Assimilation of organic matter by two benthic consumers across gradients of latitude and nutrient enrichment

Abstract

In modifying the traits of producers, coastal development and latitude may influence the assimilation of organic matter resources by consumers. The aim of the present study was to assess spatial variation across gradients of latitude and diffuse nitrogen loading in: (1) the N content of the seagrass Zostera muelleri and the mangrove Avicennia marina; and (2) the ultimate organic matter sources (inferred from δ15N and δ13C signatures) of the detritivorous mud whelk Pyrazus ebeninus and the predatory polychaete Nephtys australiensis. It was hypothesised that the organic matter sources of each of the two consumers would vary spatially, following patterns of spatial variation in the N content of primary producers. Sampling in 12 estuaries of New South Wales, Australia, spanning 7° of latitude and variable nutrient loading revealed that the nitrogen content of Z. muelleri was negatively correlated with latitude and nitrogen loading, but the nitrogen content of A. marina leaves followed only latitude. Of the four organic matter sources considered by the present study, Z. muelleri was consistently the main source passed through the trophic chain to the detritivore P. ebeninus and the predator N. australiensis. Nevertheless, the proportionate contribution of Z. muelleri and microphytobenthos to the carbon sources of N. australiensis varied with latitude, the former negatively and the latter positively. These relationships suggest that latitude may influence carbon sources of consumers by modifying producer physicochemical traits.