Temporal changes in visual organic enrichment indicators at an aquaculture site over mixed- and hard-bottom substrates

Abstract

Norway employs an environmental management system to assess benthic organic loading from aquaculture activities. Monitoring surveys, mostly performed through grab sampling, scrutinise responses to enrichment within soft-sediment macrofaunal communities. However, new methods and indicators of organic enrichment (IOE) need to be identified for mixed- and hardbottom substrates. We used image characterisation to examine temporal changes in the abundance and coverage of benthic IOE on mixed- and hard-bottom substrates (organic pellets, sulphur-oxidising bacterial mats, opportunistic polychaete complexes [OPC], polychaete tube aggregations [PTA]) in relation to changes in organic deposition pressure through 1 yr of production at a rainbow trout farm on the western coast of Norway. Rates of organic deposition on the seafloor around the farm increased towards the end of the survey period as fish biomass increased. PTA were significantly associated with low levels of organic deposition and their abundance declined in 2 of the 3 study cages as the production cycle progressed. OPC coverage significantly increased with organic deposition and was greatest, 4 mo before the end of the production cycle. Organic pellet coverage closely followed the patterns of organic deposition on the seafloor, whereas bacterial mat coverage showed no relationship with deposition. Our findings provide new knowledge on the annual impact of organic enrichment on IOE beneath fish farms over mixed- and hard-bottom substrates, highlighting image characterisation methods as a means to improve benthic monitoring. This knowledge can contribute to the development of an environmental proxy to assess the enrichment stage around aquaculture farms placed over mixed- and hard-bottom areas.