Seasonal patterns in permeable sediment and water-column biogeochemical properties on the inner shelf of the Middle Atlantic Bight

This study examines the responsiveness of permeable shelf sediments to seasonal factors affecting cycles of organic matter deposition and erosion. At a site on the inner continental shelf within the Middle Atlantic Bight (39°27′N, 74°14′W; water depth 13 m), sediment inventories of chlorophyll, phaeopigments, and fine particles enriched in organic matter are reported from SCUBA diver-collected cores retrieved during six cruises conducted over 2 years. These measurements provide little evidence for substantial seasonal variation in rates of organic matter accumulation, in spite of the potential for the sinking of large phytoplankton blooms associated with summer upwelling/downwelling events, or for wave orbital motion to cause both phytodetritus and benthic algae to be trapped in sand ripples. Sediment permeability, porosity, grain size, and extracellular proteolytic activity in both coarse and fine fractions showed little seasonality. Instead, a negative correlation between chlorophyll in surface sediments and bottom-water pools (r2=0.56) was observed. Furthermore, overlays of other water-column properties with results of a principal component analysis of sediment variables suggest benthic–pelagic biogeochemical cycling is forced primarily by sediment resuspension events. Wave and current observations made during this study and during previous long-term experiments in the study area show that sediment resuspension stems from bottom stresses produced by both persistent irregular wind-waves and by very intense storm waves that occur in all seasons.