Interaction of Climate Change and Eutrophication - Climate Change Impacts on Freshwater Ecosystems - page 142

relatively subtle, yet highly variable (H. Bennion, unpublished data). Diatoms
from the core were compared with physico-chemical conditions monitored in
the lake over the same period, 1969-2005. Only small amounts of variation in
the diatom assemblages could be attributed to climate and nutrients, with
2.2%, 3.7% and 5.7% being explained by annual mean TP, annual maximum
temperature and annual total rainfall, respectively. The sum of interactions
between variables explained less than 2%. The relatively higher proportion of
variance in the diatom data explained by rainfall is possibly linked to its impact
on flushing rate, which has been shown to influence the plankton populations
of the loch (Bailey-Watts et al . 1990).
A comparison of the planktonic diatom composition in the palaeo-record
with that in the phytoplankton records for 1996-2005 revealed a good match
for the dominant taxa (J. Wischnewski & H. Bennion unpublished data)
(Fig. 6.14). Whilst there were discrepancies between the two data sets for
some taxa (e.g. Aulacoseira ambigua ), all of the key species recorded in the
plankton were also observed in the fossil assemblages, indicating that the
sediment core almost faithfully records changes observed in the water column.
Both records show a decline in Cyclotella radiosa and Asterionella formosa ,
and more recently in Aulacoseira subarctica, A. granulata and Diatoma
elongatum , and a relative increase in Fragilaria crotonensis, Stephanodiscus
hantzschii and S. parvus , taxa associated with nutrient-rich conditions. Indeed
there is some evidence that water quality may have declined recently with an
increase in measured annual mean TP concentration from ~50

μ

g l −1 in 2000

μ

to >70 g l −1 in 2005. The degree to which this increase in nutrient

concentrations can be attributed to changes in climate remains unclear.
Increased water temperature, however, is associated with greater internal
P loading at the site (Spears et al . 2008).
The research on Loch Leven to date suggests that each of the approaches
above provides different, yet complementary, perspectives about lake responses
to environmental change. In Loch Leven, the data sets show that the lake has been
slow to recover from eutrophication and that temperature has variable effects,
both positive and negative, on its plankton community. Responses, such as changes
in chlorophyll a concentrations and Daphnia densities, are often interrelated, and
the relationships between variables may vary over the year. Overall, the study
highlights the complexity of lake responses to changes in nutrient regimes and
climate and, despite advances in analytical methods, the difficulty of unravelling
their effects.

Synthesis

This chapter started with a series of questions about the combined effects of
warming and eutrophication. We set out to answer them by using a range of
approaches and techniques, and although there will never be a complete answer
to any of the questions, the results obtained in Euro-limpacs and presented above
give at least partial ones.