Assessment of benthic trophic status of marine coastal ecosystems: Significance of meiofaunal rare taxa

Eutrophication enhances organic C inputs to the sea bottom, so that the quantity and biochemical composition of sedimentary organic matter are expected to change under different trophic status conditions. In turn, changes in the trophic status are often associated with shifts in the abundance and community structure of the benthos. We investigated the quantity and biochemical composition (protein, carbohydrate, lipid and phytopigment) of sedimentary organic matter together with abundance and community structure of meiofauna in two regions of the Mediterranean Sea characterized by different levels of primary productivity. At each region, five transects were randomly selected along the coastline. At each transect, three stations located at increasing distance from the shore and different water depth were investigated. Uni- and multivariate analyses of variance revealed that the quantity and biochemical composition of sediment organic matter displayed the most relevant differences between stations, suggesting their potential use as descriptor of the benthic trophic state at the small spatial scale (i.e. <2 km). The correlation analyses, corroborated by nMDS dispersion plots and cluster analyses highlighted that the biopolymeric C content of the sediment and the algal fraction of sediment organic matter were inversely related and able to discriminate the trophic status at the scale of region, transect and station. Uni- and multivariate analyses on meiofaunal assemblages revealed that differences at different spatial scales were less evident in terms of abundance and richness of meiofaunal taxa but were more evident in terms of taxonomic composition. Furthermore, these differences were enhanced when the analyses were restricted using the meiofaunal rare taxa (i.e. those taxa representing <1% of the total meiofaunal abundance) as an input. The results of the multivariate multiple regression analyses revealed that the taxonomic composition of meiofaunal assemblages was driven mostly by protein, biopolymeric C and chlorophyll-a concentrations but also by the algal fraction of biopolymeric C. We conclude that, the study of the quantity and biochemical composition of sediment organic matter coupled with an analysis of the rare meiofaunal taxa allows an ecosystem-oriented assessment of the trophic status of marine benthic environments.