Phosphorus loading to Lake Erie from the Maumee,Sandusky and Cuyahoga rivers: The importance of bioavailability

Lake Erie has undergone re-eutrophication beginning in the 1990s, even though total phosphorus (TP) loads to the lake continued to slowly decline. Using our 1982 and 2007–10 studies of the bioavailability of dissolved and particulate phosphorus export from major Ohio tributaries, together with our long-term TP and dissolved reactive phosphorus (DRP) loading data, we estimated long-term annual export of dissolved and particulate bioavailable phosphorus. DRP was found to adequately represent dissolved bioavailable export while 26–30% of the particulate phosphorus (PP) was extractable by 0.1 N NaOH, a frequently used indicator of PP bioavailability. During the period of re-eutrophication (1991–2012), DRP export from nonpoint sources in the Maumee and Sandusky rivers increased dramatically while NaOH-PP export had a slight decline for the Maumee and a small increase in the Sandusky. For the Cuyahoga River, both DRP and NaOH-PP increased, but these changes were small in relation to those of the Maumee and Sandusky. During this period, whole lake loading of both non-point and point sources of phosphorus declined. This study indicates that increased nonpoint loading of DRP is an important contributing factor to re-eutrophication. Although nonpoint control programs in the Maumee and Sandusky have been effective in reducing erosion and PP export, these programs have been accompanied by increased DRP export. Future target loads for Lake Erie should focus on reducing bioavailable phosphorus, especially DRP from nonpoint sources. Agricultural P load reduction programs should address both DRP and PP, and take into account the lower bioavailability of PP.     

Reporting on the status,trends, and drivers of algal blooms on Lake of the Woods using satellite-derived bloom indices (2002–2021)

Despite significant declines in external phosphorus loads, Lake of the Woods continues to experience severe recurring cyanobacterial harmful algal blooms (cHABs) covering as much as 80% of the lake surface area. Satellite-derived bloom indices were used to assess the status, trends, and drivers of cHAB conditions for the period 2002 to 2021 in support of developing ecosystem objectives and response indicators for the lake. Areas of greatest potential concern, with the most prolonged bloom occurrences, were in the southeast of the lake. Significant decreases in bloom indices suggest the lake may now be responding to historical nutrient reductions. The greatest rates of decrease were within the main water flow paths, with little change in the more isolated embayments, suggesting flushing plays a key role in regulating regional bloom severity. Significant inter-annual variability in bloom phenology was observed, with blooms peaking later in recent years, which may be in response to climate-induced changes in the lake and watershed. The absence of a direct relationship between external phosphorus loads and annual bloom severity reflects the complexity of the lake’s response to eutrophication and the potential roles of other drivers including climate and a strong legacy effect of sedimentary nutrients. A case study of the 2017 bloom season captures the compounding interaction of meteorological variability and seasonal nutrient delivery in regulating the bloom response. Results highlight the need for greater understanding of seasonal and regional variability of bloom drivers to aid in forecasting the lake’s recovery under both nutrient management and climate change scenarios.     

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.     

Detroit River phosphorus loads: Anatomy of a binational watershed

As a result of increased harmful algal blooms and hypoxia in Lake Erie, the US and Canada revised their phosphorus loading targets under the 2012 Great Lakes Water Quality Agreement. The focus of this paper is the Detroit River and its watershed, a source of 25% of the total phosphorus (TP) load to Lake Erie. Its load declined 37% since 1998, due chiefly to improvements at the regional Great Lakes Water Authority Water Resource Recovery Facility (WRRF) in Detroit and phosphorus sequestered by zebra and quagga mussels in Lake Huron. In addition to the 54% of the load from Lake Huron, nonpoint sources contribute 57% of the TP load and 50% of the dissolved reactive phosphorus load, with the remaining balance from point sources. After Lake Huron, the largest source is the WRRF, which has already reduced its load by over 40%. Currently, loads from Lake Huron and further reductions from the WRRF are not part of the reduction strategy, therefore remaining watershed sources will need to decline by 72% to meet the Water Quality Agreement target - a daunting challenge. Because other urban sources are very small, most of the reduction would have to come from agriculturally-dominated lands. The most effective way to reduce those loads is to apply combinations of practices like cover crops, buffer strips, wetlands, and applying fertilizer below the soil surface on the lands with the highest phosphorus losses. However, our simulations suggest even extensive conservation on those lands may not be enough.     

Updated phosphorus loads from Lake Huron and the Detroit River: Implications

The binational Great Lakes Water Quality Agreement (GLWQA) revised Lake Erie’s phosphorus (P) loading targets, including a 40% western and central basin total P (TP) load reduction from 2008 levels. Because the Detroit and Maumee River loads are roughly equal and contribute almost 90% of the TP load to the western basin and 54% to the whole lake, they have drawn significant policy attention. The Maumee is the primary driver of western basin harmful algal blooms, and the Detroit and Maumee rivers are key drivers of central basin hypoxia and overall western and central basin eutrophication. So, accurate estimates of those loads are particularly important. While daily measurements constrain Maumee load estimates, complex flows near the Detroit River mouth, along with varying Lake Erie water levels and corresponding back flows, make measurements there a questionable representation of loading conditions. Because of this, the Detroit River load is generally estimated by adding loads from Lake Huron to those from the watersheds of the St. Clair and Detroit rivers and Lake St. Clair. However, recent research showed the load from Lake Huron has been significantly underestimated. Herein, I compare different load estimates from Lake Huron and the Detroit River, justify revised higher loads from Lake Huron with a historical reconstruction, and discuss the implications for Lake Erie models and loading targets.     

