The impact of phytoplankton community composition on optical properties and satellite observations of the 2017 western Lake Erie algal bloom

Since the early 2000s Lake Erie has seen a dramatic increase in phytoplankton biomass, manifested in particular by the rise in the severity of cyanobacteria blooms and the prevalence of potentially toxic taxa such as Microcystis. Satellite remote sensing has provided a unique capacity for the synoptic detection of these blooms, enabling spatial and temporal trends in their extent and severity to be documented. Algorithms for satellite detection of Lake Erie algal blooms often rely on a single consistent relationship between algal or cyanobacterial biomass and spectral indices such as the Maximum Chlorophyll Index (MCI) or Cyanobacteria Index (CI). Blooms, however, are known to vary significantly in community composition over space and time. A suite of phytoplankton and optical property measurements during the western Lake Erie algal bloom of 2017 showed highly diverse bloom composition with variable absorption and backscatter properties. Elevated backscattering coefficients were observed in the Maumee Bay, likely due to phytoplankton cell morphology and buoyancy regulating gas vacuoles, compared with typically Planktothrix dominated blooms in Sandusky Bay. MCI and CI calibrated to historical chlorophyll observations and applied to Sentinel 3's OLCI sensor accurately captured the 2017 bloom in Maumee Bay but underestimated the Sandusky Bay bloom by nearly 80%. The phycoerythrin-rich picocyanobacteria Aphanothece and Synechococcus were found in abundance throughout the western and central basins, resulting in substantial biomass underestimations using blue to green ratio-based algorithms. Potential misrepresentation of bloom severity resulting from phytoplankton optical properties should be considered in assessments of bloom conditions on Lake Erie.     

Re-eutrophication of Lake Erie: Correlations between tributary nutrient loads and phytoplankton biomass

Both abiotic and biotic explanations have been proposed to explain recent recurrent nuisance/harmful algal blooms in the western basin and central basin of Lake Erie. We used two long-term (> 10 years) datasets to test (1) whether Lake Erie total phytoplankton biomass and cyanobacterial biomass changed over time and (2) whether phytoplankton abundance was influenced by soluble reactive phosphorus or nitrate loading from agriculturally-dominated tributaries (Maumee and Sandusky rivers). We found that whereas total phytoplankton biomass decreased in Lake Erie's western basin from 1970 to 1987, it increased starting in the mid-1990s. Total phytoplankton and cyanobacterial seasonal (May–October) arithmetic mean wet-weight biomasses each significantly increased with increased water-year total soluble reactive phosphorus load from the Maumee River and the sum of soluble reactive phosphorus load from the Maumee and Sandusky rivers, but not for the Sandusky River alone during 1996–2006. During this same time period, neither total phytoplankton nor cyanobacterial biomass was correlated with nitrate load. Consequently, recently increased tributary soluble reactive phosphorus loads from the Maumee River likely contributed greatly to increased western basin and (central basin) cyanobacterial biomass and more frequent occurrence of harmful algal blooms. Managers thus must incorporate the form of and source location from which nutrients are delivered to lakes into their management plans, rather than solely considering total (both in terms of form and amount) nutrient load to the whole lake. Further, future studies need to address the relative contributions of not only external loads, but also sources of internal loading.     

Assessment of Microcystis growth rate potential and nutrient status across a trophic gradient in western Lake Erie

Plankton tow samples collected from 2002 through 2009 indicate that Microcystis biovolume in western Lake Erie is often most dense in transition zone (TZ) waters between Maumee Bay and the center of the western basin. TZ waters are generally high in nutrients and turbidity, and concentrations of each decrease with distance from Maumee Bay. High Microcystis biovolume in the TZ suggests the possibility that the conditions in these waters support a greater Microcystis growth rate relative to the open lake. To test this hypothesis, during the 2008 bloom, Microcystis was collected from western Lake Erie for measurements of total protein content (TPC) as an indicator of growth rate potential and cellular nutrient content to indicate nutrient deficiencies. TPC results indicate that Microcystis in the TZ had a higher potential growth rate compared to offshore waters. TPC values in Maumee Bay were intermediate but not significantly different from the TZ and offshore. Nitrogen content of Microcystis remained high over the summer at all sites, despite very low dissolved nitrate concentrations and low total nitrogen-to-total phosphorus ratio in late summer in the lake. Ammonium level in the lake was constant during the summer, and likely provided the nitrogen source for Microcystis. Cellular phosphorus content varied between site and sample date suggesting that Microcystis was moderately phosphorus deficient. Quotas of micronutrient indicated that Microcystis was not deficient of micronutrients. Results of this study suggest the waters in and adjacent to Maumee Bay provide more favorable growth conditions for Microcystis than offshore waters.     

