Major constrains of the pelagic food web efficiency in the Mediterranean Sea Major constrains of the pelagic food web efficiency in the Mediterranean Sea

Abstract

Grazing pressure plays a key role on plankton communities affecting their biodiversity and shaping their structures. Predation exerted by 2–200 µm protists (i.e. microzoo-plankton and heterotrophic nanoplankton) influences the carbon fate in marine envi-ronments channeling new organic matter from the microbial loop toward the " classic " 5 grazing food web. In this study, we analyzed more than 80 dilution experiments carried out in many Mediterranean sites at the surface and in the meso-bathypelagic layers. Our aims were to investigate prey-predator interactions and determine selectivity among energy sources (in terms of available biomass), efficiency in the exploitation and highlight likely 10 constrains that can modulate carbon transfer processes within the pelagic food webs. Generally, microzooplankton shown higher impacts on prey stocks than heterotrophic nanoflagellates, expressing larger ingestion rates and efficiency. Through different trophic conditions characterized on the base of chlorophyll a concentration, micro-zooplankton diet has shown to change in prey compositions: nano-and picoplankton 15 almost completely covered consumer needs in oligotrophy and mesotrophy, while mi-crophytoplankton (mostly diatoms) represented more than 80 % of the consumers' diet in eutrophy, where, nevertheless, picoplankton mortality remained relatively high. Ingestion rates of both consumers (nano-and microzooplankters) increased with the availability of prey biomasses and consequently with the trophic condition of the 20 environment. Nevertheless, overall the heterotrophic fraction of picoplankton resulted the most exploited biomass by both classes of consumers. Ingestion efficiency (as the ratio between available biomass and ingestion rate) in-creased at low biomasses and therefore the highest efficiencies were recorded in olig-otrophic conditions and in the bathypelagic layers. 25 4366 BGD 12, 4365–4403, 2015