Ecology under lake ice.

Winter conditions are rapidly changing in temperate ecosystems, particularly for those that experience periods of snow and ice cover. Relatively little is known of winter ecology in these systems, due to a historical research focus on summer 'growing seasons'. We executed the first global quantitative synthesis on under-ice lake ecology, including 36 abiotic and biotic variables from 42 research groups and 101 lakes, examining seasonal differences and connections as well as how seasonal differences vary with geophysical factors. Plankton were more abundant under ice than expected; mean winter values were 43.2% of summer values for chlorophyll a, 15.8% of summer phytoplankton biovolume and 25.3% of summer zooplankton density. Dissolved nitrogen concentrations were typically higher during winter, and these differences were exaggerated in smaller lakes. Lake size also influenced winter-summer patterns for dissolved organic carbon (DOC), with higher winter DOC in smaller lakes. At coarse levels of taxonomic aggregation, phytoplankton and zooplankton community composition showed few systematic differences between seasons, although literature suggests that seasonal differences are frequently lake-specific, species-specific, or occur at the level of functional group. Within the subset of lakes that had longer time series, winter influenced the subsequent summer for some nutrient variables and zooplankton biomass.

Can submerged macrophytes influence turbidity and trophic state in deep lakes? Suggestions from a case study.

Feedback between submerged macrophytes and water transparency stabilizing the clear, macrophyte-dominated regime has been described so far for shallow lakes. Based on data of total phosphorus (TP) concentrations, underwater light supply, phytoplankton and submerged macrophyte abundance from narrow, stratified Lake Scharmützelsee (mean depth: 9 m, retention time: 16 yr) of the period 1994-2006 we hypothesize that submerged macrophytes may influence transparency and trophic state in deep lakes. The lake was characterized by summer epilimnion TP concentrations of 38 to 57 mug L(-1), turbid water due to mass development of cyanobacteria, and low abundance of few submerged macrophyte species until 2003. Thereafter, a sudden increase in water transparency was followed by a rapid submerged macrophyte colonization of the littoral down to about 5 m depth corresponding to the depth of a light supply of 3 E m(-2) d(-1). Initially, this recolonization was probably a consequence of decreased turbidity. We argue that the increase of submerged macrophyte coverage from < 10% in 1994 to 2003 to about 24% in 2005-2006 has contributed to the stabilization of the clear-water regime during the subsequent years. This is supported by the fact that earlier shifts to clear-water regimes in 1994 and 2000 without a significant spread of submerged macrophytes were not stable. We discuss potential mechanisms that may have resulted in a positive effect of plants on transparency such as P uptake and immobilization by the dominant rootless macrophyte species Nitellopsis obtusa and Ceratophyllum demersum and other macrophyte-related mechanisms such as increased zooplankton grazing and allelopathy.

Causal relationships of Raphidiopsis (formerly Cylindrospermopsis) dynamics with water temperature and N:P-ratios: A meta-analysis across lakes with different climates based on inferential modelling.

Raphidiopsis raciborskii is a tropical toxic cyanobacterium that is rapidly expanding to diverse lake habitats in different climate zones by sophisticated adaptation mechanisms. This meta-analysis investigated correlations of R. raciborskii with water temperature and N:P (nitrogen to phosphorus)-ratios across four lakes with different climates and trophic states by means of long-term time series and the hybrid evolutionary algorithm HEA. The results have shown that in the lakes with temperate and Mediterranean climate, R. raciborskii is strongly correlated with water temperature since germination and growth rely on rising water temperatures in spring. In contrast, there was a weaker correlation with water temperature in subtropical and tropical lakes where pelagic populations of R. raciborskii are overwintering, and are present all year round. However, the highest abundances of R. raciborskii coincided with highest water temperature for the Mediterranean, subtropical and tropical lakes, whilst in the temperate Langer See the highest abundances of R. raciborskii occurred at 24.1 °C, even though temperatures of up to 27 °C were recorded in 2013 and 2014. The correlation of R. raciborskii with N:P-ratios proved to be strongest for the meso- to eutrophic Lake Kinneret (r2 = 0.8) and lowest for the eutrophic Lake Paranoa (r2 = 0.16). However, the assumption has been confirmed that R. raciborskii is growing fastest when waters are N-limited regardless of trophic states. In terms of phenology, the temperate and Mediterranean lakes displayed "fastest growth" in spring and early summer. In contrast, the growing season in subtropical and tropical lakes lasted from spring to autumn most likely because of overwintering populations, and growing importance of direct and indirect biotic regulating factors such as competition, grazing, remineralisation of nutrients along warming climate. In order to carry out a meta-analysis of time series across four different lakes, HEA served as powerful tool resulting in inferential models with predictive capacity for population dynamics of R. raciborskii just driven by water temperature or N:P-ratios, whilst coefficients of determination r2 served as criteria for hypotheses testing.

Seasonal dynamics of cylindrospermopsin and cyanobacteria in two German lakes.

