Interaction of Climate Change and Eutrophication - Climate Change Impacts on Freshwater Ecosystems - page 122

Chlorophyll a Phytoplankton Diatoms Green algae Non N-fixing

cyan o phytes

Dinophytes

Cyanobacteria N-fixing

cyanobacteria 2

3.02
±

2.27

0.85 ±0.75

3.68

±0.88

1

* *

*

0

** *** * *** **** **

*** *** *

-1

-2.09
±0.79

-2

Chrysophytes Zooplankton Zooplankton:

phytoplankton

Cladoceran:
phytoplankton
ratio

Size
cladocerans

Size copepods Size Daphnia Daphnia :

cladoceran
ratio

2

1

* * * ** * *

* * *

0

* * * *

** *

-1

-2

TP TN Depth Water temperature

Figure 6.1 Slopes of multiple regressions relating various phytoplankton and
zooplankton variables (log transformed) to concentrations of total phosphorus (TP) and
total nitrogen (TN) in surface water, mean lake depth and surface water temperature (all
log transformed) measured in August in 250 lakes and over 800 lake-years in Denmark.
Significant ( p < 0.05) slopes are indicated by asterisks. (From Jeppesen et al . 2009.)

Size structure of fish populations also changes latitudinally, with a higher
proportion of small fish in warm lakes. Higher-latitude fish species are often not
only larger but grow more slowly, mature later, have longer lifespans and allocate
more energy to reproduction than populations at lower latitudes (Blanck &
Lamouroux 2007). Even within species, such changes can be seen along the
latitudinal gradient (Blanck & Lamouroux 2007).
Fish abundance, and thus predation on zooplankton and benthos, may also be
influenced by changes in the duration of ice cover (see also Chapter 3).
Comparative studies of Danish coastal lakes and continental Canadian lakes with
similar summer temperatures, but major differences during winter, have shown
much lower chlorophyll:TP ratios and higher zooplankton:phytoplankton ratios
in the winter-cold Canadian lakes, perhaps due to lower winter survival of
zooplanktivorous fish under ice (Jackson et al . 2007). Thus, reduced ice cover in
winter should enhance fish survival, which might cascade down the food web
and also reinforce symptoms of eutrophication. The effects of warming on fish
survival may be contradictory, however, as discussed below.
Enhanced predation on zooplankton at higher temperatures is also evident
from analyses of data from Danish lakes, showing a decrease in the average size
of cladocerans and copepods with increasing temperature (Fig. 6.1, Jeppesen
et al . 2009). Tendencies for decreasing zooplankton:phytoplankton biomass ratios
and lower proportions of Daphnia among the cladocerans point in the same