Bilbao Estuary Research Articles

Intertidal macrofaunal communities in an intensely polluted estuary

The Bilbao estuary (northern Spain) has suffered a serious environmental deterioration due to industrial and domestic discharges during decades. A total of 21 sampling stations were selected in order to: (1) study the intertidal fauna distribution on both hard and soft bottoms; (2) link this information with the prevalence of certain environmental variables at the sampling sites; and (3) define distinct biological zones with all the information gathered. Up to 50 taxa were identified mainly in the outer estuary with an abrupt decrease upstream. In general, fauna on hard substrates was more diverse, while soft bottom communities exhibited a more consistent structure. Sporadic hypoxic events were recorded in the water column of all the sites studied, which affects the distribution of species. Three major biotic zones are proposed in the estuary. This provides a reliable biomonitoring tool for assessing the effectiveness of a sewerage and remediation scheme ongoing on the river and evaluating the future use of each area in the urban development of the city of Bilbao and conurbation.

Analytical and thermodynamical approach to understand the mobility/retention of arsenic species from the river to the estuary. The Bilbao case study

Thermodynamical speciation and mobilization experiments were used, for the first time, to understand arsenic behavior in estuarine conditions (Bilbao estuary, north of Spain). Natural waters were used as the chemical extractant in the mobilization/retention studies. Variables that define each sampling point were measured: pH, ionic strength, conductivity, pE or As, Ca, Mg and Fe total concentrations in both natural waters and sediments (ICP–OES, HG–AAS or ET–AAS). Modeling inorganic arsenic distribution was performed for the given natural conditions of each sampling point. For that, a theoretical thermodynamic model of inorganic arsenic, defining the acid–base, complexation, precipitation and redox equilibria, together with the Modified Bromley's Methodology (MBM) for activity coefficients estimation, was used. The predictions were also confirmed by Raman spectroscopy measurements of the estuarine sediments. The influence of the tides was considered in the estuary. During low tides, the medium conditions led to significant retention of inorganic arsenic but mobilization was found as the main process in high tidal conditions, being salinity, Mg 2+ and Ca 2+ concentrations as the key factors that control the arsenate distribution in estuary conditions.

Effects of pollution on zooplankton abundance and distribution in two estuaries of the Basque coast (Bay of Biscay)

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.

Holocene environmental development of the Bilbao estuary, northern Spain: sequence stratigraphy and foraminiferal interpretation

The foraminiferal assemblages recorded in the sedimentary successions of nine boreholes drilled in the Bilbao estuary have been analyzed. Palaeoenvironmental interpretation of these and other previously studied assemblages defines the Bilbao estuary sedimentary infill as a depositional sequence within a fourth-order eustatic cycle (Holocene). This sedimentary sequence comprises a wide range of foraminiferal assemblage zones (FAZ) that are organized into three systems tracts. Each systems tract is composed by a distinct suite of FAZs separated by continuous stratigraphic surfaces. During Lateglacial low sea-level conditions, sedimentation was represented by fluviatile gravels and coarse sands barren of foraminifera (lowstand systems tract). During marine transgression (transgressive systems tract, 8500–3000 cal years BP), great volumes of mainly near-marine sediments were deposited in the lower estuary, alternation of brackish and near-marine sediments were accumulated in the middle estuary, and brackish materials sedimented in the upper estuary. The highstand systems tract deposited during the upper Holocene (3000 cal years BP-19th century human reclamation) represented brackish intertidal and supratidal conditions as the sedimentary infill was taking place under stabilized sea-level. Comparison of these results with other sedimentary sequences from different coastal areas in the Bay of Biscay indicates that, following postglacial sea-level rise, modern estuaries in the region began to develop around 8500 cal years BP. A final transgressive event was dated at around 3000 cal years BP when sea level reached approximately its present position after a second upward-shallowing sequence commenced.

