Mapping trophic continua of benthic foodwebs:: body size-δ15N relationships

The stable nitrogen isotopic composition (delta(15)N) of benthic foodwebs was analyzed to determine trophic relationships among 23 taxa composited from 4 proximal oligotrophic lakes and 35 taxa composited from 2 proximal seagrass meadows. Because omnivory was prevalent, animals in both foodwebs existed along continua of trophic positions rather than in discrete trophic levels. These trophic continua were in turn related to organism body size in support of Elton's early suppositions about foodweb structuring. These foodweb delineations developed due to feeding hierarchies being determined by size-related predation, resulting in 'upper triangular' foodweb matrices as posited by Cohen's cascade model. Expressing aquatic foodweb structure in relation to the ataxonomic continuous variables of organismal trophic position (delta(15)N) and body size has the advantage of providing a convenient common currency for comparative purposes that dispenses with the artificiality and impossibility of constraining omnivorous animals into rigidly defined trophic levels.