Search

Many Pythium spp. are causal agents of diseases of turfgrasses. Pythium spp. disseminate through irrigation systems in agricultural settings, and this study provides evidence that Pythium spp. also disseminate through golf course irrigation. Water samples were collected from irrigation heads and water sources at 10 golf courses in Missouri and Kansas, U.S.A. Samples were collected from 2018 to 2019 in April, July, and October. Phosphorus, nitrogen, and chloride concentrations were measured from irrigation head samples to determine if these parameters influence frequency of Pythium spp. detected. Pythium spp. were detected in samples through baiting and membrane filtration. Cultures were isolated on PARP media, and DNA was extracted from putative Pythium isolates. The internal transcribed spacer region was PCR-amplified and sequenced. Phylogenetic trees were constructed using representative sample sequences, sequences from seven morphologically identified reference isolates of Pythium, and similar GenBank accessions. Detected oomycete species include Lagenidium giganteum, Pythium biforme, Pythium insidiosum, Pythium marsipium, Pythium plurisporium, and Saprolegnia hypogyna. Twenty-one clades lacked species-level resolution, and 14 of these clades were associated with Pythium species. Clades A, C, D, E, I, and M contain Pythium species that cause root and crown rot on creeping bentgrass. Detected Pythium communities were dependent on the detection method used and sampling source. Pythium frequency and diversity were highest in April 2019. Sample temperature, sampling site, and chloride and nutrient concentrations did not influence Pythium frequency in samples. Irrigation systems using surface water sources contained at least three Pythium spp. over the course of 2 years.

Carbon, nitrogen, and phosphorus are critical macroelements in freshwater systems. Historically, researchers and managers have focused on inorganic forms, based on the premise that the organic pool was not available for direct uptake by phytoplankton. We now know that phytoplankton can tap the organic nutrient pool through a number of mechanisms including direct uptake, enzymatic hydrolysis, mixotrophy, and through symbiotic relationships with microbial communities. In this review, we explore these mechanisms considering current and projected future anthropogenically-driven changes to freshwater systems. In particular, we focus on how naturally- and anthropogenically- derived organic nutrients can influence phytoplankton community structure. We also synthesize knowledge gaps regarding phytoplankton physiology and the potential challenges of nutrient management in an organically dynamic and anthropogenically modified world. Our review provides a basis for exploring these topics and suggests several avenues for future work on the relation between organic nutrients and eutrophication and their ecological implications in freshwater systems.

Cyanobacterial blooms sometimes create secondary metabolites that can be transferred between trophic levels and accumulate in fish, but little is known about what time of year fish are most susceptible. Here, we examine microcystin in the muscle, liver, and kidney of bluegill and largemouth bass from an agricultural reservoir over 12 months. We identify which fish characteristics and water parameters best explain microcystin accumulation in fish tissues. Microcystin in bluegill was significantly higher than largemouth bass. In both species, microcystin was highest in livers (bluegill mean = 57.6 ng g

Large-bodied predatory sportfish from Missouri reservoirs can contain elevated methylmercury concentrations that are of concern to the health of consumers. The concentration of total mercury (tHg) in the muscle (which 95% is in the methylated-Hg form) of harvestable-sized largemouth bass (Micropterus salmoides; LMB) was examined to determine which factors contributed to the variability of tHg concentration in sportfish populations among Missouri reservoirs. Mean tHg concentrations in LMB from each reservoir were compared to physical and chemical characteristics of the reservoir and to biological attributes of each LMB population. Low concentrations of tHg (70-170 ng/g wet weight) in LMB from large reservoirs (surface area ≥ 35,680 acres) were likely related to the dilution of chemical Hg forms with water volume and depth. The highest tHg concentrations in LMB (268-542 ng/g) were from reservoirs with low particulate inorganic material ( 1.5 mg/L) and chlorophyll a concentrations ( 14.6 μg/L), and from LMB populations with a low proportion of large fish (proportional size distribution of LMB 12 inches was 33%). These relationships suggest that resource competition among LMB likely contributed to tHg bioaccumulation in reservoirs 930 acres. Small reservoirs located in northern Missouri also may have greater methylation potential due to warmer water temperatures and anoxic conditions, but more data are needed to confirm these interactions. Fish consumption advisories for reservoirs with large surface area and volume could be reduced from one fish meal per month to one per week. To improve Missouri fisheries and protect consumers, management strategies to limit methylation and improve fish growth should be considered to reduce methylmercury bioaccumulation in small- and medium-sized reservoirs.

Harmful algal blooms (HABs) in aquatic ecosystems are of concern worldwide. This review deals with how jurisdictions around the world are addressing this water quality issue to inform recommendations regarding nutrient loading and HABs in Missisquoi Bay-Lake Champlain and Lake Memphremagog; transboundary lakes located in the USA and Canada that suffer from symptoms of eutrophication. A global scan of the literature resulted in the consideration of 12 case studies of large water bodies within large watersheds, excluding in-lake geoengineering approaches. Although all of the systems experience excessive nutrient loading, they vary in two key ways: sources of nutrients and manifestations of eutrophication ranging from HABs, to limited recreational uses, to the additional complexity of internal loadings and fish kills, up to drinking water shutdowns. The case studies were analyzed with respect to four categories of approaches, namely: (i) regulatory; (ii) incentive-based; (iii) risk mitigation; and (iv) outreach, engagement, and educational. We found that the management frameworks are based on integrated watershed management planning and national standards. National water quality standards, however, are not stringent enough to prevent HABs. Overall, identified case studies did not successfully remediate HABs, they simply managed them.

