Abstract
Although numerous studies have reported the negative effects of shrimp aquaculture on water quality, little is known about the ecological effects of these practices in coastal lagoons and near-shore marine habitats. The impact of shrimp-farm effluents on the food webs of an impacted subtropical coastal lagoon in the Gulf of California was evaluated through measurements of isotopic (δ 13C, δ 15N) signatures in sediments, plants and animals, and compared with the results of a near-pristine reference site. Degradation was manifested in a strong reduction on fish diversity at the perturbed site. δ 13C signatures provided ambiguous evidence of degradation while δ 15N was a better descriptor of shrimp-farm effluent impact on coastal lagoon food webs. The site receiving nutrient-rich discharges showed significant enrichment of δ 15N (≈ 5‰) in sediments, macroalgae, benthic algae, filterfeeders and omnivorous feeders, resulting in qualitative differences in foodweb structure between both lagoons. The food web in the perturbed site was sustained by sediment detritus and dominated by opportunistic species. The lowest influence on δ 15N signatures by aquaculture discharges recorded in the upper trophic levels could be explained by the shift in the composition of biotic communities, and associated feeding strategies. While alterations in resource availability do not affect directly food chain length, trophic linkages between food web compartments can be reduced as a result of shrimp farm impacts. Our study demonstrates that nutrient-enriched discharges from shrimp-farm aquaculture generate changes in the availability of food sources, which reduce biodiversity and alter structural and functional food web characteristics.
Highlights
Despite mangrove wetlands being considered among the most productive ecosystems, they are one of the most threatened ecological systems in the world (Alongi, 2002)
The nutrient surplus is such that the annual nutrient loading from shrimp aquaculture into the Gulf of California through coastal lagoons can be 2,900 t N y-1 and 834 t P y-1, considering a scenario of ca. 26,000 ha of shrimp-farms in operation (≈110 kg N ha-1 y-1 and 30 kg P ha-1 y-1) (Páez-Osuna et al, 1999)
Numerous studies have reported the effects of shrimp aquaculture on water quality, eutrophication and biodiversity in both coastal lagoons and near-shore marine habitats (e.g. Penczak et al, 1982; Boaventura et al, 1997; Ruiz et al, 2001; Karakassis et al, 2002; Paez-Osuna, et al, 2003), little attention has been paid on the impact on ecosystem structure
Summary
Despite mangrove wetlands being considered among the most productive ecosystems, they are one of the most threatened ecological systems in the world (Alongi, 2002). Aquaculture farms generate high amount of organic wastes and unassimilated inorganic fertilizers (García-Sandoval et al, 1991). These untreated effluents discharge directly in coastal lagoons and disperse until 500 m away from the point source (Meili et al, 2000; McGhie et al, 2000; Sutherland et al, 2001). Numerous studies have reported the effects of shrimp aquaculture on water quality, eutrophication and biodiversity in both coastal lagoons and near-shore marine habitats Few studies have reported the effects of aquaculture on coastal food web structure, using either qualitative approaches or focusing on the first trophic level There is still a lack of understanding how impact of aquaculture shrimp-farms affects integrally food webs in subtropical coastal lagoons
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