Nitrogenous nutrient transfers in oyster ponds: role of sediment in deferred primary production

Abstract

The oyster pond under study has a natural sediment bottom and water is changed only once or twice every month depending on tidal height. Between successive water renewals, the pond is a closed system, equivalent to a batch culture in which sinking and biodeposition of particulate matter are the only sources of organic matter to the bottom. Particulate organic matter enrichment of the sediment in winter is followed by intensive ammonification in summer with very limited nitrification. Seasonal nitrogen budgets involving particulate and dissolved phases show that in summer during batch mode periods, ammonium enrichment from the sediment to the overlying water is an order of magnitude higher than the initial nutrient supply in the feed water. This allows a large increase of the phytoplankton biomass. We introduce the term 'deferred' primary production for this phenomenon because of the seasonal lag between particulate nitrogen deposition and dissolved nitrogen fluxes from the sediment. INTRODUCTIONIn coastal and estuarine areas, benthic organic matter mineralization and exchanges at the waterhediment interface are important processes in nutrient cycling (Nixon 1981, Balzer 1984, Hopkinson 1987. Moreover, there is now evidence that benthic nutrient flux contributes significantly to the requirements of pelagic primary producers (Boynton & Kemp 1985, Hargrave & Phillips 1986.Along the French Atlantic coast, shallow oyster ponds (less than 1 m deep) cover several thousand hectares in marsh areas. These ponds are subjected to high nutrient pulses from feed water at spring tides. As a result of their physical characteristics (small volume, well-defined water exchanges between pond and sea), their large planktonic blooms and easy access, these ponds can be used as models to study biological rate processes.Early studies of seasonal variations in phytoplankton abundance in such environments (Zanette 1980) showed maximal biomass during summer, when the nutrient supply by feed water was lowest. Similar observations were made by Robert (1983) who also reported a decrease in external nutrient supply and an increase in chlorophyll a values during summer.In the present study, seasonal variations of various elements in suspended matter, as well as exchanges between water and sediment, were followed in an oyster pond in order to determine if nutrient regeneration in sediments during summer allows the development of a phytoplankton biomass which cannot be explained by the initial nutrient supply. Conversely, we also determined if, during winter, the supply of organic matter to the sediment would allow sufficient accumulation of nitrogen reserves.Since shellfish cultures enrich the sediment through biodeposition processes (Kusuki 1981, Sornin et al. 1983) and enhance the recycling of some elements to the overlying water (Kaspar et al. 1985), we made these measurements in an oyster culture sediment area with high sedimentation rate and high organic matter content, and compared these to a control sediment area without oyster ...

Influence of shellfish farming activities on the biogeochemical composition of the water column in Thau lagoon

“…The positive linear relationship between temperature and the excess of NH 4 + , SRP and Si concentrations at shellfish farming stations should be linked to those existing between temperature and nutrient benthic fluxes reported in Thau lagoon by Mazouni et al (1996). The cyclical pattern of variation between NH 4 + , SRP and Si concentrations suggests that biodeposits accumulate at the water-sediment interface and are recycled to a large extent during the warm season due to enhanced remineralisation processes (Sornin et al 1990). The excretion rates of filter feeders are also temperature dependent and display summer maxima (Kautsky & Wallentinus 1980, Mazouni et al 1998b).…”

“…Filter feeders are efficient vehicles of particulate matter from the water column to sediments by filtration coupled with biodeposition (Kautsky & Evans 1987), which leads to the accumulation of organic matter in sediments and the enhancement of benthic fluxes of nutri-ents (Sornin et al 1986(Sornin et al , 1990. Direct release of nutrients in the water column by excretion from reared filter feeders and their epibiota is also an important pathway for remineralisation (Dame et al 1985).…”

Exoproteolytic activity in an Atlantic pond (France): estimates of in situ activity

“…enrichment of coastal water by drainage of agricultural After extraction in 100% methanol at 0°C (Holmareas (Sornin et al 1990, Delmas et al 1992. During Hansen & Riemann 1978), chl a concentrations were the first days of the batch period, nitrate decreased as measured by HPLC.…”

Chemical, isotopic and enzymatic monitoring of free and enclosed seawater implications for primary production estimates in incubation bottles

“…France ' ' Laboratoire Arago, F-66650 Banyuls-sur-mer, France primary production in bottles to direct in situ sampling during a 40 h time series in an homogenous, isolated water mass. Large differences in phytoplankton biomass changes between bulk water and incubated samples had already been documented under those conditions (Sornin et al 1990). As the use of 14C was precluded in the open study site, we used stable isotope and enzymatic estimates of carbon assimilation.…”