Many

The concentration of dissolved oxygen in aquatic systems helps to regulate biodiversity1,2, nutrient biogeochemistry3, greenhouse gas emissions4, and the quality of drinking water5. The long-term declines in dissolved oxygen concentrations in coastal and ocean waters have been linked to climate warming and human activity6,7, but little is known about the changes in dissolved oxygen concentrations in lakes. Although the solubility of dissolved oxygen decreases with increasing water temperatures, long-term lake trajectories are difficult to predict. Oxygen losses in warming lakes may be amplified by enhanced decomposition and stronger thermal stratification8,9 or oxygen may increase as a result of enhanced primary production10. Here we analyse a combined total of 45,148 dissolved oxygen and temperature profiles and calculate trends for 393 temperate lakes that span 1941 to 2017. We find that a decline in dissolved oxygen is widespread in surface and deep-water habitats. The decline in surface waters is primarily associated with reduced solubility under warmer water temperatures, although dissolved oxygen in surface waters increased in a subset of highly productive warming lakes, probably owing to increasing production of phytoplankton. By contrast, the decline in deep waters is associated with stronger thermal stratification and loss of water clarity, but not with changes in gas solubility. Our results suggest that climate change and declining water clarity have altered the physical and chemical environment of lakes. Declines in dissolved oxygen in freshwater are 2.75 to 9.3 times greater than observed in the world’s oceans6,7 and could threaten essential lake ecosystem services2,3,5,11., The concentration of dissolved oxygen in aquatic systems helps to regulate biodiversity, nutrient biogeochemistry, greenhouse gas emissions, and the quality of drinking water. The long-term declines in dissolved oxygen concentrations in coastal and ocean waters have been linked to climate warming and human activity, but little is known about the changes in dissolved oxygen concentrations in lakes. Although the solubility of dissolved oxygen decreases with increasing water temperatures, long-term lake trajectories are difficult to predict. Oxygen losses in warming lakes may be amplified by enhanced decomposition and stronger thermal stratification8,9 or oxygen may increase as a result of enhanced primary production10. Here we analyse a combined total of 45,148 dissolved oxygen and temperature profiles and calculate trends for 393 temperate lakes that span 1941 to 2017. We find that a decline in dissolved oxygen is widespread in surface and deep-water habitats. The decline in surface waters is primarily associated with reduced solubility under warmer water temperatures, although dissolved oxygen in surface waters increased in a subset of highly productive warming lakes, probably owing to increasing production of phytoplankton. By contrast, the decline in deep waters is associated with stronger thermal stratification and loss of water clarity, but not with changes in gas solubility. Our results suggest that climate change and declining water clarity have altered the physical and chemical environment of lakes. Declines in dissolved oxygen in freshwater are 2.75 to 9.3 times greater than observed in the world’s oceans and could threaten essential lake ecosystem services.

Among its many impacts, climate warming is leading to increasing winter air temperatures, decreasing ice cover extent, and changing winter precipitation patterns over the Laurentian Great Lakes and their watershed. Understanding and predicting the consequences of these changes is impeded by a shortage of winter-period studies on most aspects of Great Lake limnology. In this review, we summarize what is known about the Great Lakes during their 3–6 months of winter and identify key open questions about the physics, chemistry, and biology of the Laurentian Great Lakes and other large, seasonally frozen lakes. Existing studies show that winter conditions have important effects on physical, biogeochemical, and biological processes, not only during winter but in subsequent seasons as well. Ice cover, the extent of which fluctuates dramatically among years and the five lakes, emerges as a key variable that controls many aspects of the functioning of the Great Lakes ecosystem. Studies on the properties and formation of Great Lakes ice, its effect on vertical and horizontal mixing, light conditions, and biota, along with winter measurements of fundamental state and rate parameters in the lakes and their watersheds are needed to close the winter knowledge gap. Overcoming the formidable logistical challenges of winter research on these large and dynamic ecosystems may require investment in new, specialized research infrastructure. Perhaps more importantly, it will demand broader recognition of the value of such work and collaboration between physicists, geochemists, and biologists working on the world's seasonally freezing lakes and seas.

1. Freshwater cyanobacterial blooms have become ubiquitous, posing major threats to ecological and public health. 2. Decades of research have focused on understanding drivers of these blooms with a primary focus on eutrophic systems; however, cyanobacterial blooms also occur in oligotrophic systems, but have received far less attention, resulting in a gap in our understanding of cyanobacterial blooms overall. 3. In this review, we explore evidence of cyanobacterial blooms in oligotrophic freshwater systems and provide explanations for those occurrences. 4. We show that through their unique physiological adaptations, cyanobacteria are able to thrive under a wide range of environmental conditions, including low-nutrient waterbodies. 5. We contend that to fully understand cyanobacterial blooms, and thereby mitigate and manage them, we must expand our inquiries to consider systems along the trophic gradient, and not solely focus on eutrophic systems, thus shifting the high-nutrient paradigm to a trophic-gradient paradigm.

Storage and removal of nutrients by wetlands and riparian areas is an important process in understanding catchment nutrient fluxes and in helping to mitigate current issues of eutrophication in man...

This is one of few studies to comprehensively examine seasonality and phytoplankton nutrient deficiency in sub-tropical lakes over multiple years, to our knowledge. We tested whether phytoplankton communities in two sub-tropical impoundments in the mid-hills of Nepal were nitrogen (N)-, phosphorus (P)-, or co-deficient in N and P across pre-monsoon, monsoon, and post-monsoon seasons spanning a decade. Nutrient limitation to phytoplankton growth was assessed via in situ stoichiometry of N and P (N:P ratios) and nutrient stimulation experiments (NSEs). The experiments indicated co-deficiency of N and P in 97–100% of NSEs in all seasons. N-deficiency was common (> 60% of N:P ratios and ~ 90% of NSEs) during the rainy monsoon, but P-deficiency occurred twice as often as N-deficiency during drier seasons (pre- and post-monsoon). These findings provide perspective for the ongoing debate over N and P as limiting nutrients in freshwater ecosystems by incorporating seasonality and corresponding hydrology. We also re-visit long-standing assumptions regarding the nutrient status of sub-tropical lakes. The warm, stratified season does not fully illustrate lake processes, and more research during dry periods will inform how seasonality affects phytoplankton nutrient deficiencies across aquatic systems.

