Cyanobacteria can form dense and sometimes toxic blooms in freshwater and marine environments, which threaten ecosystem functioning and degrade water quality for recreation, drinking water, fisheries ...and human health. Here, we review evidence indicating that cyanobacterial blooms are increasing in frequency, magnitude and duration globally. We highlight species traits and environmental conditions that enable cyanobacteria to thrive and explain why eutrophication and climate change catalyse the global expansion of cyanobacterial blooms. Finally, we discuss management strategies, including nutrient load reductions, changes in hydrodynamics and chemical and biological controls, that can help to prevent or mitigate the proliferation of cyanobacterial blooms.
Artificial mixing has been used as a measure to prevent the growth of cyanobacteria in eutrophic lakes and reservoirs for many years. In this paper, we give an overview of studies that report on the ...results of this remedy. Generally, artificial mixing causes an increase in the oxygen content of the water, an increase in the temperature in the deep layers but a decrease in the upper layers, while the standing crop of phytoplankton (i.e. the chlorophyll content per m
2
) often increases partly due to an increase in nutrients entrained from the hypolimnion or resuspended from the sediments. A change in composition from cyanobacterial dominance to green algae and diatoms can be observed if the imposed mixing is strong enough to keep the cyanobacteria entrained in the turbulent flow, the mixing is deep enough to limit light availability and the mixing devices are well distributed horizontally over the lake. Both models and experimental studies show that if phytoplankton is entrained in the turbulent flow and redistributed vertically over the entire depth, green algae and diatoms win the competition over (colonial) cyanobacteria due to a higher growth rate and reduced sedimentation losses. The advantage of buoyant cyanobacteria to float up to the illuminated upper layers is eradicated in a well-mixed system.
This review summarizes and discusses the current understanding of human exposure to cyanobacterial toxins in “seafood” collected from freshwater and coastal areas. The review consists of three parts: ...(a) the existing literature on concentrations of cyanobacterial toxins in seafood is reviewed, and the likelihood of bioaccumulation discussed; (b) we derive cyanotoxin doses likely to occur through seafood consumption and propose guideline values for seafood and compare these to guidelines for drinking water; and (c) we discuss means to assess, control or mitigate the risks of exposure to cyanotoxins through seafood consumption. This is discussed in the context of two specific procedures, the food specific
HACCP-approach and the water-specific
Water Safety Plan approach by the WHO. Risks of exposure to cyanotoxins in food are sometimes underestimated. Risk assessments should acknowledge this and investigate the partitioning of exposure between drinking-water and food, which may vary depending on local circumstances.
Accumulation of cyanobacterial toxins in freshwater “seafood”.
Cyanobacteria are predicted to increase due to climate and land use change. However, the relative importance and interaction of warming temperatures and increased nutrient availability in determining ...cyanobacterial blooms are unknown. We investigated the contribution of these two factors in promoting phytoplankton and cyanobacterial biovolume in freshwater lakes. Specifically, we asked: (1) Which of these two drivers, temperature or nutrients, is a better predictor of cyanobacterial biovolume? (2) Do nutrients and temperature significantly interact to affect phytoplankton and cyanobacteria, and if so, is the interaction synergistic? and (3) Does the interaction between these factors explain more of the variance in cyanobacterial biovolume than each factor alone? We analyzed data from > 1000 U.S. lakes and demonstrate that in most cases, the interaction of temperature and nutrients was not synergistic; rather, nutrients predominantly controlled cyanobacterial biovolume. Interestingly, the relative importance of these two factors and their interaction was dependent on lake trophic state and cyanobacterial taxon. Nutrients played a larger role in oligotrophic lakes, while temperature was more important in mesotrophic lakes: Only eutrophic and hyper-eutrophic lakes exhibited a significant interaction between nutrients and temperature. Likewise, some taxa, such as Anabaena, were more sensitive to nutrients, while others, such as Microcystis, were more sensitive to temperature. We compared our results with an extensive literature review and found that they were generally supported by previous studies. As lakes become more eutrophic, cyanobacteria will be more sensitive to the interaction of nutrients and temperature, but ultimately nutrients are the more important predictor of cyanobacterial biovolume.
