On the role of nutrients on phytoplankton blooms in the skagerrak.

Einar Dahl
Institute of Marine Research
N-4817 His
Norway
email: einar.dahl@imr.no

Skagerrak is a transition area between the Kattegat/Baltic and the North Sea. It is essentially flushed through by brackish water from the Baltic via Kattegat, "polluted" water from the German Bight via the Jutland current and "pristine" Atlantic water via the northern North Sea. The overall annual current pattern is an anti-clockwise circulation with inflow in the south of Skagerrak and outflow in the north, in westward direction along the southern coast of Norway. The speeds and directions of the surface currents can, however, change a lot and rapidly, mainly due to shifts in the weather, i.e. wind and pressure. Accordingly environmental parameters such as temperature, salinity and nutrients also may change rapidly and over a wide range. This highly dynamic environment of the upper layers of the Skagerrak is reflected by a productive and diverse phytoplankton society in the area. When the phytoplankton society from nature is rich and frequently changing, it is difficult to detect possible trends, and any changes, for instance due to effects of nutrients, can be masked or mistaken by effects of climate (temperature). For about two decades, most regularly since 1989, a monitoring of chlorophyll a and selected harmful phytoplankton species has been performed in the Skagerrak with two main objectives; 1) early warning of potential harmful bloom and 2) accumulation of reliable data for scientific purposes, as trends analyses and studies of relations between occurrences of phytoplankton and environmental factors. Intensified field studies, to learn from nature, have been organised when harmful blooms, such as Karenia mikimotoi, Chrysochromulina polylepis, Chattonella spp, and Dinophysis spp., have afflicted the Skagerrak. The existing long-term data of oxygen in the depth of the central Skagerrak and in the depth of basins along the Norwegian coast are considered of relevance in understanding possible shifts in phytoplankton amounts and composition over time. Our long-term oxygen data reveal increased consumption in basins along the coast, leading to lower levels than before. This is due to increased sedimentation of organic material, and may reflect increased new production of phytoplankton along the coast, possibly combined with a shift in species composition and less grazing. Our time series on harmful species are too short to reveal any obvious trends, but analyses of occurrence of selected species in relation to the nutrient conditions suggest some interesting connections. Blooms of Chrysochromulina spp. and Chattonella spp. seems more directly dependent on the nutrient conditions than blooms of Karenia mikimotoi and Dinophysis spp.