Small, shallow waterbodies are potentially important sites of greenhouse gas release to the atmosphere. The role of ebullition may be enhanced here relative to larger and deeper systems, due to their shallow water, but these features remain relatively infrequently studied in comparison to larger systems. Herein, we quantify ebullitive release of methane (CH4) in small shallow ponds in three regions of North America and investigate the role of potential drivers. Shallow ponds exhibited open-water season ebullitive CH4 release rates as high as 40 mmol m–2 d–1, higher than previously reported for similar systems. Ebullitive release of CH4 varied by four orders of magnitude across our 15 study sites, with differences in flux rates both within and between regions. What is less clear are the drivers responsible for these differences. There were few relationships between open water–season ebullitive flux and physicochemical characteristics, including organic matter, temperature, and sulphate. Temperature was only weakly related to ebullitive CH4 release across the study when considering all observation intervals. Only four individual sites exhibited significant relationships between temperature and ebullitive CH4 release. Other sites were unresponsive to temperature, and region-specific factors may play a role. There is some evidence that where surface water sulphate concentrations are high, CH4 production and release may be suppressed. Missouri sites (n = 5) had characteristically low ebullitive CH4 release; here bioturbation could be important. While this work greatly expands the number of open-water season ebullition rates for small and shallow ponds, more research is needed to disentangle the role of different drivers. Further investigation of the potential thresholding behaviour of sulphate as a control on ebullitive CH4 release in lentic systems is one such opportunity. What is clear, however, is that efforts to scale emissions (e.g., as a function of temperature) must be undertaken with caution.

Many physical, chemical, and biological processes in freshwater ecosystems mobilize the nutrient phosphorus (P) from sediments, which in turn may contribute to the formation of harmful algal blooms. Here, we critically reviewed internal P loading in Canadian fresh waters to understand the geographic patterns and environmental drivers of this important process. From 43 publications, we consolidated 618 estimates of internal P loading from Canadian freshwater ponds, lakes, reservoirs, and coastal wetlands (n = 70). Expressed in terms of total P, short-term gross rates in sediment samples (Lgross) ranged from −27 to 54 mg·m−2·day−1(n = 461), while long-term net rates in whole ecosystems (Lnet) ranged from −1694 to 10 640 mg·m−2·year−1(n = 157). The main environmental drivers of this variation were oxygen, pH, geology, and trophic state. Internal P loading tended to be higher during the open-water season and most prominent in small prairie lakes. Priorities for future research on internal P loading should include resolving methodological problems, assessing the relative importance of different mechanisms, examining the influence of anthropogenic activities, and quantifying rates in understudied ecosystems.

Fluid fine tailings (FFT) are a principal by-product of the bitumen extraction process at oil sands mines. Base Mine Lake (BML) – the first full-scale demonstration oil sands end pit lake (EPL) – contains approximately 1.9 x108 m3 of FFT stored under a water cover within a decommissioned mine pit. Chemical mass transfer from the FFT to the water cover can occur via two key processes: (1) advection-dispersion driven by tailings settlement; and (2) FFT disturbance due to fluid movement in the water cover. Dissolved chloride (Cl) was used to evaluate the water cover mass balance and to track mass transport within the underlying FFT based on field sampling and numerical modeling. Results indicated that FFT was the dominant Cl source to the water cover and that the FFT is exhibiting a transient advection-dispersion mass transport regime with intermittent disturbance near the FFT-water interface. The advective pore water flux was estimated by the mass balance to be 0.002 m3 m−2 d−1, which represents 0.73 m of FFT settlement per year. However, the FFT pore water Cl concentrations and corresponding mass transport simulations indicated that advection rates and disturbance depths vary between sample locations. The disturbance depth was estimated to vary with location between 0.75 and 0.95 m. This investigation provides valuable insight for assessing the geochemical evolution of the water cover and performance of EPLs as an oil sands reclamation strategy.

Temperate lakes are ice covered for much of the year; however, winter lake conditions have not been well studied and are undergoing rapid change. Using data collected during ice-on periods from 4 north-temperate water bodies, we report observations of stable surface layers, solar-induced convective mixed layers, and their potential impacts on phytoplankton. The convective mixed layer is defined as the region where the convective Richardson number (Ri) is ≤1. In the absence of a convective mixed layer, peaks in chlorophyll a were near the ice–water interface. Light conditions here seemed sufficient to support phytoplankton biomass accrual in the short-term in 50% of our measurements, although snow depths >13.5 cm may lead to light limitation. When a convective mixed layer was present, light conditions were sufficient for biomass accrual in 37.5% of cases. The frictional timescale for damping averaged 15 minutes, indicative of a lack of mixing at night. Convective mixing depths and velocity increase...

Aside from a companion investigation to this study, there are currently no peer-reviewed phosphorus (P) release rate data for northern North American (i.e., Canadian) reservoirs. Using Lake Diefenbaker, Saskatchewan, Canada as a case study, we tested the effect of variation in overlying water DO conditions on the P release rates from sediment cores. Sediment cores from four down-reservoir locations in Lake Diefenbaker were incubated under high (>8 mg l−1), low (2–3 mg l−1), or anoxic (

Lake Diefenbaker (LD) is an important source of water for southern Saskatchewan. LD is characterized by numerous embayments containing anthropogenic activities (e.g., housing, marinas, cattle watering). Many of these activities are increasing on this important reservoir in association with the rapidly developing economy of Saskatchewan. These activities may reduce water quality directly or indirectly by encouraging the growth of nuisance algae (i.e. cyanobacteria). Here, we examined phytoplankton biomass and composition in eight embayments exposed to anthropogenic activities, four unexposed embayments with no perceived human activities and six main channel sites adjacent to the embayments from June to October (2011 and 2012). Phytoplankton biomass and composition was not significantly different in exposed, unexposed embayments and main channel sites (p > 0.05), with the diatoms and cryptomonads constituting 87%–91% of the total phytoplankton biomass in both years. High flows from the South Saskatc...

