Külli Kangur

Rapid and highly variable warming of lake surface waters around the globe

In this first worldwide synthesis of in situ and satellite-derived lake data, we find that lake summer surface water temperatures rose rapidly (global mean = 0.34°C decade−1) between 1985 and 2009. Our analyses show that surface water warming rates are dependent on combinations of climate and local characteristics, rather than just lake location, leading to the counterintuitive result that regional consistency in lake warming is the exception, rather than the rule. The most rapidly warming lakes are widely geographically distributed, and their warming is associated with interactions among different climatic factors—from seasonally ice-covered lakes in areas where temperature and solar radiation are increasing while cloud cover is diminishing (0.72°C decade−1) to ice-free lakes experiencing increases in air temperature and solar radiation (0.53°C decade−1). The pervasive and rapid warming observed here signals the urgent need to incorporate climate impacts into vulnerability assessments and adaptation efforts for lakes.

A global database of lake surface temperatures collected by in situ and satellite methods from 1985–2009

Global environmental change has influenced lake surface temperatures, a key driver of ecosystem structure and function. Recent studies have suggested significant warming of water temperatures in individual lakes across many different regions around the world. However, the spatial and temporal coherence associated with the magnitude of these trends remains unclear. Thus, a global data set of water temperature is required to understand and synthesize global, long-term trends in surface water temperatures of inland bodies of water. We assembled a database of summer lake surface temperatures for 291 lakes collected in situ and/or by satellites for the period 1985–2009. In addition, corresponding climatic drivers (air temperatures, solar radiation, and cloud cover) and geomorphometric characteristics (latitude, longitude, elevation, lake surface area, maximum depth, mean depth, and volume) that influence lake surface temperatures were compiled for each lake. This unique dataset offers an invaluable baseline perspective on global-scale lake thermal conditions as environmental change continues.

Nutrients and phytoplankton in Lake Peipsi during two periods that differed in water level and temperature

Data for the vegetation periods (May–November) of 1985–2003 were used to collate the nutrient content and biomass of the most important phytoplankton groups in Lake Peipsi (Estonia). Two periods differing in external nutrient load and water level were compared by analysis of variance. The years 1985–1988 were characterized by the highest loads of nitrogen and phosphorus, high water level and cool summers. The years 2000–2003 were distinguished by low or medium water levels and warm summers. The first period showed statistically significantly higher values of total nitrogen (Ntot) and a higher Ntot:Ptot mass ratio. The second period showed a higher content of total phosphorus (Ptot), a higher ratio of dissolved inorganic compounds N to P and higher phytoplankton and cyanobacterial biomasses. Comparison between parts of the lake demonstrated that the differences between the two periods were more evident in the shallower and strongly eutrophic parts, Lake Pihkva and Lake Lämmijärv, than in the largest and deepest part, the moderately eutrophic Lake Peipsi s.s. Temperature and water level acted synergistically and evidently influenced phytoplankton via nutrients, promoting internal loading when the water level was low and the temperature high. The effect of water level was stronger in the shallowest part, Lake Pihkva. The difference in Ptot content between the southern and northern parts was twofold; the Ntot:Ptot mass ratio was significantly lower in the southern parts, and phytoplankton biomass (particularly the biomass of cyanobacteria) was significantly higher for Lake Pihkva and Lake Lämmijärv than for Lake Peipsi s.s.

The role of temperature in the population dynamics of smelt Osmerus eperlanus eperlanus m. spirinchus Pallas in Lake Peipsi (Estonia/Russia)

We analysed lake smelt (Osmerus eperlanus eperlanus m. spirinchus Pallas.) population dynamics in relation to water level and temperature in Lake Peipsi, Estonia/Russia, using commercial fishery statistics from 1931 to 2004 (excluding 1940–1945). Over this period, smelt provided the greatest catch of commercial fish although its stock and catches have gradually decreased. At times, catches of smelt were quite variable with a cyclic character. Disappearance of smelt from catches in years 1973–1975 was the result of summer fish kill. Regression analysis revealed a significant negative effect of high temperature on the abundance of smelt stock, while the effect of water level was not significant. Our results suggest that critical factors for the smelt population are the absolute value of water temperature in the hottest period (≥20°C) of summer and the duration of this period. These weather parameters have increased in synchrony with smelt decline during the last 7 decades. There appeared to be a significant negative effect of hot summers on the abundance of smelt operating with a lag of one and 2 years, which can be explained by the short life cycle (mainly 1–2 years) of this species.

