Elsevier

Aquaculture

Volume 433, 20 September 2014, Pages 450-457
Aquaculture

Influence of suspended mussel lines on sediment erosion and resuspension in Lagune de la Grande Entrée, Îles-de-la-Madeleine, Québec, Canada

https://doi.org/10.1016/j.aquaculture.2014.07.006Get rights and content

Highlights

  • An erosion device was used to quantify sediment near cultured mussels.

  • Erosion rates were significantly higher beneath mussel lines.

  • Critical shear velocities compared to other studies in eastern Canada

  • The BEAST contributes to our understanding of benthic–pelagic coupling.

Abstract

Downward fluxes of organically rich biodeposits under suspended mussel lines can cause benthic impacts such as changes in benthic community structure or microbial mat production. Quantifying sediment erosion in these coastal ecosystems is important for understanding how fluxes of organic matter and mussel biodeposits contribute to benthic–pelagic coupling. Critical shear velocity (u*crit), erosion rates and particle size distributions of resuspended sediment were measured at four stations distributed along a transect perpendicular to a mussel farm in Lagune de la Grande Entrée, Îles-de-la-Madeleine (Quebec, Canada). Stations were selected underneath the outer-most mussel line (0 m) and at distances of 15, 30 m and at a reference station (500 m) further along the transect. Shear velocity was measured using a calibrated portable Particle Erosion Simulator, also referred to as the BEAST (Benthic Environmental Assessment Sediment Tool). Undisturbed sediment cores obtained by divers were exposed to shear stress to compare differences between stations. Erosion sequences indicated no significant differences in u*crit between stations, but there were significant differences in erosion rates beneath mussel lines compared to other stations. Erosion rates were the highest in cores from beneath mussel lines, but paradoxically had the lowest u*crit. Mean erosion rates at u*crit varied between 25 and 47 g m 2 min 1 and critical erosion thresholds varied between 1.58 and 1.73 cm s 1, which compare with intensive mussel culture sites elsewhere in eastern Canada. Significant differences existed in biotic and abiotic properties of sediments which could explain variation in maximum erosion rates within and between stations. Particle sizes measured by videography of resuspended sediment at different shear velocities ranged from 0.2 to 3.0 mm. Quantifying sediment erosion from intact marine sediments helps to improve our mechanistic understanding of these processes, and the BEAST further contributes to predictive capability in benthic–pelagic coupling modeling.

Introduction

Quantification of sediment erosion around coastal aquaculture operations is essential for understanding fluxes of organic rich particulate matter. Sedimentation, sinking rates and dispersion of organic and inorganic particles (comprising of phytoplankton, sediment, detritus, fecal pellets or resuspended aggregates), are dependent on particle diameter and density, and are highly variable in coastal water columns (Andersen et al., 2002, Giles and Pilditch, 2004, Miller et al., 2002, Nickell et al., 2003). Sedimentation is further compounded by filter-feeding bivalves which play an important role in coastal ecosystems through their influence on benthic–pelagic coupling and nutrient cycling (Christensen et al., 2003). Filter-feeding bivalves repackage fine suspended material into larger organic rich biodeposits (feces and pseudofeces) that sink more rapidly than their constituents, increasing fluxes of organic matter to the benthos, depending on water depth, currents and resuspension (Chamberlain et al., 2001). While dynamics of mussel biodeposition (resuspension and disaggregation) is poorly quantified, enhanced sedimentation under mussel culture is well documented (e.g., Callier et al., 2006, Hatcher et al., 1994).

Bottom sediment resuspension is affected by biostabilization, porosity, organic content, grain size, and bioturbation (Giles and Pilditch, 2004, Miller et al., 2002, Nickell et al., 2003, Walker and Grant, 2009). Quantifying sediment resuspension is important for understanding sediment erosion thresholds (critical shear velocity, u*crit) and fluxes generated by currents or waves become an important predictive tool in coastal ecosystem management. Quantifying sediment transport is possible when erosion thresholds are known, although few calibrated data exist for sediment entrainment rates (Grant et al., 2013, Tolhurst et al., 2000), especially those influenced by mussel biodeposits or microbial mats (Walker and Grant, 2009). Sediment stability (defined as increased erosion threshold) is often associated with biostabilizing microbial mats, including diatoms and/or bacteria which can physically bind cohesive and non-cohesive sediment particles via the excretion of extracellular polymeric substances (Grant and Gust, 1987, Grant et al., 1986, Tolhurst et al., 2002). Alternatively, bioturbation can destabilize sediments by increasing porosity or by grazing on stabilizing organisms (Gerdol and Hughes, 1994). Sediment erosion thresholds are therefore difficult to predict, due to varying biotic and abiotic influences. Moreover, erosion thresholds are difficult to measure for undisturbed sediments, requiring substantial effort using laboratory or field flume quantification (Widdows et al., 1998).