Winter is an important season for many limnological processes, which can range from biogeochemical transformations to ecological interactions. Interest in the structure and function of lake ecosyst ...

Primary production by phytoplankton helps fuel hypolimnetic oxygen depletion as well as food web productivity, and is thought to have been affected by dreissenid mussel colonization in Lake Simcoe. Measured by short-term 14 C uptake, areal and volumetric rates of photosynthetic carbon uptake (production) were lower in nearshore ( a (Chl a ) concentrations were also lower at nearshore sites, except in summer (June–Aug.). Production increased from May to an annual maximum in September (350 mg C m − 2 d − 1 nearshore and 650 mg C m − 2 d − 1 offshore) and remained high through November; its annual minimum ( − 2 d − 1 ) was in February. A secondary spring maximum occurred in April in the offshore (450 mg C m − 2 d − 1 ) but not the nearshore (30 mg C m − 2 d − 1 ). Chl a and production of net phytoplankton (> 20 μm) was greatest from August through November at offshore sites (median contribution > 35% of total) but was much less at nearshore sites (median contribution a were dominated by nanoplankton (2–20 μm), with median contributions > 40% in most seasons. The lower production at nearshore than offshore sites supports previous inferences that dreissenid mussels have caused differential loss of planktonic primary production, especially of net phytoplankton, in nearshore waters. The results from Lake Simcoe are consistent with evidence from other lakes that dreissenids can have major impacts on seasonal blooms of diatoms as the mussels continue to spread within and among North American lakes.

The Boreal Plains Ecozone (BPE) in Western Canada is expected to be an area of maximum ecological sensitivity in the 21st century. Successful climate adaptation and sustainable forest management require a better understanding of the interactions between hydrology, climate, and vegetation. This paper provides a perspective on the changing water cycle in the BPE from an interdisciplinary team of researchers, seeking to identify the critical knowledge gaps. Our review suggests the BPE will likely become drier and undergo more frequent disturbance and shifts in vegetation. The forest will contract to the north, though the southern boundary of the ecotone will remain in place. We expect detrimental impacts on carbon sequestration, water quality, wildlife, and water supplies. Ecosystem interactions are complex, and many processes are affected differently by warming and drying, thus the degree and direction of change is often uncertain. However, in the short term at least, human activities are the dominant source of change and are unpredictable but likely decisive. Current climate, hydrological, and ecological monitoring in the BPE are limited and inadequate to understand and predict the complex responses of the BPE to human activities and climate change. This paper provides a case study of how hydrological processes critically determine ecosystem functioning, and how our ability to predict system response is limited by our ability to predict changing hydrology. WIREs Water 2015, 2:505–521. doi: 10.1002/wat2.1098 For further resources related to this article, please visit the WIREs website.

Lake Diefenbaker (LD) is a large reservoir located in the Canadian Prairies. Approximately 98% of the inflow is from the South Saskatchewan River (SSR). The composition and ecology of the phytoplankton community along the length of LD has not been reported since the 1980s. This is a potential problem for a reservoir that serves as an important source of water for indirect domestic consumption, irrigation, and recreation. Therefore, we examined epilimnetic whole water samples along the length of LD during the open-water season (June to October) in 2011 and 2012. We estimated the trophic status from chlorophyll a (TSI chl a ) and examined the phytoplankton community in relation to environmental factors. We observed high flow events that carried large nutrient loads and associated turbidity into LD from the SSR. Despite the high flow events, mean TSI chl a placed LD as a mesotrophic system with a highly diverse phytoplankton community (72 genera). The increased nutrient and non-algal turbidity associated with high flow events may be related to the dominance of the cryptophytes and the bacillariophytes (both constituted ~ 89% of the total phytoplankton biomass). Inflow and water temperature explained 41% of the variation in cryptophyte biomass, whereas mixing depth and particulate nitrogen to particulate phosphorus molar ratios explained 38% of the variation in bacillariophyte biomass. Despite the low cyanobacterial biomass (

Increasingly, our global water storage is contained in large reservoirs that retain nutrients. Given the value of reservoirs to ecosystem services, it is important to know the risks associated with the mobilization of legacy phosphorus (P) from sediments. From 2011 to 2013, Lake Diefenbaker was a significant sink for P retaining 91% of the external total phosphorus (TP) and 41% of the dissolved reactive phosphorus (DRP) loaded from the tributaries. We investigated if this retained P has the potential to re-enter the water column through internal P loading from sediments. In 2013, we estimated the rates of internal P loading with a specific focus on measuring TP, DRP, and total dissolved iron release from sediments. We estimated that over the whole reservoir, the internal load (based on TP increases) was 169 mg TP m − 2 summer − 1 and 229 mg TP m − 2 winter − 1 . Year-round, the mean daily internal P loading rate was 1.8 mg TP m − 2 day − 1 , representing ~ 24% of the annual external TP load. Although our internal load estimates are dependent upon sedimentation rates, they do suggest winter internal P loading is important, resulting in a continuous release of P from the sediments into the overlying water year-round. Internal loading from sediments is an unaccounted-for source of P to Lake Diefenbaker and should be considered in future management of this critically important reservoir. Our recommendations for other reservoirs in obtaining realistic estimates of internal P loading include year-round estimates considering both the stratification and flow regimes.

