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Physico-chemical aspects of sediment recovery at two abandoned salmon farm sites in British Columbia Mehlenbacher, Nara Marie

Abstract

The deposition and accumulation of fish farm waste discharges within marine sediments can induce geochemical and physical changes which may inhibit the survival of the natural benthic community. There is no primary literature published on the topic of benthic habitat recovery following fish farming activity in British Columbia. Furthermore, there is no standardized set of methods that has been recommended for determining rates and patterns of benthic habitat recovery at abandoned fish farm sites. Physical and chemical indicators of benthic impact have been determined to be cost and time-effective alternatives to biological indicators. Therefore, the primary focus of this research was to investigate physico-chemical aspects of sediment recovery in the proximity of two abandoned salmon farm sites in British Columbia. The results of this investigation were used to recommend a sampling design, data analysis methods, and sensitive indicators appropriate for determining spatio-temporal recovery rates that can be used to develop fallowing criteria for sites characterized by similar substrate, hydrography, and production levels. The abandoned farm at one site operated as a low production farm for 11.5 years, and was located within an open, silty-sandy bay where the current speeds were slow, but the water column was considered to be well-mixed. The abandoned farm at another site had operated as a high production farm for 8.5 years, and was located in a sheltered, muddy bay with slow current speeds. Sediment grabs were collected at each site in order to measure physico-chemical parameters within the top two centimeters of sediment. Sediment cores were also collected at each site in order to measure profiles of sediment chemistry. Grabs and cores were collected at the low production site at 1, 3, 8, and 11 months following site abandonment. Grabs were collected at the high production site at 11, 24, and 35 months following site abandonment, and cores were collected at 36 and 40 months. Transects of stations were sampled with the objective of obtaining reasonable spatial coverage at each site. Physico-chemical parameters were compared with existing sediment quality guidelines, proposed impact and toxicity thresholds, and background values in order to determine if the sediments exhibited signs of impact or recovery. Statistical and mapping analyses were used to assess impact and recovery, as well as to establish three separate tiers of sensitive indicators (based on various time and budget allowances) that should be measured during recovery assessments at similar abandoned fish farm sites. At the low production site the actual effects to the benthic environment due to deposits of organic fish farm wastes were not significant, despite organic matter accumulations detected immediately following site abandonment. Copper levels near the farms, however, were above Environment Canada's recommended sediment quality guidelines throughout the 11-month study period. Tier one recovery indicators (all valid recovery indicators) identified at the Chained Islands site were total carbon, organic content, organic carbon, nitrogen, chlorophyll, copper, zinc, porosity, bulk density, redox potential, and sulphides. At the high production site, fish fatrning activity had significant effects on the benthic environment both within the vicinity of the farm, as well as up to several hundreds of meters away. Organic matter, copper, zinc, and anaerobic sediment conditions at the farm site had not yet fully recovered by the conclusion of the study, approximately three years after the site had been abandoned. Tier one recovery indicators identified at the Carrie Bay site were organic C:N, organic carbon, organic content, nitrogen, total carbon, inorganic carbon, copper, zinc, silicon, fine sand, very fine sand, coarse sand, silt, clay, medium sand, redox potential, and sulphides. An effective method for assessing physico-chemical aspects of recovery at abandoned fish farm sites was determined to be a combination of traditional statistics (descriptive and inferential), contour mapping, and GIS analysis. It was recommended that in order to further understand recovery at fish farm sites in British Columbia for the purposes of developing adequate fallowing times and conditions, it also would be useful to define the spatial extents of the impacts and the rate of recovery at various sites. A systematic grid was proposed as a sampling design, and contour mapping and GIS analysis were recommended as effective data analysis methods for determining the zone of impact and monitoring its variation over time. It was recommended that further research be performed into the mechanisms of sediment recovery at fish farm sites in British Columbia. Additionally, research on the toxicities of sediment sulphides and heavy metals within specific sediment environments needs to be further investigated. Confounding factors including grain size, organic matter, and sediment gases may significantly influence toxicity thresholds. If toxicity guidelines can be established for different sediment habitats, a more solid basis can be established for performing comparative analyses between recovering sites and background conditions. This study showed that the recovery rates of anoxic conditions and potentially toxic metals in sediments may be slow, and it is unclear whether or not certain sites may ever actually recover to a natural, pre-farming state.

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