Effects of CO enrichment on benthic primary production and inorganic nitrogen fluxes in two coastal sediments.

Ocean acidification may alter the cycling of nitrogen in coastal sediment and so the sediment-seawater nitrogen flux, an important driver of pelagic productivity. To investigate how this perturbation affects the fluxes of NO (nitrite/nitrate), NH and O, we incubated estuarine sand and subtidal silt in recirculating seawater with a CO-adjusted pH of 8.1 and 7.9. During a 41-day incubation, the seawater kept at pH 8.1 lost 97% of its NO content but the seawater kept at pH 7.9 lost only 18%. Excess CO increased benthic photosynthesis. In the silt, this was accompanied by a reversal of the initial NO efflux into influx. The estuarine sand sustained its initial NO influx but, by the end of the incubation, released more NH at pH 7.9 than at pH 8.1. We hypothesise that these effects share a common cause; excess CO increased the growth of benthic microalgae and so nutrient competition with ammonia oxidising bacteria (AOB). In the silt, diatoms likely outcompeted AOB for NH and photosynthesis increased the dark/light fluctuations in the pore water oxygenation inhibiting nitrification and coupled nitrification/denitrification. If this is correct, then excess CO may lead to retention of inorganic nitrogen adding to the pressures of increasing coastal eutrophication.