Phosphorus dynamics during the transition from nitrogen to phosphate limitation in the central Baltic Sea

Abstract

Phosphate uptake rate, alkaline phosphatase activity (APA), nucleotidase activity (ATP- NA), and nutrient pool sizes were investigated in the eastern Gotland Basin from May until Septem- ber 2001 to describe the phosphorus dynamics during this period, which is characterized by the tran- sition from nitrogen to phosphate limitation and the development of nitrogen-fixing cyanobacteria. Phosphate concentrations declined from 0.1 µM in May to 0.01 µM in June. However, the decrease in phosphate was not reflected in increasing organic phosphorus pools. Its concentrations remained at a relatively constant level of 0.4 µM. Development of plankton biomass in July was based on the intracellular phosphorus pool. The ( 33 P)PO4 uptake rate and hydrolytic activities increased from very low levels in May to a maximum in July, decreasing in the following months. The development of these activities was associated with the development of heterocystous cyanobacteria. However, size fractionation showed that the ( 33 P)PO4 uptake in the fraction >10 µm accounted for only a small por- tion of the total ( 33 P)PO4 uptake rate, but for 41% of APA. Due to elevated total phosphorus concen- trations of 1.0 and 0.8 µM in the buoyant surface blooms, phosphorus dynamics were different com- pared to situations when cyanobacteria were dispersed in the euphotic zone. Depending on the parameter used, a different length of the period of phosphate limitation can be estimated. Based on ( 33 P)PO4 uptake rates and hydrolytic activities, phosphate limitation was restricted to 2 to 4 wk in July. This period is shorter than that indicated by phosphate deficiency and C:P ratios.