Macrobenthic communities in a temperate urban estuary of high dominance and low diversity: Montevideo Bay (Uruguay).

Abstract

The macrobenthic subtidal community was studied between April 1997 and April 1998 in Montevideo bay, an urban estuary located in the fluvio marine system of the Río de la Plata (Uruguay) that receives a variety of industrial and sewage inputs. Monthly surveys were carried out at ten sampling stations where sediment samples were taken with a manual corer and analysed for granulometric parameters, organic matter content, chlorophyll a and phaeopigments content, redox potential, and macrobenthic fauna. The area presented high organic matter content in its sediments and several regions of the bottom were anoxic during a large part of the sampling period. The benthic macrofauna was dominated, both in numbers as well as in biomass, by the small surface deposit-feeder gastropod Heleobia cf. australis. Cluster analysis, Multidimensional Scaling and Canonical Correspondence Analysis revealed that the study area could be divided in two well-defined regions with different environmental characteristics and different faunal composition. The dissolved oxygen content in the bottom water and variables related to it were the most important factors in explaining the patterns of the benthic communities. At the phylum level, the meta-analysis of “production” showed a high disturbance status for all stations. The innerregion, the most affected by anthropogenic   activities, was the most compromised  environmentally and biologically, and was  characterised by a very low diversity and  abundance, reduced conditions in the sediments and low oxygenated bottom water. In more external places of the bay, on the other hand, perhaps due to their location at a greater distance from the sources of organic material and in a region with higher hydrodynamic energy, the conditions for the development of benthic fauna were more favourable. Spatial and temporal faunistic patterns observed and their possible causes are analysed and discussed in relation to the natural and anthropogenic factors that act in this coastal ecosystem.