In summer 2000, a research team from the Faroe Islands, Denmark, Iceland, Norway and Sweden undertook a biological investigation of five Faroese lakes. The purpose of the investigation was to gain more knowledge of the biology and ecology of the lakes. The five study lakes were Eystara Mjaavatn, Leynavatn and Saksunarvatn on the island of Streymoy, Sorvagsvatn/Leitisvatn on the island of Vagar and Toftavatn on the island of Eysturoy (Fig. 1). The lakes differ in morphometry, physical nature and biological community structure and vary in size from 0.03 to 3.56 km2, maximum depth ranging from 7 to 59 m and catchment area from 1.8 to 35.2 km2. Introduction There are many small and medium sized lakes in the Faroe Islands, located both in the low-lying valleys and in the hills and mountains (Rasmussen, 1982). The largest lakes are found on the island of Vagar, but most lakes are found on Suðuroy, Sandoy, Streymoy and Eysturoy. Some of the lakes are of glacial origin, the results of erosion by ice in valleys or in the highland basalt. Others are of tectonic origin, and yet others are inland lagoons (Rasmussen, 1982; Mortensen, 2002).
Abstract Fresh waters make a disproportionately large contribution to greenhouse gas ( GHG ) emissions, with shallow lakes being particular hot spots. Given their global prevalence, how GHG fluxes from shallow lakes are altered by climate change may have profound implications for the global carbon cycle. Empirical evidence for the temperature dependence of the processes controlling GHG production in natural systems is largely based on the correlation between seasonal temperature variation and seasonal change in GHG fluxes. However, ecosystem‐level GHG fluxes could be influenced by factors, which while varying seasonally with temperature are actually either indirectly related (e.g. primary producer biomass) or largely unrelated to temperature, for instance nutrient loading. Here, we present results from the longest running shallow‐lake mesocosm experiment which demonstrate that nutrient concentrations override temperature as a control of both the total and individual GHG flux. Furthermore, testing for temperature treatment effects at low and high nutrient levels separately showed only one, rather weak, positive effect of temperature ( CH 4 flux at high nutrients). In contrast, at low nutrients, the CO 2 efflux was lower in the elevated temperature treatments, with no significant effect on CH 4 or N 2 O fluxes. Further analysis identified possible indirect effects of temperature treatment. For example, at low nutrient levels, increased macrophyte abundance was associated with significantly reduced fluxes of both CH 4 and CO 2 for both total annual flux and monthly observation data. As macrophyte abundance was positively related to temperature treatment, this suggests the possibility of indirect temperature effects, via macrophyte abundance, on CH 4 and CO 2 flux. These findings indicate that fluxes of GHG s from shallow lakes may be controlled more by factors indirectly related to temperature, in this case nutrient concentration and the abundance of primary producers. Thus, at ecosystem scale, response to climate change may not follow predictions based on the temperature dependence of metabolic processes.
Much information can be gained from the net ecosystem production (NEP) of freshwater lakes, and NEP has attracted new interest due to climate change, which may change the importance of freshwaters as a source and sink of CO 2 . Direct measurement of NEP in freshwater lakes is, however, time‐consuming, and high frequency monitoring of diel variations in oxygen levels for metabolism estimation has only recently been commonly employed. However, midday snap‐shot oxygen data is available from numerous monitoring programs worldwide, occasionally covering decades. We hypothesize that midday oxygen saturation levels may provide information on NEP in lakes as oxygen super‐saturation, indicative of high NEP, is observed in very productive lakes, whereas very low oxygen levels may occur in lakes with great input of organic matter or in lakes experiencing sudden decline in primary producers, indicative of low NEP. By analysis of a high frequency dataset encompassing 24 fully mixed mesocosms with contrasting trophic states and temperatures, we show that midday oxygen saturation provides a reasonable description of daily NEP only marginally affected by trophic state, temperature, and season. Oxygen sampling conducted in the afternoon gave a slightly better prediction than at midday, whereas predictions based on NEP representing an average of the previous 3 days led to a 2‐fold increase in R 2 . Moreover, an analysis of high frequency data sampled in a shallow Danish lake suggests that the method is transferable to natural shallow lakes. This method may therefore allow estimation of NEP based on oxygen measurements available from monitoring programs.
