Toxicity testing of freshwater sediment collected near freshwater aquaculture facilities in the Maritimes, Canada.

In the Atlantic region of Canada, there are close to 50 land-based freshwater aquaculture facilities, most of which discharge wastewater to freshwater receiving environments. This study was designed to assess the chemical and toxicological characteristics of sediments in those receiving environments. Thirty sediment samples were collected from 3 locations (upstream, outfall and downstream) at seven freshwater aquaculture facilities. Toxicity tests conducted included amphipod growth, amphipod survival and Microtox™. Sediments were also analysed for ammonia/porewater ammonia, redox and sulphide. Porewater ammonia concentration for the amphipod survival test ranged from 0.01 to 42 mg/L at the conclusion of the 14-day survival test. Ammonia concentration in sediment ranged from 0.3-202 μg/g, sulphide concentration ranged from 0.15 to 17.4 μg/g, yet redox ranged from 32 to 594 mV. Microtox™  IC50 values ranged from 108,00 to >164,000 mg/L, yet amphipod survival ranged from 0 to 100%, depending on sampling locations. Amphipod survival was significantly related (P < 0.05) to porewater ammonia (at time = 0 and 14 days) and Microtox™  IC50 was significantly related (P < 0.05) to ammonia, sulphide and redox levels. These results indicate that discharges from some the land-based aquaculture facilities are impacting sediment dwelling benthic invertebrates at the outfall but that impact largely disappears by 100 m downstream. Furthermore those impacts were rarely detected during the early winter season, when biomass production was at the lowest.