Abstract

Normalized biomass size spectra (NBSS) and the stable nitrogen isotopic composition of food webs were analyzed to determine the inter-specific relationships between trophic level (measured as delta N-15) and body size in macrobenthic communities living on the continental shelf of the southeastern Weddell Sea (SEWS). We found that the relationship between trophic level and body size for the whole macrobenthic community was not significant (r(2) = 0.08, p > 0.05), probably associated with biomass accumulated in the larger body size fractions, which are represented particularly by suspension feeders such as sponges and tunicates. We used an alternative method of studying trophic structure of the aquatic communities based on the distribution of residuals of the NBSS. Here we demonstrate how residual distribution exhibited dome-like patterns, which may offer an additional quantitative tool for studying the relationship between trophic level and body size. These domes of biomass represent trophic positions derived from relationships between the body size distribution of the predators in one trophic position and their prey in another. We found 4 well defined domes of biomass, which appear when large benthic species are abundant, especially favored in an environment with high organic matter flux. A significant positive relationship (r(2) = 0.59, p < 0.05) between the trophic level and residuals derived from NBSS suggests that these domes might be considered as functional groups comprising organisms from different trophic levels. We suggest that dome-like patterns in the biomass size spectra can provide a robust framework for conceptualizing and statistically modeling trophic levels of macrobenthic communities.