Relationships between microbial distributions and the anaerobic decomposition of organic matter in surface sediments of long Island Sound, USA

Abstract

Relative rates of the anaerobic decomposition of organic matter in the upper 10 cm of sediment from two stations in central Long Island Sound, USA, were compared. Sediment samples from discrete depth intervas were incubated anoxically and changes in SO 4 = , NH 4 + , bacterial numbers, extractable adenosine triphosphate (ATP), organic matter, and organic carbon were measured as a function of time and temperature. At both stations (∼15 and ∼34 m water-depths, respectively), the calculated rates of SO 4 = reduction and NH 4 + production decreased exponentially (approximately) with depth below the sediment-water interface. Over the same depth interval, ATP concentrations dropped by a factor of ∼6 to 7 and bacterial numbers were lower by a factor of 2 to 3. These decreases in SO 4 = reduction, NH 4 + production, bacterial numbers, and ATP, reflect a change in the physiological state of microbial populations with depth in the sediment and are consistent with the conclusions that the quantity of easily utilizable organic matter changes rapidly below the sediment surface and that food limitation controls the basic depth distribution of microbial activity. The average rates of SO 4 = reduction, ∼29 to 39 mM year-1 (22°C), in the top 10 cm are similar at both stations studied here, as well as at an additional station from a previous study. In contrast, average NH 4 + production differs by a factor of ∼2 at the two stations, reflecting differences in the C:N ratio of the organic matter supplied to the sediment surface and differences in particle reworking by macrofauna at each site. The apparent activation energy of SO 4 = reduction was 19±1 kcal mole-1 and that of NH 4 + production, 18±3 kcal mole-1. The overall quantity of carbon required to support the calculated average SO 4 = reduction rate in the top 10 cm is 23 g C m-2 year-1 and represents ∼36% of all the carbon available to the benthos annually and ∼11% of the net primary production in the water column. Directly measured fluxes of NH 4 + from sediments to overlying water at both stations agree well with those predicted from production rates obtained by the incubation techniques.