Interaction of Climate Change and Eutrophication - Climate Change Impacts on Freshwater Ecosystems - page 127

(a) (b)

10

10

* *

Floating
Submerged

Floating
Submerged

8 8

* *

* 6 6

4

4

2 2

0 0

Heated Unheated N0 N1 N2

Figure 6.5 Effects of (a) warming by 4 °C above ambient and (b) increased nutrient
loading (N0, N1, N2) on submerged and free-floating plants in a mesocosm experiment
in the United Kingdom. Values are in kg fresh biomass per mesocosm. Asterisks indicate
significant differences at p <0.05, which occurred among treatment levels as well as
between macrophyte growth forms.

Macrophytes still dominated in the second experiment (Feuchtmayr et al . 2009),
but warming led to lower phytoplankton biomass, almost certainly because
substantial floating lemnid communities developed, especially at the higher
nutrient levels (Fig. 6.5). Warming increased phosphorus concentrations and
conductivity, decreased pH and oxygen saturation and increased the frequency
of severe deoxygenation in the first experiment. In the second experiment, raised
temperature had similar effects, but these were more extreme, except for pH,
which did not change.
Effects of nutrient addition and the presence of fish were independent of
warming in the first experiment and tended to increase and decrease
macrophyte abundance, respectively. This was not the case in the second
experiment where warming and nutrients together increased the growth of
floating lemnids, which had been negligible in the previous experiment. There
were also much more severe effects on fish in the second experiment (Moran
et al . 2009). Heating reduced the stickleback population by 60%, and nutrient
addition reduced it by approximately 80%. A combination of nutrients and heating
resulted in a complete loss of the stickleback populations. This was attributed to
the increased likelihood of hypoxia in mesocosms subject to heating and
nutrient addition, rather than direct thermal stress, though aquarium
experiments showed reduced breeding male activity at temperatures above
17 °C in well-oxygenated conditions (K. Hopkins, B. Moss & A.B. Gill
unpublished data).
Results of the two experiments thus suggest that warming exacerbates the
symptoms of eutrophication, to a small extent with a temperature increase of 3 °C
and low nutrient loading but with high severity with 4 °C and nutrient loadings
comparable with those in many European lowlands at present. These include
increased phosphorus loading owing to release from the sediments, deoxygenation,