Belgian Coastal Zone Research Articles

Validation and application of an LC-MS/MS method for the simultaneous quantification of 13 pharmaceuticals in seawater

Knowledge of the presence of micropollutants such as pharmaceuticals, in coastal areas, is very limited; therefore, the main objective of this study was to optimize and validate a new analytical method for the quantitative analysis of 13 multiclass pharmaceuticals in seawater. Target compounds included antibiotics, non-steroidal anti-inflammatory drugs, beta-blockers, lipid regulators and one psychiatric drug. A combination of solid-phase extraction and liquid chromatography coupled with multiple mass spectrometry enabled their detection at the low nanogram per litre level. The limits of quantification varied between 1 and 50 ng L(-1), for most components the linearities were more than 0.99 and the recoveries obtained in seawater (95-108%) were satisfactory. This method was applied to seawater and estuarine water samples collected in the Belgian coastal zone, to assess the prevalence of common pharmaceuticals in this marine environment. Seven pharmaceuticals, including compounds of which the presence in marine environments had not been reported earlier, were detected, with salicylic acid and carbamazepine being the most abundant, in concentrations up to 855 ng L(-1).

Effect of eutrophication on air–sea CO2 fluxes in the coastal Southern North Sea: a model study of the past 50 years

AbstractThe RIVERSTRAHLER model, an idealized biogeochemical model of the river system, has been coupled to MIRO‐CO2, a complex biogeochemical model describing diatom and Phaeocystis blooms and carbon and nutrient cycles in the marine domain, to assess the dual role of changing nutrient loads and increasing atmospheric CO2 as drivers of air–sea CO2 exchanges in the Southern North Sea with a focus on the Belgian coastal zone (BCZ). The whole area, submitted to the influence of two main rivers (Seine and Scheldt), is characterized by variable diatom and Phaeocystis colonies blooms which impact on the trophic status and air–sea CO2 fluxes of the coastal ecosystem. For this application, the MIRO‐CO2 model is implemented in a 0D multibox frame covering the eutrophied Eastern English Channel and Southern North Sea and receiving loads from the rivers Seine and Scheldt. Model simulations are performed for the period between 1951 and 1998 using real forcing fields for sea surface temperature, wind speed and atmospheric CO2 and RIVERSTRAHLER simulations for river carbon and nutrient loads. Model results suggest that the BCZ shifted from a source of CO2 before 1970 (low eutrophication) towards a sink during the 1970–1990 period when anthropogenic DIN and P loads increased, stimulating C fixation by autotrophs. In agreement, a shift from net annual heterotrophy towards autotrophy in BCZ is simulated from 1980. The period after 1990 is characterized by a progressive decrease of P loads concomitant with a decrease of primary production and of the CO2 sink in the BCZ. At the end of the simulation period, the BCZ ecosystem is again net heterotroph and acts as a source of CO2 to the atmosphere. R‐MIRO‐CO2 scenarios testing the relative impact of temperature, wind speed, atmospheric CO2 and river loads variability on the simulated air–sea CO2 fluxes suggest that the trend in air–sea CO2 fluxes simulated between 1951 and 1998 in the BCZ was mainly controlled by the magnitude and the ratio of inorganic nutrient river loads. Quantitative nutrient changes control the level of primary production while qualitative changes modulate the relative contribution of diatoms and Phaeocystis to this flux and hence the sequestration of atmospheric CO2.

