Sue B. Watson

Bloom development and phytoplankton succession in Lake Winnipeg: a comparison of historical records with recent data

Over the past 40 years, hydroelectric, agricultural and urban watershed development and changing hydrology have transformed Lake Winnipeg into a highly eutrophic reservoir with annual outbreaks of widespread surface algal blooms, shoreline and net fouling, and concerns with intermittent cyanotoxin production. To provide a better understanding of the magnitude of these changes and the major causes, we examine long-term increases in phytoplankton biomass and shifts in phytoplankton species dominance in the context of both in-lake and watershed processes. We compare phytoplankton and water quality data from early (1969) and recent (1994–2007) lake-wide surveys, and information from paleolimnological analysis of sediment cores and satellite remote sensing. Our results demonstrate a recent and dramatic rise in severe algal blooms and increased dominance of cyanobacteria beginning in the mid-1990s, coincident with a large increase in phosphorous loading to the lake. Distinct increases in sediment core accumulation of nutrients and chlorophyll, cyanobacteria and diatom microfossils coincided with hydroelectric and agricultural development, increased Red R discharge and shifts in water transparency patterns across the lake. There has been a dramatic increase in phytoplankton biomass, accompanied by marked shifts in seasonal community composition. Spring diatoms blooms are of shorter duration and increasingly dominated by more eutrophic diatom taxa while summer blooms show reduced taxonomic diversity and an increased predominance of nitrogen-fixing cyanobacteria. Satellite images showed annual development of vast summer surface blooms, mainly in the north basin, with chlorophyll highest in regions of relatively low suspended sediment concentration and high transparency. There is an increasing dominance of potentially toxic cyanobacteria taxa and high levels of microcystins in nearshore samples of surface blooms. The combined effects of nutrient increases, algal species shifts and toxin production represent a potential threat to the sustainability of ecosystem function and productivity.

Metatranscriptomic Evidence for Co-Occurring Top-Down and Bottom-Up Controls on Toxic Cyanobacterial Communities

ABSTRACT Little is known about the molecular and physiological function of co-occurring microbes within freshwater cyanobacterial harmful algal blooms (cHABs). To address this, community metatranscriptomes collected from the western basin of Lake Erie during August 2012 were examined. Using sequence data, we tested the hypothesis that the activity of the microbial community members is independent of community structure. Predicted metabolic and physiological functional profiles from spatially distinct metatranscriptomes were determined to be ≥90% similar between sites. Targeted analysis of Microcystis aeruginosa, the historical causative agent of cyanobacterial harmful algal blooms over the past ∼20 years, as well as analysis of Planktothrix agardhii and Anabaena cylindrica, revealed ongoing transcription of genes involved in microcystin toxin synthesis as well as the acquisition of both nitrogen and phosphorus, nutrients often implicated as independent bottom-up drivers of eutrophication in aquatic systems. Transcription of genes involved in carbon dioxide (CO2) concentration and metabolism also provided support for the alternate hypothesis that high-pH conditions and dense algal biomass result in CO2-limiting conditions that further favor cyanobacterial dominance. Additionally, the presence of Microcystis-specific cyanophage sequences provided preliminary evidence of possible top-down virus-mediated control of cHAB populations. Overall, these data provide insight into the complex series of constraints associated with Microcystis blooms that dominate the western basin of Lake Erie during summer months, demonstrating that multiple environmental factors work to shape the microbial community.

