Assessing the Sulfide Footprint of Mussel Farms with Sediment Profile Imagery: A New Zealand Trial.

Peter S. Wilson,Kay Vopel,Christopher W. McKindsey

doi:10.1371/journal.pone.0129894

Abstract

Growing numbers and increased stocking of marine mussel farms make reliable techniques for environmental effect assessment a priority. Previously, we showed how the color intensity of soft sediment could be used to estimate its acid volatile sulfide (AVS) content, a product of the anaerobic microbial degradation of organic matter deposits. We then proposed to include assessments of the AVS farm footprint in marine farm monitoring, in particular, to investigate temporal changes in the extent of the seafloor area of elevated sediment AVS content. Such assessment requires accurate detection of the AVS footprint boundary. Here, we demonstrate how to detect this boundary with analyses of sediment color intensity. We analyzed 182 sediment profile images taken along three transects leading from approximately 50 m inside to 200 m outside a long-line mussel farm in New Zealand and found that the mean sediment color intensity inside the farm boundary was almost one third lower than that of the sediment distant from the farm. Segmented regression analysis of the combined color intensity data revealed a breakpoint in the trend of increasing grey values with increasing distance from the farm at 56 ± 13 m (± 95% confidence interval of the breakpoint) outside the mussel farm. Statistical analyses indicated that the extent of the color intensity footprint was a function of water column depth, as was shown visually using mapping methods; organic particles disperse further in a deeper seawater column. We conclude that for soft coastal sediments, our sampling and data analysis techniques may provide a rapid and reliable supplement to existing benthic surveys that assess environmental effects of mussel farms.

Highlights

Global food production from marine farms has increased on average 6.2% per year since 2000, rising to 90.4 million tons in 2012 [1, 2]
The mean sediment color intensity inside the farm, that is, south of the northern farm boundary, was almost one third lower (61 grey values) than the background color intensity. This lower sediment color intensity defines the footprint of the farm
We demonstrated the use of in situ sediment profile imagery (SPI) to assess the sediment color intensity footprint of a long-line mussel farm

Summary

Introduction

Global food production from marine farms has increased on average 6.2% per year since 2000, rising to 90.4 million tons in 2012 [1, 2]. Documented effects include changes in local hydrodynamics [7], phytoplankton depletion [8, 9], the spread of invasive organisms [10], and the deposition of farm-derived organic matter (mussel feces and pseudofeces, [11]) The latter can increase the sulfide and ammonium content of the sediment below mussel farms altering the structure and composition of benthic species assemblages [4, 12, 13]. Other effects may result from the provision of additional hard substrate due to dropping shells: aggregation of sessile suspension feeder including ascidians, bryozoans, sponges, bivalves, and calcareous polychaete species Such alterations increase the surface roughness and heterogeneity of the seafloor and create a reef-like habitat for a variety of mobile species including fish, crustaceans, and various echinoderms [13, 14]

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Conclusion