The Tundra Northwest 1999 expedition to the Canadian Arctic included a limnological theme (D): ”Freshwater ecosystems in the high Arctic”. Although most projects under Theme D were of a basic nature, they were in several cases motivated by aspects of global change, e.g. the impact on freshwater ecosystems of greenhouse warming and increased UV-B-radiation caused by CO² emissions and stratospheric ozone thinning. During the expedition Theme D scientists worked closely together with the aquatic part of Theme E, Impact of climate change and pollution, which was more explicitly applied with respect to research objectives and included studies of various widespread pollutants (metals, pesticides, radionuclides). Theme D also co-operated with scientists studying the genetic diversity of zooplankton in lakes and ponds, formally under Theme B, Biodiversity in the Arctic tundra. Co-operation with members from Themes E and B was initially motivated by logistic advantages, e.g. helicopter transport to lakes/ponds, use of Zodiacs, polar bear protection etc. However, before, and to an even greater extent during the expedition, scientific co-operation was also initiated.

The growth rate of Arctic terrestrial vegetation is extremely low. However, Arctic terrestrial ecosystems are sites of net accumulation of organic matter within the global biosphere, and at the same time, a source of dissolved organic matter feeding the Arctic Ocean. These two biomes are connected through the hydrosphere, i.e. through surface water runoff and its temporary passage through streams, ponds, lakes and rivers. Models predicting the potential for climate change in different ecosystems as a result of the build-up of greenhouse gases point to the Arctic as the region most likely to experience large shifts in temperature and precipitation over the coming decades. If these changes come to pass, what will be the consequences for aquatic organisms, communities and ecosystems? To be able to predict future conditions, we need to know how the Arctic freshwater ecosystems function today. This was therefore the main objective of Theme D.

Low precipitation, but even lower losses through evapotranspiration; result in a wealth of shallow lakes, ponds, and bogs, spread across the tundra. Although all three of these aquatic systems were sampled, the emphasis in this study was predominantly placed on lakes and ponds. Arctic lakes and ponds are characterized by very long periods with low water temperatures (0-<10°C), thick ice cover (up to 3 m), darkness, and often nutrient-poor water. Many Arctic lakes are young, having relatively recently been covered by glaciers after the last great glaciation period. These extreme conditions result in freshwater ecosystems characterized by low biodiversity and simple food-webs, although during the short but bright Arctic summers biomass of crustacean zooplankton can reach conspicuously high levels. This is often an effect of the absence of vertebrate predators, e.g. fish.

Although Arctic lakes and ponds have received appreciable limnological interest, there has been no synoptic study encompassing such a large west-east and south-north gradient as the Tundra Northwest 1999 expedition made possible. On the other hand, the study was necessarily limited in that it did not include possible seasonal variability, as each system was sampled only once. Hence we cannot know to what extent values are representative for the total ice-free period. However, if the character of lakes and ponds can be shown to be similar over the whole, vast sampling area, this would be an indication that it might be possible to generalize from our spot samples.

Arctic/Antarctic lakes are especially well-suited for testing general limnological theories, e.g. with respect to food-web interactions (top-down versus bottom-up control of trophic levels) and the importance of externally added (allochthonous) organic matter versus internally produced (autochthonous) organic matter. This is because Arctic lakes and ponds have a very simple biotic structure, due to the harsh environmental conditions. Fish may be entirely absent, or restricted to only one species (generally the Arctic char). Although zooplankton biomass may be high, communities are species-poor. Fishless and fish-containing lakes of otherwise similar size and other characteristics may be situated very close to each other. Thus, the structuring effect of the top predator can easily be studied. Many Arctic ponds and lakes are more or less permanently ice covered, which means that autochthonous primary production is low. Another feature of Arctic lakes and ponds is that primary producers are often benthic (typically periphytic alga), while pelagic forms are less important.

During the expedition the following questions were addressed (for more detailed descriptions and some preliminary results see reports from the individual Theme D projects):

1. l. What general limnological features characterize lakes and ponds in the investigated area, e.g. with respect to ice cover, transparency, temperature and oxygen vertical stratification, sediment characteristics, conductivity, pH, nitrogen and phosphorus, chlorophyll (phytoplankton) content. Which zooplankton species can be found and which fish species? Are fish-free deeper lakes common? Are there south-north and east-west gradients in the vast area covered by the two legs and are there marked differences between shallow lakes (ponds) and deeper lakes? To what extent are the High Arctic Canadian lakes similar to lakes in other parts of the Arctic?
2. What is the quantity and quality of dissolved organic matter in various types of Arctic freshwaters? Is the dissolved organic matter predominately of autochthonous or allochthonous origin?
3. How abundant are bacteria in surface waters and what is their activity (protein production)? Are there systematic differences between lakes and ponds in this respect? Which is the limiting nutritional factor for bacterial growth potential in Arctic ponds and lakes, ear bon, nitrogen or phosphorus, or a combination of these elements? What are the short and long-term effects of temperature fluctuations on bacterial substrate utilization and growth rate in Arctic freshwaters?
4. To what extent can photochemical processes contribute to the a biotic and bacterial turnover of dissolved organic matter in Arctic lakes, and are there adaptations among Arctic lake communities for coping with high UV-radiation?
5. What is the significance of allochthonous organic matter for food chains in Arctic lakes, specifically with respect to bacterial and zooplankton production? Are Arctic ponds and lakes supersaturated with CO², indicating net heterotrophy of the system?
6. What determines the quality of zooplankton food particle assemblage in Arctic lakes? What are the most important limiting factors for zooplankton: fatty acids, phosphorus or nitrogen? To what extent does mesozooplankton (copepods, cladocerans) community composition influence bacterial production and abundance in Arctic ponds and lakes?
7. To what extent is the biotic structure in Arctic lakes controlled by fish (char)?
8. Is there a trade-off among copepods of being protected against harmful UV-radiation (pigmented) and being less vulnerable to predation (non-pigmented)? Does the presence of fish induce resorption of pigments among copepods?
9. What is the genetic diversity of char in Arctic lakes and what is the population structure? What life-history strategies can be found? Are populations generally landlocked or migratory?

Lake waters for chemical analyses including trace metals sediments, plankton and fish were sampled at all the 17 sites visited. Generally more than one lake/pond was sampled per site. The lakes were of varying sizes, from a few hectares to more than a km², and with depths ranging from a few metres to 30-40 m. Most of the lakes sampled on leg 2 were deeper than 15 m. Preliminary results from the water chemical analyses show that lakes were well buffered with pH values above 7.0, as a result of alkaline rocks in their catchments. The last site on Baffin Island was an exception showing impact of volcanic bedrock and lake water pH of 6.4. The lakes were generally poor in organic carbon and most of them moderately turbid (Secchi disc transparency 2-10 m) from colloidal clay minerals, originating from weathering and run-off from glaciers in the catchments. Chlorophyll content was generally low, while zooplankton abundance could be quite high in fish-free lakes/ponds. Most lakes contained char and the majority of the char populations showed signs of being anadromous.