21.6 Climate, water level and phytoplankton21 Lakes Peipsi and Võrtsjärv21.4 Water level changes21.5 Water level, nutrients and phytoplankton

21.5 Water level, nutrients and phytoplankton

In very shallow L. Võrtsjärv the annual mean amplitude of WL (1.4m) is equal to ½ of its mean depth, and the absolute range of WL fluctuations (3.2m) even exceeds the mean depth of the lake. Between the registered absolute minimum and maximum WL the lake area differs 1.4 times, the mean depth 2.5 times, and the volume 3.5 times (Figure 7) having a strong influence on the ecosystem [336]. In low-water years, the water column is better illuminated while both phosphorus release and denitrification increase because of more frequent resuspension of bottom sediments (Figure 8) bringing about substantially higher phosphorus concentration and lower N/P ratio (Figure 9). Such conditions favour the development of phytoplankton, first of all nitrogen-fixing cyanobacteria. In L. Võrtsjärv phytoplankton biomass is substantially higher and the share of N2-fixing species bigger in low-water years (Figure 10). In deeper L. Peipsi where seasonal and absolute WL variation ranges make up approximately 1/5 and ½ of the mean depth, respectively, the direct influence of the WL is not so obvious.

nogesFig8

Figure: In a large and very shallow lake the decrease of water level increases resuspension of bottom sediments enhancing the release of phosphorus. Intensive resuspension denudes anaerobic sediment layers and denitrification increases. The shallower is the mixed water column the better it is illuminated.

 

nogesFig9

Figure: Dependence of total nitrogen (TN) and total phosphorus (TP) concentration, of the TN/TP mass ratio and of the average water column irradiance (Imix) on the mean depth (or water level, WL) in lakes Võrtsjärv and Peipsi.

 

nogesFig10

Figure: Relationship between the mean depth (or water level, WL) and phytoplankton biomass (mg wet weight l-1) in lakes Võrtsjärv and Peipsi.

 
21.6 Climate, water level and phytoplankton21 Lakes Peipsi and Võrtsjärv21.4 Water level changes21.5 Water level, nutrients and phytoplankton