FILTER-FEEDING ASCIDIANS (CIONA-INTESTINALIS) IN A SHALLOW COVE - IMPLICATIONS OF HYDRODYNAMICS FOR GRAZING IMPACT

The grazing impact by a dense population of filter-feeding ascidians C iona intestinalis on horizontally flowing water (driven by density cir culation) in a shallow cove (Kertinge Nor, Denmark) has been described and quantified by means of a simple one-dimensional numerical model. The agreement between observations and modelled predictions was satisf actory. The applied numerical model has the following analytical solut ion in the idealized case: C-x = C(0)e(-(fx/y2)), where C-x= algal con centration at a downstream distance x, C-0 = initial concentration, f = F/v(c); F = area specific population filtration rate; v(c) = current velocity; Y-2 = depth of mixed layer below halocline. The numerical m odel quantifies the actual grazing impact while the analytical model i llustrates the governing physics in well-known terms. To describe situ ations with no current (i.e. stagnant water), we performed simulation studies in the laboratory and measured vertical profiles of algal cell s over filter-feeding C. intestinalis. The results showed that phytopl ankton became reduced in a near-bottom water layer of 20-30 cm thickne ss. Such water layers may develop in stagnant water (calm days and no advective currents), thus uncoupling the pelagic food and the fitter f eeders which within a short time will experience extremely meagre food conditions.