Light, nutrients and primary productivity in Lake Biwa: An evaluation of the current ecosystem situation

Simple correlation and multiple regression analyses were performed to exami ne the relationship between primary productivity and environmental factors in the north basin of Lake Biwa. The primary production rates used in the a nalyses were estimated monthly or bimonthly during the growing season (Apri l-November) in 1992, 1996 and 1997 with the C-13 method. Elemental(C, N and P) contents of seston were used to assess nutrient conditions. Analyses re vealed that 86% of variance in depth-integrated primary production rates (a real PP) can be explained by changes in light intensity, and sestonic C, N and P concentrations. Water temperature had no effect on areal PP. To asses s relative effects of light and nutrients on PP, the P:B ratio was estimate d by normalizing PP with sestonic C. The areal P:B ratio correlated most si gnificantly with the sestonic N:P ratio, followed by light intensity. When regression analyses were made at each depth, however, the P:B ratio correla ted significantly only with the sestonic N:P ratio at 0 and 1 m depths, whi le light intensity was also incorporated into the regressions at deeper tha n 2.5 m. In these regressions, the P:B ratio was negatively correlated with sestonic N:P ratio but positively with light intensity. The results sugges t that the primary production rate in this lake was mainly limited by P rel ative to N supply rates, but was not free from light limitation in a large part of the epilimnion. In Lake Biwa, the vertical water mixing regime as w ell as the nutrient supply seem to be important in determining the growth a nd composition of primary producers, since the surface mixing layer extends into 10-15 m depths during most of the growing season.