Retention of ciliates and flagellates by the oyster. In order to evaluate the importance of the 'protozoan trophic link' for energy transfer from the 'microbial food web' to large benthic suspension feeders, we offered a coastal pond comn~unity of ciliates and flagellates as potential prey to the oyster Crassostrea gigas. Clearance rate, filtered particles and relative retention efficiency were evaluated. In the grazing experiment, 94 % of ciliates and 86% of flagellates (size between 4 and 72 pm), were retained by the oyster. Whatever their size, protists were similarly retained by the oyster gills. In terms of carbon, oysters retain on average 126 pg C h-' g-' dry weight, a value over 4 times h~g h e r than reported for phytoplankton. These results indicate that a field community of protists can contribute in coastal oyster rearing ponds to the energy requirements of the oyster C. gigas. We report here the first experimental ev~dence of a significant retention of a protist community by oysters, supporting the role of protists as a trophic link between picoplankton and benthic filter-feeding bivalves. The importance of phytoplankton in the nutrition of oysters is well documented , Pastoureaud et al. 1996. However, in oyster rearing environments, such as the particularly light-limited turbid estuary of Marennes-Oleron, or in coastal ponds of the Charente where nutrients are quickly exhausted, phytoplankton cannot entirely account for the energy requirements of oysters .In the oceans, more than 50% of the primary production is due to unicellular organisms less than 3 pm in size (Li et al. 1983, Glover et al. 1986), which constitutes a nutrient source of particulate and dissolved organic matter for heterotrophic organisms. Dissolved organic matter (DOM) present in coastal waters (Pomeroy & Wiebe 1993) provides a potential for high bacterial production. Thus, in the 0 Inter-Research 1999Resale of full artrcle not perrnltted

The Charente River provides nutrient- and virus-rich freshwater input to the Marennes Oléron Basin, the largest oyster-producing region in Europe. To evaluate virioplankton distribution in the Charente Estuary and identify which environmental variables control dynamic of virioplankton abundance, five stations defined by a salinity gradient (0-0.5, 0.6-5, 13-17, 20-24, and higher than 30 PSU) were surveyed over a year. Viral abundance was related to bacterioplankton abundance and activities, photosynthetic pigments, nutrient concentration, and physical parameters (temperature and salinity). On a spatial scale, virus displayed a decreasing pattern seaward with abundance ranging over the sampling period from 1.4x10(7) to 20.8x10(7) viruses mL-1 making virioplankton the most abundant component of planktonic microorganisms in the Charente Estuary. A good correlation was found between viral and bacterial abundance (rs=0.85). Furthermore, bacterial abundance was the most important predictor of viral abundance explaining alone between 66% (winter) and 76% (summer) of viral variability. However, no relation existed between viral abundance and chlorophyll a. Temporal variations in viral distributions were mainly controlled by temperature through the control of bacterial dynamics. Spatial variations of viral abundance were influenced by hydrodynamic conditions especially during the winter season where virioplankton distribution was entirely driven by mixing processes.

As agents of mortality, viruses and nanoflagellates impact on picoplankton populations. We examined the differences in interactions between these compartments in two French Atlantic bays. Microbes, considered here as central actors of the planktonic food web, were first monitored seasonally in Arcachon (2005) and Marennes-Oléron (2006) bays. Their dynamics were evaluated to categorize trophic periods using the models of Legendre and Rassoulzadegan as a reference framework. Microbial interactions were then compared through 48 h batch culture experiments performed during the phytoplankton spring bloom, identified as herbivorous in Marennes and multivorous in Arcachon. Marennes was spatially homogeneous compared with Arcachon. The former was potentially more productive, featuring a large number of heterotrophic pathways, while autotrophic mechanisms dominated in Arcachon. A link was found between viruses and phytoplankton in Marennes, suggesting a role of virus in the regulation of autotroph biomass. Moreover, the virus-bacteria relation was weaker in Marennes, with a bacterial lysis potential of 2.6% compared with 39% in Arcachon. The batch experiments (based on size-fractionation and viral enrichment) revealed different microbial interactions that corresponded to the spring-bloom trophic interactions in each bay. In Arcachon, where there is a multivorous web, flagellate predation and viral lysis acted in an opposite way on picophytoplankton. When together they both reduced viral production. Conversely, in Marennes (herbivorous web), flagellates and viruses together increased viral production. Differences in the composition of the bacterial community composition explained the combined flagellate-virus effects on viral production in the two bays.