Mitigating harmful cyanobacterial blooms in a human- and climatically-impacted world

Bloom-forming harmful cyanobacteria (CyanoHABs) are harmful from environmental, ecological and human health perspectives by outcompeting beneficial phytoplankton, creating low oxygen conditions (hypoxia, anoxia), and by producing cyanotoxins. Cyanobacterial genera exhibit optimal growth rates and bloom potentials at relatively high water temperatures; hence, global warming plays a key role in their expansion and persistence. CyanoHABs are regulated by synergistic effects of nutrient (nitrogen:N and phosphorus:P) supplies, light, temperature, vertical stratification, water residence times, and biotic interactions. In most instances, nutrient control strategies should focus on reducing both N and P inputs. Strategies based on physical, chemical (nutrient) and biological manipulations can be effective in reducing CyanoHABs; however, these strategies are largely confined to relatively small systems, and some are prone to ecological and environmental drawbacks, including enhancing release of cyanotoxins, disruption of planktonic and benthic communities and fisheries habitat. All strategies should consider and be adaptive to climatic variability and change in order to be effective for long-term control of CyanoHABs. Rising temperatures and greater hydrologic variability will increase growth rates and alter critical nutrient thresholds for CyanoHAB development; thus, nutrient reductions for bloom control may need to be more aggressively pursued in response to climatic changes globally

Alteration of microbial metabolic activities in association with detritus.

Detritus serves as a microhabitat, or microzone, hosting redox gradients, altered microbial metabolism and associated biochemical nutrient transformation processes qualitatively and quantitatively distinct from the ambient planktonic environment. Distributions of microbial communities and their extracellular deposition products on detrital as well as non-detrital submersed surfaces are patchy in nature, both in terms of physical distribution and associated metabolism. This patchiness promotes the development of steep redox gradients, which in turn lead to diverse microhabitats in which microorganisms, protozoans and invertebrates, all having narrow environmental tolerances, can reside. The compact nature of gradients helps promote diffusive exchange of metabolites, gases and nutrients, thereby maintaining community diversity and structural stability of microzones. Microzones also harbor specific nutrient transformation processes (nitrogen cycle N2 fixation, ammonification, nitrification and denitrification) which may otherwise be unfavorable or inhibited in ambient waters

Tackling Harmful Cyanobacterial Blooms with Chinese Colleagues: We're All in the Same Boat

Harmful cyanobacterial blooms (CyanoHABs) are a rapidly proliferating global problem, threatening the use and sustainability of our freshwater resources. In recent decades, the United States, China, and other developed and developing countries threatened by CyanoHAB expansion have established collaborative efforts aimed at mitigating and managing this environmental and human health problem. However, an escalating negative political climate and restrictive policies on scientific exchange threaten these efforts. In this Perspective, I point to progress that has been made to counter the CyanoHAB problem on U.S.–Chinese fronts through our collaborations, which have been mutually beneficial from research and academic perspectives. Much like global efforts now needed to control pandemics, we are all “in the same boat” when to comes to countering the threat CyanoHABs pose for drinkable, swimmable, and fishable freshwater supplies and human health

Mitigating toxic planktonic cyanobacterial blooms in aquatic ecosystems facing increasing anthropogenic and climatic pressures

Toxic planktonic cyanobacterial blooms are a pressing environmental and human health problem. Blooms are expanding globally and threatening sustainability of our aquatic resources. Anthropogenic nutrient enrichment and hydrological modifications, including water diversions and reservoir construction, are major drivers of bloom expansion. Climatic change, i.e., warming, more extreme rainfall events, and droughts, act synergistically with human drivers to exacerbate the problem. Bloom mitigation steps, which are the focus of this review, must consider these dynamic interactive factors in order to be successful in the short- and long-term. Furthermore, these steps must be applicable along the freshwater to marine continuum connecting streams, lakes, rivers, estuarine, and coastal waters. There is an array of physical, chemical, and biological approaches, including flushing, mixing, dredging, application of algaecides, precipitating phosphorus, and selective grazing, that may arrest and reduce bloom intensities in the short-term. However, to ensure long term, sustainable success, targeting reductions of both nitrogen and phosphorus inputs should accompany these approaches along the continuum. Lastly, these strategies should accommodate climatic variability and change, which will likely modulate and alter nutrient-bloom thresholds

Why does N-limitation persist in the world's marine waters?