Potamoplankton of the Maumee River during 2018 and 2019: The relationship between cyanobacterial toxins and environmental factors

While algal blooms are common in eutrophic lakes, blooms can also occur in tributaries that load nutrients into the lake. We sampled six sites along a 122-km stretch of the Maumee River May through October 2018 and 2019 at weekly to biweekly intervals to determine if algal blooms occur, in particular toxic cyanobacteria, and to provide insights on potential environmental drivers of blooms. Samples were analyzed for concentrations of potamoplankton (=riverine phytoplankton), chlorophyll a, nutrients, cyanobacterial toxins, microcystins and saxitoxins, and cyanotoxin genes (mcyE and sxtA). Extreme precipitation in 2019 resulted in more high discharge events during 2019 than in 2018. Chlorophyll a ranged from 50 µg/L to 300 µg/L during periods of low discharge (<50 m³/s), and green algae and diatoms accounted for the majority of the chlorophyll a. In both years, cyanobacteria comprised a low proportion of all chlorophyll a, usually<20 %, but microcystins and saxitoxins were detectable in 38.7 % and 16.7 % samples, respectively, and mcyE and sxtA were detected in 36.2 % and 59.7 % samples, respectively. Therefore, cyanotoxins were present even when cyanobacteria were not at bloom densities. Chlorophyll a, cyanotoxin genes, and microcystins negatively correlated with discharge rate measured on the date of sample collection. Together our results suggest that cyanotoxins can occur in any portion of the Maumee River during low discharge conditions. Climate change is expected to reduce precipitation during the warm summer months in the Maumee River watershed and thus possibly increase the frequency of low discharge conditions that favor cyanobacteria.     

Relationships between environmental factors,periphyton biomass and nutrient content in Garças Reservoir,a hypereutrophic tropical reservoir in southeastern Brazil

This study focused on evaluating the factors related to seasonal variations of periphyton biomass and nutrient content (N, P) in Garças Reservoir, a tropical, shallow hypereutrophic tropical reservoir in southeastern Brazil that exhibits a permanent cyanobacteria bloom. Artificial substrata were placed in the subsurface water (20 cm) of the lake on a monthly basis (incubation time of 28 days) over a 1‐year period. Two limnological periods were characterized, including: (i) a period of decreased water clarity, higher levels of soluble reactive phosphorus and higher phytoplankton biomass; and (ii) a period of relatively higher water clarity (as measured by Secchi depth), higher nitrate concentration and lower phytoplankton biomass. The periphyton chlorophyll‐a levels were lower during the first period, being negatively correlated with phytoplankton biomass. The results of this study suggest that during the period of decreased water clarity, periphyton was primarily light‐limited. In contrast, the periphyton biomass was higher during the second period, regardless of P limitation of periphyton growth. Rehabilitation of this highly degraded tropical reservoir must consider the light regime, which is controlled by phytoplankton abundance. Thus, a reduction in the P loading to the lake should be considered to suppress its cyanobacterial blooms, thereby improving conditions for submerged macrophytes and the re‐establishment of periphyton.     

From River to Lake: Phosphorus partitioning and algal community compositional changes in Western Lake Erie

The Maumee River is an important source of phosphorus (P) loading to western Lake Erie and potentially a source of Microcystis seed colonies contributing to the development of harmful algal blooms in the lake. Herein, we quantified P forms and size fractions, and phytoplankton community composition in the river–lake coupled ecosystem before (June), during (August), and after (September) a large Microcystis bloom in 2009. Additionally, we determined the distribution and density of a newly emergent cyanobacterium, Lyngbya wollei, near Maumee Bay to estimate potential P sequestration. In June, dissolved organic phosphorus (DOP) was the most abundant P form whereas particulate P (partP) was most abundant in August and September. Green algae dominated in June (44% and 60% of total chlorophyll in river and lake, respectively) with substantial Microcystis (17%) present only in the river. Conversely, in August, Microcystis declined in the river (3%) but dominated (32%) the lake. Lake phytoplankton sequestered < 6% of water column P even during peak Microcystis blooms; in all lake samples < 112 μm non-algal particles dominated partP. Lyngbya density averaged 19.4 g dry wt/m², with average Lyngbya P content of 15% (to 75% maximum) of water column P. The presence of Microcystis in the river before appearing in the lake indicates that the river is a potential source of Microcystis seed colonies for later lake blooms, that DOP is an important component of early summer total P, and that L. wollei blooms have the potential to increase P retention in nearshore areas.     

