Long‐term ecological studies in northern lakes—challenges, experiences, and accomplishments

Abstract

We review three long-term research programs performed over the last four decades on the ecology and management of oligotrophic lake systems with different fish communities at 69 N in Norway. Through whole-lake perturbation experiments, intensive culling of stunted fish removed 35 tons (1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991) of Arctic charr Salvelinus alpinus in Takvatn (15 km 2 ) and 153 tons (1981)(1982)(1983)(2002)(2003)(2004) of European whitefish Coregonus lavaretus in Stuorajavri (25 km 2 ). In Takvatn, the overcrowded charr population decreased to 20% of the initial abundance, whereas brown trout Salmo trutta abundance increased. Somatic growth improved strongly in both species. In charr, ontogenetic habitat shifts broke down, the diet changed to more benthos, and plankton-borne parasites decreased. High abundance of juvenile, littoral charr provided new prey for trout, creating an alternative, predator-regulated stable state. Similar density reductions, positive effects on growth and reduced parasite loads occurred in whitefish in Stuorajavri. Despite the heavy culling, however, a new stable state did not occur and the fish community returned to the pre-culling situation. In the Pasvik watercourse, vendace Coregonus albula invaded around 1990 after an upstream introduction. The population of this non-native, highly specialized planktivore increased rapidly, resulting in steep density declines in zooplankton and the native planktivorous whitefish morph, and large changes in energy flow and structure and dynamics of the lacustrine food web. These programs show that long-term research is essential for understanding the ecology of manmade disturbances and providing a scientific basis for management efforts.

Climate warming is predicted to enhance the negative effects of harvesting on high‐latitude lake fish

“…Long‐term empirical studies (with study periods longer than 10 years; Lindenmayer & Likens, ) are especially important for assessing population impacts of climate warming. However, such studies are rare and often examine systems simultaneously impacted by other anthropogenic stressors such as harvesting (Amundsen et al, ). A review of long‐term empirical time series of freshwater fish in Europe indicates that declines in Arctic charr populations can be attributed to climate warming, even though somatic growth rates have often increased over time (Jeppesen et al, ).…”

“…Lake Takvatn is located about 300 km north of the Arctic Circle in northern Norway, has an area of 15 km 2 and is situated 215 m above sea level. Data on Arctic charr have been collected yearly since the early 1980s (Amundsen et al, ) and include individual age, length, weight, maturation status, sex and fecundity data (Amundsen et al, ; Henriksen et al, ). Parameters used for our model are listed in Table S1 in Appendix , and data from Lake Takvatn charr are visualized in Figure S1 in Appendix .…”

“…Arctic charr population dynamics are modelled over the period 1950–2100 for climate scenarios characterized by the representative concentration pathways (RCPs) RCP‐4.5 and RCP‐8.5, and for five different levels of size‐selective harvesting. The model is parametrized and evaluated based on long‐term data from a sub‐Arctic lake (Amundsen, Knudsen, & Klemetsen, ; Amundsen et al, ; Persson et al, ). We investigate whether climate change will increase individual growth rates of Arctic charr in high‐latitude lakes, as water temperatures approach the optimum for summer growth, resulting in larger size at age and higher stock biomass and production.…”

Seasonal dietary shifts enhance parasite transmission to lake salmonids during ice cover

“…In the present study, we address seasonal patterns in the intestinal parasite community of sympatric Arctic charr ( Salvelinus alpinus ) and brown trout ( Salmo trutta ) in subarctic Lake Takvatn, northern Norway, which is ice covered for approximately half the year. Lake Takvatn has been the subject of numerous ecological studies, including fish and food‐web analyses that comprise parasites (see Amundsen & Knudsen, 2009; Amundsen et al, 2013; Amundsen et al, 2019), thus constituting a good basis for seasonal studies addressing intestinal parasites. The intestinal parasites of Arctic charr and brown trout are typically transmitted trophically with diet as an important predictor of their community composition (Curtis, Bérubé, & Stenzel, 1995; Knudsen et al, 2008; Kuhn, Knudsen, et al, 2016).…”

High parasite diversity in the amphipodGammarus lacustrisin a subarctic lake

“…Although arctic and subarctic ecosystems are generally species poor (Hoberg et al, 2012), which suggests a low diversity of parasites (Hechinger & Lafferty, 2005), recent studies show a surprisingly high taxonomic diversity of trematode parasites in Takvatn (Soldánová et al, 2017) and Icelandic lakes (Blasco-Costa et al, 2014). Three decades of comprehensive studies of the parasite communities of snails and fishes in Takvatn indicate that G. lacustris should harbor parasites that use both fishes and birds as final hosts, owing to the lack of other potential final host taxa in the ecosystem (Amundsen et al, 2009(Amundsen et al, , 2019Knudsen et al, 1999). The main objectives of the present study were to: i) describe the parasite community of G. lacustris in a subarctic lake, and ii) describe and contrast the abundance of parasites that use fishes and birds as final hosts.…”