Internal loading of phosphate in Lake Erie Central Basin

After significant reductions in external phosphorus (P) loads, and subsequent water quality improvements in the early 1980s, the water quality of Lake Erie has declined considerably over the past decade. The frequency and magnitude of harmful algal blooms (primarily in the western basin) and the extent of hypoxic bottom waters in the central basin have increased. The decline in ecosystem health, despite meeting goals for external P loads, has sparked a renewed effort to understand P cycling in the lake. We use pore-water P concentration profiles and sediment cores incubation experiments to quantify the P flux from Lake Erie central basin sediments. In addition, the oxygen isotopes of phosphate were investigated to assess the isotopic signature of sedimentary phosphate inputs relative to the isotopic signature of phosphate in lake water. Extrapolating the total P sediment flux based on the pore-water profiles to the whole area of the central basin ranged from 300 to 1250metric tons per year and using the flux based on core incubation experiments an annual flux of roughly 2400metric tons of P is calculated. These estimates amount to 8-20% of the total external input of P to Lake Erie. The isotopic signature of phosphate in the extractable fraction of the sediments (~18‰) can explain the non-equilibrium isotope values of dissolved phosphate in the deep water of the central basin of Lake Erie, and this is consistent with sediments as an important internal source of P in the Lake.

Harmful Algal Blooms

Abstract Harmful algal blooms (HABs) are symptomatic of ecosystem imbalance, often caused by the many environmental changes that demonstrate the expanding global human footprint and climate change. HABs are a major issue in marine, brackish, and freshwater systems worldwide. HAB species include representatives from most major algal and cyanobacterial taxonomic groups, but, despite this diversity, many HAB taxa respond to similar broad environmental stimuli (e.g., nutrients, light) and employ similar growth and defense strategies to maximize growth and minimize losses. In inland waters, attention is focused on dense surface HABs caused by planktonic cyanobacteria (cHABs), and the past few decades have seen an apparent increase in cHAB frequency and geographic range. There has also been an apparent rise in blooms of eukaryotic phytoplankton, including invasive marine flagellates, while reports of thick shoreline and benthic mats of chlorophytes, cyanobacteria, and other benthic HABs are likewise increasing. Research has made tremendous advances in our understanding of HAB events, yet they remain poorly understood. This chapter provides an overview of these intriguing and challenging phenomena, which affect ecosystems and human societies. While HABs are as diverse as the water bodies they impact, we focus on some specific examples of known HAB taxa that affect inland waters in North America and highlight some of the important strategies used by these species to enable their excessive growth.

Occurrence of Waterborne Pathogens and Escherichia coli at Offshore Drinking Water Intakes in Lake Ontario

ABSTRACT The occurrence of waterborne pathogens was investigated at three drinking water intakes located about 2 km offshore in Lake Ontario. Water sampling was conducted over 3 years for Campylobacter spp., Cryptosporidium spp., Giardia spp., cultivable enteric viruses, and water quality parameters. All pathogens were detected in the offshore source water for each water treatment plant (WTP1 to WTP3), although at relatively low frequencies and concentrations. Giardia was the most common pathogen, occurring in 36% of water samples from the influent of WTP1 (n = 46), and with a maximum concentration of 0.70 cysts/liter in this influent. Cryptosporidium occurred as frequently as 15% in the WTP2 influent (n = 35), with a maximum concentration of 0.40 oocysts/liter in the WTP1 influent. The human Bacteroidales HF183 DNA marker was most common in the WTP1 influent (19%), and this was the only WTP where the Cryptosporidium hominis genotype was detected. No water quality parameter was predictive of pathogen occurrence across all three WTP influents. Escherichia coli was often below detection when pathogens were detected, and spikes in E. coli concentrations often did not coincide with pathogen occurrence. After summer rain events, river plumes had E. coli concentrations as high as 222 CFU/100 ml in surface waters 2 km offshore, without impacting drinking water intakes below the thermocline on the lake bottom. At times, prechlorination to control mussels at offshore intake cribs compromised the use of E. coli for “raw” water quality assessment, particularly for chlorine-resistant Cryptosporidium. E. coli measured by standard methods did not reliably predict pathogen occurrence at drinking water intakes in offshore ecosystems.

Sediment and nutrient distribution and resuspension in Lake Winnipeg.