Modeling of Phytoplankton-Nutrient Dynamics in Saginaw Bay,Lake Huron

A multi-class, phytoplankton simulation model was developed and calibrated to an extensive set of field data acquired on Saginaw Bay, Lake Huron, during 1974. Phytoplankton biomass was partitioned into five functional groups: diatoms, greens, non-N₂-fixing blue-greens, N₂-fixing blue-greens, and “others”. Nutrients included in the model were phosphorus, nitrogen, and silicon. The model was applied to a single spatial segment encompassing the inner portion of Saginaw Bay.Process level analyses were conducted with the calibrated model to determine the relative importance of various factors affecting phytoplankton and nutrient dynamics. The concept of a single limiting factor for phytoplankton growth was found to be overly simplistic. Results indicated that temperature and light were relatively more growth rate limiting than nutrients on an annual average basis. However, as a consequence of nutrient depletion, nutrients became relatively more important at the times of peak phytoplankton crops. Nitrogen was found to be relatively more growth rate limiting than phosphorus to the total phytoplankton crop, although important differences occurred among the individual functional groups. At various times, and for various groups, all three nutrients were important in limiting either the rates of growth and/or the maximum sizes of the phytoplankton crops. Results were consistent with the hypothesis that while nitrogen and silicon were important in phytoplankton-nutrient dynamics, the supply of phosphorus would ultimately determine the size of the blue-green component of the total crop because N₂-fixing blue-greens do not have absolute requirements for dissolved available nitrogen or silicon. Results indicated that phosphorus requirements of spring and fall diatom crops were satisfied primarily by external loadings. Phosphorus requirements of summer blue-green crops were satisfied primarily by recycle processes within the water column. Upon cell death, direct nutrient recycle to the available nutrient compartments in the water column from excess internal phytoplankton stores was found to be important for both phosphorus and nitrogen. Phytoplankton production was found to be extremely sensitive to variations in the light extinction coefficient in the water column, and relatively insensitive to variations in incident solar radiation.     

Assessment of Phosphorus-microbe Interactions in Lake Ontario by Multiple Techniques

Conventional and newly-developed techniques to determine the phosphorus (P) status of Lake Ontario phytoplankton were employed in September 2003, immediately after the passage of the storm system associated with Hurricane Isabel. Surface water (1–5 m) was collected at 29 stations, with selected stations sampled throughout the water column. Chemical estimates of total P concentrations were compared with proxies of P bioavailability: P enrichment bioassays of lake water, alkaline phosphatase activity (APA), and P-dependent bioreporter assays. Average total P (314 nM) and total chlorophyll-a (2.12 μg/L) concentrations measured in pelagic surface waters from throughout Lake Ontario suggest an oligotrophic status prevailed across much of this lake during the sample period. Autotrophic picoplankton (0.2–2 μm) displayed the highest growth rates and were grazed at the highest rate, whereas P-enrichment bioassays favored the production of autotrophic nanoplankton (2–20 μm) and autotrophic microplankton (> 20 μm) biomass. Average concentrations of bacteria (2.61 × 10¹⁰ cells/L) were higher than those measured during summer in a similar lake (Erie), whereas the average viral density (1.38 × 10¹⁰ virus particles/L) was similar. Pelagic stations exhibited higher APA than coastal stations; cyanobacterial bioreporter responses did not show high correlation with APA suggesting that proxies of P-demand based on residual effects (e.g., enzyme production) were not indicative of shorter-term biological responses related to planktonic growth (bioreporter genetic response). The combination of traditional chemical, biochemical (APA), and cutting-edge biological methods (bioreporter) provided information on nutrient concentrations and primary productivity throughout Lake Ontario, while concurrently allowing real-time assessment of P bioavailability.     