Ecologically based indicators for Phaeocystis disturbance in eutrophied Belgian coastal waters (Southern North Sea) based on field observations and ecological modelling

Recent field observations of Phaeocystis globosa colonies (number, size and cell content) and historical ecosystem model simulations have been used to define and diagnose undesirable eutrophication in the Belgian coastal zone (BCZ) as recommended by the EU Water Framework Directive and OSPAR. The proposed methodology included an initial definition of a reference abundance of Phaeocystis cells from which the disturbance could be scaled. This number was based on the Phaeocystis attribute that creates ecosystem disturbance: the ability of Phaeocystis colonies to grow and reach sizes unmanageable for the current copepods. The threshold was set at 4 × 10 6 cells L − 1 and corresponded to the maximum Phaeocystis cells contained in the grazable colonies (< 400 µm) recorded in the BCZ between 1988 and 2000. Interestingly, this cell number was similar to the maximum number of Phaeocystis colony cells simulated for pristine time by the coupled RIVERSTRAHLER-MIRO (R-MIRO) model, suggesting that natural Phaeocystis ecosystems are well balanced and efficiently transfer their production to higher trophic levels. However, both the field observations obtained during the 1988–2000 period and the 1950–2000 historical reconstruction of eutrophication in the BCZ simulated by the R-MIRO model showed Phaeocystis colony cell maxima well above the threshold value. Considering that a return to the pristine reference value is not achievable, another methodology was then proposed based on historical R-MIRO simulations of annual primary (PP) and copepod (CP) productions where the CP:PP ratio can be considered an indicator of trophic efficiency. The analysis of changing CP:PP ratios in relationship with simulated winter concentrations and annual inputs of nutrients showed that the decrease in trophic efficiency was related to the imbalanced DIN and DIP inputs (N:P > 25) explained here by the reduction in DIP loads after 1985 while DIN remained elevated. On this basis, a critical DIN load estimated at 60 kT y − 1 was proposed to re-equilibrate the N:P balance in the BCZ and to favour diatoms over Phaeocystis colonies. More generally we conclude that N:P ratios in nutrient loads (or winter stocks in the coastal area) in excess of 25 in Phaeocystis ecosystems are indicative of Phaeocystis colony dominance and weak trophic efficiency.

Changes of sedimentological patterns and morphological features due to the disposal of dredge spoil and the regeneration after cessation of the disposal activities

The interaction of dumped sediments with the morphology and sedimentology of a coastal environment and the recovery of the seafloor after cessation of dumping activities, is still poorly known, especially from a longer term perspective. This paper presents a quantitative impact study of the dumping of dredged material on two overlapping disposal sites, situated on different morphological entities: an old disposal site located on a sandy shoal, was closed in 1999; afterwards a new disposal site was designated, mainly located in a tidal gully. The approach included the interpretation of chrono-sequential single-beam echosoundings, high-resolution multibeam bathymetrical and backscatter data, ground truthed with boxcores. Dumping has caused the formation of a large-scale dump mound on both the old and new disposal site. On the new disposal site, the dump mound is superposed by individual disposal mounds and depressions that are probably formed by scouring events. On the old disposal site, also small depressions occur; which are likely associated with the former dumping process. The disposal has led to the accumulation of sand, both on the old and new disposal site, regardless of the high mud content of the dredge spoil. For the new disposal site, dumping efficiency has been estimated as 30–40%, which would lead to a yearly growth of 380,000 m 3. A different sedimentation pattern was revealed for the old and new disposal site, which is ascribed to the difference in morphological setting and differences in dredge spoil nature. After cessation of the disposal of dredged sediments on the old disposal site, a morphodynamic equilibrium was fairly quickly restored. Bedforms were formed within less than one year and their distribution and surficial sediments are again representative for the Belgian coastal zone. The consistent ebb-dominated asymmetry of the very large dunes, their SW migration and sediment accretion extending beyond the SW limit of the new disposal site, suggest an important bedload transport in the direction of the navigation channels. These findings contrast to the overall flood dominated residual current transport direction.

Diagnosis of the sediment transport in the Belgian Coastal Zone

Estimating the age of particles in marine environment constitutes an invaluable tool to understand the interactions between complex flows and sediment dynamics, particularly in highly energetic coastal areas such as the Belgian Coastal Zone (Southern Bight in the North Sea). To this end, the Constituent Age and Residence time Theory – CART – introduced by Delhez, E.J.M., Campin, J.-M., Hirst, A.C., Deleersnijder, E. [1999a. Toward a general theory of the age in ocean modelling. Ocean Modelling 1, 17–27] for passive water constituents is extended to describe the sediment dynamics. It is then used in combination with a three-dimensional coupled hydrodynamic-sediment transport model to investigate sediment processes in the Belgian Coastal Zone focusing on two complementary aspects of the sediment dynamics: the internal sediment motion and redistribution within the Belgian coast; and the horizontal transport.