Severe algal blooms in Lake Winnipeg since the late 1990s have been attributed to increased watershed nutrient loading, much of which is associated with suspended particles. Within-lake transport and fate of this nutrient fraction and the importance of internal loading via resuspension, however, are unknown. We measured radioisotopes (7Be, 210Pb, 137Cs), metal and nutrient contents of suspended solids in major tributaries and lake-water, in sediment traps and in bottom sediments to estimate sediment resuspension and mass accumulation rates using two models. Sedimentation rates calculated from 137Cs and 210Pb dated cores and sediment traps indicated that most (95-99%) suspended material is derived from bottom sediment; mixing models using7Be/210Pb and 137Cs yielded similarly high estimates (82 and 84%, respectively). 137Cs profiles in cores indicated that up to ~7cm remains actively resuspended for times up to 23years before incorporation into deeper sediments. Total and bioavailable phosphorus (TP, BAP) in this top sediment layer were generally lower in the North than the South Basin, likely reflecting inputs from the Assiniboine and Red Rivers at the southern end of Lake Winnipeg, with an average of ~30% TP as BAP. Estimates of average sediment-associated internal TP loading for the South Basin (0.264g/m2/y) were ~2× those for the North Basin (0.146g/m2/y). Together, this internal loading is comparable to the magnitude of the external loading. Our results indicate that surficial sediments in Lake Winnipeg will remain a significant and active source of internal nutrient loading for several decades, a process which may delay the response of the lake to external nutrient management.

Delineation of the role of nutrient variability and dreissenids (Mollusca, Bivalvia) on phytoplankton dynamics in the Bay of Quinte, Ontario, Canada

The Bay of Quinte, a Z-shaped embayment at the northeastern end of Lake Ontario, has a long history of eutrophication problems primarily manifested as spatially extensive algal blooms and predominance of toxic cyanobacteria. The purpose of this study was to identify the structural changes of the phytoplankton community induced by two environmental alterations: point-source phosphorus (P) loading reduction in the late 1970s and establishment of dreissenid mussels in the mid-1990s. A combination of statistical techniques was used to draw inference about compositional shifts of the phytoplankton assemblage, the consistency of the seasonal succession patterns along with the mechanisms underlying the algal biovolume variability in the Bay of Quinte over the past three decades. Based on a number of diversity and similarity indices, the algal assemblages in the upper and middle segments of the Bay are distinctly different from those typically residing in the outer segments. Our analysis also identified significant differences among the phytoplankton communities, representing the pre- and post-P control as well as the pre- and post-dreissenid invasion periods. Recent shifts in phytoplankton community composition were mainly associated with increased frequency of occurrence of toxin-producing Microcystis outbreaks and reduced biovolume of N2 fixers, such as Aphanizomenon and Anabaena. Bayesian hierarchical models were developed to elucidate the importance of different abiotic factors (light attenuation, water temperature, phosphorus, and ammonium) on total cyanobacteria, Microcystis, Aphanizomenon, and Anabaena relative biovolume. Our modelling exercise suggests that there is significant spatial heterogeneity with respect to the role of the factors examined, and thus total phosphorus alone cannot always explain the year-to-year variability of cyanobacteria succession patterns in the system. The lessons learned from the present analysis will be helpful to the water quality criteria setting process and could influence the management decisions in order to delist the system as an Area of Concern.

Integration of best management practices in the Bay of Quinte watershed with the phosphorus dynamics in the receiving waterbody: What do the models predict?

We present a modelling analysis of the management practices that could lead to significant reduction of phosphorus export from the Bay of Quinte watershed and an evaluation of the overall uncertainty associated with the assessment of the Beneficial Use Impairment Eutrophication and Undesirable Algae. Our work highlights the internal recycling as one of the key drivers of phosphorus dynamics in the Bay. The flow from the Trent River is the predominant driver of the upper Bay dynamics until the main stem of the middle area however, the sediments in the same segment release a significant amount of phosphorus and the corresponding fluxes are likely amplified by the macrophyte and dreissenid activity. From a management standpoint, the presence of a significant positive feedback loop in the upper Bay suggests that the anticipated benefits of additional reductions of the exogenous point and non-point loading may not be realized within a reasonable time frame (i.e. 5—10 years). Our analysis of nutrient loading scenarios shows that the restoration pace of the Bay could be slow, even if the riverine total phosphorus concentrations reach levels significantly lower than their contemporary values, <25 µg TP l−1. We believe that the on-going management decisions, monitoring, and modelling should also explicitly consider the role and broader ramifications of internal phosphorus loading into the system. The anticipated lessons from such a multi-faceted exercise are a unique aspect of the Bay of Quinte ecosystem because of the long history of research and monitoring data. This study can produce transferable knowledge to other systems worldwide, experiencing similar hysteresis patterns associated with internal nutrient loading.

