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

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 rest 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.