Condition and growth of ruffe Gymnocephalus cernuus (L.) in two large shallow lakes with different fish fauna and food recourse

Ruffe is one of the most numerous fishes in Estonian large Lakes Peipsi (area 3555 km2, mean depth 7.1 m) and Võrtsjärv (270 km2, 2.8 m). Owing to its small size ruffe has little value for commercial or recreational fisheries today. However, due to their high abundance, ruffe play an important role in the food webs of both lakes. The aim of the investigation was to establish the proportion of ruffe in the fish community and to assess the wellbeing of this fish in the two lakes on the basis of growth rate and condition factor. According to experimental catches made in autumn 1998–2002, the average total catch of fishes per trawl-hour (CPUE) in L. Peipsi was about two times higher than in L. Võrtsjärv (204 kg and 105 kg, respectively). CPUE of ruffe made up 16 kg in L. Peipsi and 7 kg in L. Võrtsjärv. In these lakes the fish community composition and food resource appear to be different. Benthivorous fishes were dominating in L. Võrtsjärv and the biomass of macrozoobenthos was significantly lower in this lake than in L. Peipsi. In the first year, the growth rate of ruffe was higher in L. Peipsi where the state of zooplankton was better compared with L. Võrtsjärv. Differences in the linear growth rate of older fishes in L. Peipsi and L. Võrtsjärv were not significant. The lower condition factor of ruffe in L. Võrtsjärv reflects the limited food resource of abundant benthivorous fishes in this lake. Different trophic relationships in Estonian large lakes are better reflected in the ruffes condition factor than in its linear growth rate.

A comparative study on the feeding of eel, Anguilla anguilla (L.), bream, Abramis brama (L.) and ruffe, Gymnocephalus cernuus (L.) in Lake Võrtsjärv, Estonia

The stomach contents of eel, bream and ruffe were examined in order to detect the diet overlap and possible food competition between these benthophagous fishes in the eutrophic Lake Võrtsjärv. The frequency of occurrence, mean number and mean restored weight of food items per individual were used for the description of their diet. Overlap in diet of bream (standard length >15 cm), eel (SL>30 cm) and ruffe (SL>4.5 cm) appeared mostly in respect of Chironomus plumosus(L.) larvae and pupae, which may lead to food competition between these fishes. However, these fishes can eat different chironomid instars. Unlike eel, who takes mainly large fourth-instar larvae and pupae of C. plumosus, ruffe and bream eat, in addition, small third- and second-instar larvae of this species as well as other small-sized chironomids, mainly Einfeldia carbonaria(Meig.), Microchironomus tener(Kieff.) and Procladiusspp. Obviously, even if the overlap was pronounced, serious competitive interactions would arise only in case of limited food resources in the years with a low biomass of C. plumosus. A significant correlation (r= 0.88; P< 0.05) was found between the mean biomass of fourth-instar larvae of C. plumosusin the lake sediment and in the stomach of ruffe in different years. The diet composition of ruffe was more diverse (including a wide range of invertebrates, fish ova and detritus) than that of eel and bream. Evidently, omnivorous mode of feeding provides ruffe with an advantage over other benthophagous fishes living in the lake. Unlike eel and bream, ruffe continued to take food in late autumn and winter when water temperature in L. Võrtsjärv decreased to 2–0.2°C.

Modelling lake-water photochemistry: Three-decade assessment of the steady-state concentration of photoreactive transients (OH, CO3- and 3CDOM∗) in the surface water of polymictic Lake Peipsi (Estonia/Russia)

Over the last 3-4 decades, Lake Peipsi water (sampling site A, middle part of the lake, and site B, northern part) has experienced a statistically significant increase of bicarbonate, pH, chemical oxygen demand, nitrate (and nitrite in site B), due to combination of climate change and eutrophication. By photochemical modelling, we predicted a statistically significant decrease of radicals ·OH and CO3(-·) (site A, by 45% and 35%, respectively) and an increase of triplet states of chromophoric dissolved organic matter ((3)CDOM(∗); site B, by ∼25%). These species are involved in pollutant degradation, but formation of harmful by-products is more likely with (3)CDOM(∗) than with ·OH. Therefore, the photochemical self-cleansing ability of Lake Peipsi probably decreased with time, due to combined effects of climate change and eutrophication. In different environments (e.g. Lake Maggiore, NW Italy), ecosystem restoration policies had the additional advantage of enhancing sunlight-driven detoxification, suggesting that photochemical self-cleansing would be positively correlated with lake water quality.

