Climate Change and the European Water Dimension - Agri ...

Mean depth, m
Phytoplankton biomass, mgWW m -3
Water column irradiance, %
TN, mg l -1
3.6
3.4
3.2
3.0
2.8
2.6
2.4
2.2
80
70
60
50
40
30
20
10
30
26
22
18
14
10
6
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Depth year
Depth spring
2.0
1.8
1.6
Depth year: r = 0.3194, p = 0.0041
Depth spring: r = 0.3893, p = 0.0004
-3 -2 -1 0 1 2 3 4
Year
Spring
Summer
Autumn
NAOw
Year: r = -0.59, p = 0.00009
Spring: r = -0.08, p = 0.6
Summer: r = -0.59, p = 0.00009
Autumn: r = -0.49, p = 0.0026
0
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2
Yearly average lake depth, m
< 2 2 - 3 > 3
Average lake depth, m
< 2 2 - 3 > 3
Average lake depth, m
164
TP, mg m -3
TN/TP, mg mg -1
80 Year: r = -0.02, p = 0.9
Spring: r = 0.48, p = 0.004
70 Summer: r = -0.1, p = 0.5
Autumn: r = -0.08, p = 0.6
60
0
-3 -2 -1 0 1
NAOw
2 3 4
220
180
140
100
60
20
-20
90
70
50
30
10
-10
< 2 2 - 3 > 3
Average lake depth, m
< 2 2 - 3 > 3
Average lake depth, m
Figure VI.A.12. Relationship between the mean depth and concentration of total nitrogen
(TN), total phosphorus (TP), TN/TP mass ratio and average irradiance of the water column in
L. Võrtsjärv (Nõges et al. 2003).
Several winter fish-kills have been documented in Lake Võrtsjärv during the last
century (in 1939, 1948, 1967, 1969, 1978, 1987, 1996). One reason for these fishkills
is the depletion of oxygen in low-water years during late winter when the underice
oxygen concentration dropped faster due to smaller amount dissolved in the
Phytoplankton biomass, mgWW m -3
50
40
30
20
10
Year
Spring
Summer
Autumn
Figure VI.A.11. Correlation of the lake
depth, and phytoplankton biomass in L.
Võrtsjärv with North Atlantic Oscillation
Index in winter (NAOw), and the relation
of phytoplankton biomass with lake
depth