Spatial Variability of Benthic-Pelagic Coupling in an Estuary Ecosystem: Consequences for Microphytobenthos Resuspension Phenomenon

Martin Ubertini,Francis Orvain,Sébastien Lefebvre,Karine Grangeré,Romain Le Gendre,Aline Gangnery,Simon Thrush

doi:10.1371/journal.pone.0044155

Abstract

The high degree of physical factors in intertidal estuarine ecosystem increases material processing between benthic and pelagic compartments. In these ecosystems, microphytobenthos resuspension is a major phenomenon since its contribution to higher trophic levels can be highly significant. Understanding the sediment and associated microphytobenthos resuspension and its fate in the water column is indispensable for measuring the food available to benthic and pelagic food webs. To identify and hierarchize the physical/biological factors potentially involved in MPB resuspension, the entire intertidal area and surrounding water column of an estuarine ecosystem, the Bay des Veys, was sampled during ebb tide. A wide range of physical parameters (hydrodynamic regime, grain size of the sediment, and suspended matter) and biological parameters (flora and fauna assemblages, chlorophyll) were analyzed to characterize benthic-pelagic coupling at the bay scale. Samples were collected in two contrasted periods, spring and late summer, to assess the impact of forcing variables on benthic-pelagic coupling. A mapping approach using kriging interpolation enabled us to overlay benthic and pelagic maps of physical and biological variables, for both hydrological conditions and trophic indicators. Pelagic Chl a concentration was the best predictor explaining the suspension-feeders spatial distribution. Our results also suggest a perennial spatio-temporal structure of both benthic and pelagic compartments in the ecosystem, at least when the system is not imposed to intense wind, with MPB distribution controlled by both grain size and bathymetry. The benthic component appeared to control the pelagic one via resuspension phenomena at the scale of the bay. Co-inertia analysis showed closer benthic-pelagic coupling between the variables in spring. The higher MPB biomass observed in summer suggests a higher contribution to filter-feeders diets, indicating a higher resuspension effect in summer than in spring, in turn suggesting an important role of macrofauna bioturbation and filter feeding (Cerastoderma edule).

Highlights

Estuaries are known to be among the most productive systems in the biosphere [1]
The correlation circle (Fig. 3A) showed a clear relationship between the mud fraction, Chlorophyll a (Chl a) content and the bathymetry of the intertidal area. These three variables were well represented in the 1st axis and explained 61.4% of the total variation, confirming that the distribution patterns for both the mud fraction and the Chl a concentration remained stable between the two seasons
Individual distribution was explained by the correlation circle, with a Chl a concentration gradient from north to south, and an eastern area with a lower mollusk biomass

Summary

Introduction

Estuaries are known to be among the most productive systems in the biosphere [1]. Their high productivity is mainly due to the presence of nutrients and of multiple food resources for the trophic web, coming from both riverine, marine planktonic and benthic compartments [2]. In most of these shallow water environments, the intensity of the physical factors reinforces the connections between benthic and pelagic environments by increasing material processing, nutrient cycling and erosion/ deposition exchanges Among all these processes, microphytobenthos (MPB) resuspension is a major phenomenon involved in benthic-pelagic coupling since MPB can contribute up to 50% or more of the primary production for such ecosystems [3]. Benthic-pelagic coupling and especially MPB resuspension are controlled by a complex set of interactions (Fig. 1) between biological, physical, and chemical components or processes [7] Physical processes such as waves and tidal currents are responsible for erosion of the sediment, leading to sediment resuspension in the water column [8], and modifying the properties of the sediment. As a consequence of trophic interactions, MPB can influence long-term trends in benthic macrofauna composition [15], which in turn influence differently MPB resuspension by bioturbation

Methods

Results

Discussion

Conclusion