Katrin Teubner

Cyanobacterial dominance in lakes

Cyanobacterial dominance in lakes has received much attention in the past because of frequent bloom formation in lakes of higher trophic levels. In this paper, underlying mechanisms of cyanobacterial dominance are analyzed and discussed using both original and literature data from various shallow mixed and deep stratifying lakes from temperate and (sub)tropical regions. Examples include all four ecotypes of cyanobacteria sensu Mur et al. (1993), because their behavior in the water column is entirely different. Colony forming species (Microcystis) are exemplified from the large shallow Tai Hu, China. Data from a shallow urban lake, Alte Donau in Austria are used to characterize well mixed species (Cylindrospermopsis), while stratifying species (Planktothrix) are analyzed from the deep alpine lake Mondsee. Nitrogen fixing species (Aphanizomenon) are typified from a shallow river-run lake in Germany. Factors causing the dominance of one or the other group are often difficult to reveal because several interacting factors are usually involved which are not necessarily the same in different environments. Strategies for restoration, therefore, depend on both the cyanobacterial species involved and the specific causing situation. Some uncertainty about the success of correctives, however, will remain due to the stochastic nature of the events and pathways leading to cyanobacterial blooms. Truly integrated research programs are required to generate predictive models capable of quantifying key variables at appropriate spatial and temporal scales.

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.

Changes of nutrients and phytoplankton chlorophyll-a in a large shallow lake, Taihu, China: an 8-year investigation

Inter-annual changes of total nitrogen (TN) and total phosphorus (TP) and phytoplankton chlorophyll-a (Chla) in a large shallow lake, Taihu China, were analysed using the monthly monitoring data covering the period of 1991–1999. The concentrations of TN, TP and Chla showed marked gradients in the lake from high values near the northern river input in the inner Meiliang Bay towards lower concentrations the lake centre. TN was always much greater than TP (≫7:1 by weight) indicating that nitrogen was not a limiting factor in the lake. Annual averages of TN, TP and Chla increased until 1996 declining thereafter possibly because of the controlling of wastewater discharged from the catchment area by the local government. The internal nutrient loading in the lake was low because of the well-mixed condition and the low organic content of superficial sediment. Our results suggest that the recovery process in the lake might have started in 1996.

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.

Steady state phytoplankton in a deep pre-alpine lake: species and pigments of epilimnetic versus metalimnetic assemblages

The vertical distribution pattern of algal species, chlorophylls and specific carotenoids present in the dimictic pre-alpine Ammersee (Bavaria, Germany) are given for the year 2001. A detailed taxonomic list of the phytoplankton species is recorded, along with light micrographs and detailed descriptions of the flagellates. A deep chorophyll maximum, mainly built by Planktothrix rubescens, was common in this deep mesotrophic lake. The three most dominant species among 83 identified taxa alternated seasonally and reached significant biovolumes in both the epi- and the metalimnion (Planktothrix rubescens > Ceratium hirundinella > unicellular centric diatoms > Asterionella formosa > Fragilaria spp. > Anabaena lemmermannii > Phacotus lenticularis and less frequent dominant was Rhodomonas minuta). We define a steady state phytoplankton assemblage in Ammersee as a stable community in terms of species composition and standing crop. The stability of species composition was measured by Bray-Curtis similarity between monthly samples and indicate the change of individual biovolumes of species from month to month. The stability of standing crop was evaluated by the net change of total biovolume for the same time intervals. Focussing on steady state phytoplankton assemblages we compared three spatially heterogeneous environments for vertical niche separation within the top 12 m: the euphotic epilimnion (2 and 5 m), the euphotic metalimnion (7 m) and the metalimnion below the euphotic zone with dim-light less than 1% (10 and 12 m). The definition of a steady state assemblage on both ends hold true only for metalimnetic layers at dim-light levels below 1% in Ammersee. At this metalimnetic layer more than 80% similarity in phytoplankton composition between successive monthly samples was reached, associated with almost zero net changes of total biovolume only. The greater the contribution of the three most dominant taxa to biovolume, the higher were the Bray-Curtis similarities at metalimnetic depths below the euphotic zone. Zooplankton biomass had very little effect on species assemblages in the metalimnion, while parameters related to stratification (Schmidt stability) as well as those of trophy (TP, Chl) correlated with species changes. The similarity values between successive monthly samples from all the euphotic layers never reached more than 60% and were usually significantly lower, even if biovolume net changes were around zero. Both the high fluctuations of the ratio of photosynthetic versus photo-protective carotenoids (PSC:PPC) and the statistical significance of correlations between the change of species and environmental-biotic parameters separate the euphotic layer of the top 7 m as a homogenous community from deeper strata. At all sampling depths within this euphotic zone the increase of sunshine duration was associated with an increase of the carotenoid ratio PSC:PPC, but no relationship was found for the deeper layers. The change of species in the euphotic layer was not significantly related to thermal stability, TP or the dominance structure of phytoplankton, but linked with the zooplankton biomass and therefore seemed to be top-down controlled. From our observations, we can conclude that only during stratification and only in the metalimnion below the euphotic zone steady state assemblages can be expected in the deep mesotrophic Ammersee.