The reliance on reservoirs to meet global water demands is increasing, as is the need to develop an understanding of factors influencing water quality in these understudied water bodies. This study assessed how the interaction between light and nutrients influences phytoplankton dynamics in a large, heterogeneous prairie reservoir; Lake Diefenbaker, Saskatchewan. A multiple indicator assessment of factors (light, phosphorus [P], and nitrogen [N]) influencing phytoplankton biomass, physiology, and gross primary productivity (GPP) was conducted. Light deficiency was assessed through the application of an indicator threshold for the mean daily mixed layer irradiance as well as the ratio of this irradiance to the light saturation parameter derived from photosynthesis-irradiance curves. Short-term physiological assays and long-term compositional nutrient status indicators were applied to assess the in situ nutrient deficiency of the epilimnetic phytoplankton communities. We observed regional differences in light conditions and nutrient chemistry related to incoming flow and nutrient load from the South Saskatchewan River. Light deficiency was detected 59% of the times measured during the open-water season of 2013. Both GPP and quantum efficiency of photosystem II (ϕ PSII ) were relatively high (108 ± 101 mmol O 2 m − 2 day − 1 and 0.57 ± 0.07, respectively) across all regions and months. During conditions of light sufficiency, there was no evidence of N deficiency, and P was the limiting nutrient to phytoplankton communities 90% of the times measured. Nutrient use efficiency and GPP rates were significantly higher under light sufficient conditions. The asynchrony of light and nutrient supply influences production and biomass accrual in this reservoir.

There is an up-coming global surge in dam construction. River damming impacts nutrient cycling in watersheds through transformation and retention in the reservoirs. The bioavailability of silicon (Si) relative to nitrogen (N) and phosphorus (P) concentrations, in combination with light environment, controls diatom growth and therefore influences phytoplankton community compositions in most freshwater aquatic ecosystems. In this study, we quantified reactive Si cycling and annual Si retention in Lake Diefenbaker, a 394 km 2 prairie reservoir in Saskatchewan, Canada. Retention estimates were derived from 7 sediment cores combined with high-resolution spatiotemporal sampling of water column dissolved Si (DSi). Current annual DSi retention in the reservoir is approximately 28% of the influx, or 2.5 × 10 8 mol yr − 1 . The relative retention of DSi is higher than that of bioavailable N, but lower than that of bioavailable P, indicating a decoupling of the P, N, and Si cycles in the reservoir. The largest accumulation of reactive particulate Si (PRSi) is found in sediments deposited in the mid-reservoir region, which corresponds to the region of highest epilimnetic DSi decrease during the 2013 open water season. This region corresponds to the fluvial-to-lacustrine transition zone, and represents a hot spot for Si retention. Differences in retention efficiencies of macronutrients may not only affect the in-reservoir ecology and water quality, but also modifies the N:P:Si ratios exported downstream, which has the potential to alter ecosystem processes in receiving water bodies.

The nutrient budget of the Lake Diefenbaker reservoir (SK, CAN) is being developed as part of a larger multi-faceted trophic status study. However, it is currently unknown if shoreline erosion from the near vertical banks serves as a source of nutrients to the reservoir. Thus, the objective of this study was to quantify the annual nutrient loading to the reservoir through shoreline erosion, and the relative magnitude of erosion-derived inputs versus nutrient loading through the major tributary, the South Saskatchewan River (SSR). Nutrient status measurements indicate that Lake Diefenbaker algal populations would be responsive to additional inputs of phosphorus (P). We measured the total carbon (TC), total nitrogen (TN), Mehlich 3 P, and iron oxide strip extracted (FeO) P nutrient content by weight in the bank soils along the reservoir. Combined with shoreline erosion rates and lengths, the mean annual nutrient inputs for TC, TN, M3P, and FeO P were 57,500 kg year − 1 , 2100 kg year − 1 , 49.8 kg year − 1 and 30.0 kg year − 1 , respectively. We also compared nutrient content and inputs across bank classifications and land uses to identify factors contributing to nutrient loading through erosion and to determine potential areas for mitigation efforts. Relative to the primary nutrient loading from the SSR, bank soils represent less than 1% of the overall load. These measurements of nutrient inputs will allow us to assess the relative contribution of soil nutrients to prairie reservoirs and will serve as important baseline data for the reservoir.

Agricultural runoff is an important source of phosphorus (P) to surface waters. This paper investigated the relationship between agricultural land use and the forms of P (i.e., total phosphorus [TP], total dissolved phosphorus [TDP], and soluble reactive phosphorus [SRP]) in streams draining 8 headwater subcatchments in the Beaver River subwatershed, a major inflow to Lake Simcoe, which is a large hard-water lake in south-central Canada. The time period of analysis had a strong influence on the results. There was no relationship between percent total agriculture and average TP when concen trations were averaged over the entire 12-month (Jun 2010–May 2011) monitoring period, whereas there were significant positive correlations between agricultural land use and average TP during the summer season (Jun–Aug). Significant cor relations between average stream TDP and SRP and percent pastureland were observed, although relationships were again dependent on the time period of analysis. Concentrations of TP, TDP, and SRP were highly variable over time, with maximum concentrations occurring during the winter months. This was illustrated by a single rain-on-snow event (11 mm) on 5 March 2011, when samples taken 3–4 h apart varied by as much as 100%. These results indicate that winter storm-targeted sampling is likely necessary to capture the full range of annual variability in P forms and concentrations.

