Description: While reduction in nutrient loading is a prerequisite for mitigation of harmful cyanobacterial blooms in nutrient-enriched waters, in certain surface waters eutrophication control is not always feasible due to practical and economic constraints or might be effective only in the long run. Yet, the urgent need to control cyanobacteria in water for drinking, irrigation, aquaculture, industry and recreation has spurred the development of a plethora of alternative methods that claim to be fast acting. Here, we provide a critical overview of several of these end-of-pipe measures: effective microorganisms (EM ® ), golden algae ( Ochromonas ), plant/tree extracts, ultrasound and artificial mixing of non-stratifying waters. Most of the end-of the pipe measures claim to provide sustainable control of harmful cyanobacterial blooms, while at best only targeting symptom relief rather than eutrophication relief. Support for “effective” microorganisms, golden algae, plant extracts, ultrasound and artificial mixing of non-stratifying waters to diminish eutrophication problems such that the resulting water quality meets societal and legislation demands is limited, and several proposed underlying mechanisms are doubtful. None of these curative measures seem the desired wide applicable solution to cyanobacterial nuisance; they should not be considered Columbus’s egg. A critical evaluation of end-of pipe measures is crucial for water authorities in their choice for mitigating measures.

Description: Increased external nutrient loads of anthropogenic origin, especially those of phosphorus (P), were one of the major causes of eutrophication during the first half of the twentieth century in Europe. They led to deterioration of lake ecosystems, particularly including noxious blooms of (potentially toxic) cyanobacteria. From the 1970–1980s, strategies to decrease the phosphorus loads from sewage were increasingly implemented, among them are the ban of phosphates in detergents, the expansion of sewer systems and improvement in wastewater treatment to remove nutrients. Case studies of eight lakes, whose response to point source reduction of phosphorus was observed over decades, show that a pronounced reduction of the phosphorus load from point sources can be achieved either by the diversion of inflows carrying high loads, by upgraded sewage treatment, or by phosphorus precipitation in the major tributary directly before its inflow into the water body. Outcomes demonstrate that in order to effectively control cyanobacterial blooms, the measures taken need to reduce in-lake concentrations of total phosphorus below 20–50 µg L −1 , with this threshold varying somewhat between lakes depending in particular on hydromorphological and biological conditions. Whether and when load reduction succeeds in controlling cyanobacteria depends primarily on the load remaining after remediation and on the water residence time.

Description: The aim of this paper was to summarize the current knowledge on how physical methods can reduce or control internal P release from sediments in lakes and reservoirs. Particular emphasis is given to the role of internal phosphorus load in fueling cyanobacterial blooms which are predicted to increase in frequency and intensity in response to climate change and eutrophication. We present selective case studies (both successful and unsuccessful) to assess the applicability and efficiency of major physical approaches used for decades to reduce internal loading in different systems of various morphology. In particular, we concentrate on where and when (1) hypolimnetic aeration/oxygenation, (2) hypolimnetic withdrawal and (3) sediment dredging are likely to reduce cyanobacterial blooms and whether these methods have an adverse impact on other organisms. We conclude that each method has its strength and weakness depending on the system considered. Sufficient knowledge of all lake nutrient sources and their dynamics together with detailed lake and sediment characteristics is an essential prerequisite for choosing an appropriate control method. We also report that many experiences demonstrated that a combination of restoration methods is often more successful than a single method.

Description: Macrophyte health status can influence the composition of their exudates causing different effects on zooplankton behavior and distribution in nature. We hypothesize that: (1) the release of phenolic compounds and chromophoric dissolved organic matter (CDOM) depends on macrophyte species and its health status (broken macrophytes: BM, or healthy macrophytes: HM); (2) the repellency effect depends on zooplankton species, macrophyte species and its health status; and (3) higher concentrations of phenolic compounds and CDOM produce a stronger repellency effect. Phenolic compounds and CDOM were analyzed in exudates of BM and HM of Salvinia sp., Eichhornia crassipes , Pistia stratiotes , Azolla sp. and Ludwigia peploides . Through a flow-through experiment, the repellency produced by these exudates was assessed in two copepods ( Notodiaptomus conifer and Argyrodiaptomus falcifer ) and one cladoceran ( Ceriodaphnia dubia ). Our hypotheses were partially validated. The quantity of exudated phenolic compounds and CDOM depended on macrophyte species and, to a lesser extent, on the plant health status. The repellency effect was affected by macrophyte and zooplankton species but not by the health status of plants. Only C. dubia and A. falcifer increased their evasion behavior when phenolic compound and CDOM concentrations increased. In brief, the structuring effect of repellent substances depends on different factors. Under a certain threshold concentration, zooplankton behavior might depend on the information associated with the plant odor (e.g., predation risk, structural complexity) more than on the quantity of the released chemical compounds. Above this threshold, evasion would be the only possible option to avoid damaging effects.

Description: In response to Živić et al. (Aquat Ecol 49:81–90, 2015 . doi: 10.1007/s10452-015-9506-7 ), who dismissed the previously established subdivision of the southern medicinal leech Hirudo verbana into an Eastern and Western phylogroup, we present a re-analysis of their and other available DNA sequence (COI) data. We demonstrate that their data not only perfectly support this subdivision, but also improve the understanding of the boundary between both phylogroups. As predicted previously, it lies on the Balkan Peninsula, roughly following the divide between Danubian and Adriatic drainage basins. This finding is important from a conservation perspective, as medicinal leeches are endangered by over-collecting and habitat destruction.

