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

• Published in: Biogeosciences
• Main Authors: Hrustic, Enis, Lignell, Risto, Riebesell, Ulf, Thingstad, Tron Frede
• Other Authors: Tvärminne Zoological Station
• Format: Article in Journal/Newspaper
• Language: English
• Published: COPERNICUS GESELLSCHAFT MBH 2018
• Subjects: ALKALINE-PHOSPHATASE ACTIVITY; NORTHERN BALTIC SEA; NUTRIENT-LIMITATION; N-P; PHYTOPLANKTON; OCEAN; MECHANISMS; RESPONSES; FIXATION; KINETICS; 1181 Ecology; evolutionary biology; Ocean acidification
• Online Access: http://hdl.handle.net/10138/231633

The balance in microbial net consumption of nitrogen and phosphorus was investigated in samples collected in two mesotrophic coastal environments: the Baltic Sea (Tvarminne 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 springearly summer situation, the Tvarminne assays (n = 5) indicated N limitation with an average P+ = 0.30 +/- 0.10 mu M-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+ approximate to 0.9 mu M-N. The N : P ratio (molar) of nutrient net consumption varied considerably between investigated locations: from 2.3 +/- 0.4 in the Tvarminne 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. Peer reviewed