Temporal change in the spatial distribution of visual organic enrichment indicators at aquaculture sites in Newfoundland, Canada

Abstract

Growing salmon in sea cages results in the deposition of uneaten feed and wastes which can smother organisms, increase biological oxygen demand, and alter seafloor communities. At some sites with mainly hard-bottom substrates, opportunistic annelids and mat-forming bacteria are used as visual indicators of aquaculture waste and are monitored via drop camera imaging. Additionally, high organic matter sedimentation rates can result in the proliferation of microbes that release gas; this offgassing could also serve as a useful visual enrichment indicator. Little is known of how the distribution of opportunistic annelids, microbial mats, and offgassing might change over time and space, although coverage has important repercussions in some regulatory schemes. Here, we explore spatiotemporal distributions of offgassing, bacterial mats, annelids, and epibenthic taxon richness based on existing video-monitoring data collected along transects at four Newfoundland aquaculture sites in 2011–2012. A high degree of spatial and temporal patchiness was observed at both fallow and production sites, and combinations of time, transect, and distance to cage explained the distribution of bacterial mats, annelids, and taxon richness. Expected successional patterns resulting from organic enrichment or remediation were evident in some patches. Offgassing was observed only at fallow sites and may signal a distinct and persistent phase in organic matter degradation. Results highlight the spatial heterogeneity of benthic organic enrichment indicators across transects at Newfoundland aquaculture sites, with indicators appearing to form a mosaic of patches undergoing different stages of enrichment or recovery.