Sedimentologisch onderzoek van de Spuikom te Oostende

Thirty sediment samples of the Scour Basin in Ostend were analysed for the boulder content, detric elements, carbonates, organic material and their Zn, S, Cu, Mn and P concentration were determined. Then correlations between all these factors were determined. The spread of most factors appeared bound to those of the boulder

Discrete suspended particles of barite and the barium cycle in the open ocean

Barite particles are a universal component of suspended matter in the Atlantic and Pacific Oceans. This is demonstrated by scanning electron microscope and electron microprobe analyses of samples collected during the GEOSECS program. These discrete particles, about 1 µm in diameter, account for by far the greatest part of the total particulate barium of most of the filters collected at different depths. Total particulate barium (mean value: 20 ng/kg seawater) was measured on the same filters by instrumental neutron activation analysis.Several observations indicate that biochemical, rather than purely chemical, processes are involved in the formation of the BaSO4 mineral in the water column. Sr/Ba molar ratios among the individual barite grains, particularly from surface waters are extremely variable, which would not be anticipated for purely chemical interactions. Barite crystals occurring within fecal debris have been observed throughout the water column. Within such debris decomposition of the abundant organic matter may provide the micro-environment predicted as necessary for the precipitation of BaSO4. Finally, a strong correlation between nutrient content and particulate barium is found in the upper 1000 m of the water column, which also suggests a control of barite formation by biota.Some of the barite dissolves at depth in the water column. Dissolution rates were calculable for two GEOSECS stations, from which a dissolved barium flux of 0.4 µg/cm2 yr was deduced. This figure is of the same order as the dissolved barium flux calculable from the barium content and known dissolution rates of calcareous and siliceous tests: approximately 0.5 µg/cm2 yr. These fluxes represent the largest source of dissolved barium in the water column, the other being river input (0.6 µg/cm2 yr). This supports the contention that the barium in the water column is mostly recycled. The residual flux of barite-Ba reaching the sea floor is of about equal importance as the flux of barium associated with fast-settling fecal material. These two sources together are almost sufficient to account for the total sedimentation rate of barium

Carbon sources supporting benthic mineralization in mangrove and adjacent seagrass sediments (Gazi Bay, Kenya)

The origin of carbon substrates used by in situ sedimentary bacterial communities was investigated in an intertidal mangrove ecosystem and in adjacent seagrass beds in Gazi bay (Kenya) by d13C analysis of bacteria-specific PLFA (phospholipid fatty acids) and bulk organic carbon. Export of mangrove-derived organic matter to the adjacent seagrass-covered bay was evident from sedimentary total organic carbon (TOC) and d13CTOC data. PLFA d13C data indicate that the substrate used by bacterial communities varied strongly and that exported mangrove carbon was a significant source for bacteria in the adjacent seagrass beds. Within the intertidal mangrove forest, bacterial PLFA at the surface layer (0-1cm) typically showed more enriched d13C values than deeper (up to 10cm) sediment layers, suggesting a contribution from microphytobenthos and/or inwelled seagrass material. Under the simplifying assumption that seagrasses and mangroves are the dominant potential end-members, the estimated contribution of mangrove-derived carbon to benthic mineralization in the seagrass beds (16-74%) corresponds fairly well to the estimated contribution of mangrove C to the sedimentary organic matter pool (21-71%) across different seagrass sites. Based on the results of this study and a compilation of literature data, we suggest that trapping of allochtonous C is a common feature in seagrass beds and often represents a significant source of C for sediment bacteria - both in cases where seagrass C dominates the sediment TOC pool and in cases where external inputs are significant. Hence, it is likely that data on community respiration rates systematically overestimate the role of in situ mineralization as a fate of seagrass production

Estimating turnover rates of d¹³C and d¹⁵N in muscle, heart and liver tissue of juvenile sand gobies (<i>Pomatoschistus minutus</i>)

Large numbers of marine fish typically enter and remain within estuaries during their juvenile life stage. Stable isotopes of carbon and nitrogen can serve to trace these individual movements due to the food web differences among marine and estuarine habitats. Here, we present the background for the utilization of d13C and d15N to analyze the migration dynamics of juvenile sand gobies (Pomatoschistus minutes), between the North Sea and the Scheldt estuary. The isotopic turnover of P. minutes, defined as the change in isotopic composition due to growth and metabolic tissue replacement, was examined for muscle, liver and heart tissue. A diet switch experiment simulating natural conditions for the Scheldt estuary was conducted during 90 days. Fish were fed a commercial pellet diet, which was isotopically different from the initial goby tissue d13C and d15N. Chopped polychaetes (Arenicola sp.) and mussels (Mytilus edulis) were used as control diets to test for effects other than diet. Fish were sacrificed for stable isotope analysis (CF-IRMS) on regular time intervals depending on the diet. Trophic fractionation was estimated for the different tissues and the effect of food deprivation on stable isotope composition was also evaluated. Heart and liver tissues had a faster isotopic turnover than muscle tissue as a result of their higher metabolism. However, growth was found to explain most of the variation in isotopic composition within a single tissue. There was no significant effect of 20 days of food deprivation on d13C and d15N for the tissues. The isotopic assay of muscle, liver and heart tissue within and among individuals will allow a better delineation of those individuals not in equilibrium with their isotopic environment. Therefore new arrivals in the estuary will be identified on a finer temporal resolution than feasible with muscle tissue alone