The invasion of dreissenid mussels to Lake Simcoe coincided with notable changes in the ecosystem, including a period of lower phytoplankton biovolume, particularly at shallower sites. Dreissenid grazing effects are typically most prominent during the summer season in the nearshore areas of lakes. Grazing effects in the winter are small because dreissenid filtration rates, especially those of zebra mussels ( Dreissena polymorpha), decrease in cold temperatures. Phytoplankton dynamics in the winter season are poorly characterized, particularly in lakes that experience ice cover. The purpose of this study was to examine the effects of dreissenids on the seasonality of phytoplankton dynamics in the nearshore waters of the lake using long-term monitoring data from unchlorinated water treatment plant (WTP) intake pipes. The long-term dataset from the WTPs showed significant and sustained declines in phytoplankton biovolumes and chlorophyll a (Chl-a) concentrations in the 12-year period following the invasion of dreissenid mussels to Lake Simcoe. The decline was smallest during the ice-covered winter months (Jan–Mar). The fall phytoplankton peak observed in the pre-dreissenid years shifted to a smaller peak during the winter in post-dreissenid years. We hypothesized that reduced dreissenid grazing pressure during the winter, and possible improved under-ice light conditions that we attribute to climate change, may be contributing to this shift. Phytoplankton biovolume data collection continues to be important in Lake Simcoe as a measure of phytoplankton biomass. Our results also indicate that winter phytoplankton biovolumes should be considered when managing lakes experiencing both climate change and dreissenid mussel effects.

Lake Simcoe in Ontario, Canada, is a large lake surrounded by a mix of urban, agricultural, and less developed areas and is showing adverse effects from excess nutrient inputs, including low hypolimnetic oxygen concentrations. Knowledge of both the quantity and quality of nutrients and seston entering the lake is important because large reductions in phosphorus (P) loads have been proposed to help restore the lake and its coldwater fishery. We examined land use effects on P quality (i.e., bioavailability) and its relationship to seston in the tributaries of Lake Simcoe. Indicators of agricultural impacts were examined in 13 tributaries of Lake Simcoe, which were selected to represent a range of land use types. Bioavailability of P was assessed through analysis of different forms of P and stoichiometric indicators of nutrient status in seston. Nutrient sources were examined using the δ 15 N of seston. The percentage of cropland in the subwatershed had a strong relationship with P as reflected in higher soluble reactive P concentrations and lower indicators of P deficiency. Cropland land use effects were complicated; they contributed highly bioavailable P to a P deficient lake, and at the same time, contributed high seston loads causing turbidity, resulting in light deficiency. In the Lake Simcoe watershed, animal manure application on cropland could be a source of nutrients related to the δ 15 N variability and, correspondingly, bioavailable P. Management efforts should therefore include best management practices to reduce manure application to croplands and to prevent runoff from areas where manure is stored.

Bacteria serve an important function in aquatic environments and are associated with nutrient regeneration, carbon (C) metabolism, and secondary production. This is the first study to measure heterotrophic bacterial production, abundance, biomass, and biovolume in Lake Simcoe, Ontario. Excessive phosphorus loading resulting in low hypolimnetic oxygen concentrations has impacted the cold-water fishery in Lake Simcoe. We tested the hypothesis that bacteria contributed to dissolved oxygen declines in Lake Simcoe and examined the environmental factors impacting bacterial activity. Spatial and temporal variations in the microbial community were measured from June 2010 to July 2011. A dual-isotope method ( 3 H-TdR and 14 C-leu) was used, resulting in mean (± standard deviation) annual epilimnetic bacterial production estimates of 0.130 ± 0.173 and 0.268 ± 0.304 µg C L −1 h −1 , respectively. The mean annual bacterial abundance was 1.49 ± 1.53 cells × 10 9 L −1 , with a mean biomass of 1.21 ± 1.34 µg C L −1 and a mean biovolume of 0.0043 ± 0.0030 µm 3 cell −1 . These estimates had distinct seasonal patterns, with consistently lower bacterial activity in the winter relative to the spring, summer, and fall. Differences between epilimnetic and hypolimnetic bacterial activity were inconsistent for the 4 bacterial parameters measured. Lake temperature, chlorophyll a, and dissolved organic C concentrations were the most significant factors influencing the annual epilimnetic patterns in the examined bacterial parameters. Annual bacterial production was low in Lake Simcoe and does not seem to be a major contributor to the low hypolimnetic oxygen concentrations in the lake.

This special section of Inland Waters contains the final installment of a collection of papers on Lake Simcoe, Ontario, Canada. Collectively, the 4 papers in Volume 3, Issue 1 and the 13 papers in the current issue demonstrate the integrative and collaborative monitoring and research efforts underway to protect this large, multi-stressed lake. In the first preface to the special sections (Palmer et al. 2013), we detailed legislative and financial initiatives that have been implemented by the federal and provincial governments to support science in the Lake Simcoe watershed. Here, we summarize the resultant science reported in these special sections and highlight priority areas for future work.

Lake Simcoe, the largest lake in southern Ontario outside of the Laurentian Great Lakes, is affected by numerous stressors including eutrophication resulting from total phosphorus (TP) loading, climate change, and invasions of exotic species. We synthesized the long-term responses of Lake Simcoe to these stressors by assessing trends in water quality and biological composition over multiple trophic levels. Evidence for climate change included increasing thermal stability of the lake and changes in subfossil diatom communities over time. Although the deep water dissolved oxygen (O 2 ) minimum has increased significantly since TP load reductions, it is still below estimated historical values and the Lake Simcoe Protection Plan end-of-summer target level of 7 mg O 2 L −1 . Low deep water O 2 concentrations corresponded with a decline in coldwater fish abundance. Since 1980, some nutrient concentrations have decreased (spring TP) while others have increased (silica), but many show no obvious changes (ice-free TP, nitrate, ammonium). Increases in water clarity, combined with declines in chlorophyll a and phytoplankton biovolumes in Cook’s Bay, were temporally consistent with declines in TP loading and the lake-wide establishment of dreissenid mussels as a major component of the Lake Simcoe ecosystem. Using an investigative tool, we identified 2 periods when abrupt shifts potentially occurred in multiple parameters: 1986 and 1995–1997. Additional ecosystem level changes such as declines in zooplankton, declines in offshore benthic invertebrate abundance, and increased nearshore invertebrate abundance likely reflect the effects of invasive species. The interaction of these multiple stressors have significantly altered the Lake Simcoe ecosystem.

