Acetate cycling in the water column and surface sediment of Long Island Sound following a bloom

Abstract

A number of workers have suggested that rates of acetate cycling in sediments may approximate the rates of anaerobic decomposition of organic matter. However, few studies have examined whether this relationship holds over very short time scales, such as those encountered during the deposition of the spring bloom into coastal sediments. In this study, we measured acetate concentrations and uptake rate constants in surface and bottom waters and in the top few centimeters of sediment in Long Island Sound. Samples were collected every 2-3 weeks from early spring to early summer of 1993. Acetate concentrations varied from 85 to 530 nM in the water column and from <2 to 29.8 μM(scd) in pore water. Acetate concentrations in pore water showed a strong depth variation, with a concentration maximum in the surface few millimeters, as well as a strong temporal variation. The concentration of acetate averaged over the top 4 cm varied from about 5 μM(scd) in the early spring to 14.3 μM(scd) following the addition of spring bloom-derived organic carbon. Acetate uptake rate constants in the water column varied from 0.0012 to 0.037 h-1 (turnover time 33.4 to 1.13 d). The average uptake rate constants in the sediment over the top 4 cm varied from 0.07 to 0.59 h-1 (turnover time 0.59 to 0.07 d) and tended to increase over the course of the experiment in both the sediment and the water column. In the water column, acetate uptake rates (calculated as rate constant times concentration) varied from less than one nanomole per liter per hour in the early spring to tens of nanomoles per liter per hour in the early summer. In the sediment over the same period, acetate uptake rates averaged over the top 4 cm varied from 0.13 to 5.25 μM(scd) -1 h-1. Rates of acetate metabolism to CO2 agreed within a factor of two to estimates of the ΣCO2 flux from the sediments, and both increased sharply following the deposition of the spring bloom. However, because temperatures were also increasing during this period, we are unable to separate unambiguously the effects of supply of labile carbon and of temperature on the acetate uptake rate.