Reaction of large and shallow lakes Peipsi and Vortsjarv to the changes of nutrient loading

More than 20-year monitoring of Estonian rivers reveals that the loading of nitrogen to large shallow lakes Peipsi (3,555 km(2), mean depth 7.1 m) and Vortsjarv (270 km(2), mean depth 2.8 m) decreased substantially in the 1990s. Phosphorus loading decreased to a much smaller extent than nitrogen loading. In L. Vortsjarv both N and P concentrations followed the decreasing trends of loading, which show the high sensitivity of large shallow lakes to catchment processes. Our study showed a positive relationship between P content in sediments and the relative depth of the lake. Assumingly the resilience of a lake in responding to the reduction of nutrient loading decreases together with the decrease of its relative depth. In L. Peipsi the concentration of P has not decreased since the 1990s. Our data show indirectly that P loading from Russia to L. Peipsi may have increased. The N/P ratio has decreased in both lakes. Cyanobacterial blooms have been common in both lakes already at the beginning of the 20th century. The blooms disappeared during heavy nitrogen loading in the 1980s but started again in L. Peipsi in recent years together with the drop of the N/P ratio. In L. Vortsjarv the N/P ratio is higher and the ecosystem is more stable although the share of N-2-fixing cyanobacteria increased from the 1990s. Reappearing cyanobacterial blooms in L. Peipsi have caused fish-kills in recent years. In L. Peipsi summer/autumn fish-kills during water-blooms are a straightforward consequence of reduced nitrogen level at remaining high phosphorus level while in L. Vortsjarv the climatic factors affecting water level are more critical-at low water level winter fish-kills may occur. In L. Vortsjarv nutrient loading has decreased and water quality has improved, present ecological status seems to be mostly controlled by climatic factors through changes of water level. The most important measure to improve water quality in L. Peipsi would be the reduction of phosphorus loading from both Estonian and Russian subcatchments.