Reconstructing historical trends in metal input in heavily-disturbed, contaminated estuaries: studies from Bilbao, Southampton Water and Sicily

Estuaries may be important reservoirs for contaminants as they tend to act as sinks for fine, contaminant-reactive sediments, and, historically, they have acted as centres for industrial and urban development. Analysis of dated sediment cores from these areas may allow historical trends in heavy metal input to be reconstructed, and recent and historical inputs of metal contaminants to be compared. Undisturbed saltmarsh settings have been used widely in the reconstruction of historical trends in metal input as saltmarshes provide a stable, vegetated substrate of dominantly fine sediments, and are less prone to erosion and reworking than adjacent mudflat areas. In comparison, much less research on historical pollution trends has been undertaken at estuarine sites which are prone to severe local disturbance, such as intertidal areas which are routinely dredged or where sedimentary processes have been modified by human activities such as shipping, salt working, port activities, land claim etc. This paper assesses the usefulness of 210Pb and 137Cs dating, combined with geochemical studies, in reconstructing historical trends in heavy metal input and sediment accretion in 3 heavily-modified, industrialised estuarine areas in Europe: the Bilbao estuary (Spain), Southampton Water (UK), and the Mulinello estuary (Sicily). Of these sites, only a salt marsh core from the Mulinello estuary provides a high-resolution record of recent heavy metal inputs. In Southampton Water only a partial record of changing metal inputs over time is retained due to land-claim and possible early-diagenetic remobilisation, while at Bilbao the vertical distribution of heavy metals in intertidal flats is mainly controlled by input on reworked sediment particles and variations in sediment composition. Where 137Cs and 210Pb distributions with depth allow a chronology of sediment deposition to be established, and early-diagenetic remobilisation has been minimal, mudflat and saltmarsh cores from even the most heavily-disturbed estuarine sites can provide useful information on variations in historical contaminant input. When the sediments have been vigorously mixed or reworked, large-scale compositional variations are present, or significant early-diagenetic remobilisation has occurred, however, only general information on the scale of contamination can be obtained.

Recovery simulations of grossly polluted sediments in the Bilbao Estuary

The Bilbao Estuary is one of the most contaminated estuaries on the north coast of Spain, and vast efforts have been made to abate pollution there. In fact, the local water authority has forecast a biological recovery of the native fauna after a substantial increase in dissolved oxygen to normoxic levels. In order to assess this prediction by evaluating the extent of natural regeneration of these polluted sediments, two long-term bioassays ( t=90 d) were performed. In both of them, lethal (differences in survival) and sublethal (differences in length and weight growth) effects were measured by using juvenile individuals of the autochthonous clam, Scrobicularia plana (Da Costa, 1778). The sediments tested differed in pollution levels, as measured by a set of indicators including PAHs, PCBs, heavy metals, volatile organic matter and coprostanol. All sediments were finally exposed to normoxic conditions in situ in the Bilbao Estuary (DO ∼ 6.3 mg l −1). In the first experiment, concerning moderately polluted sediments from the Bilbao Estuary and reference sediments from the “pristine” Plentzia Estuary, no significant differences ( P>0.05) were found regarding animal survival (∼94.5%) or growth in length or weight between the sediments tested. In the second experiment, also involving grossly polluted sediments (GPS) from the Bilbao Estuary, survival (24.5%) was statistically lower ( P<0.05) than in the other sediments (∼93%). No significant differences were found in growth (length, weight) between animals exposed to moderately polluted or reference sediments. We interpret this dramatic difference in survival as the lethal effect on the animals tested of the GPS of the Bilbao Estuary, indicating a situation where biological recovery is not possible due to the adverse consequences of contaminants sorbed into sediments. The extensive use of this inexpensive bioassay could help to distinguish sediments in which homeostatic recovery is possible from grossly polluted “hot spots”, which need costly remedial actions.

