By Episodic Input of Organic Matter

determined from the downward vertical flux of pheopigments. Improved extraction of chlorophyll a and b from algae using dimethylsulfoxide. Limnol. zooplankton reproduction: Implications for diel vertical migration theory. Abstract-We report the first seasonal time-series observations of porewater composition obtained for deep-sea sediments. We observed considerable temporal variability of O,, NO,-, and Mn2+ profiles at the French JGOFS site DYFA-MED (western Mediterranean) in response to a pulsed input of organic matter. A delivery of reactive organic matter representing only 1% of the average organic C content of surface sediments (0.6% wt/wt) was required in order to fuel the observed changes in porewater composition. The perturbation resulted in dramatic changes in integrated reaction rates and sediment water fluxes. O2 uptake rates increased from 1.26 to 1.82 mmol m-* d-l, while nitrification rates rose from 0.13 to 0.18 mmol m-* d-l. These changes were paralleled by a decrease in NO,-effluxes across the sediment-water interface from 0.07 to 0.01 mmol m-' d-l and a concomitant rise of denitrification rates from 0.06 to 0.17 mmol m-2 d-l. Pore-water profiles responded rapidly to the sediment pulse depo-sition, returning to their steady-state values within a few months. The main driving force appeared not to be the seasonal variability of pelagic production but rather the down-slope transport of resuspended upper-shelf sediments. Numerous studies during the last decade have demonstrated that the deep-sea superficial sediment is an environment of considerable temporal variability. Seasonality of pelagic processes, coupled to rapid delivery of largely unaltered particulate organic matter, and episodic events like benthic storms and turbidity currents generated by downslope transport were identified as potential driving forces of deep-sea temporal variability because of the ephemeral nature of the perturbations and logistical constraints, little is known of the geochemical response of surface sediments. Pulsed delivery of organic matter to the deep-sea floor may provide an important energy source to deep-sea benthic communities (Lochte and Turley 1988; Graf 1989; Gooday 1988). The benthic response to variations in particulate organic carbon (POC) flux has been assessed through time-series analyses of sediment community oxygen consumption. These studies have yielded divergent results. While a North Pacific site indicated varying rates of benthic organic carbon mineralization (Smith et al. 1992), only minor temporal variations in metabolic rates in response to variations in POC flux of similar amplitude were documented for an Atlantic site near Bermuda (Sayles et al. 1994). Deep-sea benthic communities have been documented to react …