Bottom-up control on macrobenthic communities in a guanotrophic coastal system.

Soft bottom macrobenthic assemblage and sediment characteristics were studied from autumn 2008 to summer 2009 in three coastal ponds (Marinello ponds, Sicily, Italy) at increasing distances from a gull (Larus michahellis Naumann, 1840) colony to investigate the effect of seabird-induced eutrophication (i.e. guanotrophication) on macrobenthic communities. We hypothesized that enhanced nutrient concentration and organic load caused by guano input significantly alter the sedimentary condition of ponds, affecting benthic fauna through a bottom-up control. Polychaetes were the dominant taxon in the system, followed by amphipods, gastropods and bivalves. Macrobenthic patterns showed high variability between ponds and were significantly influenced by confinement, sediment texture and trophic condition (i.e. phaeopigments, dissolved oxygen, nitrogen concentration, organic load). Overall, the lowest benthic abundances and functional group diversity were found in deep sites, especially in the pond characterised by severe guanotrophication, where the higher disturbance resulted in a decline of suspension feeders and carnivores in favour of deposit feeders. Increase of opportunistic/tolerant species (e.g. chironomid larvae and paranoids) and totally azoic sediments were also found as effect of the harshest environmental conditions, resulting in a very poor ecological status. We conclude that macrobenthic assemblages of the brackish coastal ponds of Marinello displayed a clear impoverishment of abundance and trophic functional groups as a consequence of the bottom-up control caused by a chronic guanotrophication condition. This harsh effect decreased at low depth and at increasing distance from the gull colony.