Decrease in Lithothamnion sp. (Rhodophyta) primary production due to the deposition of a thin sediment layer

Coralline algae are important reef-builders which can form nodules, known as rhodoliths, occurring worldwide in beds sustaining a high biodiversity. Although considered a non-renewable resource, they are exploited as a source of calcium carbonate used mainly for agricultural purposes. In Brazil between 96,000 and 120,000 metric tonnes of rhodoliths are extracted per year. Besides the direct impact caused by removal on the coralline bed, the dredge process may also produce a plume of fine sediment, which can change the primary production of the remaining organisms. In this study, four treatments, with three replicates, were used to acquire Lithothamnion sp. net photosynthetic rates with and without a sediment layer using a Clark-type oxygen microelectrode and micromanipulator. The results demonstrated that, under controlled conditions, the addition of a thin sediment layer resulted in a 30% reduction of the irradiance, decreasing the Lithothamnion sp. net production in 70%. For this reason direct and indirect effects of mechanical exploitation of the rhodolith beds should be included in future studies that focus on environmental impacts of dredging activity, whether it is linked to the extraction of these algae.