The Impact of Climate Change on Lakes in Central Europe

◦by 2071‐2100. The associated projections for the rainfall give even more cause for concern with the reductions in some areas approaching 50% in summer. In this chapter we analyse impacts of changing weather conditions on lakes in Central Europe. Long-term data sets from a number of lakes are used to link measured variables to climate signals. Particular attention is paid to the lakes in the perialpine region which are known to be very sensitive to short-term changes in the weather (Psenner, 2003; Thompson et al., 2005). Here, the topography and the steep orography enhance the water cycle, and result in flooding, debris flows, avalanches, vertical plant migration etc. The Alps also form a barrier to the mass movement of air and are responsible for the sharp climatic divide between Atlantic, Continental and Mediterranean influences. Central Europe is a variously and vaguely defined region. Rather than a physical entity, it is more a reflection of a shared history. The results summarized here are based on the analysis of long-term climatological and limnological data from the countries shown in Fig. 20.1. These include Germany (DE), Poland (PL), the Czech Republic (CZ), Slovakia (SK), Switzerland (CH), Lichtenstein (LI), Austria (AT) and Hungary (HU). The Central European countries are geographically diverse with landforms ranging from the North-German Lowlands, through the Alps to the Hungarian plain. The pannonian plain in the eastern part is also a major climatic ‘crossing point’ and is affected by the Eastern-European continental, the WesternEuropean oceanic and the Mediterranean influence.

The Impact of Variations in the Climate on Seasonal Dynamics of Phytoplankton

Phytoplankton, an assemblage of suspended, primarily autotrophic single cells and colonies, forms part of the base of the pelagic food chain in lakes. The responses of phytoplankton to anthropogenic pressures frequently provide the most visible indication of a long-term change in water quality. Several attributes related to the growth and composition of phytoplankton, such as their community structure, abundance as well as the frequency and the intensity of blooms, are included as indicators of water quality in the Water Framework Directive. The growth and seasonal succession of phytoplankton is regulated by a variety of external as well as internal factors (Reynolds et al., 1993; Reynolds, 2006). Among the most important external factors are light, temperature, and those associated with the supply of nutrients from point and diffuse sources in the catchment. The internal factors include the residence time of the lakes, the underwater light regime and the mixing characteristics of the water column. The schematic diagram (Fig. 14.1) shows some of the ways in which systematic changes in the climate can modulate these seasonal and inter-annual variations. The effects associated with the projected changes in the rainfall are likely to be most pronounced in small lakes with short residence times (see George et al., 2004 for some examples). In contrast, those connected with the projected changes in irradiance and wind mixing, are likely to be most important in deep, thermally stratified lakes.

