Functional adaptations of benthic communities to organic matter enrichment at the edge of an allowable zone of effect (AZE)

Abstract

In a changing environment, it is important to understand the contribution of faunal and microbial communities to ecosystem functioning. In this context, the present study aimed to explore the effect of organic matter inputs due to aquaculture on the interaction between microphytobenthos and macrofaunal traits related to bioturbation. The study was conducted in the vicinity of a fish farm in a semi-enclosed bay in Cephalonia (Eastern Mediterranean). Two different disturbance zones were compared – a control area and an area close to fish cages and, more specifically, at the edge of the Allowable Zone of Effect (AZE) subject to intermediate stress from aquaculture waste discharge. Bioturbation potential was the main driver shaping microphytobenthic community composition. While Euglenophytes prevailed in the benthic communities close to fish farms, Cyanobacteria were more abundant in the control area, indicating a shift from a microbially driven functioning to a more macrofaunal one near fish cages. Indeed, a shift from suspension feeding to predation and scavenging was recorded close to fish cages. This shift was caused by the interaction of different trophic groups, food availability, as well as the interference of the increased bioturbation in suspension feeding near fish cages. High bioturbation and bioirrigation potentials recorded near fish cages probably acted as moderators of the negative effects of organic matter deposition caused by fish farming. In conclusion, the benthic community close to fish cages was more diverse in terms of biological traits than at the control sites. This indicated the co-existence of species with different ecological strategies near fish cages.