Our project aims at a better understanding of Arctic lake and pond ecosystems, in particular the role of food quality for the zooplankton community structure, and vice versa. Arctic waters are of interest because they may favour such studies due to low ecological complexity and minimal human disturbance. They also represent a climatic endpoint (light, temperature, and water renewal). The supply of inorganic and organic nutrients is often small. Combined with the relatively recent glacial withdrawal, these extreme conditions result in freshwater ecosystems characterized by low diversity and simple food-webs.

Life in Arctic lakes and ponds experiences long periods of darkness, low temperatures, thick ice cover (limited gas exchange) or even complete freezing. Nonetheless, the short but bright Arctic summers can be very productive and especially crustacean zooplankton often reaches conspicuously high biomass. The demand for rapid growth in Arctic waters supports investigations into the relationship between zooplankton and the suspended particles they feed on, since they constitute the basis and a major link in aquatic food chains. Apart from ecological properties, food quality and food-web structure as well as extreme a biotic conditions are likely to influence the biomagnification of air-borne pollutants in Arctic waters.

Our central questions can be divided in to three different but interrelated themes:

• What determines the quality of the food particle assemblage in Arctic lakes? The importance of food quality for zooplankton production has received much scientific attention during the past decade. High food quality promotes higher zooplankton production and leads to a more efficient utilization and transfer of carbon in the food-web. Hence, food quality has implications both for animal populations and for carbon cycling. Essential fatty acids and inorganic nutrients, particularly phosphorus and nitrogen, have been put forward as growth limiting factors. Our approach is to quantify different nutrients in different particle fractions and to compare these data with the growth rate of zooplankton feeding on natural particle assemblages.
• What is the zooplankton community structure in Arctic lakes of different types? In other words, what species are there and what do they eat? How do factors such as climate, the presence of fish, or food quality influence the zooplankton community? For estimating the dietary composition of each species, lipid biomarkers as well as stable isotopes of carbon and nitrogen are used. Some fatty acids are specific for certain algal groups, bacteria, or protozoa, and are incorporated without changes in to the animal body. Stable nitrogen isotopic signatures provide indications about the trophic level of an organism. Thus, with these modern tools we can obtain information about food choice and trophic relationships among zooplankton that otherwise would have been very difficult to obtain in situ.
• What is the primary source of the organic carbon that enters the plankton food-web? Is it photosynthesized by plants in the water, e. g. phytoplankton, or does it originate from terrestrial vegetation? In some regions the tundra is rich and can potentially supply lakes with dissolved organic carbon. At other locations terrestrial vegetation is virtually absent. Not only the supply but also the bioavailability of nutrients may vary substantially. How does this influence their partitioning in lakes? Carbon isotope signatures often provide good indications about the primary source of organic carbon in animals.

Many of the aspects above are likely to change or vary along with climate. Among others, this applies to the runoff of both organic and inorganic nutrients. Moreover, surface water chemistry is known to vary with geology and climate, e.g. pH, inorganic carbon, organic carbon, nutrients, ionic strength and composition, and water colour and turbidity. Can we observe any regional trends in ecological patterns that follow the wide environmental gradients covered by Tundra northwest 1999?

Several independent methods are used, ranging from natural tracers to bioassays: lipid biomarkers and stable isotopes of carbon and nitrogen for assessing structural aspects (communities, food-webs, nutrient sources); standardized growth experiments, nutrient elemental ratios (stoichiometry) and essential fatty acids for assessing food quality. Lakes (and ponds, streams, rivers) of different types are compared: e.g. presence/absence of fish, terrestrial/pelagic carbon sources, high/low transparency. Simultaneously, a multitude of other chemical, microbial, and ecological data of relevance were and are produced within the framework of the Tundra Northwest 1999 expedition.

Field and laboratory work

At each of the 17 main sites at least one lake was sampled. Often additional lakes and ponds within walking distance were also sampled. Apart from basic lake descriptions and measurement, our field-work consisted mainly of collecting zooplankton, phytoplankton and water. The plankton was collected by trawling using nets of different mesh sizes, which at some locations with low plankton densities was quite time-concerning, whereas animal abundance in other ponds was extremely high. Water was collected either from different depths using a Ruttner water sampler or directly from the surface.

Back on board, intensive work was required to process the fresh sample s during the short trips between sites. The zooplankton sample s were sorted according to species and placed in tin capsules or cryotubes for later analyses. The phytoplankton suspensions were transferred to glass fibre filters. Water from each lake was prefiltered through a 11μm sieve and sequentially filtered through a series of glass fibre filters of different pore size. The combined field and laboratory procedures yielded planktonic particle size fractions with the following boundaries: 0.7-1.2-3-10-40- 200 μm. All samples were frozen for later analyses.

For an optimal comparison between the sampled lakes, zooplankton growth rates were estimated in standardized experiments, using Daphnia galeata isolated from Lake Ånnsjön, Sweden, as an ”independent” indicator of food quality. Lake water, prefiltered through 40 μm, was used as a food suspension in which the Daphnia were allowed to grow for four days in a light incubator at 12°C. After the experiments the animals were put in tin capsules and dried. Growth will be estimated by measuring the change in individual carbon content.

First results

Altogether we sampled 55 lakes and ponds, covering a wide variability of systems ranging in area from 1 m² to several km² and in depth from 0.05 to about 50 m. Water temperatures on leg 2 varied from about 1 to 13°C, while air temperatures showed even greater variation. Also water chemical data showed wide ranges (collective data from leg 2): from soft to hard waters (measured inorganic carbon 0.2- 105 mg/L) and from clear to humic waters (estimated organic carbon 0.3-30 mg/L). Conductivity ranged from 20-3 200 μS/cm, and pH from 3.7 to almost 9. Some areas were apparently influenced by sea salt. Although most chemical and biological analyses remain to be completed, some preliminary results and general observations can be presented.

Crustacean zooplankton diversity ranged from zero to six taxa. It is noteworthy that at the northernmost site on Ellef Ringnes Island, we found no sign of zooplankton, except for rotifers, in any of the lakes and ponds visited. At other places seemingly as barren as Ellef Ringnes Island, the abundance of crustaceans was sometimes staggering. Possibly the productive season is just too short at Ellef Ringnes for successful completion of crustacean life cycles.

The presence of fish appeared to have a profound impact on the species composition. In lakes with fish, zooplankton was dominated by small copepods and sometimes also by very small Daphnia or Bosmina species. Without fish, large Daphnia, anostracans and larger copepods dominated. This pattern was expected, but interestingly, the presence of fish also appeared to have an influence on the composition of fine particulate matter. For example, in two very similar lakes not more than 200 m from each other, particles less than 10 μm were very different in colour between the lakes. One lake had a yellow-brown colour while the other was green, indicating a different phytoplankton composition. While chemical analyses showed similar results between the lakes, one of them contained fish and the other did not.

In conclusion, the expedition has fulfilled or even exceeded most of our expectations. We were able to visit a large number of systems showing high variability between and within sites, both biologically and chemically (see above). This is promising for our upcoming results, since our part of the investigation has a correlative approach and benefits from a wide range in the variables studied. Research projects can now proceed according to plan, starting with various chemical analyses of collected samples. Standardized growth experiments have already been performed. Given the vast coverage of the Tundra Northwest 1999 expedition, the large number of lakes sampled, the basic lake variable programme shared with other Theme D projects, the use of independent methods, and the high resolution in both particle size fractionation and zooplankton species separation, we can test some important hypotheses regarding zooplankton composition, food quality and primary carbon sources.