Modeling approach of nitrogen and phosphorus exchanges at the sediment–water interface of an intensive fishpond system
Introduction
The sediment plays an important role in aquaculture ponds as it is both an important source of various dissolved substances and a sink for particulate materials (Masuda and Boyd, 1994). Gas and nutrients exchanged at the sediment–water interface are potentially toxic to cultivated species and represent a critical aspect of aquaculture pond management (Boyd, 1995). Among these substances, the accumulation of reduced substances, such as ammonia, is one of the major factors that can explain fish growth retardation in earthen ponds (Avnimelech and Zohar, 1986). In such systems, biogeochemical processes, and consequently sediment to water exchanges, depend mostly on food and feeding strategies, water temperature variations, water circulation and aeration, and pond depth (Hargreaves, 1998). Anaerobic zones often occur a few millimeters under the sediment surface of aquaculture ponds or directly at the sediment water interface (Avnimelech et al., 1981). Anaerobic soils are known to contain high concentrations of ammonia, phosphates and sulfide. Therefore, the molecular diffusion from sediment and excretion by cultivated species are the main sources of ammonia in aquaculture ponds (Hargreaves, 1998).
The aim of this study was to assess the ammonia and phosphate fluxes from the sediment of earthen ponds used to culture seabass (Dicentrarchus labrax). Fluxes were determined at different seasons and the contribution of sediment flux to the total dissolved inorganic nitrogen and phosphate was defined. This estimation was achieved by independent evaluation of the sinks (sedimentation) and the sources (mineralization–diffusion) of ammonia and phosphate.
Section snippets
Farm site and sampling strategy
The experiments were carried out in a pond of a land-based seabass (D. labrax) farm located on the French Atlantic coast, from January 1997 to December 1998. Fish were cultured up for 3 years in floating cages in five earthen ponds. The average pond area was 2600 m2, its depth 3.5 m, and it contained a fish biomass of 100 tons. Fishponds were shaded and hence primary production was absent in the pond. A previous study indicated that macrofauna was not present in the fishpond sediment (Hussenot
Sediment spatial and temporal variability
Table 4 gives the results of the sediment survey. A two-way ANOVA was performed on the effect of temperature and station, which revealed significant differences within the factors for all variables (Table 5). ANOVA analysis also revealed significant interactions between the factors, except for organic matter (OM), ammonia and phosphate bottom water concentrations. Most of the variability of sediment porewater ammonia and phosphate concentrations and diffusive fluxes was explained by temperature
Discussion
The comparison of sedimentation and diffusive fluxes provides an independent evaluation of the processes occurring at the sediment–water interface in such an intensive marine system where data and descriptions are scarce in literature (Hargreaves, 1998). According to the classification of Krom et al. (1989), the system under study can be considered as a high water flow rate pond where water is replaced several times a day. In such a system, phytoplankton is flushed out, making the
Acknowledgements
We thank Drs. Mario Laima, Maurice Héral, Michèle Feuillet-Girard and Dominique Gouleau for their interesting comments on our study. We would also like to thank Robert Knutsen for his english assistance, Lucette Joassard, Marcel Guillaut and Michel Prineau for technical assistance. Special gratitude is extended to the private farm company and its managers Bernard Houin and André Zwaga, who allowed Sébastien Lefebvre access to their facilities and gave financial support for the study. This study
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