Combining limnological and palaeolimnological approaches in assessing degradation of Lake Pskov

Limnological monitoring data and palaeorecords from large shallow Lake Pskov were used to study the long-term dynamic pattern of the lake ecosystem and to identify the start of its degradation. Lake Pskov is the southern part of Lake Peipsi s.l., the largest transboundary lake in Europe. The limnological monitoring data collected in the years 1956–2005 show that the water quality of L. Pskov has deteriorated and caused adverse changes in the whole ecosystem (e.g. excessive growth of algae, increased cyanobacterial blooms, silting of the lake bottom, fish kills). Doubled total phosphorus (Ptot), dissolved inorganic P (PO4-P) and chlorophyll a (Chl-a) contents, increased total alkalinity (HCO3 −) and pH, as well as decline in water transparency and oxygenation conditions, indicate a clear increase in the trophic level of the lake. However, the limnological studies do not show when the degradation started. To understand long-term dynamics of the lake ecosystem, a 52 cm sediment core taken from one monitoring station of L. Pskov was studied and dated by the 210Pb method. Palaeodata show that substantial changes in the L. Pskov ecosystem started already in the 1930s when mesotrophic conditions in the lake turned increasingly eutrophic. Since that time, the content of P, nitrogen (N), carbon (C) and relative abundance (RA%) of planktonic diatoms in the sediment have increased significantly (P 0.9 or r <-0.9, P < 0.001) were observed between the contents of P, N, C and sulphur (S) in the sediment and the mean water level (WL) and temperature (WT) registered one to five years earlier in the lake. The C content of the sediment showed a negative relationship with WL and a positive correlation with WT. A significant positive relationship was recorded also between WT, and N and P content in the sediment. It is hypothesized that a higher WT and lower WL result in an increased organic mater accumulation in the sediment in the coming years.

Lake Peipsi: Changes in nutrient elements and plankton communities in the last decade

In the 1990s, as a consequence of a decline in agricultural production in the watershed and a decrease in the amount of waste water discharged into rivers, the nutrient load carried from the catchment area into large, shallow, eutrophic Lake Peipsi (area 3,555 km 2 ) decreased. The aim of the present study was to analyze the in-lake response of key physical and chemical variables and the biota to large-scale changes in the nutrient load. Yearly changes in water transparency, nutrient elements of surface water, chlorophyll a content, as well as in phyto- and zooplankton biomass were studied during the growth season of 1992-2000. A clear decline in total nitrogen, ammonium ion, and orthophosphate ions was revealed in the northern part of the lake over the studied years. Beginning from 1996-1997, a decreasing tendency was revealed for the nitrate ion and total phosphorus. A significant decline of the total nitrogen:total phosphorus ratio was also observed from 1992-2000. However, the biomass of phytoplankton (particularly cyanobacteria) and chlorophyll a concentration did not follow the dynamics of nutrients, but displayed an increasing trend. Concentrations of nutrients in the lake during the last decade were not so low as to limit phytoplankton growth directly. In Lake Peipsi, strong and long-lasting algal blooms were observed in recent years, despite a definite decline in the nutrient content of surface water. Weather conditions appear to be very important factors in causing algal blooms in Lake Peipsi.

Widespread increases in iron concentration in European and North American freshwaters

Recent reports of increasing iron (Fe) concentrations in freshwaters are of concern, given the fundamental role of Fe in biogeochemical processes. Still, little is known about the frequency and geographical distribution of Fe trends or about the underlying drivers. We analyzed temporal trends of Fe concentrations across 340 water bodies distributed over 10 countries in northern Europe and North America in order to gain a clearer understanding of where, to what extent, and why Fe concentrations are on the rise. We found that Fe concentrations have significantly increased in 28% of sites, and decreased in 4%, with most positive trends located in northern Europe. Regions with rising Fe concentrations tend to coincide with those with organic carbon (OC) increases. Fe and OC increases may not be directly mechanistically linked, but may nevertheless be responding to common regional-scale drivers such as declining sulfur deposition or hydrological changes. A role of hydrological factors was supported by covarying trends in Fe and dissolved silica, as these elements tend to stem from similar soil depths. A positive relationship between Fe increases and conifer cover suggests that changing land use and expanded forestry could have contributed to enhanced Fe export, although increases were also observed in nonforested areas. We conclude that the phenomenon of increasing Fe concentrations is widespread, especially in northern Europe, with potentially significant implications for wider ecosystem biogeochemistry, and for the current browning of freshwaters.