Glyphosate influence on phytoplankton community structure in Lake Erie

In this study we investigated the effect of the phosphonate herbicide glyphosate (N-(phosphonomethyl)glycine) on the phytoplankton community structure in Lake Erie using lake water incubations, laboratory growth experiments and phylogenetic analysis of phosphonate metabolism genes. In microcosms, addition of glyphosate to Sandusky Bay water resulted in a significant increase in phytoplankton abundance, specifically causing an increase in the abundance of Planktothrix spp. In microcosms using Maumee Bay water, glyphosate did not stimulate phytoplankton growth but caused a decrease in Microcystis spp. abundance. The difference in the ability of Planktothrix spp. and Microcystis spp. to grow in the presence of glyphosate was confirmed in laboratory growth experiments. Further, an examination of the molecular pathways involved in phosphonate metabolism demonstrated that heterotrophic bacteria may be critical in allowing this proliferation. The results indicate that glyphosate has both positive and negative influences on phytoplankton community structure, serving as a nutrient source to microbes able to tolerate the herbicidal effects of the compound while killing those less tolerant. Moreover, this work highlights that in natural environments microorganisms function as communities, and the metabolic abilities of individual species are often less important than the collective ability of the community.     

渤海湾浮游细菌分布特征及环境影响因素

于2011年5月、2012年5月和2012年11月分别对渤海湾33个站位表层水体中的浮游细菌及环境因子进行了调查,探讨渤海湾浮游细菌生态分布特征及其与环境影响因子的关系。结果表明:在渤海湾表层水体中,2011年5月的海洋浮游细菌丰度为(2. 51~28. 39)×108L-1,2012年5月的浮游细菌丰度为(2. 62~87. 26)×108L-1,2012年11月浮游细菌丰度为(2. 18~18. 15)×108L-1;浮游细菌数量在空间上都是近岸站位高于远岸站位; 2011年5月和2012年5月,浮游细菌生长所需有机碳分别来源于浮游植物胞外分泌的溶解有机碳和陆源输入有机碳,浮游细菌与氨氮都有极显著相关性关系,与磷酸盐均呈显著负相关关系; 2012年11月,浮游细菌除了与亚硝酸盐氮、水温呈正相关外,与其他环境因子都呈负相关关系。     

Lyngbya wollei in western Lake Erie

We report on the emergence of the potentially toxic filamentous cyanobacterium, Lyngbya wollei as a nuisance species in western Lake Erie. The first indication of heavy L. wollei growth along the lake bottom occurred in September 2006, when a storm deposited large mats of L. wollei in coves along the south shore of Maumee Bay. These mats remained intact over winter and new growth was observed along the margins in April 2007. Mats ranged in thickness from 0.2 to 1.2 m and we estimated that one 100-m stretch of shoreline along the southern shore of Maumee Bay was covered with approximately 200 metric tons of L. wollei. Nearshore surveys conducted in July 2008 revealed greatest benthic L. wollei biomass (591 g/m² ± 361 g/m² fresh weight) in Maumee Bay at depth contours between 1.5 and 3.5 m corresponding to benthic irradiance of approximately 4.0–0.05% of surface irradiance and sand/crushed dreissenid mussel shell-type substrate. A shoreline survey indicated a generally decreasing prevalence of shoreline L. wollei mats with distance from Maumee Bay. Surveys of nearshore benthic areas outside of Maumee Bay revealed substantial L. wollei beds north along the Michigan shoreline, but very little L wollei growth to the east along the Ohio shoreline.     

Assessment of physico‐chemical properties of water and phytoplankton bloom formation in a eutrophic pond using multivariate analysis

An investigation of the water chemistry and phytoplankton bloom formation characteristics of a seasonal earthen pond located in the Alappuzha District, Kerala (India) was carried out during November 2011 to May 2012. Twelve important physico‐chemical parameters (temperature, Secchi disc transparency, pH, electrical conductivity EC), dissolved oxygen (DO), biochemical oxygen demand (BOD), ammonia‐nitrogen, nitrite‐nitrogen, nitrate‐nitrogen, total phosphorus (TP), dissolved iron and chlorophyll a related to phytoplankton growth in the pond were the focus of the present study. Chlorophyll a was used as a measure of phytoplankton biomass. The pond developed subsurface phytoplankton blooms of Chlorella vulgaris, Aphanothece sp., Leptosira sp., Lepocinclis globulus and Lepocinclis fusiformis. Visible scums of Lepocinclis globulus and Euglena proxima was also observed during March 2012. Principal component analysis was performed to understand the biochemical processes in the pond ecosystem leading to the Euglena spp. bloom.     

