Organic Enrichment at Cold Water Aquaculture Sites—the Case of Coastal Newfoundland

Abstract

Benthic organic matter (OM) enrichment is a frequent environmental effect of coastal aquaculture. There is a need for simple, general methods of predicting and monitoring such effects. One approach uses a geochemical relationship between total sulfides and redox potential to define organic enrichment. This empirical relationship developed for shallow, macrotidal aquaculture sites in Southwest New Brunswick (SWNB), Canada, is used to determine organic matter (OM) loading and to monitor shifts in benthic quality. A similar relationship is seen for salmonid farm sites in coastal Newfoundland but not for sediments under mussel farms or at nearby reference sites. Overall, redox potentials and total sulfides are not correlated and 70% of observations fall below the relationship documented for SWNB. Sediments in Newfoundland coastal waters are often rich in OM, [1.5 to >30%, median 8% loss on ignition (LOI)]. In addition, they are seasonally (<0°C) or always cold (−1.8 to <5°C). Much of the coast is exposed to high-energy conditions often with an effective fetch exceeding 700 km. Inner basins of some bays and fjords are protected from the waves associated with such exposure and may naturally experience seasonal anoxia that will significantly influence sediment-water exchange processes. All these factors will moderate sedimentation, remobilization and eventual remineralization of organic matter. The application of simple geochemical indices of organic enrichment as monitoring and assessment tools must be tempered with understanding of the environmental processes that regulate these factors under diverse geomorphological conditions.