This special section of Inland Waters features the first of numerous papers that highlight trends and insights emerging from decades of ecological monitoring and research activities on Lake Simcoe, Canada. Lake Simcoe is the largest lake in southern Ontario after the Laurentian Great Lakes. Like most large lakes, Simcoe has been negatively impacted over the past century by human activities, which accelerated dramatically around the 1930s (Hawryshyn et al. 2012). Phosphorus (P) loading from point and nonpoint sources caused excessive growth of plants and algae that consume hypolimnetic oxygen during decomposition, which limited coldwater fish habitat and contributed to the recruitment failure of popular sportfish such as lake trout ( Salvelinus namaycush ) and lake whitefish ( Coregonus clupeaformis ; Evans et al. 1996). The establishment in recent decades of invasive fish, invertebrates, and plants is changing lake habitat, food webs, and native species dynamics (Evans et al. 2011, Ginn 2011, Ozersky et al. 2011). Increasing air temperature associated with climate change has prolonged thermal stratification and shortened the period of ice cover (OMOE et al. 2009, Stainsby et al. 2011). Metals and organic pollutants originating from urban and industrial sources have accumulated in lake and tributary sediments (Helm et al. 2011, Landre et al. 2011), potentially affecting aquatic biota and increasing the risk associated with human fish consumption (Gewurtz et al. 2011, Lembcke et al. 2011). Additionally, the cumulative effects of these and other stressors have drastically altered aquatic communities (Depew et al. 2011, Ginn 2011, Jimenez et al. 2011, Winter et al. 2011). In response to public concern about the ecological health of the lake, the Lake Simcoe Protection Act was approved by the Government of Ontario in 2008 with a mandate to protect and restore the Lake Simcoe watershed (Government of Ontario 2008). The Act established the Lake Simcoe Protection Plan (LSPP; OMOE et al. 2009) that identifies a number of targets and indicators to characterize environmental health in the Lake Simcoe watershed and details 119 policies and actions to achieve these targets. Scientific monitoring and research play an integral role in the success of the LSPP, which supports an ecosystem approach to informing policies and actions, taking into account the interconnectedness of the lake and watershed. The LSPP mandates the enhancement of current monitoring programs, development of new monitoring programs, and the promotion and implementation of research projects that build upon existing science to continually update management decisions as part of an adaptive management approach. The challenges posed by the LSPP necessitate collaborative research efforts and sharing of responsibilities, resources, and knowledge among federal, provincial, and local governments, academics, conservation authorities, agricultural, commercial, and industrial sectors, First Nations communities, the general public, and other stakeholders. The collection of papers shows the value of a collaborative approach and demonstrates how strong partnerships can facilitate integrative approaches to scientific monitoring and research efforts being used to protect Lake Simcoe.

Increased human population growth, reduction of phosphorus (P) loading, and the invasion of dreissenid mussels may have changed the spatial pattern and relationships between the nearshore and the offshore seston and nutrient concentrations in the eastern basin of Lake Erie over the past 30 years. We compared seston characteristics, nutrient concentrations, and phytoplankton nutrient status between nearshore and offshore zones in years before (1973–1985) and after (1990–2003) the dreissenid invasion. In 1973 (the only pre-dreissenid year nearshore data was collected), chlorophyll a (chl a ) and nutrient concentrations were higher nearshore than offshore. In post-dreissenid years, nearshore chl a concentrations became significantly lower than the offshore, while carbon (C):chl a ratios became higher, which was related to mussel grazing and possibly photoacclimation. Phosphorus deficiency in the phytoplankton increased over the 30-year period, and in the post-dreissenid years was less acute in the nearshore than offshore. Mean water column irradiance became higher in the nearshore relative to the offshore in the post-dreissenid years. The nutrient changes and phytoplankton physiology were consistent with the expected effects of nutrient cycling by mussels and diminished demand by phytoplankton despite increased demand from benthic algae in the nearshore. This basin-scale study suggests that dreissenid mussel invasion can be associated with alterations in the spatial pattern of water column properties in large lakes even on open coasts with vigorous circulation and exchange.

Phytoplankton interactions with iron (Fe) were examined in surface waters of Lake Erie during summer thermal stratification. Lake-wide sampling in June and September 2005 was conducted using a continuous surface water sampler (1 m sampling depth) and in July at 18 hydrographic stations (5 m sampling depth). In situ measurements of photosynthetic efficiency (maximum quantum yield of photosystem II) and phytoplankton community composition were measured using fast repetition rate fluorometry and a phytoplankton pigment-specific fluorometer, respectively, during June and September. High ratios (73%–85%) of intracellular Fe to particulate Fe coincident with increases in chlorophyll a (Chl a) concentrations in the western and central basins in June and July imply that the majority of Fe in these regions was associated with intracellular pools. Correlations between intracellular Fe and Chl a were frequently observed when Heterokontophyta and Pyrrophyta dominated the phytoplankton community. Assimilation of Fe by the phytoplankton strongly influenced its partitioning between the dissolved and particulate phase. Dissolved iron (

(2008). Nitrogen, phosphorus, and iron colimitation of phytoplankton communities in the nearshore and offshore regions of the African Great Lakes. SIL Proceedings, 1922-2010: Vol. 30, No. 2, pp. 259-264.

