Exploring the distance between nitrogen and phosphorus limitation in mesotrophic surface waters using a sensitive bioassay
- 1University of Dubrovnik, Institute for Marine and Coastal Research, Kneza Damjana Jude 12, P.O. Box 83, 20000 Dubrovnik, Croatia
- 2Marine Research Centre, Finnish Environment Institute, Mechelininkatu 34a, P.O. Box 140, 00251 Helsinki, Finland
- 3GEOMAR Helmholtz Center for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany
- 4Department of Biology and Hjort Centre for Marine Ecosystem Dynamics, University of Bergen, P.O. Box 7803, 5020 Bergen, Norway
Abstract. The balance in microbial net 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 nutrient, but also gives a quantitative estimate for the excess of the secondary limiting element (P+ or N+, respectively), as well as the ratio of balanced net consumption of added N and P over short timescales (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 net 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.