Exploring patterns of variation in amphipod assemblages at multiple spatial scales: natural variability versus coastal aquaculture effect

Abstract

A 5-factor design survey was carried out to examine the spatial distribution at different scales of amphipod assemblages and sedimentary variables in soft bottoms adjacent to coastal aquaculture installations. Natural variability of sediment variables showed the highest values at the scales of sites (10s of meters) and locality (1 to 10 km), while the greatest component of variation of amphipod assemblages occurred among replicates (on the scale of meters). Regarding the influence of coastal aquaculture, the highest variability of the environmental variables was observed among the different fish farms. On sandy localities, the influence gradient of coastal aquaculture was determined by total free sulphides, whilst, in muddy localities, the main variable was δ15N. This study has important consequences for the establishment of a clear and effective methodology for studying and monitoring the impact of fish farming, highlighting the complicated establishment of a widespread pattern of effects by coastal aquaculture. The necessity to apply a high replication effort at several spatial scales, especially at the scales of meters and 10s of meters, to increase the precision of estimates of assemblage composition should be taken into consideration.