Optimizing the complexity of phytoplankton functional group modeling: An allometric approach

Abstract Elucidating patterns and mechanisms that shape phytoplankton assemblages is a popular area of research for empirical and theoretical ecologists. Despite the daunting complexity of phytoplankton dynamics, much of our current understanding has been based on simple models describing food-web interactions with few differential equations. Skeptical views in the literature raise concerns about the increasing model complexity and advice to seek parsimony rather than simplicity. To address this controversy (simple versus complex models), we propose the introduction of an extra layer of causality into plankton models by connecting algal processes (maximum growth rates, nutrient kinetics, settling velocities, metabolic rates) with species-specific morphological features (cell volume, surface-to-volume ratio, shape). In this study, we demonstrate the capacity of a size-based plankton model to reproduce observed water quality patterns (phosphate, total phosphorus, nitrate, total ammonia, total nitrogen, chlorophyll a, and total zooplankton biomass) in the Hamilton Harbour, Ontario. Consistent with empirical evidence, our modeling analysis showed that small algal species have a distinct competitive advantage in summer epilimnetic environments across the range of cell volume and nutrient loading conditions examined; especially, when they are characterized by higher optimal temperature for growth. Strong top-down pressure mediated by high zooplankton abundance effectively controls the standing biomass of phytoplankton species that can otherwise realize high growth rates under the conditions typically prevailing in the end-of-summer epilimnetic environments (e.g., higher temperature optima, higher tolerance in low water clarity). Under high zooplankton control, the secondary variations of phytoplankton are modulated by the ambient phosphorus levels and the size-based strategies for resources procurement, such as the regulation of nutrient transport kinetics. By contrast, when the summer algal assemblage is released by the zooplankton grazing, the exceedance of critical phytoplankton biomass levels and the likelihood of harmful algal blooms are determined by the multitude of factors that shape inter-specific competition patterns (e.g., relative abundance of competing species, nutrient uptake kinetics). Our study evaluates the strengths and weaknesses of this approach and identifies future directions that would provide operational models founded upon concepts of allometry.

Modelling phosphorus dynamics in Cootes Paradise marsh: Uncertainty assessment and implications for eutrophication management

Cootes Paradise marsh, a hypereutrophic wetland draining into the western end of Hamilton Harbour, Ontario, has historically been considered an important regulatory factor of the severity of local eutrophication phenomena. In this study, we present a modelling exercise that aims to draw inference on the relative contribution of various external and internal flux rates to the phosphorus budget of Cootes Paradise. We first examined the capacity of a phosphorus mass-balance model, accounting for the interplay among water column, sediments and macrophytes, to reproduce the observed total phosphorus dynamics over a 17-year period (1996–2012). Water level fluctuations were one of the key challenges for balancing the phosphorus budget during model calibration. Our analysis shows that the model satisfactorily reproduced the average seasonal patterns, as well as the year-to-year total phosphorus variability (coefficient of determination = 0.20, relative error = 26.8%, root mean square error = 62.2 μg P l−1, model efficiency = 0.15). However, our model failed to capture two years of the study period (1997 and 2007), when ambient TP levels significantly deviated from the typically prevailing conditions. Model sensitivity analysis identified the sedimentation of particulate material and diffusive reflux from sediments as two critical processes to characterize the phosphorus cycle in the wetland. Based on the current parameter specification, our model postulates that the sediments still act as a net sink, whereas macrophyte processes (respiration rates, nutrient uptake from interstitial water) appear to play a minor role. We conclude by discussing the various sources of uncertainty and additional remedial actions required in Cootes Paradise marsh to realize a shift from the current turbid-phytoplankton dominated state to its former clear-macrophyte dominated state.