VARIATION OF SUSPENDED AND SEDIMENTARY ORGANIC MATTER WITH DEPTH IN SHALLOW COASTAL WATERS

I tested the hypothesis that the quantity and quality of suspended and sediment organicmatter in shallow coastal waters is affected by wind-induced resuspension at a smaller depth scale (,1 m) than usually assumed. Water and sediment surface (0 to 1 cm) samples were collected on a seasonalbasis and analyzed for total suspended matter, organic fraction, and phytopigments at 12 shallow sitesrepresenting a depth gradient from 0.2 to 2.0 m along the western shore of Sicily. Water columnconcentrations of all measured variables decreased rapidly with increasing water column depth, andconcentrations levelled off at about 1 m water column depth. The likelihood of sediment resuspension bywind for various combinations of water column depth and fetch length was modelled using the CERC(U.S. Army Corps of Engineers, Coastal Engineering Centre, Washington D.C. US) model for 10 yearsof local wind data. The simulations indicated that even light winds (2 m s21) increased the likelihood ofresuspension in the shallowest basin (0.2 m depth) with an effective fetch of 250 m. This study providesevidence that shallow water systems should not be investigated by considering the entire water column asa single homogenous layer. Investigations of shallow water ecosystems should consider 2 main layers withdifferent ecological conditions and functions: a shallow surface turbulent layer from the surface to adepth of 1 m that is strongly affected by wind-driven physical forces and a deeper, below 1 m to thesediment surface, layer where wind-driven turbulence has less impact.