Fish activity: A major mechanism for sediment resuspension and organic matter remineralization in coastal marine sediments

We quantify sediment resuspension due to groundfish activity in a partly anoxic basin using acoustic backscatter sensors, transmissometers, and remotely operated cameras on stationary and mobile platforms. Where these fish were present, a distinct benthic nepheloid layer (BNL) developed despite minimal bottom currents (<10 cm s^-1). In contrast, water clarity was markedly higher over the adjacent anoxic and fishless zone. Sediment resuspension events, mostly by flatfish, occurred at a rate of >100 disturbances m^-2 d^-1 resulting in complete surface reworking every 2.5 d and a daily resuspension of 1.3 ± 0.7 l bulk sediment m^-2 d^-1. Preliminary geochemical measurements suggest substantial impact of fish resuspension activity, the immediate effect being an instantaneous increase in nutrient concentration in the benthic boundary layer and a drop in oxygen concentration. Over longer time scales (hours to days), the freshly exposed organic matter is oxidized and additional nutrients (silica, ammonium, phosphate) are regenerated and released to the water. The increase in benthic oxygen demand suggests that fish activity reduces organic carbon sequestration by 9 ± 5 mmol C m^-2 d ^-1, equivalent to ∼40% of its downward flux in this basin. To date, these processes are missing from geochemical models and require further investigation, particularly considering the depletion of groundfish stocks and the likely effects on global biogeochemical cycles.