Nutrient and suspended-sediment concentrations in the Maumee River and tributaries during 2019 rain-induced fallow conditions

Above average precipitation from October 2018 through July 2019 in the Maumee River (R.) Basin resulted in 29% of cropland left fallow, providing a glimpse of potential effects from decreased nutrient application. Ongoing monitoring at 15 water-quality sites on the Maumee R. upstream from Defiance enabled comparison with 2017, which was hydrologically similar to 2019 in precipitation and streamflow. In 2019, nitrate (as nitrogen; NO₃-N) for March-July was significantly less than previous years (2015–2018), but the response for phosphorus (P) was more complicated. Relative to 2017, total P (TP) was lower at 7 of 15 sites, but higher at 7, reflecting higher suspended sediment (SS). Dissolved P (DP) was generally lower, but less different than NO₃; DP was higher at 3 sites. DP-P:NO₃-N was generally higher in 2019, DP-P:TP was lower, and there was less TP relative to SS. Overall, less P was in the system in 2019. However smaller streams showed a large range of difference between 2019 and 2017 for all constituents, indicating variability in land management and physiography. In contrast, all constituents were lower in 2019 in larger (>5000 km²) streams, including the Maumee R. near Defiance, where the difference in NO₃ (−37%) exceeded that for TP (−16%), DP (−10%), and SS (−20%). Differences in these relations among N, P, and SS indicate that P was available from legacy sources that are more difficult to distinguish during typical agricultural production years and that some material from 2019 was stored in the system upstream from the largest sites.     

Identification and quantification of microcystins in western Lake Erie during 2016 and 2017 harmful algal blooms

Liquid chromatography-mass spectrometry (LC-MS) and tandem mass spectrometry (MS/MS) were used to provide qualitative and quantitative information about microcystin (MC) congeners in western Lake Erie. Samples were collected at eight open-water locations on selected days during harmful algal blooms (HABs) in 2016 and 2017. Seven MCs were identified and 20 MCs were tentatively identified using high-resolution mass accuracies and a unique fragment (Adda m/z 135). The most abundant MC was MC-LR, followed by MC-RR, MC-YR, and MC-LA, and these congeners were quantified. Total (extracellular and intracellular) MC concentrations ranged from 0.068 to 14.88 µg/L in 2016, and from 0.050 to 10.15 µg/L in 2017, with averages of 2.71 and 1.86 µg/L, respectively. Near-shore sites in Lake Erie had higher MC concentrations and Microcystis biovolumes than off-shore sites. This implies that nutrient loading from the Maumee River greatly influences Maumee Bay, and this influence decreases with distance from the river. Consequently, six MCs (MC-LR, MC-RR, MC-LA, MC-YR, MC-LW, and MC-LF) were quantified in water samples collected from the Maumee River and the Maumee Bay shore of Lake Erie in 2017, and MC-RR was the most abundant. The total MC concentrations in river samples ranged from 0.17 to 305.03 µg/L. Additionally, an MC degradation product (linear MC-LR) was detected at all open-water locations, and data indicated an increase in its concentration towards the end of the bloom. The trends for 2016 and 2017 HABs are comparable in terms of spatial distribution and MC congeners produced, though the intensity and peak dates change.     

Effects of temperature and nutrients on phytoplankton biomass during bloom seasons in Taihu Lake

Long-term variations of phytoplankton chlorophyll-a (Chl-a), nutrients,and suspended solids (SS) in Taihu Lake, a large shallow freshwater lake in China, during algal bloom seasons from May to August were analyzed using the monthly investigated data from 1999 to 2007. The effective accumulated water temperature (EAWT) in months from March to June was calculated with daily monitoring data from the Taihu Laboratory for Lake Ecosystem Research (TLLER).The concentrations of Chl-a and nutrients significantly decreased from Meiliang Bay to Central Lake. Annual averages of the total nitrogen (TN), total phosphorus (TP), and Chl-a concentrations, and EAWT generally increased in the nine years. In Meiliang Bay, the concentration of Chl-a was significantly correlated with EAWT, ammonia nitrogen (NH4+-N ), TN, the soluble reactive phosphorus (SRP),TP, and SS. In Central Lake, however, the concentration of Chl-a was only correlated with EAWT, TP, and SS. Multiple stepwise linear regression revealed that EAWT, dissolved total phosphorus (DTP), and TP explained 99.2% of the variation of Chl-a in Meiliang Bay, and that EAWT, NH4+-N, and TP explained 98.7% of the variation of Chl-a in Central Lake. Thus EAWT is an important factor influencing the annual change of phytoplankton biomass. Extreme climate change, such as extremely hot springs or cold springs, could cause very different bloom intensities in different years. It is also suggested that both nutrients and EAWT played important roles in the growth of phytoplankton in Taihu Lake. The climate factors and nutrients dually controlled the risk of harmful algal blooms in Taihu Lake. Cutting down phosphorus and nitrogen loadings from catchments should be a fundamental strategy to reduce the risk of blooms in Taihu Lake.     

