FILTRATION CAPACITY OF THE ASCIDIAN CIONA-INTESTINALIS AND ITS GRAZING IMPACT IN A SHALLOW FJORD

Filtration capacity of the ascidian Ciona intestinalis (L.) was measured in the laboratory and determined from the exponential reduction in algal cell (Rhodomonas sp.) concentration as a function of time. Filtration rate (F, ml min-1) as a function of dry weight (W, g) was found to be F = 118W(total)0.68 and F = 199W(organ)0.67 for total dry weight and dry weight of organs, respectively. Ascidians starved at least 12 h had, after addition of algal cells, an initial lag-phase (1 to 2 h) with low filtration rates before constant high rates were attained. The duration of the initial lag-phase was longer at low algal cell concentrations. At algal cell concentrations above 10 to 15 x 10(3) cells ml-1, the filtration rate declined to a lower level after some time, possibly correlated with the filling of the gut. In the temperature range 4 to 21-degrees-C, maximum filtration rate (F(max), ml min-1 ind.-1) increased linearly with increasing temperature (T, degrees-C) according to F(max) = 1.46T-1.21. Above 21-degrees-C filtration rate declined rapidly with increasing temperature. The population density of C. intestinalis in the shallow cove Kertinge Nor, Fyn, Denmark, was estimated using analysis of stereophotographs. Population density varied greatly over the year with maximum densities in autumn. The filtration potential of the C intestinalis population varied during the year between 0. 1 and 1.0 times a volume equivalent to the total water volume of the cove per day. The results suggest that the population of C. intestinalis in Kertinge Nor may have an important grazing impact on the phytoplankton in late summer-early fall.