Environmental transformation of the Bilbao estuary, N. Spain: microfaunal and geochemical proxies in the recent sedimentary record

The distribution and abundance of benthic foraminifera and a range of elements have been analysed in six long cores (up to 10 m long) from the Bilbao estuary, N. Spain, to document environmental transformation of this estuarine ecosystem and to determine sediment contamination levels. Three different environmental zones could be distinguished in the recent sedimentary record based on its microfaunal and geochemical contents. Initially, a pre-industrial zone containing very abundant and diverse foraminiferal assemblages together with baseline metal levels. Secondly, an older industrial zone exhibiting co-existence of abundant and diverse foraminiferal assemblages with high concentrations of metals. Finally, a younger industrial zone with extreme concentrations of metals and barren of indigenous foraminifera. This environmental transformation has been caused by the discharge of untreated domestic and industrial effluents during the last 150 years. The occurrence of foraminifera in the two industrial zones and along the estuary is not related to defined levels of metals, and this seems to confirm oxygen limitation as the key factor to explain complete estuarine defaunation during deposition of the younger industrial zone (period 1950–2000). Effectiveness of current regeneration schemes could be assessed using microfaunal and geochemical proxies as environmental quality indicators.

Recent Anthropogenic Impacts on the Bilbao Estuary, Northern Spain: Geochemical and Microfaunal Evidence

The distributions of a range of elements (including137Cs and210Pb) have been studied in surface sediments and 0·5m cores from the Bilbao Estuary (Cantabrian coast, northern Spain) to determine recent and historical sediment contamination. Benthic foraminifera have also been examined to infer recent ecosystem changes. High concentrations of a range of metals are observed in three cores and in surface-scrapes. Observed concentrations depend on the proximity to sources of pollutants. Living foraminifera are absent from surface sediments in the upper estuary, and are scarce in the middle and lower estuary, due to persistent anoxia in the estuarine channel, and possibly, high pollutant concentrations. High metal concentrations in sediment core profiles, combined with the sporadic presence of foraminifera, indicate that environmental degradation has persisted in this estuary for at least the last 40 years. This degradation has been caused by the discharge of untreated industrial and domestic effluents. Surface sediments show a few transported living foraminifera of species that have been identified as dominant in nearby, less-polluted estuarine environments. Successful recolonization by these species may occur if the sediment conditions improve sufficiently. Foraminiferal assemblages could therefore be used as environmental quality indicators to assess the effectiveness of current regeneration schemes implemented under strategic local authority plans.

Extreme variation in the concentration of trace metals in sediments and bivalves from the Bilbao estuary (Spain) caused by the 1989–90 drought

Intertidal sediments and bivalves Scrobicularia plana from Bilbao estuary (Spain) were repeatedly sampled during and after the 1989–90 drought. The organic content (OC) and the concentration of most of the 10 metals analysed in sediments were extremely variable (e.g. Cd ranged from 4 to 112 μg g −1, ppm dry wt), and they were strongly related to an index applied to estimate the magnitude of the drought (namely the deficit flow of the main river). After OC normalisation, the correlation versus deficit was lost for some elements (Cr, Fe, Ni) but not for others (Cd, Co, Cu, Pb, Zn). This leads to the conclusion that both a natural and a man-induced process account for the observed rise and fall of the contamination pattern: the reduced river flow raised the metal-binding capacity of sediments, and restrictions in the water supply increased the corrosion of sewers and their leaching of some elements. The concentration of trace metals in local bivalves (Cd reached 100 and 458 ppm dry wt, respectively, for whole tissues and digestive gland) mirrored the sediment contamination dynamics, but at a slower pace. The means by which S. plana is able to tolerate such high Cd tissue levels remain unknown. These results constitute some prime field evidence of drought effects that should be born in mind when pollution and risk from climate change are assessed in estuaries.

