Rita Adrian

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.

Environmental stability and lake zooplankton diversity – contrasting effects of chemical and thermal variability

Environmental variability in space and time is a primary mechanism allowing species that share resources to coexist. Fluctuating conditions are a double edged sword for diversity, either promoting coexistence through temporal niche partitioning or excluding species by stochastic extinctions. The net effect of environmental variation on diversity is largely unknown. We examined the association between zooplankton species richness in lakes and environmental variability on interannual, seasonal and shorter time scales, as well as long-term average conditions. We analyzed data on physical, chemical and biological limnology in 53 temperate zone lakes in North America and Europe sampled over a combined 1042 years. Large fluctuations in pH, phosphorus and dissolved organic carbon concentration on different time scales were associated with reduced zooplankton species richness. More species were found in lakes that showed greater temperature variation on all time scales. Environmental variability on different time scales showed similar or, in some cases, stronger associations with zooplankton species richness compared with long-term average conditions. Our results suggest that temporal fluctuations in the chemical environment tend to exclude zooplankton species while temperature variability promotes greater richness. The results indicate that anthropogenic increases in temporal variability of future climates may have profound effects on biodiversity.

Drivers and stressors of freshwater biodiversity patterns across different ecosystems and scales: a review

The present review with focus on the last decade (2000–2010) aims to (i) collecting the major hypotheses explaining freshwater biodiversity patterns, (ii) identifying the main stressors affecting freshwater biodiversity, and (iii) revealing information gaps regarding ecosystem types, organism groups, spatial and temporal scales to highlight research needs to better propose sound conservation measures. The comparative analysis addresses six organism groups ranging from microorganisms to fish in basins, rivers, lakes, wetlands, ponds and groundwater. Short-term studies at ecoregion and catchment scale focusing on invertebrates, macrophytes and fish in Palaearctic and Nearctic regions dominated. The most frequent hypotheses tested were the landscape filter concept, the species–area relationship, the metacommunity concept. Dominating natural drivers were area, heterogeneity and disturbance. Land use, eutrophication and habitat destruction were identified as most important stressors. Generally, freshwater biodiversity declined in response to these stressors in contrast to increasing biodiversity determined by natural drivers across all ecosystems. Preferred organism groups were fish and invertebrates, most frequently studied in rivers, in contrast to smaller organisms (e.g. bacteria) and, e.g. groundwater being underrepresented. Hypotheses originating from the last century are still tested in freshwater research, while novel concepts are either missing or untested. Protection of freshwater biodiversity is the ultimate challenge since it supports valuable ecosystems services ensuring perpetuation of mankind. For that, comprehensive large-scale studies with holistic approaches are urgently needed.

Effects of Climate Warming, North Atlantic Oscillation, and El Niño-Southern Oscillation on Thermal Conditions and Plankton Dynamics in Northern Hemispheric Lakes

Impacts of climate warming on freshwater ecosystems have been documented recently for a variety of sites around the globe. Here we provide a review of studies that report long-term (multidecadal) effects of warming trends on thermal properties and plankton dynamics in northern hemispheric lakes. We show that higher lake temperatures, shorter periods with ice cover, and shorter stagnation periods were common trends for lakes across the hemisphere in response to the warmer conditions. Only for shallow dimictic lakes was it observed that deep-water temperatures decreased. Moreover, it became evident that phytoplankton dynamics and primary productivity altered in conjunction with changes in lake physics. Algal spring blooms developed early and were more pronounced in several European lakes after mild winters with short ice cover periods, and primary productivity increased in North American lakes. Effects of elevated temperatures on zooplankton communities were seen in an early development of various species and groups, as is documented for cladocerans, copepods, and rotifers in European lakes. Furthermore, thermophile species reached higher abundance in warmer years. Obviously, the nature of responses is species specific, and depends on the detailed seasonal patterning of warming. Complex responses such as effects propagating across trophic levels are likely, indicating that observed climate—ecosystem relationships are not generally applicable. Nonetheless, the picture emerges that climate-driven changes in freshwater ecosystems may be synchronised to a certain extent among lakes even over great distances if climatic influences are not masked by anthropogenic impacts or differences in lake morphology. Macro-scale climatic fluctuations — such as the North Atlantic Oscillation or the El Niño-Southern Oscillation — were identified as the most important candidates responsible for such coherence, with the former predominating in Europe and the latter in North America. We emphasise, however, that the driving mechanisms and the future behaviour of these oscillations are rather uncertain, which complicates extrapolation of observed effects into the future. Thus, it is necessary to quantify the most important climate—ecosystem relationships in models of appropriate complexity. Such models will help elucidate the multiple pathways climate affects freshwater ecosystems, and will indicate possible adverse effects of a warmer future climate.

