Biodiversity and biological structure of arctic lakes

Kirsten Christoffersen

Freshwater Biological Laboratory, University of Copenhagen, Denmark

Lakes and ponds are so numerous in many arctic areas that it would be more precise to talk about “lakescapes” than landscapes! It is intuitively obvious that such abundant ecosystems must play an important role in a number of biological and bio-geochemical processes.

Permanent water bodies indeed act as habitats for breeding, resting and foraging birds, serve as spawning and overwintering habitats for migratory fish (Arctic char and salmons), provide supporting stocks of stationary fish (e.g. Arctic char, sticklebacks) and act as a decomposition place for substantial amounts of organic matter of terrestrial origin.  Several studies from, for example, Alaska have shown that the aquatic primary production is set by nutrient availability which again is a result of the geo-chemical conditions, climatic conditions (precipitation/aridity) and catchment area, while community structures of heterotrophs are controlled by both  resources and predation control. Studies from west and north-east Greenland suggest that predation by Arctic char controls the zooplankton communities, as has been found in temperate regions for various other planktivorous fish, but to a more extreme extent due to the low density and poor diversity of zooplankton species. Fish can, however, not inhabit lakes and ponds that freeze to the bottom (usually those with a depth of less than 2.5 m) and predation in such systems is therefore less intensive but may still operate through invertebrates (Cyclops and Lepidurus) or birds.

There are only few large zooplankton species and they tend to grow slowly due to low temperatures and low availability of algal food. On the other hand, it is often found that a substantial input of dissolved organic matters (DOM) from the drainage area supports a well-developed bacterial community. These factors imply that a microbial community of bacteria and protozoans can arise and subsequently provide the larger heterotrophic organisms with food. Information on these types of interactions is scarce and limited to very few localities, and most suggestions are conceptual models describing arctic aquatic food webs very speculatively.

Arctic lakes and ponds are characterized by having fewer planktonic and benthic species than found in their temperate counterparts. A short growing season (1-2 months), low summer temperatures (4-12oC), poor nutrient availability and dispersal barriers are the main agents for a limited biodiversity. This creates, however, simple and short food chains that facilitate excellent scenarios for studying the importance of central pools and processes as well as serves as keystone organisms for the biological structures at low nutrient and temperature regimes.

Arctic freshwater systems are extremely vulnerable to changes due to their low food web complexity, poor nutrient regimes and highly adapted species. Whether such changes are imposed by, for instance, eutrophication, acidification, climate or dispersal of alien species, they may change the systems dramatically and probably irreversibly. It is therefore of uttermost importance that biological patterns and trophic interactions of arctic lakes and ponds can be described in detail and hence be understood and protected as whole ecosystems.