae is very similar to that reported for Engraulis encrasicolus larvae in the Mediterranean, indicating a possible competition for food between the first-feeding larvae in situations of low prey abundance., This research was funded by the CTM2004-03 510-C02-02/MAR Project. E.M. acknowledges the predoctoral FPI Fellowship support from Spain’s Ministry of Education and Science.