Global impacts of the 1980s regime shift

Abstract Despite evidence from a number of Earth systems that abrupt temporal changes known as regime shifts are important, their nature, scale and mechanisms remain poorly documented and understood. Applying principal component analysis, change‐point analysis and a sequential t‐test analysis of regime shifts to 72 time series, we confirm that the 1980s regime shift represented a major change in the Earths biophysical systems from the upper atmosphere to the depths of the ocean and from the Arctic to the Antarctic, and occurred at slightly different times around the world. Using historical climate model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and statistical modelling of historical temperatures, we then demonstrate that this event was triggered by rapid global warming from anthropogenic plus natural forcing, the latter associated with the recovery from the El Chichón volcanic eruption. The shift in temperature that occurred at this time is hypothesized as the main forcing for a cascade of abrupt environmental changes. Within the context of the last century or more, the 1980s event was unique in terms of its global scope and scale; our observed consequences imply that if unavoidable natural events such as major volcanic eruptions interact with anthropogenic warming unforeseen multiplier effects may occur.

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.

Tube‐dwelling invertebrates: tiny ecosystem engineers have large effects in lake ecosystems

There is ample evidence that tube-dwelling invertebrates such as chironomids significantly alter multiple important ecosystem functions, particularly in shallow lakes. Chironomids pump large water volumes, and associated suspended and dissolved substances, through the sediment and thereby compete with pelagic filter feeders for particulate organic matter. This can exert a high grazing pressure on phytoplankton, microorganisms, and perhaps small zooplankton and thus strengthen benthic-pelagic coupling. Furthermore, intermittent pumping by tube-dwelling invertebrates oxygenates sediments and creates a dynamic, three-dimensional mosaic of redox conditions. This shapes microbial community composition and spatial distribution, and alters microbe-mediated biogeochemical functions, which often depend on redox potential. As a result, extended hotspots of element cycling occur at the oxic-anoxic interfaces, controlling the fate of organic matter and nutrients as well as fluxes of nutrients between sediments and water. Surprisingly, the mechanisms and magnitude of interactions mediated by these organisms are still poorly understood. To provide a synthesis of the importance of tube-dwelling invertebrates, we review existing research and integrate previously disregarded functional traits into an ecosystem model. Based on existing research and our models, we conclude that tube-dwelling invertebrates play a central role in controlling water column nutrient pools, and hence water quality and trophic state. Furthermore, these tiny ecosystem engineers can influence the thresholds that determine shifts between alternate clear and turbid states of shallow lakes. The large effects stand in contrast to the conventional limnological paradigm emphasizing predominantly pelagic food webs. Given the vast number of shallow lakes worldwide, benthic invertebrates are likely to be relevant drivers of biogeochemical processes at regional and global scales, thereby mediating feedback mechanisms linked to climate change.

Trophic Characterization, Pelagic Food Web Structure and Comparison of Two Mesotrophic Lakes in Brandenburg (Germany)

We conducted comprehensive field investigations to characterise the trophic status and food web structure of Grosser Vatersee (GV) and Kleiner Vatersee (KV), two stratified lakes in the Baltic lake region of northeastern Germany. The lakes will be used as manipulation (GV) and reference (KV) ecosystems in food web studies. Both lakes are mesotrophic to slightly eu rophic with mean epilimnetic total phosphorus (P) concentrations of 24 and 21 μg L -1 in GV and KV, respectively. Mean phytoplankton biomass was 0.9 and 0.5 cm 3 m - in GV and KV. During summer, the algal community of both lakes was dominated by cyanobacterial picoplankton and Ceratium spp. Filamentous cyanobacteria (mostly Planktothrix sp.) prevailed in a deep chlorophyll maximum found within the chemocline of both lakes. Bacterial biomass in the epilimnion of GV usually ranged between 1 and 3 cm 3 m 3 . Mean ciliate abundance was 13 ind. mL -1 in GV and 14 ind. mL -1 in KV. While Oligotricha dominated in GV, Scuticociliata were found most frequently in KV. Mean annual crustacean biomass was high in both lakes (GV 2.5 cm 3 m -3 , KV 1.2 cm 3 m -3 ); Eudiaptomus gracilis, E, graciloides and Daphnia spp. were the most prominent species. Total fish biomass of both lakes was dominated by roach (GV 52%; KV 57%) and perch (GV 24%; KV 34%) Vaucheria dichotoma and Chara spp. dominated the macrophyte community in GV; Chara spp. dominated in KV. The slightly more eutrophic character of GV was most likely a result of greater P accumulation in the sediments due to external loading, which resulted in greater hypolimnetic P concentrations throughout the summer in GV. However, on an annual basis these differences were only occasionally reflected at higher trophic levels. The differences in fish biomass, however, might have been a consequence of higher structural diversity in GV provided by submersed macrophytes rather than its more eutrophic character. We conclude that the trophic characteristics and food web structure of Grosser Vatersee and Kleiner Vatersee are similar enough to be suitable as manipulation and reference lakes in food web experiments.

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.

Filtering and feeding rates of cyclopoid copepods feeding on phytoplankton

The algal biomass ingested by omnivorous cyclopoid copepods (Cyclops kolensis and C. vicinus) was measured by two methods in the hypertrophic Heiligensee in Berlin (West Germany). The clearance and ingestion rates inferred from measurements of natural populations of 14C labelled phytoplankton were compared with those obtained from chlorophyll a determinations using the presence/absence method (observed chlorophyll a content of natural lake phytoplankton with and without addition of cyclopoids). Both methods gave similar results. Nevertheless, the radio tracer method is preferred, mainly because the short feeding duration excludes high variations in both the food composition and food concentration that limit the presence/absence method.