The environmental conditions for freshwater organisms in the Arctic regions are harsh, with short ice-free periods and low nutrient availabilities in the water column, as well as low temperatures and high ultraviolet (UV) radiation that reaches deep into the often clear waters (picture 1). These situations call for specific adaptations, for example the conspicuous red pigmentation for UV protection among herbivorous zooplankton (Hairston, 1979, Hansson, 2000). Most adaptations in nature are constitutive, i.e. the animal or plant is born with the adaptation and carries it throughout its lifetime. Other adaptations, however, can be turned on or off depending on whether there is use for them or not. Such adaptations are called ”plastic” or ”induced” and represent a way to reduce the cost of having the adaptation. During the Beringia 2005 expedition I specifically addressed the plastic response in pigmentation by some zooplankton in response to threats from UV radiation.

In a variable and unpredictable environment, phenotypic plasticity in morphology or behaviour may considerably improve an organism ’s protection against environmental threats, and thereby its fitness. Among copepods, a common zooplankter in Arctic waters, pigmentation may vary from pale white to bright red (picture 2). The red pigment, astaxanthin (a carotenoid of the same type that can be bought in health food stores!), reduces damage caused by UV radiation but also makes the organism more conspicuous, thereby exposing it to higher predation pressure from example fish. Hence the level of pigmentation in copepods may be an inducible and adjustable defence governed by the aim to improve individual protection against prevailing threats from both predation and UV radiation (Hansson, 2004). A large scale implication of such inducible defences is that these organisms may become dominant compared to competitors and predators in the future, when UV radiation, especially in Arctic clearwater lakes, may increase considerably.

The aim of my project was to assess the level of pigmentation among zooplankton in Beringian lakes and compare with the levels found in temperate regions. Another aim was to experimentally test the hypothesis that zooplankton can adjust their level of pigmentation in relation to UV radiation. My third aim was to perform monitoring sampling of Beringian lakes in order to add data to an international database on Arctic lakes.

Methods

In the field I took samples for zooplankton pigment analysis as well as for determination of community composition, including organisms varying in size from viruses to zooplankton with traditional methods (Hansson 2000, Bertilsson et al., 2003). The animals were frozen on board the icebreaker Oden for later analysis of pigments according to earlier tested methods (Hansson 2000, Hansson 2004). In total I sampled 17 lakes, of which some were very low productive meltwater lakes and some highly productive eutrophic lakes (picture 3). In lake 10 and lake 11 at Kolyuchinskaya Bay, Siberia (67°04,107’N, 173°23,200’W) live zooplankton were also sampled for the experiment performed onboard Oden. In this experiment I used aquaria with different types of plexiglass lids either letting all radiation, including UV, through, or allowing only visible light to enter the water in the aquaria. UV lamps were put above the aquaria, and in this way I created one environment that was free from UV and one where the zooplankton were exposed to about the same UV environment as in their natural environment (472 μW cm^-2). The experiment was performed in one of Oden’s laboratory containers and was allowed to run for 9 days. During the cruise through Beringia the natural levels of UVA radiation were measured on the deck of Oden 4 to 6 times a day for 10 days.

Results

The mean UV-A radiation in the Beringian area was 535 μW cm^-2, which corresponds well to the experimental intensity (472 μW cm^-2). At the time of writing this report, the pigment analyses of the zooplankton are not yet finished, but the behavioural response to UV in one of the zooplankton groups (Daphnia) was recorded in the experiment and found to be very strong. Hence in the treatment with ambient UV radiation the Daphnia showed no response in behaviour when the plexiglass lid was removed. However in the treatment where the animals, which originated from the same population, were released from UV radiation during the experiment, 98% of the individuals immediately swam towards the bottom of the aquaria upon removal of the protective plexiglass (figure). This means that those Daphnia which were used to UV continued to feed in the upper parts of the aquaria, whereas those that had been protected from UV during about a week had lost their ability to protect themselves, e.g. by photoprotective pigments. Hence besides pigmentation, some zooplankton may also have behavioural responses to protect themselves from dangerous radiation.

The other part of my studies, monitoring Arctic freshwater systems for the international database previously included data from the whole Arctic hemisphere, except Siberia. The successful work during the Beringia 2005 expedition now allows us to include these very important data in the database and to make them available for future scientific studies. Hence the data gathered during the Beringia 2005 expedition considerably improved our international knowledge of freshwater biology. In a broader context Arctic freshwater lakes are ecosystems we know very little about, but which can be predicted to respond strongly to large scale changes in for example climate, including both global warming and the increase in UV radiation due to atmospheric ozone depletion.