Description: Harmful algal blooms (HABs) pose substantial health risks to seafood consumers, drinking water supplies, and recreationalists with apparent increases associated with anthropogenic eutrophication of freshwaters and coastal areas. Attempts to intervene in these blooms can be met with reticence by citizens, non-governmental organizations, and officials, often due to local perceptions and beliefs. Hence, the social sciences have an important role to play in HAB research and mitigation. Much of the social science HAB research to date has focused on how best to communicate associated risks and appropriate behavioral responses to affected local communities. The emphasis has been on the direct human impacts, particularly in the areas of health outcomes and identification of any sociocultural and economic barriers to proposed mitigation. While this focus is warranted and valuable, there is also a need to understand HABs as part of a larger human–environmental coupled system, where blooms trigger a wide range of cultural and behavioral responses that are driven by how blooms impact other social and ecosystem dynamics. The research presented here describes a case study of a Microcystis aeruginosa bloom in a lake in the Chesapeake Bay watershed where anthropologists worked with HAB researchers. The results of this interdisciplinary collaboration show that approaching the bloom and mitigation within a ‘socio-ecological systems’ framework provides stakeholders with a range of rationales and approaches for addressing HAB mitigation, enhancing both short-term successes and longer-term opportunities, even if M. aeruginosa is still present in the lake.

Description: Climate change can promote harmful cyanobacteria blooms in eutrophic waters through increased droughts or flooding. In this paper, we explore how water-level fluctuations affect the occurrence of cyanobacterial blooms, and based on the observations from case studies, we discuss the options and pitfalls to use water-level fluctuations for lake and reservoir management. A drawdown in summer causes an increase in retention time and increased water column nutrient concentrations and temperature of shallow water layers, which may lead to severe cyanobacterial blooms. This effect can potentially be counteracted by the positive response of submerged macrophytes, which compete for nutrients with cyanobacteria, with a higher chance of cyanobacterial blooms under eutrophic conditions. The balance between dominance by submerged macrophytes or cyanobacteria is temperature sensitive with stronger positive effects of drawdown as inhibition of cyanobacterial blooms expected in colder climates. Complete drying out reduces the amount of cyanobacteria in the water column after refilling, with lower water nutrient concentrations, lower fish biomass, lower abundance of cyanobacteria, higher transparency, and higher cover of submerged plants compared to lakes and reservoirs that did not dry out. Water-level rise as response to flooding has contrasting effects on the abundance of cyanobacteria depending on water quality. We conclude that water-level fluctuation management has potential to mitigate cyanobacterial blooms. However, the success will depend strongly on ecosystem properties, including morphometry, sediment type, water retention time, quality of inlet water, presence of submerged vegetation or propagules, abundance of fish, and climate.

Description: Blooms affect water quality mainly due to the release of microcystins (MCs) by cyanobacteria. The synthesis of MCs is influenced by factors such as nutrient concentration, temperature, pH, light intensity, salinity, turbidity, and the presence of xenobiotics. In this study, we evaluated the effect of environmental factors (temperature and light intensity), the concentration of three nutrients (NaNO 3 , K 2 HPO 4 , and FeCl 3 ), and the N:P ratio on the growth of two Microcystis aeruginosa strains (Ch10 and UTEX LB2385), as well as on mcyA gene expression and intracellular MC concentration. Under similar conditions, the population growth and chlorophyll a concentration per cell of both strains were different. The mcyA gene was significantly up-regulated from the early growth phase (5 days) to the stationary phase (15 days) in most cases, whereas intracellular MC concentrations varied depending on the assessed factor. The N:P ratio affected the development of both strains and MCs production differently. High concentration of intracellular MCs was recorded at low nitrogen and iron concentrations, low temperature, and high light intensity. The response in mcyA gene expression, related to the incubation time, of both strains was different, because strain Ch10 responded in most cases starting at 5 days of growth, whereas UTEX LB2385 responded until 10 and 15 days. This difference reflects physiological plasticity that could help to understand the permanence and dominance of Microcystis genus blooms in eutrophic freshwaters. The variability in response to the tested environmental factors confirms that population growth, genetic expression, and microcystin production are not related to a single factor but to an array of conditions that, when combined, stimulate MCs production. These conditions can be both stress-causing and favorable; hence, monitoring of environmental factors aimed at alerting against health risks provoked by cyanotoxins is a very complex task.

Description: Many notorious invasive plants are clonal, and clonal integration may improve their ability to cope with disturbances such as herbivory. We hypothesize that clonal integration positively affects the growth, physiology and biomass allocation of the invasive aquatic plant Eichhornia crassipes and thus improves its performance in response to defoliation. In a 7-week greenhouse experiment, we subjected E. crassipes ramets to three levels of experimental defoliation, i.e., 20 % (mild), 50 % (moderate) and 80 % (heavy) leaf clipping and an unclipped control, and the stolon connections to the mother plants were either severed or kept intact. Compared with the control, defoliation significantly decreased the growth (total biomass and ramet number), but increased the maximum quantum yield of photosystem II ( F v / F m ) and chlorophyll content of the target ramets. Stolon connection (clonal integration) greatly increased the growth, F v / F m and contents of non-structural carbohydrates (soluble sugar and total non-structural carbohydrates) of E. crassipes , and these effects were larger under heavier defoliation. Moreover, stolon connection significantly reduced the shoot/root ratio of E. crassipes ramets, and such effects were greater under heavier leaf clipping. These results support our hypothesis that E. crassipes can benefit from clonal integration in response to defoliation, suggesting that clonal integration may enhance the invasion potential of E. crassipes and potentially reduce the efficacy of biological control.