Phosphorus speciation, transformation and retention in the Scheldt estuary (Belgium/The Netherlands) from the freshwater tidal limits to the North Sea

Nutrient delivery to the Belgian coastal zone is to a great extent through the Scheldt estuary. We have measured the phosphate (PO 4), poly-phosphate (poly-P), dissolved organic phosphorus (DOP), particulate inorganic phosphorus (PIP) and particulate organic phosphorus (POP) concentrations at the freshwater tidal limits of the Scheldt estuary, along the salinity gradient and in the coastal zone during one year. In addition, nitrate (NO 3), nitrite (NO 2), ammonium (NH 4), dissolved organic nitrogen (DON) and particulate nitrogen (PN) were determined in the freshwater Scheldt sub-basin and NO 3, NO 2 and NH 4 in the Rupel sub-basin, the Scheldt estuary and the coastal zone. The behaviour of each P species along the continuum from the freshwater tidal limits to the coastal zone is discussed. Phosphorus (P) and nitrogen (N) budgets were made for the Scheldt sub-basin during the productive period (May–September). The main P and N transformation processes were identified as PO 4 sorption, algal uptake and nitrification. Retention of P and N in the entire freshwater tidal area was estimated. In the brackish part of the estuary, there was no net source or sink of dissolved inorganic nitrogen (DIN). Desorption of PO 4 was the most important transformation process for P along the salinity gradient. The PO4 pool increased and the PIP pool decreased, while total P behaved conservatively. Mass balance calculations show that the transformation of particulate P to PO 4 enhances the PO 4 flux from the Scheldt to the coastal zone.

Suspended particulate matter dynamics and aggregate sizes in a high turbidity area

Measurements of aggregate size, suspended particulate matter (SPM) concentration and current velocity have been carried out in the Belgian coastal zone (southern North Sea). Two stations were situated in the coastal turbidity maximum zone; another station was located more offshore at the edge of this turbidity maximum. The data have been collected using a LISST 100, OBS sensors, water samples and a bottom mounted ADCP. Turbulence (Kolmogorov microscale) has been modelled for the same period using a 3D numerical model. The results show that the size of the aggregates is significantly smaller in the coastal turbidity maximum area. The processes responsible for the occurrence of smaller aggregate size in the coastal zone compared with the more offshore location are: the higher turbulence; the smaller time available for the aggregates to grow up to an equilibrium size; the higher deposition of mud, resulting in a break-up of the flocs and the lower availability of organic matter, which may limit the size of the flocs.

Causes of variability in diatom and Phaeocystis blooms in Belgian coastal waters between 1989 and 2003: A model study

Massive blooms of Phaeocystis colonies usually occur in the Belgian coastal zone (BCZ) between spring and summer diatom blooms but their relative magnitude varies between years. In order to understand this interannual variability, we used the biogeochemical MIRO model to explore the link between diatom and Phaeocystis blooms and changing nutrient loads and meteorological conditions over the last decade. For this application, MIRO was implemented in a simplified 3-box representation of the domain between the Baie de Seine and the BCZ. MIRO was run over the 1989–2003 period using actual photosynthetic active radiation (PAR), seawater temperature and riverine nutrient loads as forcing. The water mass residence time was calculated for each box based on a monthly water budget estimated from 1993–2003 water flow simulations of the three-dimensional hydrodynamical model COHSNS-3D. Overall MIRO simulations compare fairly well with nutrient and phytoplankton data collected in the central BCZ but indicate the importance of the hydrodynamical resolution frame for correctly describing the extremely high nutrient concentrations and biomass observed in the BCZ. Analysis of model results suggests that while interannual variability in diatom biomass depends on both meteorological conditions (light and temperature) and nutrient loads, Phaeocystis blooms are mainly controlled by nutrients. Further sensitivity tests with varying N and P loads suggest that only N reduction will result in significantly decreased Phaeocystis blooms without negative affects on diatoms, while P reduction will negatively affect diatoms. Moreover, Atlantic nutrient loads play such a great role in BCZ enrichment that reduction of Scheldt nutrient loads only is not sufficient to significantly decrease phytoplankton blooms in the BCZ. It is concluded that future nutrient reduction policies aimed to decrease Phaeocystis blooms in the BCZ without impacting diatoms should target the decrease of N loads in both the Seine and the Scheldt rivers.

