Extracellular enzyme activity associated with suspended matter in the River Rhine

Abstract

SUMMARY. 1. Spatial and temporal variations in exoenzymatic proteolysis and alkaline phosphatase activity in the River Rhine were measured fluorometrically using L‐leucyi (ß‐naphthylamide and methylumbelliferyl phosphate (MUFP), respectively, as test substances. 2. Extracellular proteolytic activity measured in the lower 150‐km reach of the river during 1988 followed the changes in the concentration of particulate nitrogen associated with the occurrence of phytoplankton blooms. Alkaline phosphatase activity was not related to the concentration of particulate phosphate and correlated weakly with particulate nitrogen and phytoplankton concentrations. 3. Seasonal observations (1989) in the lower River Rhine and transport studies in a 660‐km‐long reach of the river (1990) indicated that the variations in the exoenzyme activities were unrelated (or not closely related) to fluctuating bacterial numbers. Exoproteolytic activity correlated well with thymidine incorporation and algal density, confirming indications in the literature that the exoenzyme activity is related to the overall metabolic rate of the bacterial populations. 4. The exoproteolytic activities in the Rhine are similar to the highest observed in eutrophic freshwaters, indicating a rapid turnover of proteins in the river. Bacterial phosphatase activities, measured with MUFP, were similar to those in the few marine and inland waters studied so far. However, phosphate‐limited algal blooms, such as reported for the plume of the Rhine in the North Sea, are likely to contribute substantially to hydrolysis of natural organic phosphates.