Three nutrient enrichment experiments involving the addition and removal of iron (Fe) alone, as well as in combination with phosphorus (P) and/or nitrogen (N), were conducted in the offshore and nearshore waters of the eastern basin of Lake Erie. Lake Erie phytoplankton are at times colimited by P, N, and Fe. This was most clearly demonstrated in the offshore, strongly stratified waters, where the Fe concentration was below detection (DL 5 2.0 nmol L21), and nutrient limitation indicators (APA, P debt, C : P, N : P, N debt, and C : N) indicated strong P and moderate N limitation. Enrichment with Fe alone did not result in a significant increase in phytoplankton biomass, but the combined addition of Fe, P, and N yielded greater biomass increases than the addition of P and N alone. Phosphorus and Fe were both required to stimulate biomass increases. Nutrient limitation indicators and dissolved nutrient measurements showed that P and Fe additions facilitated the uptake of nitrate (NO 2 ), which is the most abundant source of N because of the low ammonium (NH þ ) concentrations in Lake Erie. Our results support a colimitation hypothesis wherein the addition of Fe reduces N limitation by allowing the phytoplankton communities to use NO 2 , take up more P, and become more strongly P limited. Phytoplankton communities of the eastern basin of Lake Erie and numerous other lakes experience colimitation by P, N, and Fe during the summer period of thermal stratification. Iron may be an important limiting nutrient in some freshwaters as well as in some areas of the oceans. Recent

Reduction of phosphorus (P) inputs to Lake Erie since the 1970s have resulted in lower mean total phosphorus (TP) concentration in the lake and lower mean chlorophyll a concentration. In this study we examine indicators of phytoplankton nutritional status to determine whether the observed decrease in TP concentration has resulted in a strongly P deficient phytoplankton community. Phytoplankton nutrient status measurements including alkaline phosphatase activity (APA), P and nitrogen (N) debt, PO 4 turnover time and particulate carbon (C), N and P stoichiometric ratios, were made in all three basins of Lake Erie from May through October in 1997. The phytoplankton in the western basin only infrequently showed signs of nutrient deficiency over the May through October sampling season. Phytoplankton in the large central basin were moderately P deficient during the stratified season except for a period of strong deficiency in July, the time of maximum heat content and strong stratification. The eastern basin became moderately P deficient with the onset of stratification and remained moderately P deficient over the entire stratified season and no indications of extreme deficiency were observed. Compared to several other lakes in the same geoclimatic region as Lake Erie, including Lake Superior, the phytoplankton community was not strongly nutrient deficient.

Lake Diefenbaker (LD) is a large reservoir on the South Saskatchewan River used for agricultural irrigation, drinking water, and recreation. Our objectives were to determine the distribution and abundance of bacterial indicators in embayments and the main channel of LD and to relate these to environmental factors. Total coliforms (TCs), fecal coliforms (FCs), and fecal indicator bacteria (i.e., Escherichia coli) were measured concurrently with water quality parameters. Although TCs, FCs, and E. coli were present in LD, they rarely exceeded the TC and FC Canadian Council of Ministers of the Environment (CCME) water quality standards for agricultural use (1000 colony-forming units (CFU) per 100 mL and 100 CFU per 100 mL, respectively). The correlation between the bacterial indicators in the sediments and the water column indicates that higher embayment abundances may be related to sediment loading and (or) resuspension events in these frequently mixed embayments. With higher water temperatures and water levels, as well as higher microbial activity, CCME bacterial limits may be exceeded. The greatest contributor to bacterial indicator abundance was water temperature. We predict that water quality standards will be exceeded more frequently with climate warming.

The (Lower) Lake of Zurich provides an ideal system for studying the long-term impact of environmental change on deep-water hypoxia because of its sensitivity to climatic forcing, its history of eutrophication and subsequent oligotrophication, and the quality and length of its data set. Based on 39 years (1972-2010) of measured profiles of temperature, oxygen concentration and phosphorus (P) concentration, the potentially confounding effects of oligotrophication and climatic forcing on the occurrence and extent of deep-water hypoxia in the lake were investigated. The time-series of Nurnberg's hypoxic factor (HF) for the lake can be divided into three distinct segments: (i) a segment of consistently low HF from 1972 to the late-1980s climate regime shift (CRS); (ii) a transitional segment between the late-1980s CRS and approximately 2000 within which the HF was highly variable; and (iii) a segment of consistently high HF thereafter. The increase in hypoxia during the study period was not a consequence of a change in trophic status, as the lake underwent oligotrophication as a result of reduced external P loading during this time. Instead, wavelet analysis suggests that changes in the lake's mixing regime, initiated by the late-1980s CRS, ultimately led to a delayed but abrupt decrease in the deep-water oxygen concentration, resulting in a general expansion of the hypoxic zone in autumn. Even after detrending to remove long-term effects, the concentration of soluble reactive P in the bottom water of the lake was highly correlated with various measures of hypoxia, providing quantitative evidence supporting the probable effect of hypoxia on internal P loading. Such climate-induced, ecosystem-scale changes, which may result in undesirable effects such as a decline in water quality and a reduction in coldwater fish habitats, provide further evidence for the vulnerability of large temperate lakes to predicted increases in global air temperature.

High rates of nutrient loading from agricultural and urban development have resulted in surface water eutrophication and groundwater contamination in regions of Ontario. In Lake Simcoe (Ontario, Canada), anthropogenic nutrient contributions have contributed to increased algal growth, low hypolimnetic oxygen concentrations, and impaired fish reproduction. An ambitious programme has been initiated to reduce phosphorus loads to the lake, aiming to achieve at least a 40% reduction in phosphorus loads by 2045. Achievement of this target necessitates effective remediation strategies, which will rely upon an improved understanding of controls on nutrient export from tributaries of Lake Simcoe as well as improved understanding of the importance of phosphorus cycling within the lake. In this paper, we describe a new model structure for the integrated dynamic and process-based model INCA-P, which allows fully-distributed applications, suited to branched river networks. We demonstrate application of this model to the Black River, a tributary of Lake Simcoe, and use INCA-P to simulate the fluxes of P entering the lake system, apportion phosphorus among different sources in the catchment, and explore future scenarios of land-use change and nutrient management to identify high priority sites for implementation of watershed best management practises.