Evidence for better microphytobenthos dynamics in mixed sand/mud zones than in pure sand or mud intertidal flats (Seine estuary, Normandy, France).

Jérôme Morelle,Francis Orvain,Pascal Claquin,Vanesa Magar

doi:10.1371/journal.pone.0237211

Abstract

Understanding the dynamics of microphytobenthos biomass and photosynthetic performances in intertidal ecosystems will help advance our understanding of how trophic networks function in order to optimize ecological management and restoration projects. The main objective of this study was to investigate microphytobenthic biomass and photosynthetic performances as a function of the sedimentary and environmental variabilities in the range of intertidal habitats in the downstream Seine estuary (Normandy, France). Our results highlight higher biomass associated with more stratified biofilms and better photosynthetic performances in areas characterized by a sand/mud mixture (40–60% of mud) compared to pure sand or pure mud environments. This type of sediment probably offers an efficient trade-off between the favorable characteristics of the two types of sediments (sand and mud) with respect to light penetration and nutrient accessibility. Moreover, the large quantities of exopolysaccharides produced in sand/mud mixtures emphasizes the functional role played by microphytobenthos in promoting sediment stability against erosion. This allows us to show that despite the strong increase in sand content of the downstream Seine estuary, intertidal flats are still productive since microphytobenthic biomass, photosynthetic performances and exopolysaccharides secretion are highest in sand-mud mixtures. This study also underlines the impact of ecosystem modifications due to human disturbance and climate change on the dynamics of key primary producers in estuaries.

Highlights

Microphytobenthos (MPB) is an assemblage of photosynthetic microalgae and cyanobacteria that form biofilms in intertidal and subtidal areas [1]
Despite being located on the northern mudflat, sites C and E in September, and B and H in April can be considered as sandy sites with less than 40% of fine particles
Evidence for better microphytobenthos dynamics in mixed sand/mud zones cause fine particles to be swept away by the strong flow thereby explaining the presence of these sandy areas

Summary

Introduction

Microphytobenthos (MPB) is an assemblage of photosynthetic microalgae and cyanobacteria that form biofilms in intertidal and subtidal areas [1]. In areas with cohesive sediment, biofilms are typically composed of epipelic diatoms able to migrate vertically to access nutrient supplies and let light penetrate the sediment [2]. Evidence for better microphytobenthos dynamics in mixed sand/mud zones sediment grains in areas characterized by higher hydrodynamic stress and intrinsically mobile sands, where MPB diversity and biomass stocks are lower [3]. MPB constitutes a large pool of photosynthetically competent organisms that significantly contribute to autochthonous primary production [11,12,13]. Understanding the spatial and temporal dynamics of intertidal MPB in relation with biological and environmental variability is crucial to understand coastal ecosystems, and their restoration and management

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Conclusion