Fernando Villate

Plankton responses to hydrological changes induced by freshets in a shallow mesotidal estuary

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.

Day-to-day variability in the plankton community of a coastal shallow embayment in response to changes in river runoff and water turbulence

Abstract 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.

Dissolved oxygen in the rehabilitation phase of an estuary: influence of sewage pollution abatement and hydro-climatic factors.

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.

Reproductive response of Euterpina acutifrons in two estuaries of the Basque Country (Bay of Biscay) with contrasting nutritional environment

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 <20 μm seston fraction. Data on female size (prosome length) and abiotic factors, such as temperature, salinity and oxygen saturation in water, were also obtained to relate them to reproductive and nutritional variables. Correlation and stepwise regression analyses indicated that temperature via female size was the main factor controlling brood volume in a temporal context, whilst the nutritional environment had a secondary role. Nevertheless, the nutritional environment caused noticeable between-estuary differences in the reproductive response of this species. ANOVA tests showed that E. acutifrons clutch size and brood volume were significantly bigger in the estuary of Bilbao, which was richer in both total (POM) and labile (PGL) organic particulate matter than the estuary of Mundaka. In contrast, egg size, female size and phytoplanktonic food quantity did not differ significantly between estuaries. These results suggest that the organic enrichment with particles of detritical–heterotrophic origin enhanced E. acutifrons clutch size and brood volume in Bilbao. Egg size was closely related to phytoplankton in both estuaries, but a clear trade-off between egg size and number per clutch was observed only in Mundaka. Thus, we suggest that the reduction of egg number in order to increase egg size might happen only under food-limiting conditions, but not in organically rich and quite constant nutritional environments such as found those in Bilbao.

Temporal variability of the spatial distribution of the zooplankton community in a coastal embayment of the basque country in relation to physical phenomena

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.

Composition, vertical distribution and age of zooplankton benthic eggs in the sediments of two contrasting estuaries of the Bay of Biscay

The effects of natural and anthropogenic factors on the vertical distribution of zooplankton benthic eggs of the sediments were analysed in the estuaries of Bilbao and Mundaka, both located in the Bay of Biscay. These estuaries differ largely in their pollution level, so in the estuary of Bilbao the human impact by physical perturbation (canalisation, dredging... ) and by introduction of pollutants (urban and industrial wastes) have been considerably greater than in Mundaka. Core samples were obtained in the intertidal soft sediments at the middle zone of each estuary using a hand core sampler 2 m length. Sediments grain size composition, and organic matter, chloropigments, lipid, carbohydrate and protein contents were analysed. Eggs were extracted from the sediment and their age was estimated by means of 210Pb and 137Cs analyses. Results showed that the benthic eggs abundance, composition and vertical distribution differed largely between estuaries. The total density of eggs recovered from the sediment was on average one order of magnitude higher in the estuary of Mundaka (105 eggs m−2) than in the estuary of Bilbao (104 eggs m−2), in which sediments anthropogenic organic enrichment was evident. Rotifer eggs dominated in Bilbao and calanoid copepod eggs in Mundaka, in agreement with the actual dominance of rotifers and calanoids in their estuarine zooplankton communities. In the estuary of Mundaka, benthic eggs and the majority of environmental variables were closely related to depth, calanoid eggs decreasing with depth while rotifer eggs increased. The results on 210Pb and the 137Cs allow us to know the date of the sediment layers, and to estimate at around 100 years the entire sequence analysed. Calanoid eggs dominated in recent sediments and rotifer eggs in older ones suggesting noticeable changes in the zooplankton community during the last century. In contrast, the age estimation was difficult in the estuary of Bilbao because 210Pb and 137Cs values indicated that more recent sediments were mixed with older sediments. This indicates that sedimentary processes are strongly affected by the human physical perturbation of sediments probably due to the periodical dredging and intense navigation in the estuary, and accounted for the rather homogeneous distribution of eggs with depth.

Insights on the origin of invasive copepods colonizing Basque estuaries; a DNA barcoding approach

BackgroundThe 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.ResultsWhile 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.ConclusionsPhylogeographic 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.

Dissolved Oxygen in a Temperate Estuary: the Influence of Hydro-climatic Factors and Eutrophication at Seasonal and Inter-annual Time Scales

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.

Zooplankton variability at four monitoring sites of the Northeast Atlantic shelves differing in latitude and trophic status

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.

Assessment of the climate and human impact on estuarine water environments in two estuaries of the Bay of Biscay

Abstract 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.