Stress in estuarine communities: Lessons from the highly-impacted Bilbao estuary (Spain)

The highly-impacted Bilbao estuary was studied toquantify the response of estuarine communities tostress. Total heterotrophic biomass, a measure ofsediment infauna, declined sharply from the mouth tothe upper reaches of the estuary. Differentstatistical methods selected the depletion ofdissolved oxygen (DO) at the bottom of the watercolumn as the main correlate with the observedpattern. Biomass size-spectra (BSS) of estuarinecommunities suggested that `unimpacted' sites at themouth of the estuary had a high proportion of biomassin the larger size classes under regular andpredictable environmental conditions. In contrast, inthe impacted middle and inner parts of the estuarybiomass was concentrated in the smaller size classes,indicating a clear effect of stressful environmentalconditions on larger bodied organisms. A progressive increase of DO values has beenforecasted by the local Water Authority after thecompletion of a sewerage plan for the whole estuary,and predictions of water quality criteria were used toestimate community biomass figures following thismanagement. In order to predict the recovery at thelevel of individual species, several bioassays withthe native species Nereis diversicolor andScrobicularia plana were performed. Results obtainedwith the latter, more-sensitive species, suggest thatchemical contamination in sediments from the Bilbaoestuary has the potential to cause deleterious effectson indigenous benthic organisms, which could delayfaunal recovery in some parts of the estuary oncewater-oxic conditions improve.

Fitting response models of benthic community structure to abiotic variables in a polluted estuarine system

Abstract Models of the macrozoobenthic community responses to abiotic variables measured in the polluted Bilbao estuary were obtained by multiple linear regression analyses. Total, Oligochaeta and Nematoda abundance and biomass were considered as dependent variables. Intertidal level, dissolved oxygen at the bottom of the water column (DOXB) and organic content of the sediment were selected by the analyses as the three principal explanatory variables. Goodness-of-fit of the models was high ( x =71.3% ). Total abundance and biomass increased as a linear function of DOXB. The principal outcome of the vast sewage scheme currently in progress in the study area is an important contributor of increasing DOXB levels. The models exposed in this paper will serve as a tool to evaluate the expected changes in the near future.

Loss of Heterotrophic Biomass Structure in an Extreme Estuarine Environment

Macrozoobenthic assemblages inhabiting estuarine mudflats in areas located at high intertidal levels or close to the mouth had: (1) higher values of abundance, biomass and production; (2) larger mean body sizes; and (3) lower turnover rates than at the remaining stations located elsewhere in the organically enriched, severely polluted Bilbao Estuary. All these community descriptors were similar to those from the nearby and relatively unpolluted Plentzia Estuary, considered here as a reference location.Biomass was irregularly distributed among logarithmic classes of individual body dry weight (DWi). However, a tendency to increase biomass with mean DWiwas found at the reference estuary and at the seaward end of the polluted estuary. This indicated the prevalence of more stable and regular environmental conditions which favoured larger-bodied,K-selected species. In fact, a biomass ‘sink’ was identified in the adult individuals of the bivalveScrobicularia plana(Da Costa), which were only found at the reference estuary (DWi=256 to 512mg) and close to the mouth of the polluted estuary (DWi=128 to 256mg). However, biomass was strongly centred around the smallest size classes (DWi<0·250mg) in the middle and upper reaches of the polluted estuary. Onlyr-selected species of Oligochaeta and Nematoda were favoured under the influence of catastrophic events, indicating the existence of heavily disturbed habitats.Oxygen depletion in the water column was causing a clear segregation of two ecological strategies along the longitudinal axis of the polluted estuary. On the one hand, large sized ‘persisters’ were successful close to the mouth of the polluted estuary. On the other hand, small sized ‘invaders’ (or even the absence of macrofauna) characterized the other extreme of the gradient. Spectral methods revealed a reliable tool for quantifying faunal responses in relation to the stress induced by adverse environmental conditions.

Short-term spatio-temporal changes in urban pollution by means of faecal sterols analysis

Changes in the spatial and temporal distribution of faecal sterols in sediments of the Bilbao Estuary, one of the most densely urbanized areas on the Atlantic coast of Spain, have been investigated. The results indicate elevated concentrations of faecal sterols, including coprostanol, in surface sediments throughout the estuary. Highest concentrations are associated with two major point sources within the estuary, where up to 293 μg−1 of coprostanol were measured. On the other hand, temporal patterns in faecal sterols concentrations are presumably related to weather conditions at the time of the sampling, implying that more samples are required for a meaningful record of the actual variation of sewage contaminated sediments. This study provides baseline data for future examination of the efficacy of the ongoing sewerage plan promoted by the Local Water Authority.

