Hydrography, near-bottom currents, and grazing impact of the filter-feeding ascidian ciona intestinalis in a danish fjord

Hans Ulrik Riisgård*, Annika Sundberg Jensen, Carsten Jürgensen

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Abstrakt

Grazing exerted by a bed of filter-feeding ascidians, Ciona intestinalis, was examined in Kertinge Nor, the innermost shallow cove of a Danish fjord connected to the Great Belt. The aim was to study the dynamic interactions between the water column and benthic Filter feeders in order to account 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 video technique, and the downstream reduction in algal concentration over the ascidian bed documented and mathematically modelled. Relatively high algal concentrations developing in the surface layers during stratification may only be available to benthic grazers if a density-driven circulation, due to a salinity change in the Great Belt, carries these algae down to the bottom. Though wind is a crucial force for vertical mixing and transport of phytoplankton to the bottom, the frequent salinity changes in the Danish straits may give rise to hitherto disregarded density--driven currents of considerable importance for the nourishment of filter-feeding benthos in most coastal Danish waters.

OriginalsprogEngelsk
TidsskriftOphelia
Vol/bind49
Udgave nummer1
Sider (fra-til)1-16
Antal sider16
ISSN0078-5326
DOI
StatusUdgivet - 1. okt. 1998

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