Effect of Thermal Stratification on Phytoplankton and Nutrient Dynamics in a Regulated River (Saar,Germany)

The weir pool Serrig is the deepest one along the impounded river Saar. Damming caused massive changes in the river's hydrodynamics. We analyzed the spatio‐temporal variability of thermal density stratification in the weir pool and its effect on phytoplankton and nutrient dynamics. In the analysis, continuous measurements from the years 2014 and 2015 were combined with three two‐day sampling campaigns in spring 2015. Thermal stratification occurred regularly in the downstream section of the weir pool during spring and summer and showed a diurnal rhythm. Temperature differences >1 K between the 1 and 2 m water layer were observed during 34 out of 217 days (16%) in 2014, with maximum temperature gradients up to 3.71 K. Whereas the influence of thermal stratification on phytoplankton biomass and distribution was low during the algal bloom in early spring, stratification events between May and July promoted temporary algal blooms in the surface layer with chlorophyll a concentrations up to 98 µg Chla l^?1 and a maximum difference between the 1 and 2 m water layer of 36 µg Chla l^?1. Some of the stratification events resulted in reduced concentrations of dissolved nutrients in the surface layer as a result of increased uptake by algae, with maximum gradients between the surface and the 8 m water layer of 0.070 mg ortho‐PO₄^3‐‐P l^?1, 0.136 mg NH₄⁺?N l^?1 and 0.24 mg NO₃¯?N l^?1. These vertical gradients should be considered in sampling protocols for the assessment of the water quality of temporarily stratified river sections. Copyright © 2016 John Wiley & Sons, Ltd.     

Limnological characteristics, eutrophication and cyanobacterial blooms in an inland reservoir, Australia

Ben Chifley Reservoir, the only potable water supply for Bathurst, New South Wales, Australia, has been experiencing recurrent cyanobacterial bloom problems since 1991. A study was undertaken from June 1998 to July 1999 to assess the limnological characteristics pertinent to eutrophication and the associated cyanobacterial blooms. From January–May 1999, the reservoir exhibited significant numbers of cyanobacterial cells, totalling > 9000 cells mL^?1. The highest number of cells (> 27 000 cells mL^?1) was recorded during April 1999. The water quality characteristics of the reservoir, and the river inflow and climatic data, were grouped into three distinct periods; before, during and after cyanobacterial blooms. High water temperature (15–22°C), thermal stratification (ΔT = 2.7–2.8°C), depletion of dissolved oxygen and high nutrient concentrations, all of which are conducive to enhanced cyanobacterial blooms, were evident before and during the bloom periods. Based on its nitrogen to phosphorus molar ratio, Ben Chifley Reservoir can be considered as being phosphorus‐deficient, in contrast to nitrogen, which is readily available from a number of sources in its drainage basin, including atmospheric fixation. Thus, it is recommended that adopting management strategies to reduce the quantity of bioavailable phosphorus in the reservoir would be the most effective way to minimize the occurrence of algal blooms.     

High nutrient loading and climatic parameters influence the dominance and dissimilarity of toxigenic cyanobacteria in northern bays of Lake Victoria

