Influence of environmental variables on the spatio-temporal dynamics of bentho-pelagic assemblages in the middle slope of the Balearic Basin (NW Mediterranean)

Abstract

The spatio-temporal dynamics of benthic and bentho-pelagic assemblages of megafauna from the middle slope of the Catalan Sea (NW Mediterranean Sea) were studied. Nineteen bottom trawls were performed to collect megafauna on 5 cruises between February 2007 and February 2008. Samples were obtained from three depth strata, including a submarine canyon. Simultaneously, environmental variables were recorded and three levels of bottom-boundary layer macrofauna (infauna, suprabenthos and zooplankton), potential prey of megafauna, were sampled. Fish and decapod assemblages were explored individually. Depth was the main factor structuring megafaunal assemblages, with larger organisms usually observed at greater depths (based on mean individual weight). Nevertheless, in April, coinciding with the reproductive period of the dominant species, larger fish and decapods were observed inside the submarine canyon rather than at greater depths. Assemblages inside and outside the submarine canyons differed in species abundance and biomass, rather than in species composition; larger densities of endobenthic species were found inside the canyon. Also, higher abundance and biomass of megafauna were recorded inside the canyon on all cruises, generally linked to higher prey availability there. A seasonal trend related to size was also observed, associated with the stratification (ΔTsurface-bottom water=8.37±0.33°C for summer–autumn) versus homogeneity (ΔTsurface-bottom water=2.09±1.76°C for winter–spring) of the water column. Larger amounts of smaller species feeding at lower trophic levels were found in periods of homogeneity, while assemblages during periods of stratification were characterised by larger species feeding at higher trophic levels. Environmental variables most strongly explaining changes in megafaunal assemblages (from Spearman rank correlation and generalised linear models) were temperature (T) and salinity (S) close to the sea bottom, river discharges and turbidity above the sea bottom. Also, micronekton biomass explained changes in fish assemblages, while Polychaeta biomass and surface primary production were explanatory for decapods. All these variables directly or indirectly affect availability of trophic resources at bathyal depths. T and S are intrinsically related to depth, and are the variables to which organisms may be actually responding. Prey biomass plays an important role in the bathymetric and the topographic distribution of megafauna. T and S are also linked to the stratification or homogeneity of water masses. Peaks of surface primary production (SPP) in February and maximum river discharge in April corresponded to a maximum in megafaunal biomass in summer after ca. 3–5 months. High river discharges, channelled to bathyal depths through submarine canyons, were related to increases in water turbidity, associated with resuspension of organic matter (food for low trophic levels). On the other hand, low turbidity coincided with minimum river discharges, favouring deposition of organic matter in the sediments. Peaks in abundance and biomass of megafauna were observed after peaks in key prey (zooplankton, micronekton and benthic infauna) with a lag of ca. 2 months. Diversity of megafaunal assemblages was coupled to food availability.