Flux of Particulate Matter in the Tidal Marshes and Subtidal Shallows of the Rhode River Estuary

There was a net influx of suspended particulate matter to the uppermost part of the Rhode River estuary during the several years of this study. Most of the influx was due to episodic discharges of suspended sediment from the watershed during heavy rains. In contrast, tidal exchange of particulate matter was not related to rainstorms. Sediment composition data and historical records indicate that marsh accretion accounts for only 13% of the sediment trapping although marshes occupy 60% of the study area. Influx of particulate matter to the marshes is directly related to the amount of time they are submerged during tidal cycles. study suggested that the marshes of Chesapeake Bay could trap 15% of the annual influx ofsuspended matter, even though the total area of marshes is only 4% of the area of open water (Nixon 1980). However, Officer et al. (1984) found that sediment deposition in subtidal areas could more than account for the sediment influx to the upper Chesapeake Bay. The sediment trapping ability of marshes is generally attributed to the presence of emergent macrophytes. Plant stems promote sedimentation by slowing current velocities and by providing surfaces for sediment adhesion, while plant roots tend to bind sediment against erosion (Redfield 1972; Frey and Basan 1978; Stumpf 1983). However, the amount of suspended matter that marshes receive may be limited by the amount of time they are submerged. This could partly explain why sediment accretion may decrease with increase in elevation in salt marshes (Richards 1934; Richard 1978). Even unvegetated mudflats have been found to accrete faster than nearby vegetated areas of slightly higher elevation (Richard 1978). 310 0160-8347/86/0480310-10$01.50/0 © 1986 Estuarine Research Federation Introduction Knowledge of the flux of suspended matter in estuaries is crucial to understanding turbidity, shoaling, and transport of nutrients and pollutants. Estuaries can filter most of the suspended matter they receive from rivers. This is especially true for partially mixed estuaries with outward flowing surface water and inward flowing bottom water (Schubel and Carter 1984). For example, Chesapeake Bay is such an effective sediment trap that it actually imports suspended matter from the ocean as well as from rivers (Schubel and Carter 1984). The efficiency of sediment trapping by an estuary may be determined in part by the ratio of its volume capacity to the annual freshwater inflow (Biggs and Howell 1984). Tidal marshes are also considered sediment trapping environments (e.g., see review by Frey and Basan 1978), but little is known about their importance as sediment traps in estuaries. One study suggested that South Carolina marshes could trap 85% of the suspended matter in terrestrial runoff (Settlemyre and Gardner 1977). Another Thus, subtidal environments might actually trap more sediment per unit area than marshes. Episodic events can be very important to the flux of suspended matter in both estuaries and marshes. For example, 70% of the annual discharge of suspended matter from the Susquehanna River to Chesapeake Bay typically occurs during a few weeks in the spring (Gross et al. 1978). In marshes direct measurements ofthe flux ofsuspended matter are often confined to a few tidal cycles throughout the year (Nixon 1980), so relatively rare storm events are not often sampled. However, in instances when sampling coincided with storms, unusually high fluxes of suspended matter have been observed (Settlemyre and Gardner 1977; Stumpf 1983; Chalmers et al. 1985; Stevenson et al. 1985). In the present study we used a network of automated samplers to measure the flux of suspended matter into and out of the uppermost section ofthe Rhode River estuary. The automated samplers provided a nearly continuous record of the flux of suspended matter and revealed the importance of storms. We also investigated the relative importance of tidal marshes and subtidal areas in sediment trapping.