Organic carbon flux through a Delaware Bay salt marsh: tidal exchange, particle size distribution, and storms

Abstract

Organic carbon exchange via tidal transport between a 190 ha Spartina alterniflora marsh and Delaware Bay was quantified during 5 seasonal sampling periods in April, June, August, October, 1980 and January, 1981. Based on hourly samples collected over 3 consecutive tidal cycles, mean ebb tide concentrations of particulate organic carbon (POC) were higher than flood tide concentrations, except in January when the marsh surface and creeks were frozen. Mean ebb tide dissolved organic carbon (DOC) concentrations were greater than flood during all seasons, although the difference was only significant in August and January. Hourly concentrations were variable, ranging from 0.4 to 11.4 mg POC l⁻¹ and 1.5 to 15.5 mg DOC l⁻¹. The relative degree of marsh surface flooding, current velocity and phytoplankton concentration are important factors influencing seasonal differences in mean flood and mean ebb concentrations of POC. A broad size spectrum of particulate detritus is available to estuarine consumers, with 60 to 84 % of the total POC on both flood and ebb tides being < 20 μm in size and the remaining material distributed fairly equally among larger fractions (> 243 μm, 243–63 μm, 63–20 μm). There was a net export of POC from Canary Creek marsh during all seasonal studies, except in January when exchange processes were much reduced. DOC was exported during all studies, except in June when the water volume transport estimate was significantly biased toward the flood direction. Annually, 1.05 × 10⁵ kg was exported as POC and 1.97 × 10⁵ kg as DOC. The influence of storms was not included in the annual organic carbon exchange budget. However, during an October tropical storm ebb-directed transports of POC and DOC were 5 to 6 times greater than during the normal ebb cycles monitored in October, suggesting that major coastal storms are significant to salt marsh organic material exchange processes. Elevated POC concentrations during 2 ebb cycles following an intense rainstorm in April suggest that these episodic events also represent important marsh-estuarine exchange mechanisms.