Sources of suspended organic matter and selective feeding by zooplankton in an estuarine mangrove ecosystem, as traced by stable isotopes

Between January 1995 and August 1996, suspended matter and zooplankton were sampled at different locations in a mangrove ecosystem located in the Gautami Godavari Estuary and adjacent Kakinada Bay (Andhra Pradesh, India). Suspended matter was sampled at 13 different stations, and was found to have a highly variable carbon stable isotope composition, with delta 13C values ranging overall between -30.94 and -19.18 ppt, and a highly variable elemental (C:N) composition. Our data suggest that the phytoplankton component has a seasonally and spatially variable delta 13C signature, which is surpressed by the terrestrial signal but may at times fall in the same range as the delta 13C of the allochtonous matter. It is argued that the phytoplankton delta 13C decreases after the onset of the monsoon rains, most likely due to the 13C-depletion of the DIC pool caused by the microbial respiration of the allochtonous organic matter. At each of the 4 sites selected for concurrent zooplankton sampling, the zooplankton showed a much wider range of delta 13C than did the suspended matter, with overall delta 13C values between -30.14 and -16.45 ppt. In addition, spatial differences in average delta 13C were much more pronounced for zooplankton than for total suspended matter. These data indicate that zooplankton feed on a component of the suspended matter pool, which has more pronounced seasonal and spatial delta 13C variations than the total suspended matter. Thus, despite the large amounts of terrestrial and mangrove detritus present in the water column, the locally produced phytoplankton appears to be a more important carbon source for the zooplankton

d¹³C and d¹⁵N composition of suspended particular organic matter (SPOM) and zooplankton in the eutrophic Scheldt estuary (Belgium) (poster)

d13C and d15N were used to study food sources of zooplankton in the Scheldt Estuary. The SPOM carried by the river is a mixture of antropogenic and terrestrial detritus with associated bacteria and locally produced autotrophic organic matter. Samples of SPOM and zooplankton were taken monthly from June 1999 to April 2000 at four stations located along a salinity gradient (0-14 ppt). We investigated the relative importance of detritus, heterotrophic and autotrophic organic matter in the diet of zooplankton.Both d15NSPOM and d13CSPOM varied seasonally, paralleling the change in biomass of autotrophic organisms. Generally, phytoplankton bloom periods were characterised by high d15NSPOM (maximum +12.9‰) and low d13CSPOM values (minimum -31.1‰). Winter d15NSPOM and d13CSPOM values were characteristic for pure antropogenic/terrestrial detritus (d15N = +2.5‰; d13C = -26.8‰). The summer increase of d15NSPOM was attributed to autotrophic consumption of NH4+ strongly enriched in 15N due to intense nitrification. However, the seasonal pattern between the stations differed reflecting differences in the timing, duration and intensity of the phytoplankton bloom. d13Czooplankton and d15Nzooplankton co-varied with d13CSPOM and d15NSPOM. The high deviation from the original d13CSPOM and d15NSPOM suggested selective feeding on specific components of the SPOM

Migration of juvenile herring (<i>Clupea harengus</i>) and sprat (<i>Sprattus sprattus</i>) between the North Sea and the Schelde estuary proved by stable C en N isotopes

The temporal changes in abundance of juvenile herring (Clupea harengus, Linnaeus, 1758) and sprat [(Sprattus sprattus, Linnaeus, 1758)] in the Schelde Estuary were examined by means of stable isotopes. Juvenile herring and sprat typically overwinter in the estuary. Herring exhibits a second, smaller density peak in the summer. A similar density peak for sprat is not observed. The temporal use of the estuary by clupeoid fish has previously been attributed to seasonal migrations of juveniles between the North Sea and the estuary. Using stable isotopes of carbon and nitrogen we have tried to elucidate these migration patterns. Herring and sprat were sampled between May 2000 and April 2001. Samples were taken every month in the cooling water of the Doel Nuclear Power Plant (in the brackish part of the estuary) and the Borssele Nuclear Power Plant (at the mouth of the estuary). Using cluster analysis on the d13C and d15N values of individual muscle tissue, fish which recently immigrated from the North Sea (marine group with typical marine isotope values) could be distinguished from individuals which had resided in the estuary (estuarine group with typical estuarine isotope values). The analysis showed that herring and sprat had very similar migration dynamics in the Schelde Estuary, characterized by immigration and emigration almost throughout the year and an intensive migration activity during the winter. Net upstream immigration (i.e. the majority of fish enter the estuary) started in September and peaked in November. During December immigration remained high but had already decreased, which probably explains lower fish densities recorded at Doel. Although the density of herring and sprat further declined in February and March, net seaward emigration sensu strictu (i.e. the majority of the fish leave the estuary) was not demonstrated using the stable isotope technique. During the winter larger proportions of individuals with a typical marine isotope signature at Doel were not only associated with migration but also with a slower tissue turnover rate