Eutrophication and Climate Change: Present Situation and Future Scenarios

Anthropogenic eutrophication still is a major threat to inland waters in large parts of the world although re-oligotrophication has largely progressed in several regions. Climate change now poses a new risk effectively endangering inland waters. Impacts of climate change on inland waters are already well documented and established. Direct effects are mainly through changes in temperature and associated physico-chemical alterations. Indirect effects mediated through processes such as extreme weather events in the catchment include increased nutrient loading among others. The combination of all impacts will lead to severe and significant changes in the physical structure and the biological configuration of the waters depending on future climate scenarios. Since water quality and quantity will be affected, concern is necessary. Possible adaptation and mitigation mechanisms are discussed.

Does pigment composition reflect phytoplankton community structure in differing temperature and light conditions in a deep alpine lake? An approach using HPLC and delayed fluorescence techniques1

In vivo delayed fluorescence (DF) and HPLC/CHEMTAX pigment analyses were used to investigate seasonal and depth distributions of phytoplankton in a deep alpine mesotrophic lake, Mondsee (Austria). Using chl a equivalents, we determined significant relationships with both approaches. Community structure derived from pigment ratios of homogenous samples was compared with microscopic estimations using biovolume conversion factors. An advantage of the HPLC/CHEMTAX method was that it gave good discrimination among phytoplankton groups when based on a pigment ratio matrix derived from multiple regression analysis. When a single algal group was dominant, such as epilimnetic diatoms or hypolimnetic cyanobacteria in the deep chl maxima, HPLC/CHEMTAX results were significantly correlated with microscopic estimations (diatoms: r = 0.93; cyanobacteria: r = 0.94). Changes in the composition of photosynthetically active pigments were investigated with DF and benefited from excitation spectra that considered all light‐harvesting pigments, which made it possible to assess the enhancement of accessory photosynthetically active pigments relative to active chl a (chl aDF672). Changes in similarity index, based on normalized DF spectra, confirmed compositional shifts observed by microscopy. At chosen wavelengths of DF spectra, 534 and 586 nm, we generally observed a significantly inverse relationship between normalized DF intensities and temperature and light along both seasonal and depth gradients. The relative increase in photosynthetically active pigments other than chl aDF672 under low light and temperature was caused by an increasing dominance of diatoms and/or phycobilin‐rich cyanobacteria and Cryptophyta. DF spectra provided a more accurate picture of community pigments acclimated to light and temperature conditions than the β‐carotene:chl a ratio derived from HPLC.

Photosynthetic efficiency as a function of thermal stratification and phytoplankton size structure in an oligotrophic alpine lake

Allometric relationships of phytoplankton communities were studied on the basis of a five-year data-set in a deep oligotrophic alpine lake in Austria. The seasonal phytoplankton succession in Mondsee is characterised by diatoms during winter mixing and a distinct metalimnetic population of Planktothrix rubescens during stratification in summer. The variation of phytoplankton photosynthetic efficiency between seasons was assessed using in situ carbon-uptake rates (5 years data) and Fast Repetition Rate Fluorometry (FRRF) (2 years data). The light-saturated, chlorophyll-specific rate of photosynthesis (P*max), irradiance at the onset of saturation (Ek) and maximum light-utilisation efficiency (α*) were determined for winter mixing and summer stratification. Fluorescence-based parameters as the functional absorption cross section of Photosystem II (σPSII) and the photochemical quantum yield (Fv/Fm) were additionally analysed in 2003 and 2004 to study the underlying physiological mechanisms for the variability in photosynthetic performance. Beyond their sensitivity to changing environmental conditions like thermal stratification, phytoplankton populations differ in their photosynthetic behaviour according to their size structure. Therefore Photosynthesis vs. Irradiance (P/E)-relationships were analysed in detail within a 1-year period from size fractionated cell counts, chlorophyll-a and carbon-uptake.