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TECHNICAL REPORTS

Surface Water Quality

Algal-available Phosphorus Entering the Gulf of Finland as Estimated by Algal Assays and Chemical Analyses

^a Finnish Environment Institute, P.O. Box 140, FI-00251 Helsinki, Finland
^b Present address of P. Rantanen: Helsinki Univ. of Technology, P.O. Box 5200, FI-02015 TKK, Finland
^c Tampere Univ. of Technology, Environmental Engineering and Biotechnology, P.O. Box 541, FI-33101 Tampere, Finland

^* Corresponding author (petri.ekholm{at}environment.fi).

Received for publication August 11, 2008. Algal-available phosphorus (P_aa) in river water and wastewater entering the Gulf of Finland (a Baltic Sea sub-basin) was estimated by a fresh-water and a brackish-water modification of the dual-culture algal assay. The assay results were further related to those obtained by routine chemical analyses. According to the brackish-water assay, an average of 44% (range, 9–88%) of total phosphorus (TP) in water samples from the Neva, Kymijoki, and Narva rivers consisted of P_aa, whereas the mean value given by the fresh-water assay was 22% (range, 0–48%). Phaeodactylum tricornutum Bohlin, which was used as the test alga in the brackish-water assay, had higher phosphoesterase activity and P affinity than did Pseudokirchneriella subcapitata Korschikov, which was used in the fresh-water assay. This difference may explain the higher values of P_aa shown by the brackish-water assay. Of the analytical P forms, total dissolved P best approximated, yet underestimated, the P_aa in river water samples. As for the biologically purified wastewaters of the city of St. Petersburg, both assays suggested that about 80% of TP (range, 59–103%) was available. That the assays gave similar results was probably due to the fact that most of the P in the wastewater samples was in the form of readily available dissolved reactive P. In untreated urban wastewaters, the mean proportion of P_aa in TP was 46% (range, 19–76%). Although the true P_aa may not be obtained by any assay, our findings corroborate the view that severe underestimation may occur if the test conditions are suboptimal for the release and uptake of P.

Abbreviations: DRP, dissolved reactive phosphorus • DUP, dissolved unreactive phosphorus • P_aa, algal-available phosphorus • PME, phosphomonoesterase • PP, particulate phosphorus • PSU, practical salinity unit • TDP, total dissolved phosphorus • TP, total phosphorus