Ph.D. thesis
This thesis is electronically published on the publications
server
at Potsdam University, Germany. This work was carried out at
the Leibniz-Institute of Freshwater Ecology and Inland
Fisheries (IGB)
in Berlin within the EU-Project "Climate and Lake Impacts in
Europe" (CLIME).
Summary
There
is already strong evidence that temperate lakes have been highly
vulnerable to human induced climate warming during the last century.
Hitherto climate impact studies have mainly focussed on the impacts of
the recent long-term warming in winter and spring and little is known
on the influence of climate warming on temperate lakes in summer. In
the present thesis, I studied some aspects, which may have been
strongly involved in determining the response of a lake to climate
warming in summer. Thereby I have focussed on climate induced impacts
on the thermal characteristics and the phenology and abundance of
summer plankton in a shallow polymictic lake (Müggelsee,
Germany).
First, the influence of climate warming on the phenology and
abundance
of the lake plankton was investigated across seasons. Fast-growing
spring phytoplankton and zooplankton (Daphnia) advanced largely
synchronously, whereas long-term changes in the phenology of
slow-growing summer zooplankton were clearly species-specific and not
synchronised. The phenology and/or abundance of several summer copepod
species changed according to their individual thermal requirements at
decisive developmental stages such as emergence from diapause in
spring. The study emphasises that not only the degree of warming, but
also its timing within the annual cycle is of great ecological
importance.
To analyse the impact of climate change on the thermal
characteristics
of the lake, I examined the long-term development of the daily
epilimnetic temperature extrema during summer. The study demonstrated
for the first time for lakes that the daily epilimnetic minima (during
nighttime) have increased more rapidly than the daily epilimnetic
maxima (during daytime), resulting in a distinct decrease in the daily
epilimnetic temperature range. This day-night asymmetry in epilimnetic
temperature was likely caused by an increased nighttime emission of
long-wave radiation from the atmosphere. This underlines that not only
increases in air temperature, but also changes in other meteorological
variables such as wind speed, relative humidity and cloud cover may
play an important role in determining the lake temperature with respect
to further climate change.
Furthermore, a short-term analysis on the mixing regime of
the
polymictic lake was conducted to examine the frequency and duration of
stratification events and their impacts on dissolved oxygen, dissolved
nutrients and summer phytoplankton. Even during the longest
stratification events (heatwaves in 2003 and 2006) the thermal
characteristics of the lake differed from those typically found in
shallow dimictic lakes, which exhibit a continuous stratification
during summer. Particularly, hypolimnetic temperatures were higher,
favouring the depletion of oxygen and the accumulation of dissolved
nutrient in the hypolimnion. Thermal stratification will be very likely
amplified in the future, thus, I conclude that polymictic lakes will be
very vulnerable to alterations in the thermal regime with respect to
projections of further climate change during summer.
Finally, a long-term case study on the long and short-term
changes in
the development of the planktonic larvae of the freshwater mussel Dreissena polymorpha
was performed to analyse the impacts of
simultaneous changes in the thermal and in the trophic regime of the
lake. Both the climate warming and the decrease in external nutrient
load were important in determining the abundance of the pelagic larvae
by affecting different features of the life-history of this species
throughout the warm season. The long-term increase in the abundance and
length of larvae was related to the decrease in external nutrient
loading and the change in phytoplankton composition. However, the
recent heatwaves in 2003 and 2006 have offset this positive effect on
larval abundance, due to unfavourable low oxygen concentrations that
had resulted from extremely long stratification events, mimicking the
effects of nutrient enrichment. Climate warming may thus induce
counteracting effects in productive shallow lakes that underwent lake
restoration through a decrease in external nutrient loading.
I conclude that not only the nature of climate change and
thus the
timing of climate warming throughout the seasons and the occurrence of
climatic extremes as heatwaves, but also site-specific lake conditions
as the thermal mixing regime and the trophic state are crucial factors
governing the impacts of climate warming on internal lake processes
during summer. Consequently, further climate impact research on lake
functioning should focus on how the different lake types respond to the
complex environmental forcing in summer, to allow for a comprehensive
understanding of human induced environmental changes in lakes.
|