Downward fluxes of organic biodeposits under suspended mussel culture operations have been reported to have local adverse benthic impacts, decreasing biodiversity and increasing sulfate reduction leading to anaerobic conditions (see review by McKindsey et al., 2011). While some modeling studies have considered erosion and dispersion around mussel aquaculture sites (Giles et al., 2009, Weise et al., 2009), combined field measurements of sediment erosion rates from suspension-feeding bivalves have rarely been investigated (Giles and Pilditch, 2004, Miller et al., 2002, Walker and Grant, 2009, Widdows et al., 1998). Recent studies associated with mussel aquaculture in Lagune de la Grande Entrée (LGE) have documented benthic impacts associated with biodeposition, including nutrient and particle fluxes (Callier et al., 2006, Callier et al., 2009, Richard et al., 2006, Richard et al., 2007a, Richard et al., 2007b). Ecosystem models of aquaculture carrying capacity on the basis of mussel grazing have also been conducted there (Filgueira et al., 2012, Grant et al., 2007). However, the fate of biodeposits through dispersion and resuspension events remains unclear in LGE (Callier et al., 2006, Weise et al., 2009). This is especially important if benthic microalgae are resuspended as an additional mussel food source (Frechette and Grant, 1991).

We re-designed and calibrated a portable erosion chamber called the ‘Benthic Erosion Assessment Sediment Tool’ (BEAST) (Grant et al., 2013) to measure erosion in undisturbed sediment cores in an attempt to field verify biodeposit dispersion model predictions in LGE. The following objectives were undertaken: (1) quantify erosion thresholds, erosion rates, and resuspended particle size distributions along a SW transect perpendicular to a mussel line in the direction of main current flow; (2) determine sediment organic quality; and (3) compare erosion features to a separate study by Callier et al. (2006) who measured downward fluxes of biodeposits and spatial extent of dispersion investigated via sediment traps located along the same transect.

Section snippets

Study site

This study was conducted below and adjacent to a mussel farm in LGE, Îles-de-la-Madeleine, Quebec, in August 2004. Îles-de-la-Madeleine is in the Gulf of St. Lawrence in eastern Canada, with LGE (58 km2) located in the northeast of the largest island (47° 37′ N, 61° 31′ W). Mean currents are weak (< 5 cm s 1) occasionally increasing to 10 cm s 1 during strong wind events, resulting in a well-mixed water column (Koutitonsky et al., 2002). A deep navigation channel (8 m) separates LGE into a shallow (1–3

Sediment properties

Sediment grain size composition and significance tests for %TOC, %OM and C:N ratios are shown in Fig. 2. Sediments were comprised of silty sand at 0 m or fine sand at remaining stations. Median grain sizes (D50) at 0 m were 90 μm, with similar slightly coarser sizes at 15 and 30 m, increasing to 180 μm at 500 m. Percent TOC and %OM varied between stations with the lowest values measured at 500 m. This was expected of the coarser sediment found in the far field. However, the 0 m farm sediments were the

Discussion

One of the primary impacts of mussel culture is enhanced biodeposition of fecal and pseudofecal material (Cranford et al., 2009). Many studies have documented environmental effects due to this increased sedimentation including impacts associated with eutrophication, e.g., sediment hypoxia, increased sulfate reduction, and greater effluxes of ammonium (Callier et al., 2007, Callier et al., 2008, Danovaro et al., 2004, Hartstein and Rowden, 2004, McKindsey et al., 2011, Richard et al., 2007a,

Acknowledgments

We thank MAPAQ and B. Hargrave for the collaboration and C. Éloquin and associates for the permission to use their site. Funding was provided by the Aquaculture Collaborative Research and Development Program (ACRDP), the Société de Dévelopment de l'Industrie Maricole (SODIM) and the Fisheries and Oceans Canada. We thank B. Schofield and M. Merrimen for the fabrication of the BEAST which formed part of the equipment necessary for the Canadian Arctic Shelves Exchange Study (CASES), a Research

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