Probability and threshold values for recognizing eutrophication in Lake Songhua

Eutrophication of lakes or reservoirs as a consequence of human activity is a world-wide problem.The process of eutrophication was investigated in Lake Songhua in the Northeast China.Chlorophyll a concentrations and 18 physicochemical(Water depth,Water temperiture,EC,pH,Transparency,DO,COD,TOC,TP,TN,PO~(3-)-4-P,NO~--3-N,NH~+-4-N,NO~--2-N,Organic nitrogen,T-Fe,SiO-2,Chlorophyll a) parameters were monitored and analyzed during 2002-2004 in the lake water.At the same time,a predominant algal species(unialgal strain Microcystis aeruginosa) from the summer planktons of the lake was separated and purified.Multiple correlation and stepwise multiple regression analysis were applied to the monitoring data.The results indicated that the total phosphorus concentration was the first impact factor for eutrophication followed by total nitrogen.The effects of available phosphorus and available nitrogen on algal growth were studied in the laboratory by a culture experiment.The relationships between maximum specific growth rate of algal and available phosphorus and available nitrogen concentrations in water were expressed graphically.An inflexion in each curve identified the threshold values of available phosphorus and nitrogen to inititate eutrophication in the lake: these values were p=0.025 mg·L~(-1) and N=0.55 mg·L~(-1).A relationship was established between the concentrations of total aqueous P and and N and their available values and hence with those of chlorophyll a.Finally,appropriate threshold eutrophication values were derived for Lake Songhua,namely,TP=0.065mg·L~(-1),TN=0.843mg·L~(1) and chlorophyll a=11.9μg·L~(-1).The frequency distribution and cumulative frequency distributions were computed for the three analytes.For chlorophyll a,69% of values exceeded 11.9μg·L~(-1).Thus the probability of eutrophication in 20022004 for lake Songhua was 0.69.A Monte Carlo random simulation model yielded 6 classes.Using these as limits in a probability map derived from Arc/info 8.01 showed the risk-free area was 19.21%,first class area was 9.79%,second class area was 20.31%,3~(rd) class area was 16.5%,4~(th) class area was 25.8%,and 5~(th) class area was 8.39% Therefore,most of Lake Songhua water is probably at the slight eutrophication stage.In contrast, eutrophication at the mouths of rivers Huifahe and Jiaohe is serious only.