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

Abstract

<p><strong>Abstract.</strong> 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. microzooplankton and heterotrophic nanoplankton) influences the carbon fate in marine environments channeling new organic matter from the microbial loop toward the "classic" grazing food web. <br><br> 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 constrains that can modulate carbon transfer processes within the pelagic food webs. <br><br> 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 <i>a</i> concentration, microzooplankton diet has shown to change in prey compositions: nano- and picoplankton almost completely covered consumer needs in oligotrophy and mesotrophy, while microphytoplankton (mostly diatoms) represented more than 80% of the consumers' diet in eutrophy, where, nevertheless, picoplankton mortality remained relatively high. <br><br> Ingestion rates of both consumers (nano- and microzooplankters) increased with the availability of prey biomasses and consequently with the trophic condition of the environment. Nevertheless, overall the heterotrophic fraction of picoplankton resulted the most exploited biomass by both classes of consumers. <br><br> Ingestion efficiency (as the ratio between available biomass and ingestion rate) increased at low biomasses and therefore the highest efficiencies were recorded in oligotrophic conditions and in the bathypelagic layers.</p>

Changes in the Trophic Pathways within the Microbial Food Web in the Global Warming Scenario: An Experimental Study in the Adriatic Sea

“…This study suggests that CIL are important predators of not only HNF and picoplankton groups with larger cells (SYN and PE), but also picoplankton groups with smaller cells (HNA, LNA, and PROC). This is consistent with studies reporting a higher impact of microzooplankton than HNF on PICO prey stocks [89,90]. CIL can exert powerful control on the PICO community, both through direct grazing and trophic cascading [91,92].…”

Changing Ecological Conditions in the Marine Environment Generate Different Microbial Food Web Structures in a Repeatable Manner

“…These results suggest that CIL are important predators not only of HNF and picoplankton groups with larger cells (PE), but also of picoplankton groups with smaller cells (PROC and SYN). This is consistent with studies reporting a greater impact of CIL than HNF on picoplankton prey (Zingel et al, 2007;Zoccarato and Fonda Umani, 2015). One possible explanation for the high grazing influence of CIL on picoplankton prey is the change in community size structure of CIL to better adapt to the consumption of faster growing picoplankton groups with smaller cells (Šolić et al, 2020a).…”