HYDROGRAPHY, NEAR-BOTTOM CURRENTS, AND GRAZING IMPACT OF THE FILTER-FEEDING ASCIDIAN CIONA-INTESTINALIS IN A DANISH FJORD

Grazing exerted by a bed of filter-feeding ascidians, Ciona intestinal is, was examined in Kertinge Nor, the innermost shallow cove of a Dani sh fjord connected to the Great Belt. The aim was to study the dynamic interactions between the water column and benthic filter feeders in o rder to ac count for the variability of phytoplankton biomass seen in the fjord. The moderate tidal- and density-driven near-bottom currents were both estimated and directly measured by means of an underwater v ideo technique, and the downstream reduction in algal concentration ov er the ascidian bed documented and mathematically modelled. Relatively high algal concentrations developing in the surface layers during str atification may only be available to benthic grazers if a density-driv en circulation, due to a salinity change in the Great Belt, carries th ese algae down to the bottom. Though wind is a crucial force for verti cal mixing and transport of phytoplankton to the bottom, the frequent salinity changes in the Danish straits may give rise to hitherto disre garded density-driven currents of considerable importance for the nour ishment of filter-feeding benthos in most coastal Danish waters.