Validation of the 3D biogeochemical model MIRO&CO with field nutrient and phytoplankton data and MERIS-derived surface chlorophyll a images

This paper presents results obtained with MIRO&CO-3D, a biogeochemical model dedicated to the study of eutrophication and applied to the Channel and Southern Bight of the North Sea (48.5°N–52.5°N). The model results from coupling of the COHERENS-3D hydrodynamic model and the biogeochemical model MIRO, which was previously calibrated in a multi-box implementation. MIRO&CO-3D is run to simulate the annual cycle of inorganic and organic carbon and nutrients (nitrogen, phosphorus and silica), phytoplankton (diatoms, nanoflagellates and Phaeocystis), bacteria and zooplankton (microzooplankton and copepods) with realistic forcing (meteorological conditions and river loads) for the period 1991–2003. Model validation is first shown by comparing time series of model concentrations of nutrients, chlorophyll a, diatom and Phaeocystis with in situ data from station 330 (51°26.00′N, 2°48.50′E) located in the centre of the Belgian coastal zone. This comparison shows the model's ability to represent the seasonal dynamics of nutrients and phytoplankton in Belgian waters. However the model fails to simulate correctly the dissolved silica cycle, especially during the beginning of spring, due to the late onset (in the model) of the early spring diatom bloom. As a general trend the chlorophyll a spring maximum is underestimated in simulations. A comparison between the seasonal average of surface winter nutrients and spring chlorophyll a concentrations simulated with in situ data for different stations is used to assess the accuracy of the simulated spatial distribution. At a seasonal scale, the spatial distribution of surface winter nutrients is in general well reproduced by the model with nevertheless a small overestimation for a few stations close to the Rhine/Meuse mouth and a tendency to underestimation in the coastal zone from Belgium to France. PO 4 was simulated best; silica was simulated with less success. Spring chlorophyll a concentration is in general underestimated by the model. The accuracy of the simulated phytoplankton spatial distribution is further evaluated by comparing simulated surface chlorophyll a with that derived from the satellite sensor MERIS for the year 2003. Reasonable agreement is found between simulated and satellite-derived regions of high chlorophyll a with nevertheless discrepancies close to the boundaries.

Testing an integrated river–ocean mathematical tool for linking marine eutrophication to land use: The Phaeocystis-dominated Belgian coastal zone (Southern North Sea) over the past 50 years

