Land-based salmonid farm effluents enhance heterotrophy in headwater stream food webs

Land-based salmonid farm effluents enhance heterotrophy in headwater stream food websLand-based salmonid farm effluents enhance heterotrophy in headwater stream food web

Rejets de pisciculture intensive d'eau douce : quelle influence sur les chaînes trophiques des milieux récepteurs ?

Session : Système d'élevage et environnementabsen

Feeding the river: The fate of feed-pellet-derived material escaping from land-based trout farms

International audienceEnvironmental impacts of intensive fish cultivation on receiving food webs have been investigated mainly for cage culture systems in lake and marine ecosystems; however, the potential influence of land-based salmonid farms on rivers is less documented. Stable C and N isotopes were used to examine the influence of aquaculture waste on aquatic organisms in three streams with different discharge rates (1–5 m3·s−1) that receive effluents from rainbow trout (Oncorhynchus mykiss) farms with different rearing capacities (100–600 t wet weight per year). Feed pellets from each farm were significantly 13C-enriched (up to +8‰ δ13C) compared to the isotopic backgrounds of the receiving streams. Benthic invertebrates (detritivores and predators) and small-bodied fish were consistently 13C-enriched downstream the fish farm effluents. This pattern was stronger for low stream discharges and high-production farms. The trophic niche breadths of these organisms, estimated by the spread of their isotopic values. decreased in downstream these farms, suggesting a reduction in the diversity of dietary sources. Upstream-downstream comparisons based on multiple-source mixing models revealed high contributions (40–88%) of pellet-derived material in their diet, which indicates that waste from land-based salmonid farms enhanced the detritus-based food chain via this particulate route

Isotope and fatty acid trends along continental shelf depth gradients: Inshore versus offshore hydrological influences on benthic trophic functioning

This work was financially supported by the CNRS-Insu (EC2CO RETROCAN Project) and the Regional Council of Brittany (RECOMAB Project).International audienceAnthropogenic activities and land-based inputs into the sea may influence the trophic structure and functioning of coastal and continental shelf ecosystems, despite the numerous opportunities and services the latter offer to humans and wildlife. In addition, hydrological structures and physical dynamics potentially influence the sources of organic matter (e.g., terrestrial versus marine, or fresh material versus detrital material) entering marine food webs. Understanding the significance of the processes that influence marine food webs and ecosystems (e.g., terrestrial inputs, physical dynamics) is crucially important because trophic dynamics are a vital part of ecosystem integrity. This can be achieved by identifying organic matter sources that enter food webs along inshore-offshore transects. We hypothesised that regional hydrological structures over wide continental shelves directly control the benthic trophic functioning across the shelf. We investigated this issue along two transects in the northern ecosystem of the Bay of Biscay (north-eastern Atlantic). Carbon and nitrogen stable isotope analysis (SIA) and fatty acid analysis (FAA) were conducted on different complementary ecosystem compartments that include suspended particulate organic matter (POM), sedimentary organic matter (SOM), and benthic consumers such as bivalves, large crustaceans and demersal fish. Samples were collected from inshore shallow waters (at similar to 1 m in depth) to more than 200 m in depth on the offshore shelf break. Results indicated strong discrepancies in stable isotope (SI) and fatty acid (FA) compositions in the sampled compartments between inshore and offshore areas, although nitrogen SI (delta N-15) and FA trends were similar along both transects. Offshore the influence of a permanently stratified area (described previously as a "cold pool") was evident in both transects. The influence of this hydrological structure on benthic trophic functioning (i.e., on the food sources available for consumers) was especially apparent across the northern transect, due to unusual carbon isotope compositions (delta C-13) in the compartments. At stations under the cold pool, SI and FA organism compositions indicated benthic trophic functioning based on a microbial food web, including a significant contribution of heterotrophic planktonic organisms and/or of SOM, notably in stations under the cold pool. On the contrary, inshore and shelf break areas were characterised by a microalgae-based food web (at least in part for the shelf break area, due to slope current and upwelling that can favour fresh primary production sinking on site). SIA and FAA were relevant and complementary tools, and consumers better medium- to long-term system integrators than POM samples, for depicting the trophic functioning and dynamics along inshore-offshore transects over continental shelves

Comparative study of isotopic trends in two coastal ecosystems of North Biscay: A multitrophic spatial gradient approach

In coastal estuarine embayments, retention of water masses due to coastal topography may result in an increased contribution of continental organic matter in food webs. However, in megatidal embayments, the effect of topography can be counterbalanced by the process of tidal mixing. Large amounts of continental organic matter are exported each year by rivers to the oceans. The fate of terrestrial organic matter in food webs of coastal areas and on neighboring coastal benthic communities was therefore evaluated, at multi-trophic levels, from primary producers to primary consumers and predators. Two coastal areas of the French Atlantic coast, differing in the contributions from their watershed, tidal range and aperture degree, were compared using carbon and nitrogen stable isotopes (delta C-13 and delta N-15) during two contrasted periods. The Bay of Vilaine receives large inputs of freshwater from the Vilaine River, displaying N-15 enriched and C-13 depleted benthic communities, emphasizing the important role played by allochtonous inputs and anthropogenic impact on terrestrial organic matter in the food web. In contrast, the Bay of Brest which is largely affected by tidal mixing, showed a lack of agreement between isotopic gradients displayed by suspended particulate organic matter (SPOM) and suspension-feeders. Discrepancy between SPOM and suspension-feeders is not surprising due to differences in isotopes integration times. We suggest further that such a discrepancy may result from water replenishment due to coastal inputs, nutrient depletion by phytoplankton production, as well as efficient selection of highly nutritive phytoplanktonic particles by primary consumers. (C) 2013 Elsevier Ltd. All rights reserved