LAUSR

Abstract Permeable sandy sediment acts as an active biogeochemical reactor, but its role in the circulation of elements in coastal areas is not yet clear. Here, repacked flow-through reactor experiments were conducted at 5 stations adjacent to coral reefs around Weizhou Island in the northern South China Sea in May 2018 to investigate the carbon and nutrient dynamics in permeable carbonate and silicate sands. The potential rates of organic metabolism ranged from 1.82 to 12.08 mmol m−2 h−1, which impacted carbon and nutrient dynamics in permeable sands. CaCO3 calcification and dissolution indicated by the flux of total alkalinity (−0.43–0.59 mmol m−2 h−1) were controlled by the saturation state of the inflow seawater. Most dissolved inorganic nutrients were taken up by the permeable sediments, but a small amount of dissolved organic nitrogen and phosphorus was released from the sediments. The uptake rates of dissolved inorganic phosphorus and dissolved silicate were higher in the carbonate sands than in the silicate sands, while the mineralogical characteristics did not seem to play a relevant role in the fluxes of dissolved inorganic carbon and nitrogen. Our results show that the uptake of nutrients in permeable sediments resulted in a decreased ratio of nitrogen to phosphorus in seawater, which may benefit reef ecosystems. However, we still need a better understanding of the role of permeable sediments in elemental cycling under natural conditions and of their interactions with environmental factors, such as the input of terrigenous organic matter.