The RIVERSTRAHLER model, an idealized biogeochemical model of the river system has been coupled to MIRO, a complex biogeochemical model describing diatom and Phaeocystis blooms in the marine domain, to investigate the link between changing human activities over the past 50 years and coastal eutrophication in the eastern Channel and Southern Bight of the North Sea with a focus on the Belgian coastal zone (BCZ). The whole area, submitted to the influence of two main rivers (Seine and Scheldt) is invaded each spring by massive blooms of Phaeocystis colonies which impact on the structure and functioning of the coastal ecosystem. For the present application MIRO is implemented in a simplified multi-box description of the coastal zone and is run over the last 50 years using average 1989–1999 meteorological conditions (global solar radiance and temperature) and RIVERSTRAHLER simulations of Seine and Scheldt nutrient loads as forcing conditions. Model validation is obtained by visual and statistical comparison of nutrients, Chl a, diatom and Phaeocystis simulations with data collected in the central BCZ (51°26.05 N; 02°48.50 E) over the 1992–1998 period. The simulated Phaeocystis and diatom trends (yearly average and maxima reached) in BCZ, over the past 50 years are discussed with respect to changing nutrient loads, especially NO 3 and PO 4. Historical reconstruction back to the fifties suggests that Phaeocystis colonies were already blooming in BCZ in the early 1950's and were sharing almost equally with diatoms, the bulk of annual primary production. Over the 1960–1992 period, the increased loads of both N and P were beneficial to both Phaeocystis colonies and summer diatoms with however a little advantage to the latter. Since 1989, the decrease in P loads subsequent to the removal of PO 4 in washing powders and the maintenance of elevated NO 3 loads had a positive feedback to both diatoms and Phaeocytis production with however a significantly larger impact on diatoms than on Phaeocytis. This is due to the fact that Phaeocystis has strong affinity for low PO 4. Consequently Phaeocystis development is mainly controlled by NO 3 loads. We conclude that future management of nutrient emission reduction aiming at decreasing Phaeocystis blooms in BCZ without impacting on diatoms would target decrease of NO 3 loads by both the rivers Seine and Scheldt.

Hydroclimatic modulation of diatom/Phaeocystis blooms in nutrient-enriched Belgian coastal waters (North Sea)

Statistical analysis of 14 yr (1988–2001) of intensive phytoplankton monitoring at Station 330 in the central Belgian Coastal Zone (BCZ, Southern Bight of the North Sea) indicates that the long-term diatom biomass trend and the spring dominance of Phaeocystis colonies over diatoms are determined by the combined effect of the North Atlantic Oscillation (NAO) and freshwater and continental nitrate carried by the Scheldt. The strong correlation between diatoms and the NAO index is largely explained by the modulating effect of the latter on the water budget at the monitoring station. The relationship between Phaeocystis spring blooms and winter NAO (NAOw) is indirect, better expressed by springtime Phaeocystis dominance over diatoms because of the higher response of the latter to the NAO. The spring Phaeocystis : diatom bloom ratio is negatively (or positively) linked to positive (or negative) NAOw values. A complex cascade of events links large-scale NAO index variations with those local meteorological conditions (wind strength and direction, rainfall) that drive the hydrography and water budget of the BCZ. Local meteorological conditions in turn modulate the geographical spread of Scheldt nutrient loads in the coastal zone and ultimately regulate the magnitude of Phaeocystis spring blooms by determining winter nitrate enrichment. Hence, the absence of a linear relationship between Phaeocystis spring blooms and NAOw is explained by the nonlinear response of river-based nitrate pulses to NAO due to local wind-driven hydrodynamical forcing.

Spatial variation in phytoplankton dynamics in the Belgian coastal zone of the North Sea studied by microscopy, HPLC-CHEMTAX and underway fluorescence recordings

Spatial variation in the succession of phytoplankton in the Belgian Coastal Zone (BCZ) was investigated by monitoring phytoplankton biomass and community composition using microscopical cell counts, HPLC pigment analyses and in vivo fluorescence recordings. Monthly monitoring of phytoplankton community composition at five stations revealed a succession of three distinct diatom communities. The succession of these three communities was the same at each site, but the succession from the winter-spring to the summer community occurred one month earlier and the succession from the summer to the autumn community one month later at the SW than at the NE stations of the BCZ. Monthly monitoring of chlorophyll a at ten fixed sites and inspection of in vivo fluorescence recordings during various cruises of RV ‘Zeeleeuw’ indicated that the spring bloom started about one month earlier in the SW part of the BCZ than in the NE part. The spatial difference in the onset of the spring bloom was ascribed to the higher water column turbidity at the NE coast compared to the SW coast. Although a Phaeocystis bloom occurred at all monitoring stations, a clear spatial variation in the magnitude of such blooms was observed, with more intense blooms at the NE coast than at the SW coast. A close relation was observed between the intensity of the Phaeocystis bloom and the availability of inorganic nutrients (N and P) before the onset of the bloom. Comparison of microscopical cell counts and CHEMTAX analysis of accessory pigment data indicated that HPLC analysis may be a useful tool for monitoring Phaeocystis in the North Sea. The presence of chlorophyll c 3 containing diatoms, however, probably resulted in the detection of small quantities of Phaeocystis by HPLC-CHEMTAX analysis when microscopical analyses showed that the species was absent.

