engleski

Exploring the distance between nitrogen and phosphorus limitation in mesotrophic surface waters using a sensitive bioassay

The balance in microbial consumption of nitrogen and phosphorus was investigated in samples collected in two mesotrophic coastal environments: the Baltic Sea (Tvärminne field station) and the North Sea (Espegrend field station). For this, we have refined a bioassay based on the response in alkaline phosphatase activity (APA) over a matrix of combinations in nitrogen and phosphorus additions. This assay not only provides information on which element (N or P) is the primary limiting, but also gives a quantitative estimate for the excess of the secondary limiting element (P* or N*, respectively), as well as the ratio between N and P consumption over short time scales (days). As expected for a Baltic Sea late spring-early summer situation, the Tvärminne assays (n=5) indicated N-limitation with an average P*=0.30 ± 0.10 µM-P, when incubated for 4 days. For short incubations (1–2 days), the Espegrend assays indicated P-limitation, but the shape of the response surface changed with incubation time, resulting in a drift in parameter estimates toward N-limitation. Extrapolating back to zero incubation time gave P-limitation with N* ≈ 0.9 µM-N. The N:P ratio (molar) of nutrient consumption varied considerably between investigated locations ; from 2.3 ± 0.4 in the Tvärminne samples to 13  ± 5 and 32 ± 3 in two samples from Espegrend. Our assays included samples from mesocosm acidification experiments, but statistically significant effects of ocean acidification were not found by this method.

alkaline phosphatase activity; bioassays; mesotrophic temperate seas; nutrient limitation; phytoplankton

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