Laboratory studies of oxic and anoxic degradation of chlorophyll-a in Long Island Sound sediments

Laboratory incubations of Long Island Sound (LIS) sediments show that chlorophyll-a (Chla) degradation generally follows first-order kinetics, but that the rates and mechanisms of degradation differ under oxic and anoxic conditions. Chl-a in fresh sediment can be operationally defined as being present in three pools: bound Chl-a (nonextractable by acetone), free (acetone extractable) anoxically degradable Chl-a, and free anoxically stable Chl-a. A major pathway of degradation is initial release of bound Chl-a into the free pool. The release rate constants of bound Chl-a into free Chl-a are remarkably faster (> 10×) under oxic (~4 d^-1) compared to anoxic (~0.1 d^-1) conditions, and a much greater fraction of total Chl-a degrades under oxic conditions. However, the apparent degradation rate constants of free Chl-a are two to three times higher in the absence of oxygen in sediments with high bound Chla (~0.2-0.3 d^-1 for anoxic; ~0.1 d^-1 for oxic) and about seven times higher in sediments without bound Chl-a (~0.3 d^-1 for anoxic; ~0.04 d^-1 for oxic). The differences between anoxic and oxic decomposition in incubation experiments, together with naturally observed concentration profiles which reach low levels of Chl-a, imply that Chl-a in natural sediments degrades during the oscillation between oxic and anoxic conditions caused by physical and biological mixing processes. Oscillation experiments (oxic vs. anoxic and anoxic vs. oxic) suggest that the activities of aerobic organisms may be an important factor affecting Chl-a degradation. Examination of effects of meiofauna on Chl-a degradation under oxic conditions implies that microorganisms may play a stronger role in Chl-a degradation than meiofauna (> 0.125 mm). The relative temperature independence of anoxic degradation and temperature dependence of oxic degradation suggest that anoxic degradation may be controlled largely by chemical factors, while oxic degradation may be more strongly controlled by biophysical and biochemical processes.