Small- to large-scale geographical patterns within the macrobenthic Abra alba community

The Abra alba community is widely spread in the coastal zone of the English Channel and the Southern Bight of the North Sea. The community is located on shallow, fine muddy sands. Its spatial distribution can be broken up into a number of isolated patches (Atlantic French, British and German coast) and one large continuous distribution area (northern France up to the Netherlands). The aim of this study is to investigate the geographical patterns within the macrobenthic A. alba community at different scales: the community's full distribution range (i.e. large scale) and a selected area with a continuous distribution of the A. alba community (i.e. small scale) in relation to structuring environmental variables. Therefore, an analysis of newly collected samples along the Belgian coastal zone was combined with available information on the A. alba community throughout its distribution range. Although the community structure shows a high similarity across the full distribution range of the A. alba community, large- as well as small-scale changes in community composition were observed: the Belgian Continental Shelf (BCS) should be considered as a major transition from the rich southern to the relatively poorer northern distribution area of the A. alba community. At a large scale (i.e. full distribution range), the differences in community structure are expected to result from (1) the specific hydrodynamic conditions in the English Channel (Atlantic ocean waters) and the Southern Bight of the North Sea, with a consequent differential connectivity between the different areas and (2) the climatological and related faunal shift from temperate (English Channel) to boreal conditions (German Bight). At a small scale (i.e. within the continuous distribution area), structural and functional community aspects may result from geographic differences in (1) detrital food availability, related to riverine input and pelagic productivity, along and across the coastline and (2) the amount of suspended matter, impoverishing the A. alba community when excessively available.

Seasonal cycling of phosphorus in the Southern Bight of the North Sea

Abstract. We have investigated the seasonal cycle of nutrients, chlorophyll a, dissolved organic nitrogen (DON), and dissolved and particulate organic carbon (DOC and POC) for 10 stations in the Belgian coastal zone. Special attention was given to phosphorus speciation, i.e. dissolved inorganic and organic phosphorus (DIP and DOP) and particulate inorganic and organic phosphorus (PIP and POP). Spatial and temporal trends of the inorganic nutrients and the organic species are discussed. The Belgian part of the southern North Sea is strongly influenced by the river plumes of the Rhine, Meuse and Scheldt resulting in northeast-southwest salinity gradient. Extrapolation of winter nutrient and DOC concentrations versus salinity plots yielded zero-salinity values typical of the river Scheldt. The polyphosphate concentration did not show any temporal trend, whereas both the phosphate and the DOP concentrations exhibited distinct seasonal variations. Silica or phosphorus was the potential limiting nutrient for phytoplankton growth based on elemental ratios.

Carbon dynamics and CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; air-sea exchanges in the eutrophied coastal waters of the Southern Bight of the North Sea: a modelling study