Here we aim to incorporate trait-based information into the modern coexistence framework that comprises a balance between stabilizing (niche-based) and equalizing (fitness) mechanisms among ...interacting species. Taking the modern coexistence framework as our basis, we experimentally tested the effect of size differences among species on coexistence by using fifteen unique pairs of resident vs. invading cyanobacteria, resulting in thirty unique invasibility tests. The cyanobacteria covered two orders of magnitude differences in size. We found that both niche and fitness differences increased with size differences. Niche differences increased faster with size differences than relative fitness differences and whereas coexisting pairs showed larger size differences than non-coexisting pairs, ultimately species coexistence could not be predicted on basis of size differences only. Our findings suggest that size is more than a key trait controlling physiological and population-level aspects of phytoplankton, it is also relevant for community-level phenomena such as niche and fitness differences which influence coexistence and biodiversity.
Climate change scenarios predict that rivers, lakes, and reservoirs will experience increased temperatures, more intense and longer periods of thermal stratification, modified hydrology, and altered ...nutrient loading. These environmental drivers will have substantial effects on freshwater phytoplankton species composition and biomass, potentially favouring cyanobacteria over other phytoplankton. In this Review, we examine how several cyanobacterial eco-physiological traits, specifically, the ability to grow in warmer temperatures; buoyancy; high affinity for, and ability to store, phosphorus; nitrogen-fixation; akinete production; and efficient light harvesting, vary amongst cyanobacteria genera and may enable them to dominate in future climate scenarios. We predict that spatial variation in climate change will interact with physiological variation in cyanobacteria to create differences in the dominant cyanobacterial taxa among regions. Finally, we suggest that physiological traits specific to different cyanobacterial taxa may favour certain taxa over others in different regions, but overall, cyanobacteria as a group are likely to increase in most regions in the future.
► Climate change is predicted to favour cyanobacteria over other phytoplankton in freshwater ecosystems. ► We examined how cyanobacterial physiology may provide a competitive advantage. ► Different taxa, because of their traits, will respond to different aspects of changed climate. ► Cyanobacteria will most likely increase, but with noted taxa and geographical differences.
Abstract
David Schindler and his colleagues pioneered studies in the 1970s on the role of phosphorus in stimulating cyanobacterial blooms in North American lakes. Our understanding of the nuances of ...phosphorus utilization by cyanobacteria has evolved since that time. We review the phosphorus utilization strategies used by cyanobacteria, such as use of organic forms, alternation between passive and active uptake, and luxury storage. While many aspects of physiological responses to phosphorus of cyanobacteria have been measured, our understanding of the critical processes that drive species diversity, adaptation and competition remains limited. We identify persistent critical knowledge gaps, particularly on the adaptation of cyanobacteria to low nutrient concentrations. We propose that traditional discipline-specific studies be adapted and expanded to encompass innovative new methodologies and take advantage of interdisciplinary opportunities among physiologists, molecular biologists, and modellers, to advance our understanding and prediction of toxic cyanobacteria, and ultimately to mitigate the occurrence of blooms.
This review advocates for interdisciplinary research on cell physiology, molecular biology, and ecological modelling to improve predictions of cyanobacterial blooms in response to variability in phosphorus supply rates in situ.
