Titrating excess nitrogen content of phosphorous‐deficient eastern Mediterranean surface water using alkaline phosphatase activity as a bio‐indicator

From both an academic and a management point of view, not only the question of which element limits production, but also the excess amount of the next potentially limiting element, is of importance. Activity of the enzyme alkaline phosphatase (APA) produced during phosphorous (P) starvation has been used as a technically easy and sensitive indicator of P limitation. We explore an extension of this technique to estimate also the excess of nitrogen (N) by titrating samples with phosphate until APA disappears. In ultra‐oligotrophic eastern Mediterranean surface water, this technique indicated P limitation with a nitrogen excess of 230 ± 60 nM N. Adding ammonia as internal standard gave a molar ratio of 15 ± 2 for balancing amounts of added N and P. Taking advantage of a Lagrangian experiment with in situ phosphate addition, we also used the mirror technique of titrating with ammonia until APA appeared. Excess P inside the experimental patch was estimated to be 33 ± 2 nM 4 days after the addition of 110 nM PO 4 P. In this case, however, a molar N:P ratio of only 3.0 ± 0.2 was determined for balancing amounts of added ammonia and phosphate. The technique should, in principle, estimate excess nutrients accessible on a time scale of ca. 3 d (the incubation time used here). The method applies regardless of the form the excess nutrients are in as long as they are bioavailable. It does not presume that the organisms exhibiting the limitation belong to any particular taxon or functional group as long as they produce APA when P‐limited.