Nitrogen loads influence trophic organization of estuarine fish assemblages

Increased nutrient inputs to aquatic ecosystems can disrupt ecological processes (e.g. primary production, nutrient cycling, energy transfer), which, in turn, can alter trophic organization and the trophic niche of consumers.... [ view full abstract ]

Increased nutrient inputs to aquatic ecosystems can disrupt ecological processes (e.g. primary production, nutrient cycling, energy transfer), which, in turn, can alter trophic organization and the trophic niche of consumers. Isotopic niches have been used increasingly to infer trophic niches because the stable isotope composition of consumer tissues provides a cumulative signal of assimilated diet. We sampled stable isotope compositions of nitrogen and carbon (δ15N and δ13C) in muscle tissues of fish collected from 14 estuaries spanning a gradient of nutrient loadings to determine if the trophic niches of fish assemblages depended on local nutrient conditions. Isotope values were used to calculate isotopic niche indices and we used a novel standardization approach to correct for ecosystem-specific differences in isotope conditions at the bases of each food web. Model selection was used to relate isotopic niche indices of fish assemblages to nitrogen and phosphorous loading and to biotic (predation potential) and abiotic (hydrological flushing) characteristics of estuaries. Inorganic nitrogen loading to estuaries increased trophic diversity and decreased trophic redundancy within fish assemblages. Understanding and predicting the effects of nitrogen loading on coastal ecosystems is important for environmental researchers and managers and our results underscore the value of assessing functional as well as structural responses of estuarine communities to environmental disturbances.