Anticipated climatic changes combined with eutrophication are predicted to enhance the dominance of several notorious cyanobacterial taxa. Cyanobacteria have many key ecological traits that may allow ...them to thrive under foreseen scenarios of environmental change. Understanding the ecophysiological traits of harmful species has proven important for their successful control and management. Indeed, if the links between key cyanobacterial traits and the specific environmental conditions that allow expression of these traits can be disrupted, we could identify (novel) means for operational control and mitigate or prevent water quality problems. A good example is artificial mixing of a lake that breaks down the water column stability on which fast floating, buoyant cyanobacteria depend. Based upon Reynolds’ functional phytoplankton classification, we focused on five groups of cyanobacteria that from a management point of view can be seen as homogeneous and have comparable environmental sensitivities. For each group, we present (1) its key traits, (2) how these characteristics will maintain their function under future environmental change, (3) explanation of how understanding the function of these traits can reveal the “Achilles heel” of the particular functional group and (4) which (combination of) control measures is most likely to be successful. Despite looking for specific environmental sensitivities of individual groups, we maintain that controlling nutrients remains the basis for managing blooms, no matter which functional type dominates. Providing further ecological knowledge to lake management could be the key to effective bloom control and healthier, sustainable freshwater ecosystems even in a warmer future.
All living organisms modify their biotic and abiotic environment. Niche construction theory posits that organism-mediated modifications to the environment can change selection pressures and influence ...the evolutionary trajectories of natural populations. While there is broad support for this proposition in general, there is considerable uncertainty about how niche construction is related to other similar concepts in ecology and evolution. Comparative studies dealing with certain aspects of niche construction are increasingly common, but there is a troubling lack of experimental tests of the core concepts of niche construction theory. Here, we propose an operational framework to evaluate comparative and experimental evidence of the evolutionary consequences of niche construction, and suggest how such research can improve our understanding of ecological and evolutionary dynamics in ecosystems. We advocate for a shift toward explicit experimental tests of how organism-mediated environmental change can influence the selection pressures underlying evolutionary responses, as well as targeted field-based comparative research to identify the mode of evolution by niche construction and assess its importance in natural populations.
Climate warming is causing changes in the physics of deep lakes, such as longer summer stratification, increased water column stability, reduced ice cover, and a shallower depth of winter overturns. ...An ultimate consequence of warming would be a transition to a different mixing regime. Here we investigate the role of physical, chemical, and biological feedback mechanisms that unfold during a shift in mixing regime, and whether these feedbacks could prompt and stabilise the new regime. Although climate, interannual temperature variation, and lake morphometry are the main determinants of a mixing regime, when climate change causes shifts in mixing regime, internal feedback mechanisms may gain in importance and modify lake ecosystem functioning.
We review the role of these feedbacks in three mixing regime shifts: from polymictic to seasonally stratified, from dimictic to monomictic, and from holomictic to oligomictic or meromictic.
Polymictic lakes of intermediate depth (c. 3–10 m mean depth) could experience seasonal stratification if a stratification event triggers phytoplankton blooms or dissolved organic matter release, reducing transparency and therefore further heating the surface layer. However, this feedback is only likely to have influence in small and clear lakes, it would be easily disturbed by weather conditions, and the resulting stratified state does not remain stable in the long term, as stratification is lost in winter.
The ice‐albedo feedback might cause an accelerated shift from ice‐covered (dimictic) to ice‐free (monomictic) winters in sufficiently deep (mean depth 50 m or more) lakes, where temperature memory is carried over from one winter to the next. Nevertheless, there is an ongoing debate into whether this process can persist during natural weather variations and overcome self‐stabilising mechanisms such as thermal insulation by snow. The majority of studies suggest that a gradual transition from dimictic to monomictic is more likely than an abrupt transition.
A shift from a holomictic to a meromictic regime can occur if anoxia is triggered by incomplete mixing and an increase in deep‐water density—through the accumulation of solutes—exceeds a density decrease by hypolimnetic warming. A shift to meromixis would strongly alter the biology of a lake and might be difficult to reverse. If solutes accumulate only minimally in the hypolimnion, an oligomictic regime is formed, in which years with complete and incomplete mixing alternate.
Understanding the importance of feedback mechanisms and the role of biogeochemistry when lakes shift in mixing regime could lead to a better understanding of how climate change affects lake ecosystems.
PDF