Primary production of vast regions of the world's estuarine, coastal and pelagic ocean waters is limited by availability of fixed nitrogen; this despite the fact that a highly diverse suite of microorganisms potentially capable of fixing N2 (eubacteria and cyanobacteria), inhabit these waters. Theoretically, diazotrophs should supply the N needs to balance the N required to support primary production, assuming other key limiting nutrients, phosphorus and iron, are available and energy requirements are met. In practice however, N2 fixation often does not meet ecosystem-scale N demands, even when these nutrients are replete. The problem lies with the fact that optimal rates of N2 fixation are often controlled by additional environmental factors, including light and organic matter availability, turbulence, and high levels of dissolved oxygen which can suppress this process in N-deplete surface waters. In addition, rates of N loss via denitrification and anammox can exceed N2 fixation and external N inputs on annual scales in coastal and pelagic waters, including those experiencing eutrophication. This creates a situation where chronic N limitation persists, even in the presence of anthropogenic nitrogen enrichment. Many aquatic ecosystems exhibit a perpetual “hunger” for fixed N to support primary and higher levels of production and this is likely to continue over forseeable biological and geologic timescales

The cyanobacterial nitrogen fixation paradox in natural waters

Nitrogen fixation, the enzymatic conversion of atmospheric N (N 2) to ammonia (NH 3), is a microbially mediated process by which "new" N is supplied to N-deficient water bodies. Certain bloom-forming cyanobacterial species are capable of conducting N 2 fixation; hence, they are able to circumvent N limitation in these waters. However, this anaerobic process is highly sensitive to oxygen, and since cyanobacteria produce oxygen in photosynthesis, they are faced with a paradoxical situation, where one critically important (for supporting growth) biochemical process is inhibited by another. N 2-fixing cyanobacterial taxa have developed an array of biochemical, morphological, and ecological adaptations to minimize the "oxygen problem" however, none of these allows N 2 fixation to function at a high enough efficiency so that it can supply N needs at the ecosystem scale, where N losses via denitrification, burial, and advection often exceed the inputs of "new" N by N 2 fixation. As a result, most marine and freshwater ecosystems exhibit chronic N limitation of primary production. Under conditions of perpetual N limitation, external inputs of N from human sources (agricultural, urban, and industrial) play a central role in determining ecosystem fertility and, in the case of N overenrichment, excessive primary production or eutrophication. This points to the importance of controlling external N inputs (in addition to traditional phosphorus controls) as a means of ensuring acceptable water quality and safe water supplies. Nitrogen fixation, the enzymatic conversion of atmospheric N 2 to ammonia (NH 3) is a microbially-mediated process by which "new" nitrogen is supplied to N-deficient water bodies. Certain bloom-forming cyanobacterial species are capable of conducting N 2 fixation; hence they are able to circumvent nitrogen limitation in these waters. However, this anaerobic process is highly sensitive to oxygen, and since cyanobacteria produce oxygen in photosynthesis, they are faced with a paradoxical situation, where one critically-important (for supporting growth) biochemical process is inhibited by another. Diazotrophic cyanobacterial taxa have developed an array of biochemical, morphological and ecological adaptations to minimize the "oxygen problem" however, none of these allows N 2 fixation to function at a high enough efficiency so that it can supply N needs at the ecosystem scale, where N losses via denitrification, burial and advection often exceed the inputs of "new" N by N 2 fixation. As a result, most marine and freshwater ecosystems exhibit chronic N-limitation of primary production. Under conditions of perpetual N limitation, external inputs of N from human sources (agricultural, urban, industrial) play a central role in determining ecosystem fertility and in the case of N-overenrichment, excessive primary production, or eutrophication. This points to the importance of controlling external N inputs (in addition to traditional phosphorus controls) as a means of ensuring acceptable water quality and safe water supplies

Planktonic trophic transfer in an estuary: Seasonal, diel, and community structure effects

The high tertiary production of estuaries is largely supported by phytoplank- ton primary production. An important question thus concerns how much phytoplankton production enters the food web through planktonic grazing and what physical, chemical, or biological factors influence this trophic transfer. We conducted a 2-yr, diel investigation of planktonic trophic transfer and the factors influencing it in the Neuse River Estuary, North Carolina. Zooplankton community grazing rates were generally lowest in winter and highest spring through late summer, ranging from 0.1 to 310 mL L-1 h-1. There were few significant diel differences in community grazing rates. The overall daytime mean (± 1 SE) rate was 3.30 ± 0.62 mL L-1 h-1 while the night mean rate was 3.07 ± 0.60 mL L-1 h-1. Post- naupliar copepods were usually more abundant at night than day, but tintinnids were often more abundant by day, while total zooplankton, copepod nauplii, and rotifers displayed no significant diel abundance differences. Community grazing was positively correlated with primary productivity and the abun- dance of total phytoplankton, centric diatoms, dinoflagellates, and the small centric diatom Thalassiosira. Community grazing was also positively correlated with upstream river flow and negatively correlated with salinity. However, there were no significant correlations with water temperature, nutrient concentrations, or grazer abundance variables. On an annual basis, the zooplankton community grazed ≈ 38-45% of daily phytoplank- ton production. Planktonic trophic transfer was coincidentally greatest in late spring through summer, during the period when anadromous fish larvae migrating from the open ocean reach their estuarine primary nursery areas. The fish arrive when planktonic trophic coupling is strongest and depart in fall, when planktonic trophic transfer, zooplankton abundance, and phytoplankton productivity all decrease

