Lake ecosystems under ice

Frozen Lochan Urr in the Scottish Highlands. Image: John McSporran | Flickr Creative Commons

Around half of the world’s lakes (slightly more than 50 million) are periodically frozen and (partially or fully) covered in ice. However, ongoing climatic changes are causing reductions in ice coverage in lakes across the world. Despite this, there is comparatively little information on the ecology of under-ice conditions in lakes, and how changes to winter conditions are likely to affect their health and functioning all year round.

Recent research suggests that the timing and extent of winter ice cover can have ‘cascading‘ effects on spring and summer lake ecology, for example on algal growth. As such, winter ice cover may act as more than simply a seasonal ‘pause’ in lake productivity,  and instead play a significant role in shaping lake ecosystems all year round.

A large team of freshwater scientists from 42 research institutes across the northern hemisphere have recently collaborated to address the shortfall in knowledge of under-ice lake ecology. Writing in the journal Ecology Letters (open access), the team, led by Stephanie E Hampton at Washington State University, USA, carried out the first global synthesis of data on under-ice lake ecosystems, drawing on research from 101 lakes in Antarctica, Canada, Greenland, Europe and the USA.

The research team used the new global dataset to explore two key questions. First, they wanted to know about the ecological changes that happen in lakes between winter and summer. Second, they wanted to understand how winter and summer seasons were connected, and through which ecological variables these connections were made.

One major finding discussed in the paper is that whilst primary producers (algae) and consumers (zooplankton) are typically less abundant under ice than in summer, they maintain significant populations in many lakes through winter. This suggests that zooplankton actively feed and reproduce under ice. Light availability is likely to be an important limiting factor to winter algae and plankton populations, depending on variations in ice thickness and opacity and snow cover.

Another of the research team’s key findings is that dissolved nitrogen was consistently higher in winter ice conditions than in summer. This may be the result of winter nutrient mineralisation providing continued inputs of nitrogen into lakes through cold seasons.

Ice forms on the frozen Lake Michigan, USA. Image: Kamil Dziedzina

The research team found evidence for strong winter-summer linkages in some lakes, particularly those which had long historical datasets, such as the Laurentian Great Lakes, Wisonsin lakes, northern European lakes and Canadian lakes. Here, whilst the influence of winter conditions on the following summer differed among variables, winter and summer conditions were often negatively related.

This relationship means that high winter values (e.g. for zooplankton density or chlorophyll levels) resulted in low values in the following summer. In the case of chlorophyll, it is suggested that high winter levels may limit available nutrients for the following summer. For zooplankton, it may be the case that high abundances reduce the availability of readily ingestible phytoplankton at the beginning of the next season. However, given that previous studies have suggested that overwintering populations can boost summer populations and vice versa, there is clearly the need for further research on the seasonal dynamics of lake ecosystems.

“We are losing ice without a deep understanding of what ecological processes are at stake” is how the authors begin their conclusion. Whilst this synthesis has offered new insights into under-ice lake ecology, and how it may influence ecosystems year-round, there remains the need for significant further research. Studying long-term ecological data from sediment records may be one means of broadening our understanding of these dynamics.

The study suggests that lake conditions are not simply result of prevailing seasonal weather conditions but can also depend upon external and internal forces operating on the ecosystem in previous seasons. Predicting the ecological effects of shorter winters and longer summers, then, calls for an increased focus on winter lake ecosystem monitoring. As the authors wryly state, “In the future, we predict that there will be no more ‘off-seasons’ for freshwater ecologists.”

+++