Ecological implications of life-forms in intertidal benthic diatoms in macrotidal estuaries

Diatom communities inhabiting intertidal estuarine sediments are composed of different life-forms, ranging from adnate epipsammic to free-living epipelic and tychoplanktonic forms. A detailed study of the spatial and temporal distribution patterns of various life-forms in the macro-tidal Westerschelde estuary (South-West Netherlands) revealed that the specific life-form composition of a community has an important influence on its temporal dynamics and might also have major implications for the transfer of diatom-fixed carbon to higher trophic levels. The importance of substrate structure and availability, stochastic (e.g. climate-induced and anthropogenic) hydrodynamic events and the nature and rate of sediment development in regulating the dynamics of these communities are evaluated

Diversity, ecology and the role of protist communities in the Schelde estuary: research by the "Protistology & Aquatic Ecology" group, University Gent (poster)

The section ‘Protistology & Aquatic Ecology’ has been studying the diversity and ecology of planktonic and benthic communities of unicellular eukaryotes (or protists) in the Schelde estuary. The study area involves the entire estuarine gradient, going from the freshwater tidal reaches in the Belgian part of the estuary to the estuary mouth in The Netherlands. Large research efforts were invested in the diversity of protistan communities (diatoms and other micro-algae, heterotrophic flagellates and ciliates) and the biotic and abiotic factors that regulate their structure and composition. In addition, several studies dealt with the role of microbial plankton and benthos in the functioning of estuarine food webs. In these studies, attention was paid to the importance of diversity of microbial communities for fluxes of matter and energy through the estuarine microbial ecosystem. This poster aims at illustrating certain aspects of this research

Ciliate predation by nematodes in tidal flat sediments

The present study investigates the possibility of a trophic link between ciliates and nematodes in fine sandy sediments of the Molenplaat intertidal flat (Schelde estuary, SW Netherlands). Grazing experiments were conducted under controlled laboratory conditions, with ciliate species isolated from enrichment cultures and nematodes directly collected from the field. Significant reductions in ciliate numbers were found in the presence of the predatory nematode Enoploides longispiculosus, a prominent species (and genus) in fine to medium sandy sediments of the North Sea and adjacent estuaries. No such effects were found when ciliates were inoculated with a mix of mainly deposit-feeding nematodes from the same sampling site. Based on these results, ciliate predation by E. longispiculosus was tested for several benthic ciliate species and abundances, at a range of predator abundances and temperatures, and in the presence of alternative prey (in casu nematodes). E. longispiculosus was capable of significantly reducing densities of 5 out of 6 ciliate species offered as prey. Depending on the experimental conditions and the prey species, predation rates ranged from 0.19 to 10.8 ciliates predator-1 hour-1, corresponding to a biomass consumption of 0.001-0.33 µg C predator-1 day-1. An overall positive relation between available ciliate biomass and predation rate was found. Comparison of experimental data with field conditions suggests that a considerable part of the ciliate production in fine sandy sediments of the Molenplaat is likely to be consumed by E. longispiculosus, which largely dominates meiofaunal biomass there. Estimated carbon requirements for the predator and production estimates of ciliate and nematode prey at the study site, strongly suggest that ciliates are probably a far more important carbon source for E. longispiculosus than nematode prey, at least between late spring and autumn. This implies that carbon transfer from primary producers and bacteria to predatory nematodes may to a large extent be mediated through the microbial food web. In view of the generally high densities/biomasses of ciliates as well as predacious nematodes in fine sandy sediments, similar patterns are to be expected in many other estuarine and marine sediments

Trophic interactions between ciliates and nematodes from an intertidal flat

The present study investigated the possibility of a trophic link between ciliates and nematodes in fine sandy sediments of the Molenplaat intertidal flat (Schelde estuary, SW Netherlands). Grazing experiments were conducted under controlled laboratory conditions, with ciliate species isolated from enrichment cultures and nematodes collected directly from the field. Significant reductions in ciliate numbers were found in the presence of the predatory nematode Enoploides longispiculosus, a prominent species (and genus) in fine to medium sandy sediments of the North Sea and adjacent estuaries. No such effects were found when ciliates were inoculated with a mix of mainly deposit-feeding nematodes from the same sampling site. On the basis of these results, ciliate predation by E. longispiculosus was tested for several benthic ciliate species and abundances, at a range of predator abundances and temperatures, and in the presence of alternative prey (in casu nematodes). E. longispiculosus significantly reduced the densities of 5 out of 6 ciliate species offered as prey. Depending on the experimental conditions and the prey species, predation rates ranged from 0.19 to 10.8 ciliates predator-1 h-1, corresponding to a biomass consumption of 0.001 to 0.33 µg C predator-1 d-1. An overall positive relation between available ciliate biomass and predation rate was found. Comparison of experimental data with field conditions suggests that a considerable part of the ciliate production in fine sandy sediments of the Molenplaat is likely to be consumed by E. longispiculosus, which largely dominates meiofaunal biomass there. Estimated carbon requirements for the predator and production estimates of ciliate and nematode prey at the study site strongly suggest that ciliates are probably a far more important carbon source for E. longispiculosus than nematode prey, at least between late spring and autumn. This implies that carbon transfer from primary producers and bacteria to predatory nematodes may to a large extent be mediated through the microbial food web. In view of the generally high densities and biomasses of ciliates as well as predacious nematodes in fine sandy sediments, similar patterns are to be expected in many other estuarine and marine sediments

