Potential for Local Fertilization: A Benthocosm Test of Long-Term and Short-Term Effects of Mussel Excretion on the Plankton.

Stéphane Plourde,Pauline Robert,Vani Mohit,Bérangère Péquin,Connie Lovejoy,Monica Granados,Mehdi Cherif,Philippe Archambault,Bruno Myrand,Gregor F Fussmann,Sean Duffy,Réjean Tremblay,Christopher W Mckindsey

doi:10.1371/journal.pone.0156411

Abstract

Mussel aquaculture has expanded worldwide and it is important to assess its impact on the water column and the planktonic food web to determine the sustainability of farming practices. Mussel farming may affect the planktonic food web indirectly by excreting bioavailable nutrients in the water column (a short-term effect) or by increasing nutrient effluxes from biodeposit-enriched sediments (a long-term effect). We tested both of these indirect effects in a lagoon by using plankton-enclosing benthocosms that were placed on the bottom of a shallow lagoon either inside of a mussel farm or at reference sites with no history of aquaculture. At each site, half of the benthocosms were enriched with seawater that had held mussels (excretion treatment), the other half received non-enriched seawater as a control treatment. We monitored nutrients ([PO43-] and [NH4+]), dissolved oxygen and plankton components (bacteria, the phytoplankton and the zooplankton) over 5 days. We found a significant relationship between long-term accumulation of mussel biodeposits in sediments, water-column nutrient concentrations and plankton growth. Effects of mussel excretion were not detected, too weak to be significant given the spatial and temporal variability observed in the lagoon. Effects of mussels on the water column are thus likely to be coupled to benthic processes in such semi-enclosed water bodies.

Highlights

Aquaculture, and bivalve farming in particular, has seen explosive growth worldwide [1, 2]
The influence of mussel farming on the environment is often seen through the lens of its effects on the benthic environment [30,31,32]
Eutrophication and associated degradation of functional and species richness have been documented for the benthos, even at moderate densities of mussel farming [33, 34]

Summary

Introduction

Aquaculture, and bivalve farming in particular, has seen explosive growth worldwide [1, 2]. Among the reasons behind this expansion are fishery collapses and increasing human demand for highquality protein, and because bivalve aquaculture is generally considered a low-impact activity, in comparison to caged fed aquaculture [3, 4]. In order to maximize production, high abundances of filter feeders, including mussels, are suspended in the water column where they graze on natural plankton. This practice may alter the food web around the farms, with potential detrimental consequences [6]. Organic matter enrichment results from the accumulation of biodeposits (faeces and pseudofaeces) on the seafloor around mussel farms [9] Decomposition of this organic matter increases nutrient effluxes from the sediments to the water-column. Mussels may have indirect positive effects on plankton community production in two ways: 1. Long-term enrichment of the seafloor (on the timescale of years) with biodeposits that result in a larger nutrient efflux from the sediments to the water column

Methods

Results

Conclusion