The recovery of benthic communities following organic enrichment : examples from caged finfish aquaculture
Creators: name: MacLeod, Catriona
Publisher: name: University Of Tasmania
Year: 2023

description: A clear understanding of the factors affecting recovery processes is required for effective management of the impacts of organic enrichment on marine sedimentary environments. Using finfish aquaculture as an example this study investigated the recovery response in southern temperate sediments, identifying critical processes and indicators that could be used to improve management practices. Ecological and geochemicg responses and the effect of regional differences in the extent and rate of recovery were also examined over a range of timescales of recovery (i.e. short and long-term). In general the pattern of recovery followed established successional principles. Initial recovery was rapid following cessation of organic input. After only 2 months a marked improvement in sediment condition could be discerned. Benthic communities responded more slowly than sediment geochemistry. After 36 months the benthic infauna below the cages still differed from the references even though other sediment measures had recovered. Nonetheless, the long-term study indicated that the system had functionally recovered after only 12 months and benthic communities contained a diverse range of species with broad ecological sensitivities. Once the ecological function of the sediment was restored subsequent changes in the community structure were relatively minor, reflecting the addition of rarer climax species with longer reproductive cycles and more sensitive larval stages. Comparison of different approaches for evaluation of recovery revealed that the physico-chemical measures routinely employed in measurement of impact are of limited use in assessment of recovery. In a comparison of recovery response over 3 months at two different study locations it was found that rate and extent of recovery were affected by location, initial impact of the sediments, and length of fallow period. Initial recovery was faster at the more sheltered site than at the more exposed site, possibly reflecting differences in environmental resilience with the more sheltered location better able to assimilate organic inputs. Sediments at the more sheltered site had naturally high organic carbon content and there was greater similarity in ecological function between unimpacted and impacted conditions. In contrast, at the more exposed site the sediments had a very low organic content and ecological function was significantly altered after impact. The natural fauna at this site was less able to re-establish directly by immigration, and relied to a greater extent on interim remediation of the sediments by transitional species. This has important implications for environmental management, as it suggests that the sediments in some areas have a greater natural resilience to organic inputs. Together the results of the present study have increased our understanding of the recovery processes associated with organic enrichment in southern temperate regions and indicate that, since recovery response differs depending on the background environmental conditions, establishment of baseline conditions and local benchmarks is essential in evaluation of impact and recovery, for establishment of a regulatory framework and for ongoing environmental management. However, these baselines and the subsequent management protocols must be established at a spatial scale relevant to the community (ecological) changes.