Effect of algal bloom deposition on sediment respiration and fluxes

Using sediment cores collected in November 1989 from Aarhus Bight, Denmark, the fluxes of O2, ΣCO2 (total CO2), NH +4 , NO −3 +NO −2 and DON (dissolved organic nitrogen) across the sediment-water interface were followed for 20 d in an experimental continous flow system. On day 7, phytoplankton was added to the sediment surface, to see the result of simulated algal bloom sedimentation. Benthic O2 consumption and ΣCO2 efflux, 38 to 41 mmol O2 m-2 d-1 and 25 to 30 mmol CO2 m-2 d-1, respectively, immediately increased by 39% and 100% after the algal addition, but gradually declined to the orginal level. Fluxes of NH +4 (1.0 to 1.2 mmol m-2 d-1) and DON (0.3 to 0.9 mmol m-2 d-1) increased due to the organic substrate addition. NH +4 and NO −3 flux changed direction, becoming an efflux and influx, respectively, for a few days and a large amount of DON (max. rate 4.0 mmol m-2 d-1) was immediately produced either by bacterial hydrolytic activity or from autolysis of the algae. DON was the most significant nitrogen component in pore water and in terms of N-flux from sediment. A temporary stimulation of anaerobic respiration processes (sulfate reduction and denitrification) and a decrease in nitrification were indicated. After the effect of the organic addition had declined, the fluxes gradually reverted to the original rates. The “halflife” of the added algal material, of which 20 to 30% was very labile, was estimated to be 2 to 3 wk.