Abstract

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 (∼15 m) than at the deeper station (∼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 (kr = 0.14 – 0.19 d–1 and kd = 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 (DB) calculated using Chl-a profiles are roughly comparable with those estimated from 234Th distributions, and estimated carbon fluxes agree reasonably well with total benthic O2 uptake.

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