Phosphorus and nitrogen limitation of phytoplankton in the inner Oslofjord (Norway)

Abstract

Abstract Several criteria of potential phosphorus and nitrogen limitation were applied throughout 1986 to the phytoplankton of the inner Oslofjord, a brackish-marine fjord in southeastern Norway. The criteria were hased on information on concentrations and distribution of nutrients, on analyses of the carbon (C), nitrogen (N), and phosphorus (P) content of particulate matter in the water column, and on results of deck experiments on the plankton. Indications of nutrient limitation were obtained from seasonal trends in C:N, C:P and N:P ratios, as well as from further shifts in these ratios in plankton kept in a container on deck for 24 hours. Nitrogen limitation was also deduced from an increase in the uptake capacity for ammonium in plankton during 24 hours of containment, and phosphorus limitation from an increase in alkaline phosphatase activity. During diatom blooms. a lowering of the ratio of light to dark rates of nitrate uptake provided one further test for nitrogen limitation. Two other tests (a stimulation of dark carhon dioxide assimilation, for nitrogen limitation, and alkaline phosphatase activity in the water column, for phosphorus limitation) proved inconclusive in the present investigation. According to the criteria used, the first diatom bloom in spring was limited by nitrogen, while phosphorus limitation was the rule in flagellate-dominated plankton during an extended period in summer and autumn. In the summer, the plankton grew partly in the nutrient-depleted upper mixed layer and partly in the pycnocline, where there was a large excess of nitrate relative to phosphate. Phosphorus limitation in the summer may have resulted from recent improvements in local sewage treatment.