Coprostanol levels and organic enrichment in sediments of the Bilbao estuary (north of Spain)

An assessment was made of the distribution of the faecal sterol coprostanol (COP) in sediments in the Bilbao estuary. Concentrations of COP ranged from 135.15 to 10.77 μg g −1 dry weight, from the inner to the open estuary, with a mean value of 51.43 μg g −1; the highest concentrations were found near the mouths of several tribularies flowing into the estuary. Comparisons with other published values indicate that sediments from the Bilbao estuary are affected by sewage pollution in a similar degree to that reported for other estuaries in the world. Organic enrichment and anaerobic sediment conditions prevail in three-fourths of its watercourse bed. Since the spatial distribution of several organic sedimentary parameters could be clearly correlated with COP levels, we can conclude the existence of major pollution by the addition of untreated sewage, coming from mainly domestic sources. The implementation of a sewerage treatment plan by the local water authority needs the monitoring of reliable sewage tracers, such as faccal sterols, to assess the effectiveness of the plan in reducing the effects of pollution.

Holocene Sea-Level Change in the Bilbao Estuary (North Spain): Foraminiferal Evidence

Benthic foraminiferal assemblages in Holocene sedimentary sequences of 4 boreholes from the Bilbao estuary (Bay of Biscay, northern Spain) have been analyzed. Estuarine indigenous and open-sea exotic foraminifera have been distinguished in order to interpret the environments of deposition (inter/supratidal sand dune and beach, low marsh, brackish intertidal, near-marine intertidal and fresh water) with varying amounts of open-sea influence. Two distinct upward-shallowing sequences, with a variable succession of envi- ronments depending on the distance to the estuary entrance, have been identified (- 8000 BP and - 3000 BP by comparison with other coastal deposits) and interpreted as showing episodic increase of sea level.

Enhanced growth in juvenile Nereis diversicolor after its exposure to anaerobic polluted sediments

Organic enrichment and anaerobic sediment conditions are the dominant anthropogenic influences on the Bilbao Estuary, resulting in the elimination of all benthic aerobic activity along 3/4 parts of its watercourse bed. To simulate the recolonization of azoic sediments by the native polychaete Nereis diversicolor, once water-oxic conditions in the estuary improve, two series of juvenile survival and growth assays were conducted, both, respectively, with and without the addition of exogenous food. Long-term exposures (about 30 days) of juveniles to azoic sediments caused no mortalities. On the contrary, an increased growth occurred in both types of tests, indicating the existence of a higher nutritional quality in the anaerobic sediments of the estuary. The ecological implications of our findings are clear; azoic sediments may be re-populated by juvenile invaders of N. diversicolor, provided that water-oxic conditions improve, through the implementation of a sewerage treatment plan promoted by the local Water Authority.

Nereis diversicolor: an Unreliable Biomonitor of Metal Contamination in the ‘Ría de Bilbao’ (Spain)

Abstract. Samples of the polychaete Nereis diversicolor O.F. Müller and surficial sediments from a clean and a polluted estuary were taken during a year at monthly intervals to determine, by comparison, the range of temporal fluctuations in trace metal concentrations. Whereas metal variations in surficial sediments were more pronounced in the polluted estuary, concentrations in N. diversicolor in both estuaries showed a wide range of temporal variation that surpassed that of surficial sediments. A clear decline in all metals studied (except Mn) from polluted sediments was noted after episodes of strong rainfall. In the reference estuary the fluctuations were less pronounced. Metal body burdens in N. diversicolor from the two estuaries were usually quite similar even though total levels in sediments differed considerably, indicating a lack of worm‐sediment relationship. The low level of Cu, Cr and Pb in the tissues of N. diversicolor suggest a mechanism which prevents the polychaete from being exposed to these metals in highly polluted sediments. Therefore, it is concluded that N. diversicolor cannot be considered to be an ideal biomonitor of metal contamination in the polluted Bilbao Estuary.

