Hu. Riisgard et al., HYDROGRAPHY, NEAR-BOTTOM CURRENTS, AND GRAZING IMPACT OF THE FILTER-FEEDING ASCIDIAN CIONA-INTESTINALIS IN A DANISH FJORD, Ophelia, 49(1), 1998, pp. 1-16
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.