Duelling 'CyanoHABs': Unravelling the environmental drivers controlling dominance and succession among diazotrophic and non-N2-fixing harmful cyanobacteria

Eutrophication often manifests itself by increased frequencies and magnitudes of cyanobacterial harmful algal blooms (CyanoHABs) in freshwater systems. It is generally assumed that nitrogen-fixing cyanobacteria will dominate when nitrogen (N) is limiting and non-N2 fixers dominate when N is present in excess. However, this is rarely observed in temperate lakes, where N2 fixers often bloom when N is replete, and non-fixers (e.g. Microcystis) dominate when N concentrations are lowest. This review integrates observations from previous studies with insights into the environmental factors that select for CyanoHAB groups. This information may be used to predict how nutrient reduction strategies targeting N, phosphorus (P) or both N and P may alter cyanobacterial community composition. One underexplored concern is that as N inputs are reduced, CyanoHABs may switch from non-N2 fixing to diazotrophic taxa, with no net improvement in water quality. However, monitoring and experimental observations indicate that in eutrophic systems, minimizing both N and P loading will lead to the most significant reductions in total phytoplankton biomass without this shift occurring, because successional patterns appear to be strongly driven by physical factors, including temperature, irradiance and hydrology. Notably, water temperature is a primary driver of cyanobacterial community succession, with warming favouring non-diazotrophic taxa

Ecology of Blue‐Green Algae in Aquaculture Ponds

Cyanobacteria (blue‐green algae) in the genera Anabaena, Aphanizomenon, Microcystis, and Oscillatoria often form extensive and persistent blooms in freshwater aquaculture ponds. Bloom‐forming cyanobacteria are undesirable in aquaculture ponds because: 1) they are a relatively poor base for aquatic food chains; 2) they are poor oxygenators of the water and have undesirable growth habits; 3) some species produce odorous metabolites that impart undesirable flavors to the cultured animal; and 4) some species may produce compounds that are toxic to aquatic animals. Development of cyanobacterial blooms is favored under conditions of high nutrient loading rates (particularly if the availability of nitrogen is limited relative to phosphorus), low rates of vertical mixing, and warm water temperatures. Under those conditions, dominance of phytoplankton communities by cyanobacteria is the result of certain unique physiological attributes (in particular, N2 fixation and buoyancy regulation) that allow cyanobacteria to compete effectively with other phytoplankton. The ability to fix N2 provides a competitive advantage under severe nitrogen limitation because it allows certain cyanobacterial species to make use of a source of nitrogen unavailable to other phytoplankton. The ability to regulate cell buoyancy through environmentally‐controlled collapse ad reformation of intracellular gas vacuoles is perhaps the primary reason for the frequent dominance of aquaculture pond phytoplankton communities by cyanobacteria. Cyanobacteria that can regulate their position in the water column gain a distinct advantage over other phototrophs in poorly mixed bodies of water. In addition to the physicochemical interactions that influence phytoplankton community dynamics, cyanobacterial‐microbial associations may play an important regulatory role in determining community structure. Cyanobacteria are always found in close association with a diverse array of microorganisms, including eubacteria, fungi, and protozoans. These associations, which in the past have often been viewed as antagonistic, are increasingly seen as mutualistic and may function in a positive manner during bloom development

Picophytoplankton dynamics in a large temperate estuary and impacts of extreme storm events

Picophytoplankton (PicoP) are increasingly recognized as significant contributors to primary productivity and phytoplankton biomass in coastal and estuarine systems. Remarkably though, PicoP composition is unknown or not well-resolved in several large estuaries including the semi-lagoonal Neuse River Estuary (NRE), a tributary of the second largest estuary-system in the lower USA, the Pamlico-Albemarle Sound. The NRE is impacted by extreme weather events, including recent increases in precipitation and flooding associated with tropical cyclones. Here we examined the impacts of moderate to extreme (Hurricane Florence, September 2018) precipitation events on NRE PicoP abundances and composition using flow cytometry, over a 1.5 year period. Phycocyanin-rich Synechococcus-like cells were the most dominant PicoP, reaching ~ 106 cells mL−1, which highlights their importance as key primary producers in this relatively long residence-time estuary. Ephemeral “blooms” of picoeukaryotic phytoplankton (PEUK) during spring and after spikes in river flow were also detected, making PEUK periodically major contributors to PicoP biomass (up to ~ 80%). About half of the variation in PicoP abundance was explained by measured environmental variables. Temperature explained the most variation (24.5%). Change in total dissolved nitrogen concentration, an indication of increased river discharge, explained the second-most variation in PicoP abundance (15.9%). The short-term impacts of extreme river discharge from Hurricane Florence were particularly evident as PicoP biomass was reduced by ~ 100-fold for more than 2 weeks. We conclude that precipitation is a highly influential factor on estuarine PicoP biomass and composition, and show how ‘wetter’ future climate conditions will have ecosystem impacts down to the smallest of phytoplankton