Abstract. A description of the carbonate system has been incorporated in the MIRO biogeochemical model to investigate the contribution of diatom and Phaeocystis blooms to the seasonal dynamics of air-sea CO2 exchanges in the Eastern Channel and Southern Bight of the North Sea, with focus on the eutrophied Belgian coastal waters. For this application, the model was implemented in a simplified three-box representation of the hydrodynamics with the open ocean boundary box ‘Western English Channel’ (WCH) and the ‘French Coastal Zone’ (FCZ) and ‘Belgian Coastal Zone’ (BCZ) boxes receiving carbon and nutrients from the rivers Seine and Scheldt, respectively. Results were obtained by running the model for the 1996–1999 period. The simulated partial pressures of CO2 (pCO2) were successfully compared with data recorded over the same period in the central BCZ at station 330 (51°26.05′ N; 002°48.50′ E). Budget calculations based on model simulations of carbon flow rates indicated for BCZ a low annual sink of atmospheric CO2 (−0.17 mol C m-2 y-1). On the opposite, surface water pCO2 in WCH was estimated to be at annual equilibrium with respect to atmospheric CO2. The relative contribution of biological, chemical and physical processes to the modelled seasonal variability of pCO2 in BCZ was further explored by running model scenarios with separate closures of biological activities and/or river inputs of carbon. The suppression of biological processes reversed direction of the CO2 flux in BCZ that became, on an annual scale, a significant source for atmospheric CO2 (+0.53 mol C m-2 y-1). Overall biological activity had a stronger influence on the modelled seasonal cycle of pCO2 than temperature. Especially Phaeocystis colonies which growth in spring were associated with an important sink of atmospheric CO2 that counteracted the temperature-driven increase of pCO2 at this period of the year. However, river inputs of organic and inorganic carbon were shown to increase the surface water pCO2 and hence the emission of CO2 to the atmosphere. Same calculations conducted in WCH, showed that temperature was the main factor controlling the seasonal pCO2 cycle in these open ocean waters. The effect of interannual variations of fresh water discharge (and related nutrient and carbon inputs), temperature and wind speed was further explored by running scenarios with forcing typical of two contrasted years (1996 and 1999). Based on these simulations, the model predicts significant variations in the intensity and direction of the annual air-sea CO2 flux.

Bioconcentration and biomagnification of mercury and methylmercury in North Sea and Scheldt estuary fish.

Total Hg and MMHg concentrations were assessed in more than 350 fish and shellfish samples. Hg concentrations in Greater North Sea fish of prey range from 0.039 mg kg(-1) wet weight (ww; for ray) to 0.61 mg kg(-1) ww (for dogfish) and for all other fish species, from 0.045 mg kg(-1) ww (for plaice) to 0.33 mg kg(-1) ww (for sand sole), with 95 +/- 2% of the Hg content in the MMHg form. In Belgian coastal zone, fish concentrations range from 0.063 mg kg(-1) ww for plaice to 0.13 mg kg(-1) ww for flounder, with 82-87% of the Hg content in the MMHg form. In fish of the Scheldt, which is a very polluted estuary, Hg levels, as well as the percent MMHg of the total Hg, were lower than in the two zones previously mentioned. The intraspecies variability is of the order of 50% in each of the three zones. In liver tissue, a much larger variability was observed than in muscle tissue, except for fish species of the Scheldt. In most cases, the MMHg fraction in a particular fish species is inversely related to the intraspecies variability. Bioconcentration and biomagnification factors (BCF and BMF, respectively) were assessed. MMHg-BMFs were a few orders of magnitude higher than Hg(inorganic)-BMFs, and for the same species were always highest in the Greater North Sea and lowest in the Scheldt. For each of the Belgian coastal zone four species, a weak positive correlation between Hg content and fish length was found; however, the larger the size-range, the better the correlation. Taking fish length into account, a statistically significant difference in contamination level was observed for species sampled from the different geographical zones.

Very high-resolution seismic mapping of shallow gas in the Belgian coastal zone

Very high-resolution reflection seismic investigations off the Belgian coast have revealed an extensive area marked by poor seismic penetration that is probably caused by the presence of shallow gas. The gas is believed to be of biogenic origin, and its geographical distribution is bound to a wide band oriented more or less parallel to the coast. The main origin of the gas could to some extent be linked to the presence of a shallow, thin peat-rich layer of Late Pleistocene/Early Holocene age. Local high sedimentation rates furthermore favoured gas formation in the shallow fine-grained Holocene sediments. The gas-related features observed on the seismic profiles include acoustic turbidity and blanking, strong multiple reflections, and to a lesser extent bright spots and phase reversal. The sea-floor morphology does not reveal any clear gas escape from the sea bed, although there are some indications of local seepage of small bubbles or dissolved gas into the water column. The top of the acoustically turbid layer is located between 0 and 7 m below the sea-bed surface. It generally forms a sharp boundary, often marked by a varying offset probably due to different levels of gas penetration which could be related to the lithology of the overlying sediments. Seismic characteristics and velocity data seem to suggest a low concentration of gas, most likely less than 1%.

