Phytoplankton biomass and potential nutrient limitation of phytoplankton development in the southeastern North Sea in spring 1985 and 1986

The vernal phytoplankton bloom was observed during cruises to the southeastern part of the North Sea (east of 6°20' E, south of 56°50'N) in 1985 and 1986. Maximum phytoplankton biomass expressed as phytoplankton carbon was similar in both years (14.5 and 17 g PPCm⁻² respectively). In 1985 the bloom was located in the less saline coastal water close to the North Frisian coast. Phytoplankton was dominated by Coscinodiscus concinnus and Thalassiosira nordenskiöldii. In 1986, highest phytoplankton biomass was observed northwest of the island of Sylt, where Thalassiosira nordenskiöldii was the dominant phytoplankton species. Within the areas of high phytoplankton standing stock, concentrations of the inorganic dissolved nutrients phosphate and silicate had dropped to nearly undetectable concentrations, whereas both in 1985 and 1986 the water was still rich (10 – 20 μM.dm⁻³) in inorganic nitrogen (DIN). This, as well as the high ratios of DIN:PO₄ and DIN:Si(OH)₄ (> 50) point to phosphate and silicate as potential limiting nutrients during the spring phytoplankton bloom. The ratios of total nitrogen (TN) to total phosphorous (TP) (> 30) indicate also that phosphorous might then be in short supply. Phosphate and silicate have to be considered as potentially limiting nutrients during the vernal plankton bloom in the coastal waters of the southeastern North Sea, with nitrogen being present in surplus at that time of the year. However, in the more offshore areas nitrogen may be considered the potentially limiting element at the same time.