Cylindrospermopsin (CYN) is a potent hepatotoxin produced by different cyanobacteria of the order Nostocales. Questions of major concern are: which species produce CYN, which are the seasonal patterns of CYN dynamics and how are they regulated? Therefore, we studied for the first time the seasonal dynamics of particulate and dissolved CYN concentrations, cyanobacterial abundance and environmental factors in two German lakes over 2 years. Total CYN reached maximum concentrations of 0.34 and 1.80 microg L(-1) in Melangsee and Langer See, respectively. In both lakes, maxima of the dissolved CYN fraction occurred later in the season than those of the particulate fraction, and it reached higher concentrations. This indicates that CYN is poorly decomposed and accumulates in the water. The cyanobacterial community in both lakes included several potentially CYN-producing species that did not correlate with CYN concentrations. Significant correlations between the particulate CYN concentrations and species biovolume were only found for Aphanizomenon gracile (r(s)=0.803) in Langer See indicating that this species is a CYN producer. Different correlations of CYN with abiotic factors in the two lakes indicate the presence of further undetected CYN producers as well as different regulation mechanisms of their dynamics and the variability of CYN.

Concentrations of particulate and dissolved cylindrospermopsin in 21 Aphanizomenon-dominated temperate lakes.

The cyanobacterial toxin cylindrospermopsin (CYN) is widely distributed in German lakes, but volumetric data for risk assessment are lacking and it is unclear which cyanobacterial species produce CYN in Europe. We therefore analyzed CYN concentration and cyanobacterial composition of 21 German lakes in 2005. CYN was detected in 19 lakes (102 of 115 samples). In total, 45 samples contained particulate CYN only, and 57 contained both dissolved and particulate CYN. The concentrations were 0.002-0.484 microg L(-1) for particulate CYN and 0.08-11.75 microg L(-1) for dissolved CYN with a maximum of 12.1 microg L(-1) total CYN. A drinking water guideline value of 1 microg L(-1) proposed by Humpage and Falconer [2003. Oral toxicity of the cyanobacterial toxin CYN in male Swiss albino mice: determination of no observed adverse effect level for deriving a drinking water guideline value. Environ. Toxicol. 18, 94-103] was exceeded in 18 samples from eight lakes due to high concentrations of dissolved CYN. CYN occurrence in the German lakes could not be ascribed to the three known CYN-producing species Cylindrospermopsis raciborskii, Anabaena bergii and Aphanizomenon flos-aquae, which were detected in some lakes in low abundances. The highest correlation coefficients were observed between particulate CYN and the native Aphanizomenon gracile. It occurred in 98 CYN-positive samples, was the most abundant Nostocales and was the only Nostocales in five samples. This indicates that A. gracile is a potential CYN producer in German lakes.

Climate change affects timing and size of populations of an invasive cyanobacterium in temperate regions.

Cylindrospermopsis raciborskii, an invasive freshwater cyanobacterium, originated from the tropics but has spread to temperate zones over the last few decades. Its northernmost populations in Europe occur in North German lakes. How such dramatic changes in its biogeography are possible and how its population dynamics in the newly invaded habitats are regulated are still unexplained. We therefore conducted a long-term (1993-2005) study of two German lakes to elucidate the mechanisms behind C. raciborskii population dynamics and to identify the abiotic constraints on its development. Our data revealed that pelagic populations of C. raciborskii thrived for three months during the summer, contributing up to 23% of the total cyanobacteria biovolume. Population sizes varied greatly between years without exhibiting any distinct long-term trends. In the annual lifecycle, C. raciborskii filaments emerged in the pelagic habitat when the temperature rose above 15-17 degrees C. At that time, mean photosynthetically active radiation in the mixed water column (I (mix)) overstepped its maximum. Rates of population net increase were highest at the beginning of the season (0.15-0.28 day(-1)), declined continuously over time, and were significantly positively correlated with I (mix). This indicates that the onset of the pelagic population is temperature-mediated and that I (mix) controls its growth. Since I (mix) peaks before the population onset, the time of germination is of crucial importance for successful development. To test this hypothesis, we designed a model to simulate pelagic population size, starting at different dates in the annual cycle. Moving the population onset forward by 30 days resulted in a doubling of the population size. We therefore conclude that an earlier rise in water temperature associated with climate change has promoted the spread of C. raciborskii to the temperate zone. Earlier warming permits earlier germination, thereby shifting the pelagic populations to a phase with higher I (mix), which advances growth and the population establishment.

Occurrence of the cyanobacterial toxin cylindrospermopsin in northeast Germany.

The frequent occurrence of the cyanobacterial toxin cylindrospermopsin (CYN) in the (sub)tropics has been largely associated with cyanobacteria of the order Nostocales of tropical origin, in particular Cylindrospermopsis raciborskii. C. raciborskii is currently observed to spread northwards into temperate climatic zones. In addition, further cyanobacteria of the order Nostocales typically inhabiting water bodies in temperate regions are being identified as CYN-producers. Therefore, data on the distribution of CYN in temperate regions are necessary for a first assessment of potential risks due to CYN in water used for drinking and recreation. A total of 127 lakes situated in the north-eastern part of Germany were investigated in 2004 for the presence of the toxin CYN and the phytoplankton composition. The toxin could be detected in half of the lakes (n = 63) and in half of 165 samples (n = 88). Concentrations reached up to 73.2 microg CYN/g DW. CYN thus proved more widely distributed than previously demonstrated. The analyses of phytoplankton data suggest Aphanizomenon sp. and Anabaena sp. as important CYN producers in Germany, and confirm recent findings of Aphanizomenon flos-aquae as CYN-producing species frequently inhabiting water bodies in temperate climatic regions. The data shown here suggest that CYN may be an important cyanobacterial toxin in German water bodies and that further data are needed to assess this.