The water quality of city park ponds must be kept at certain levels because such ponds are important facilities for the comfortable life of citizens, and hence must be improved if heavy algal blooms occur frequently. Nitrogen, phosphorus, and chlorophyll concentrations were measured from 1999 to 2003 for four park ponds in Osaka city to investigate eutrophication and its causes. The means TN, TP and Chl.a concentrations ranges were 0.89-5.4 mg-N · l-1, 0.12-0.67 mg-P · l-1, and 24-310 μg · l-1 respectively. In the comparison of water quality with those of irrigation ponds and other park ponds, three of the four ponds showed intermediate levels, and the remaining one showed much progressed levels. Treated sewage water, effluent water from a landfill site, and water from a pumping well accelerated the trophic state of ponds easily. A pond where a low Chl.a concentration but with high TN and TP concentrations existed. Thus, it was surmised that treated sewage water flowing into a pond indirectly influences pond water quality. Also, there was a park pond that decreases its TN concentration markedly in summer, and denitrification in the emergent plant zone was suspected. Since such characteristics may also contribute to water quality improvement, further studies are required.