Conservation of diatom biodiversity: issues and prospects

The diatoms are microscopic unicellular plants, which in spite of their tiny size (normally within the range of 0.01-0.1 mm) play an enormous role in the functioning of the biosphere. Their contribution to the global production of organic matter created on Earth through photosynthesis is estimated as 20-25%. Very recently, the results of sensitive, fine-grained taxonomical, biological and biogeographical studies have provided strong evidence that the widely accepted dogma that microorganisms are predominantly cosmopolitan does not apply in case of the diatoms. Many diatom species may be endemics and some of them seem to be restricted to a small geographical area, which makes conservation of diatoms a significant issue. It is time to realise that efforts to develop realistic conservation strategies for aquatic environments, both at the local and global scale should include the diatoms and possibly also other groups of microorganisms. Some ways of how the diatoms could be involved in this process are presented for discussion

Influence of salinity and light climate on phytoplankton in the Schelde estuary

In this study, we tested the effects of light intensity and salinity on estuarine phytoplankton communities of the Schelde River and Estuary. We used HPLC in order to identify and quantify the different phytoplankton pigments. In two experiments (summer-spring), water was collected in the river and at two locations in the estuary (freshwater tidal and brackish). The water was mixed with an equal amount of filtered water (GF/C filters) from the same or a different site and was exposed to different light levels to test for the effects of the chemical composition of the water (including salinity) and the light climate on phytoplankton growth. In spring as well as in summer, diatoms dominated the phytoplankton community, while green algae were relatively more important in summer. A reduction in light intensity strongly reduced growth (as evaluated by changes in chl a concentration) of phytoplankton from the freshwater tidal estuary as well as the river. In spring, at high light intensity, a relative increase of green algae was observed although diatoms remain dominant (as evaluated by fuc/chl a and lut/chl a ratios). In summer, on the contrary, green algae were negatively influenced by the high light intensity. Exposure of phytoplankton from the river to water from the freshwater tidal estuary had no effect on phytoplankton biomass and community composition. Phytoplankton from the river as well as from the freshwater tidal estuary was negatively influenced by an increase in salinity. Phytoplankton from the brackish part of the estuary, on the other hand, was not significantly influenced by a decrease in salinity

Tidal wetland restoration at Ketenisse polder (Schelde Estuary, Belgium): developments in the first year

Ketenisse polder is a former intertidal brackish marsh (30ha) situated in the mesohaline part of the Schelde Estuary. In the 19th century its central part was embanked as a polder. In the mid 1980’s the area was raised above intertidal level when it was used as a dumping site for the excavated soil from the Liefkenshoek tunnel. In 2002 the area was restored, it was levelled with a weak slope below mean high water level, creating the optimal starting conditions for the development of intertidal mudflats and marshes. Geomorphological changes, sediment characteristics and colonisation by phytobenthos, vegetation, zoobenthos, water birds and breeding birds at the restored site are monitored. The monitoring results of the first year after tidal restoration are presented. Sedimentation as well as erosion between 0 and 30cm was observed in the first year. Local changes in stream current patterns caused erosion on parts of the former mudflats; sheltered depressions filled up relatively fast. Median grain size showed large variation. Organic carbon content of the sediment varied between 0.5 and 15% and was closely related to sediment medium grain size. Chlorophyll a concentrations were negatively correlated with median grain size and tended to increase from the low water line to the shore. They were comparable to nearby intertidal areas and displayed similar seasonal variability with a maximum in spring. The large surface covered wtithVaucheria was indicator of initiated succession towards tidal marsh. Scirpus maritimus and transitional vegetations to Chenopodiaceae-vegetations established with increasing altitude. The Chenopodiaceae-vegetations were relicts of earlier vegetations before the tidal restoration, and will probably disappear. The macrobenthos community was dominated by Oligochaetes, which were present in 73% of all samples and attained an average density of about 40*103 ind. m-2. Other macrobenthos species found were nematods, copepods and Corophium. On the sheltered sampling stations macrobenthic densities were high compared to those on nearby intertidal areas. In the first season, 15 breeding bird species were recorded, the most common species being the Pied Avocet (Recurvirostra avosetta). The most common waterbirds were Common Shelduck (Tadorna tadorna), Greylag Goose (Anser anser), Pied Avocet (Recurvirostra avoset) and Lapwing (Vanellus vanellus), typical species for the mesohaline part of the estuary. The first year’s results suggest that Ketenisse polder has the potential to develop towards a varied and normal functional intertidal area