Eutrophication of Lake Victoria led to changes in its phytoplankton communities. However, different levels of eutrophication exist in the open lake and the bays, and between embayments. This study utilized spatial and temporal sampling of Napoleon Gulf and Murchison Bay, exhibiting different trophic conditions. Over one year, we investigated phytoplankton biomass, richness, diversity and dissimilarity, and related the dynamics of the dominant species to the limnological and climatic conditions. The results confirmed that Napoleon Gulf and Murchison Bay showed large differences in eutrophication status, with lower nutrient concentrations in Napoleon Gulf than in Murchison Bay, where a strong gradient was observed from inshore to offshore areas. These nutrient dynamics resulted in a 4 to 10 fold higher chlorophyll-a in Murchison Bay than in Napoleon Gulf. From the embayments, 135 phytoplankton taxa were recorded with no significant differences in alpha diversity. However, high dissimilarity in community structure was observed in beta diversity, mostly due to a turnover among the dominant toxigenic species. Thus, from a similar species pool, there was a shift in the dominant toxigenic cyanobacteria from Microcystis flos-aquae and M. aeruginosa in Murchison Bay, Dolichospermum circinale and Planktolyngbya circumcreta in Napoleon Gulf to D. circinale in the offshore stations. These cyanobacteria are toxigenic taxa with known health hazards. Using partial least square models, we showed that both climatic variables (e.g. wind, solar radiation) and levels of inorganic dissolved nutrients (e.g. SRP, NO₃^–, and NH₄⁺) are the main drivers of differences and dominance in cyanobacteria communities in northern Lake Victoria.     

Recent Changes and Patterns in the Water Chemistry of Lake Simcoe

After the Great Lakes, Lake Simcoe is the largest lake in southern Ontario. Located within a 1 hour drive of half the population of Ontario, there is currently major concern over the impact of rapid urbanization on the nutrient status of the lake. However, despite a more than doubling of the human population in the Lake Simcoe watershed over the past two decades, average total phosphorus (TP) levels were lower in 2000–2003 compared with 1980–1983 at six of eight lake stations, and declines were significant at the most nutrient-enriched near-shore sites located in Kempenfelt Bay (0.29 μg/L/year) and Cook's Bay (0.31–0.41 μg/L/year). Total P concentrations varied two-fold across the lake, and phytoplankton bio-volume followed a similar pattern, with greatest phytoplankton abundance occurring at high TP sites, particularly in Cook's Bay. Nevertheless, steep declines in bio-volume occurred at all sites beginning in the mid-1990s, and water clarity (Secchi disk depth) improved concurrently such that Secchi depths were 40–80% greater in 2000–2003 compared with 1980–1983. Zebra mussels, which became established in Lake Simcoe around 1995, likely contributed to decreased phytoplankton bio-volume and related improvements in water clarity, which began during the mid-1990s. Despite major reductions in phytoplankton bio-volume, average rates of dissolved oxygen (DO) depletion (18 m-bottom zone; normalized to 4°C) did not change substantially over time, and were similar in 2000–2003 (average 1.25 ± 0.21 g O₂/m³/month) compared with 1980-1983 (average 1.26 ± 0.19 g O₂/m³/month). In contrast, minimum, end-of-summer DO levels (18 m-bottom) increased slightly over the period of record. Although there has been some improvement in end-of-summer DO availability, DO concentrations continue to decline to levels that are limiting to many fish species (e.g., 3.8 mg O₂/L in 2001) by the end of the summer stratified period.     

Seasonal nutrient export dynamics in a mixed land use subwatershed of the Grand River,Ontario, Canada

Algal blooms in the Great Lakes are a concern due to excess nutrient loading from non-point sources; however, there is uncertainty over the relative contributions of various non-point sources under different types of land use in rural watersheds, particularly over annual time scales. Four nested subwatersheds in Southern Ontario, Canada (one natural woodlot, two agricultural and one mixed agricultural and urban) were monitored over one year to identify peak periods (‘hot moments’) and areas (‘hot spots’) of nutrient (dissolved reactive phosphorus, DRP; total phosphorus, TP; and nitrate, NO₃⁻) export and discharge. Annual nutrient export was small at the natural site (0.001 kg DRP ha⁻¹; 0.004 kg TP ha⁻¹; 0.04 kg NO₃^—N ha⁻¹) compared to the agricultural and mixed-use sites (0.10–0.15 kg DRP ha⁻¹; 0.70–0.94 kg TP ha⁻¹; 9.15–11.55 kg NO₃^—N ha⁻¹). Temporal patterns in P concentrations were similar throughout the sites, where spring was the dominant season for P export, irrespective of land use. Within the Hopewell Creek watershed, P and N hot spots existed that were consistently hot spots across all events with the location of these hot spots driven by local land use patterns, where there was elevated P export from a dairy-dominated sub-watershed and elevated N export from both of the two agricultural sub-watersheds. These estimates of seasonal- and event-based nutrient loads and discharge across nested sub-watersheds contribute to the growing body of evidence demonstrating the importance of identifying critical areas and periods in which to emphasize management efforts.