SUSPENDED SEDIMENT CONCENTRATION AND SEDIMENT FLUX OVER A RESTORED OYSTER REEF

The Mullica River - Great Bay estuary hosts one of the last remaining Eastern Oyster (Crassostrea virginica) populations in southern New Jersey. In an effort to increase oyster populations, oysters were transplanted from natural beds in the Mullica River to a four acre transplant area in 2006. Oysters are very efficient at clearing suspended sediment from the water column, and very tolerant of high suspended sediment concentration (SSC). Very high SSC over long time periods can, however, be detrimental to their health, as can burial during periods of increased sediment deposition. Suspended sediment concentration in estuaries varies with tide, freshwater discharge, and meteorological conditions, and increased SSC can increase the rate of sediment deposition. The ambient suspended sediment conditions and deviations therefrom are therefore important to the health of oyster reefs. The goals of the study were to document SSC and sediment flux, and to determine how storms will affect these parameters. An observational study was conducted over a month long period from November 2 to December 6, 2017 to address these goals. An ADCP was deployed in a restored oyster reef at the mouth of the Mullica River. It was programmed to collect a two minute burst average every fifteen minutes. A YSI was also deployed to help with calibrating the ADCP backscatter to SSC. Water samples were collected to calibrate the YSI turbidity to SSC. The mean water depth during the deployment period was 2.6 m and the mean tide range 0.89 m. Water level was distorted, with the flooding tide being shorter in duration than ebbing. Depth-averaged velocity was roughly symmetrical. Maximum flooding velocity was 26 – 52 cm/s, and maximum ebbing velocity 29 – 53 cm/s. Near-surface SSC varied between 25 and 32 mg/L during clear weather, but increased during weather events. Preliminary results indicate that spikes of SSC correlate with cold fronts with winds above 6 m/s. Suspended sediment flux over the oyster reef will also be discussed.