Lakes are distributed widely across the globe, and depending on their location, they can be particularly sensitive to changing climate. For example, global warming is having a significant impact on lakes in polar and alpine environments, while variation in rainfall patterns impacts on lakes in semi-arid regions. In order to understand the climate history that can be obtained from lakes, we need to briefly consider how temperature and precipitation affect different types of lakes.
In cold, polar-regions, lakes are normally covered by ice for many months of the year. During spring, the ice melts and winds help mix the water up, which also ensures that nutrients are widely available throughout the lake for organisms to feast on. During summer, the lake water becomes warmer and less easy to mix. Nutrients are quickly used up, and only certain types of organisms thrive under these conditions. However, during autumn, winds get stronger, and because the water becomes cooler, it is also easy to mix up again. And then in winter the lake freezes over once more. Each of these phases in the annual cycle of a polar or alpine lake supports different types of species. As we shift into a warmer world, the amount of time lakes are covered in ice declines, and we see a shift in species that can tolerate these new ice-free conditions.
In semi-arid regions, lakes are more sensitive to changes in precipitation and evaporation especially when those lakes do not have an out-flowing river (these are commonly known as ‘closed’ lakes). When the climate is very wet, and precipitation amounts are high, the amount of water entering into the lake increases, making the lake larger and deeper. During drier climate, lake-levels decline, and lakes get smaller and generally shallower. When this occurs, the chemistry of the lake water changes, and salts become much more concentrated. So much so that these lakes can become quite saline, and are then commonly called salt lakes. Only certain species can tolerate increased salt levels, and so as lakes change in size, flora and fauna changes too. If a lake becomes shallower, then deep-water habitats become less available and instead we see a relative increase in species that either grow on the lake bottom or around the lake margins. Habitat changes can easily be determined by looking at the proportions between deep-water (planktonic) species and shallow-water (benthic) species.
Reconstructing climate history from lakes can be done directly by studying past shorelines of lakes sensitive to changes in precipitation and evaporation. For example, the Aral Sea in central Asia is renowned as an environmental catastrophe because since the 1950s, water has been diverted away from entering the lake, to instead being used to irrigate agricultural land. This has led to a decline in lake volume by over 90%, and the devastation of ecosystem services such as provision of clean water and fisheries and catastrophic decline in biodiversity. However, studies of previous shorelines shows a very complex interaction between the lake and natural variability, such that climate also played a significant role in controlling the size of the Aral Sea further back in the lake’s history.
The most common method of reconstructing climate histories from lakes is by studying the contents of the lake mud (sediment) that accumulates as a natural archive over time. Using specialised equipment such as corers, sediments are extracted which extend back in time, beyond any monitoring records.
Lake sediments are made up of a complex set of organic and inorganic materials, such as biological microfossils, silts and sands. Different microfossils contain substances that aid their preservation in lake sediments. For example, pollen contains a compound called sporopollenin, diatoms shells are make of silica, and even small parts of aquatic insects can preserve (e.g. water fleas and midges) because they contain chitin. Biological microfossils can also be used to quantitatively reconstruct climate history directly, e.g. annual precipitation, temperature of the warmest and coldest months, or indirectly by reconstructing past water chemistry (e.g. palaeosalinity), which has a strong association with precipitation trends. These techniques are called transfer function models, and are based on the premise of collecting knowledge on environmental requirements of species represented as fossils in sedimentary records (read more). Relationships between species and their environment are modelled numerically, based on strong statistical and theoretical bases.
The case study below is a sediment core extracted from a shallow lake in semi-arid SE Siberia. Species changes are plotted against an age scale (determined by radiocarbon dating of organic matter in several sediment samples). We can see that the core spans the last 5000 years and is dominated by varying proportions of coniferous and deciduous forests. Quantitative reconstructions show that the earlier period (before 2700 years ago) was wetter with warmer winter temperatures but cooler summer temperature. In this region, global temperatures influenced by local geography and topography drive such changes in climate.