Ecological studies in Hopavågen, a landlocked bay at Agdenes, Sør-Trøndelag, Norway
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The results of a study on the physical and biological conditions in Hopavågen and Straumen are presented. The data were collected during the period 1983-1994. The hydrographic data show that the deep water of Hopavågen is stagnant during most of the year. The tidal current is not strong enough to generate turbulence and vertical convection in the deepest parts of Hopavågen. This results in oxygen depletion and the formation of hydrogen sulphide. Total convection may take place during cold periods, but it is not certain whether this occurs every winter. The inflow and outflow of the tide through the narrow and shallow channel gives rise to a tidal movement in Hopavågen with an amplitude and timing which differs significantly from the normal tidal movement in the main fjord. It is shown that the mean water level in Hopavågen rises and falls, following the spring/neap cycle in the main fjord. The littoral zone in Hopavågen differs from the normal pattern found in very sheltered localities. There are no sublittoral laminarians, no distinct fucoid zone, and the abundance of mussels and barnacles is much less than is normal for sheltered localities. It is suggested that the abnormal tidal movement and grazing by sea urchins are the major causes of these unusual distribution patterns.