Integrated Modelling of the Belgian Coastal Zone

The management of the water resources in coastal or delta plains asks for an integrated modelling of the water system at a regional scale. In the SALMON project, detailed descriptions of the groundwater, river and marine domains are provided by coupling appropriate numerical models of these different sub-systems.The application of this three-fold model to the Scheldt and Belgian Coastal Zone reveals a marked river plume extending along the Belgian Coast with strong offshore gradients. This plume cannot be explained by the Eulerian residual transport velocity field but requires Lagrangian aspects to be taken into account. The computation of the age of this water mass shows that the different parts of the coastal zone are influenced by a discharge in the Grote Schijn River after a delay that varies between a few days (for the marine part of the estuary) to about 100 days (for the centre of the residual gyre). The computation shows also that the groundwater around Antwerp can also be contaminated by a pollution in the river but the resulting mass loss from the river does not lead to a significant reduction of the flux to the coastal zone.

Mesozooplankton grazing during the Phaeocystis globosa bloom in the southern bight of the North Sea

During spring blooms 1998 and 1999, three complementary methods were used to evaluate the in situ feeding activities of the dominant copepod species of the Belgian coastal zone: gut pigment content analysis using HPLC, the 14C tracer method, and cell count experiments. The results obtained by all three methods consistently showed that Phaeocystis globosa is not an adequate food source for the spring copepods in the Belgian coastal zone. Our results demonstrated that, among the potential prey, copepods strongly selected diatoms and microzooplankton, and that these types of prey accounted for the major part of the ingested carbon. However, diatoms and microzooplankton ingestion did not always seem sufficient in terms of carbon to avoid food limitation. Comparison of clearance rates exerted on different potential prey types during the P. globosa peak with those before and after the P. globosa peak showed that the copepods' feeding pressure on diatoms was reduced during the P. globosa peak while that on microzooplankton was not. The low grazing pressure on P. globosa, together with the preferential grazing on diatoms, which reduces the competition for nutrients, and the predation on microzooplankton organisms, which reduces the microzooplankton grazing pressure on P. globosa cells, are likely to favour the P. globosa bloom in the Southern Bight of the North Sea.

Winter distribution of phytoplankton and zooplankton around some sandbanks of the Belgian coastal zone

The distribution of phytoplankton and zooplankton around three sandbanks (Gootebank, Westhinder and Buitenratel sandbank) off the Belgian Coast was investigated in February 1994. The abundance of phytoplankton taxa was significantly different between the sandbanks. Community analysis using TWINSPAN resulted in a clear separation of clusters corresponding to the different sandbanks. The zooplankton community analysis, on the contrary, showed a rather indistinctive division of the sandbank stations. This was due to the omnipresence of three dominant copepod species (Temora longicornis, Pseudocalanus elongatus and Centropages hamatus) . When these species were excluded from the analysis, a clearer distinction between the different sandbanks was found. The observed differences in phyto- and zooplankton species distribution could be explained by the position of the sandbanks. Westhinder is positioned further fromthe coast than Buitenratel, while Gootebank has an intermediate position. Buitenratel and Gootebank harbour typical coastal plankton communities, while the plankton community over Westhinder is clearly influenced by the Atlantic current penetrating the southern North Sea from the English channel. More phyto-benthic species were found at Buitenratel than at Gootebank, probably because of its limited depth. Thus, the Belgian coastal zone, which is considered as one box in most spatial descriptions of the North Sea plankton, in fact harbours heterogeneous plankton communities at the end of winter.