Predators play a key role in the functioning of shallow lakes. Differences between the response of temperate and subtropical systems to fish predation have been proposed, but experimental evidence is scarce. To elucidate cascading effects produced by predators in contrasting climatic zones, we conducted a mesocosm experiment in three pairs of lakes in Uruguay and Denmark. We used two typical planktivorous-omnivorous fish species (Jenynsia multidentata + Cnesterodon decemmaculatus and Gasterosteus aculeatus + Perca fluviatilis) and one littoral omnivorous-predatory macroinvertebrate (Palaemonetes argentinus and Gammarus lacustris), alone and combined, in numbers resembling natural densities. Fish predation on zooplankton increased phytoplankton biomass in both climate zones, whereas the effects of predatory macroinvertebrates on zooplankton and phytoplankton were not significant in either climate zone. Macroinvertebrates (that freely colonized the sampling devices) were diminished by fish in both climate areas; however, periphyton biomass did not vary among treatments. Our experiments demonstrated that fish affected the structure of both planktonic and littoral herbivorous communities in both climate regions, with a visible positive cascading effect on phytoplankton biomass, but no effects on periphyton. Altogether, fish impacts appeared to be a strong driver of turbid water conditions in shallow lakes regardless of climatic zone by indirectly contributing to increasing phytoplankton biomass.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
'%3e%3cg id='c'%3e%3cg id='b'%3e%3cg id='a'%3e%3cpath stroke-linecap='square' d='m-2.058 8.889 3.01 4.494c-12.367 8.949-4.817 14.24-13.58 17.105 5.45-5.32-.859-5.767 3.733-16.854'/%3e%3cpath fill='none' d='M-4.21 10.163c-6.822 11.245-2.348 17.428-8.419 20.325'/%3e%3cpath d='M0 0h6L0 33-6 0h6v33'/%3e%3c/g%3e%3cuse xlink:href='%23a' transform='rotate(45)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(90)'/%3e%3c/g%3e%3cuse xlink:href='%23c' transform='rotate(180)'/%3e%3ccircle r='11'/%3e%3c/g%3e%3cg transform='scale(.011)'%3e%3cg id='d'%3e%3cpath d='M81-44c-7 8-11-6-36-6S16-35 12-38s21-21 29-22 31 7 40 16m-29 9c7 6 1 19-6 19S26-28 32-36'/%3e%3cpath d='M19-26c1-12 11-14 27-14s23 12 29 15c-7 0-13-10-29-10s-16 0-27 10m3 2c4-6 9 6 20 6s17-3 24-8-10 12-21 12-26-6-23-10'/%3e%3cpath d='M56-17c13-7 5-17 0-19 2 2 10 12 0 19M0 43c6 0 8-2 16-2s27 11 38 7c-23 9-14 3-54 3h-5m63 6c-4-7-3-5-11-16 8 6 10 9 11 16M0 67c25 0 21-5 54-19-24 3-29 11-54 11h-5m5-29c7 0 9-5 17-5s19 3 24 7c1 1-3-8-11-9S25 9 16 7c0 4 3 3 4 9 0 5-9 5-11 0 2 8-4 8-9 8'/%3e%3c/g%3e%3cuse xlink:href='%23d' transform='scale(-1 1)'/%3e%3cpath d='M0 76c-5 0-18 3 0 3s5-3 0-3'/%3e%3c/g%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='scale(-1 1)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(72)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(-72)'/%3e%3cuse xlink:href='%23c' transform='rotate(144)'/%3e%3c/svg%3e)
