Trophic Dynamics of Deep-Sea Megabenthos Are Mediated by Surface Productivity

Samuele Tecchio,Joan Navarro,Karline Soetaert,Eva Ramírez-Llodra,Dick Van Oevelen,Fabiano Thompson

doi:10.1371/journal.pone.0063796

Abstract

Most deep-sea benthic ecosystems are food limited and, in the majority of cases, are driven by the organic matter falling from the surface or advected downslope. Species may adapt to this scarceness by applying a wide variety of responses, such as feeding specialisation, niche width variation, and reduction in metabolic rates. The Mediterranean Sea hosts a gradient of food availability at the deep seafloor over its wide longitudinal transect. In the Mediterranean, broad regional studies on trophic habits are almost absent, and the response of deep-sea benthos to different trophic conditions is still speculative. Here, we show that both primary and secondary production processes taking place at surface layers are key drivers of deep-sea food web structuring. By employing an innovative statistical tool, we interpreted bulk-tissue δ13C and δ15N isotope ratios in benthic megafauna, and associated surface and mesopelagic components from the 3 basins of the Mediterranean Sea at 3 different depths (1200, 2000, and 3000 m). The trophic niche width and the amplitude of primary carbon sources were positively correlated with both primary and secondary surface production indicators. Moreover, mesopelagic organic matter utilization processes showed an intermediate position between surface and deep benthic components. These results shed light on the understanding of deep-sea ecosystems functioning and, at the same time, they demand further investigation.

Highlights

Heterotrophic, bottom-up controlled, food webs are the most abundant ecosystem structure on the planet; i.e. they are the most frequent food webs found in the deep sea, which is the largest biome on Earth [1,2]
Recent advances in benthic ecology identified that deep-sea food webs present a complex trophic structure, with a high number of trophic levels and various processes of niche adaptation [4,5,6,7]
We sampled the southern Balearic region, in the central basin we sampled the western Ionian Sea, while the southern Cretan Sea was considered for the eastern basin

Summary

Introduction

Heterotrophic, bottom-up controlled, food webs are the most abundant ecosystem structure on the planet; i.e. they are the most frequent food webs found in the deep sea, which is the largest biome on Earth [1,2]. The eastern basin is the most oligotrophic area in terms of organic matter input to bathyal depths [12] while, in the western basin, high fluvial inputs, increased surface productivity and other mesoscale oceanographic events generate a higher quantity of organic matter reaching the deep seafloor [13,14]. The central basin presents intermediate environmental conditions between the west and the east, and its continental slope hosts a diverse benthic megafaunal community, more similar to the one present in the western basin [15] This gradient spans the entire Mediterranean region and provides an interesting benchmark to test for macro-ecological patterns in numerous processes such as biodiversity and trophic relationships

Methods

Results

Conclusion