EARLY DIAGENESIS OF CHLOROPHYLL-A IN LONG-ISLAND SOUND SEDIMENTS - A MEASURE OF CARBON FLUX AND PARTICLE REWORKING

Sedimentary chlorophyll distributions reflect supply from primary production in overlying waters, transport during sedimentation/bioturbation, and alteration due to decomposition/transformation reactions. In Long Island Sound sediments, seasonal depth profiles of chlorophyll-a (Chl-a) often decreased exponentially within a few centimeters of the sediment-water interface, implying that initial decomposition rates of Chl-a were faster than surface sediment mixing rates. The highest surface sediment concentrations of Chl-a occurred in early spring, shortly after the spring bloom; the lowest concentrations occurred in summer. Chl-a was more concentrated at the shallow station (approximately 15 m) than at the deeper station (approximately 40 m) implying greater water column degradation or generally lower supply to deeper regions. Anoxic incubation experiments revealed that the degradation of Chl-a in fresh sediment apparently involves at least two stages. We operationally defined two pools of Chl-a as "free" and "bound" by their ease of extraction using a freeze-thaw technique. Thus, we hypothesized for the sake of a mathematical model that an initial degradation stage exists where Chl-a is released from a bound state, and a second stage where the released Chl-a degrades. These processes can be described by first-order kinetics (k(r) = 0.14 - 0.19 d-1 and k(d) = 0.02 - 0.04 d-1). The release rate is larger than the degradation rate, so that the release process dominates initial degradation behavior. Bound Chl-a may also degrade before being released. A simple, one-dimensional transport-reaction model shows that the largest Chl-a fluxes occurred in spring and the smallest in summer, while higher particle mixing rates occurred in summer than spring. Sediment mixing coefficients (D(B)) calculated using Chl-a profiles are roughly comparable with those estimated from Th-234 distributions, and estimated carbon fluxes agree reasonably well with total benthic O2 uptake.