Tidal zonation on mud flats in a polluted estuary caused by oxygen-depleted water

Tidal banks from the middle reaches of the Bilbao Estuary impacted by industrial and domestic waste discharges show a prominent horizontal banding of the estuarine organisms as they are confined to the high shore, whereas an azoic sediment covered by a black finely laminated layer dominates the low shore. The purpose of this study was to infer the immediate causes of this peculiar zonation. To this end, two trials of a field experiment were performed to measure biotic responses of caged organisms exposed both to dissolved oxygen deficiency and to polluted azoic sediments along a tidal gradient of air emersion. Results in relation to different controls illustrate that extensive mortalities were clearly linked to the extended periods of hypoxia occurring at the low shore of the polluted site. At increasing heights above the low water mark the shore is covered with depressed oxygen water for shorter periods of time, a circumstance which limits the inconveniences of anaerobiosis for organisms. Further, a long-term exposure (55 days) of caged Nereis diversicolor in the laboratory to azoic sediments caused no mortality. On the contrary, growth was substantially promoted, suggesting either the existence of hormetic responses by exposure to low levels of contaminants, or a higher nutritional quality within the sediment in terms of particulate organic material or microorganism abundance. The possible ecological consequences of abating pollution through the implementation of a sewerage treatment plan in the Bilbao Estuary are discussed.

Evaluation of adverse biological effects induced by pollution in the Bilbao Estuary (Spain)

The physical nature of the Bilbao Estuary (Spain) has been highly modified since the end of the last century, mainly by means of dyking, canalization, and dredging to meet the needs of navigation. Together with its industrial use, the estuary is also used for the disposal of domestic sewage. Until the completion of a sewerage plan in the year 2002, the sewage of about 1 million people and the wastes from industries will continue to be discharged into this 15-km-long estuary with a limited freshwater flow of 25 m 3 s −1. An abrupt decrease in faunal abundance followed by defaunation characterized the biotic response to the disturbance in almost three-quarters of its tideway. Relationships between biotic and environmental variables were investigated using the BIO-ENV procedure of the PRIMER computer package. Gross organic enrichment, water oxygen depletion and anaerobic sediment conditions appear to be the dominant anthropogenic influences on the Bilbao Estuary. It is concluded that the level of wastewater discharges into the estuary may substantially drop below that concentration which assures the protection and propagation of native heterotrophic resources within sediments. Biological monitoring is thus essential to assess the efficacy of the sewerage plan promoted by the local Water Authority.

7-ethoxyresorufin O-deethylase (EROD) and antioxidant enzyme activities in larvae of sardine ( Sardina pilchardus) from the north coast of Spain

Sardine larvae ( Sardina pilchardus) and their major food source (mixed zooplankton) were sampled from four sites along a transect out of the Bilbao estuary and a fifth (reference) site further along the north coast of Spain (23.5.92-10.6.92). Whole body sardine larvae were assayed for 7-ethoxyresorufin O-deethylase (EROD) activity (measure of CYP1A) as a biomarker of exposure to organic pollution, and the antioxidant enzymes catalase (EC 1.11.1.6) and superoxide dismutase (SOD; EC 1.15.1.1) as potential biomarkers of oxidative stress. EROD activity was lowest at a site closest to the Bilbao estuary, and increased along the transect until it reached levels recorded from samples for a reference site. Both catalase and SOD activities were highest at the inshore site of the transect decreasing offshore before increasing again at the site furthest from the coastline. Levels of organic contaminants (polycyclic aromatic hydrocarbons (PAHs), polychlorobiphenyls and pp'DDE) in mixed zooplankton were generally similar at the different sites, with the exception of higher PAHs at the extremes of the transect and at the reference site. The results are discussed in relation to the biomarker application of fish larvae, CYP1A and antioxidant enzymes.