Proceedings of the Back Bay Ecological Symposium
Back Bay Ecological Symposium
Back Bay is the northernmost section of the Albemarle-Pamlico lagoon-estuary system. Back Bay lagoon and its associated barrier (Currituck Spit) are moving landward in response to post-glacial sea level rise (2.6 mm yr-1). The long term (100 year time scale) landward migration rate of Currituck Spit may be on the order of a meter per year.
Sediment accumulation, resuspension and bioturbation are processes in Back Bay that control the residence time of organic matter in the bay floor. and therefore, effect the rate of nutrient release. As burial proceeds, nutrients in the zone of mixing may be remineralized and recycled back to to water column, or may pass downwards into the zone of permanent burial.
X-radiographs indicate that Back Bay sediments are bioturbated by the community of insect larvae, polychaetes and oligochaetes that constitute the benthic infauna of this oligohaline water body. However, analysis of wind records suggests that in some respects, wave resuspension is a more important mixing process. Under mild to moderate conditions, waves in the bay are fetch limited. However, under hurricane conditions the bay surface saturates with breaking waves before peak winds are attained. For a 6 km fetch (a typical long fetch for the Bay), the resuspension threshold is 6 ms-1 (13.5 knots). This value is exceeded 35.7 percent of the time, and sediment is resuspended in about 40 events in a year. Radiogeochemical analyses suggest that long term (100 yr) accumulation rates are of the order of 2-3 mm yr-1.
The Bay is floored by mud (silt and clay), with an admixture of sand. Sediment introduction probably occurs largely as a result of 'wind pumping'. During winter storms, strong southerly winds set down southern Back Bay, and drive turbid water from Currituck Sound through the Knotts Island Passage. As the storm progresses, the wind shifts to the north and northwest, sets up lower Back Bay against the Knotts Island Passage, and flushes sediment and water back into Currituck Sound.
In this model, Back Bay is a sediment-accumulating sink. The shallow (1-2 m) floor of Back Bay is controlled by an equilibrium between the rate of sediment supply and mean annual wave power. Concentration profiles of 210Pb and 137Cs measured in 1984 indicate that the short term (30 year) accumulation rate was then twice that of sea level rise. The period of record corresponds with Eurasian Milfoil invasion. The historically dense growth of this plant would have modified the equilibrium by damping wave currents, accelerating the sedimentation rate and shifting the Bay floor to a shallower equilibrium depth. The Bay floor appears to presently be undergoing a reduced rate of sedimentation with some local erosion, perhaps in conjunction with a return to an earlier regime.
Swift, Donald J.P. and Wong, George T.F., "Rates of Sediment Accumulation, Bioturbation and Resuspension in Back Bay, Virginia, a Coastal Lagoon" (1991). I. Water Quality. 2.