Aquatic Ecosystem Variability and Climate Change – A Palaeoecological Perspective - Climate Change Impacts on Freshwater Ecosystems - page 24

Although the later Holocene in the Saharan−Sahelian region was dry with
desert or semi-arid conditions prevailing and water bodies distributed and
behaving substantially the same as today, there have been continuing changes in
lake level on centennial timescales across the region over the last 3-5000 years
(Gasse 1977; Holmes et al . 1997). It is difficult to document the full variability
in any detail as the evidence is poor, limited by both the number and the quality
of sediment records. For example, lake sediment sequences in the Saharan region
in the later part of the Holocene are truncated by droughts, and previously
deposited sediment is often removed by wind (cf. Kröpelin et al . 2008). Long
core records, where they exist, therefore may contain hiatuses and become
difficult to date and interpret. Nevertheless, there are a few exceptionally good,
continuous records that do provide insights into the regional story. These (e.g.
for Lake Edward (Russell & Johnson 2005) and Lake Naivasha (Verschuren et al .
2000)) suggest that centennial- and decadal-scale droughts, marked by strong
fluctuations in lake level, have occurred regularly across East Africa. However,
while there is evidence to suggest that some of these excursions were associated
with solar forcing (cf. Verschuren & Charman 2008), there are also lake-level
changes, recorded by the sediments, that are not proportional to the forcing
expected from changes in solar activity, and some inferred that lake-level changes
have no equivalent in the solar activity record (Verschuren & Charman 2008).
Although the mechanisms remain uncertain, the palaeo-record indicates clearly
that relatively small changes in precipitation in this sensitive region can cause
pronounced, rapid responses in water chemistry and aquatic biodiversity on these
relatively short as well as long timescales. In particular, a small decrease in effective
moisture may lead to salinization and/or complete desiccation. Such changes are
not restricted to North and East Africa. Similar evidence for alternating periods of
high and low lake levels is available for lakes in many sub-humid and semi-arid
regions of the world. Some of the best documented are from the northern Great
Plains of North America where many lakes have often switched back and forth
between open and closed drainage regimes through the Holocene on multi-
decadal and centennial timescales (e.g. Fritz et al . 2000). However, within regions,
individual lakes respond to the same climate forcing differently, depending on
local patterns of groundwater influence, their position in drainage networks and
on differences in catchment and lake morphometries (Fritz 2008). Predicting the
future response of individual lakes to future climate change, therefore, is not easy,
although it is clear that a small decrease in moisture balance can quickly trigger
threshold switches between fresh and saline conditions and that water levels can
be rapidly lowered to the extent that even relatively deep lakes may become
completely desiccated, exacerbated in populated regions by the withdrawal of
freshwater surface flows and groundwater for irrigation and water supply.
Temperature
Long instrumental records together with evidence from documentary and
palaeoclimate data also show that temperature has varied continually throughout
the Holocene on centennial scales (Mackay et al . 2003). However, in contrast to
low-latitude moisture variability, the amplitude of change has been relatively