Application of a three-dimensional ecological model to develop nutrient management plans for Lake of the Woods

This paper presents the development of a three-dimensional hydrodynamic-ecological model for Lake of the Woods (LoW) to assess the impact of nutrient inputs on the lake’s ecological processes. LoW is a large bi-national water body with complex geometry and topography and receives significant nutrients mainly from the Rainy River, with additional inputs from a few other smaller tributaries, and suffers from degraded water quality with seasonal cyanobacterial and harmful algal blooms. A high-resolution model developed here has a horizontal grid resolution of 250 m with a variable vertical grid resolution and can simulate hydrodynamics, in-lake nutrients dynamics, and phytoplankton biomass. Our model reproduced observed temporal and spatial distribution of nutrient and chlorophyll-a concentrations reasonably well. The calibrated model is used to explore simulating the spatial and temporal variability of ecological conditions of the lake and its response to nutrient load reductions. Based on a range of potential nutrient loadings, the model results suggest that different areas within LoW may respond differently to decreased phosphorus loadings. The model simulations predict that as nutrient loads into LoW decrease, water quality conditions will improve in most of the segments. In addition to naturally reducing internal load, external load reductions of 160 MTA from the baseline conditions (877 MTA) are necessary to reduce late summer average TP concentrations to 0.03 mg/L and total chlorophyll-a concentrations in the range of 7–12 μg/L.     

A total phosphorus budget for the Lake of the Woods and the Rainy River catchment

The overall goal of this study was to quantify the major and minor sources and losses of total phosphorus (TP) to the Lake of the Woods (LOW), summarized as a nutrient budget. This research was initiated in response to degradation in lake water quality, including elevated TP concentrations and increased cyanobacterial blooms, which has resulted in LOW's classification as an “Impaired Waterbody” in Minnesota. The whole-lake LOW TP budget shows that tributary inflow is largely dominated by a single source, the Rainy River, draining 79% of the LOW catchment by area. Currently, there is only a small TP contribution from shoreline residential developments (6 t; ~ 1%) at a whole-lake scale, relative to the large TP loads from atmospheric deposition (95 ± 55 t; 13%) and the Rainy River (568 ± 186 t; 75%). Overall, the annual TP load to LOW was ~ 754 t with ~ 54% TP retained within the lake. The nutrient budget for the Rainy River catchment revealed that contributions from point sources along the river constitute the largest anthropogenic TP source to the Rainy River and eventually to LOW. Historical load calculations along the Rainy River show that this load has been significantly reduced since the 1970s, and presently just over 100 t of P enters LOW from anthropogenic point sources. These TP budgets provide insights into the major sources of TP influencing the overall LOW water quality and with future refinement may provide a greater understanding of linkages between TP loading and spatial and temporal water quality changes in the LOW.     

Assessment of site-specific agricultural Best Management Practices in the Upper East River watershed,Wisconsin, using a field-scale SWAT model

The Great Lakes “Priority Watershed” effort targeted the Upper East River watershed, a 116.5-km² tributary watershed to Wisconsin's Green Bay, to reduce its sediment and nutrients loads from agricultural sources. A Soil and Water Assessment Tool (SWAT) model was created to determine the effectiveness of agricultural Best Management Practices (BMPs) funded through the Great Lakes Restoration Initiative. The model was calibrated at the monthly time-step for flow, sediment, dissolved reactive phosphorus (DRP), total phosphorus (TP), nitrate, and total nitrogen (TN). Field- and watershed-scale sediment and nutrient reductions were calculated due to the implementation of 74 BMP combinations on dairy and cash grain rotations. Modeling results indicated that when multiple BMPs were placed on a field, especially those including filter strips and grassed waterways, sediment and nutrient loads generally were reduced more than single BMP implementation. The most effective in-field practice at reducing DRP and TP on dairy fields was a combination of 5 different BMPs: cover crops, crop rotation, nutrient management plan, reduced tillage, and a filter strip. Conservation cover was the single most effective practice at reducing sediment and nutrient yields. Sediment and nutrient loads decreased at the watershed scale as the quantity and coverage of BMPs increased. When all contracted BMPs were simulated at the watershed scale, sediment loads were reduced 2%, while TP, DRP, TN and nitrate loads were reduced 20%, 9%, 24%, and 17%, respectively. Modeling scenarios also indicated that over-winter manure storage was important to keep soluble nutrients out of waterways.     

Phosphorus and nitrogen deposition within a large transboundary watershed: Implications for nutrient stoichiometry and lake vs watershed budgets

Atmospheric deposition is an important source of both phosphorus (P) and nitrogen (N) to lakes and their watersheds, but the two nutrients are rarely reported together. For the first time, we measured total P (TP) and N (total, ammonium, NH₄-N and nitrate, NO₃-N) bulk deposition to the Lake of the Woods (LoW) watershed, a large international lake that experiences frequent, lake-wide cyanobacterial blooms. Phosphorus deposition was highly variable both spatially and seasonally, with on average >75 % of annual deposition occurring in the spring and summer months, associated with local biogenic input. In contrast, winter TP deposition was relatively low and spatially invariant, and not affected by local sources. Importantly, these results suggest that P deposition may be a much larger source of input to the LoW as a result of its tortuous shoreline and abundance of forested islands (>5000), which are concentrated in the northern, Canadian waters and greatly extend the ‘shoreline influence’ within this lake. We suggest that a single annual TP load estimate is not appropriate for the LoW because winter deposition is likely much lower than past estimates whereas spring/summer deposition is substantially higher and could be an important, but previously unrecognized source, of bioavailable P to the nearshore waters. Further research is needed on the spatial and temporal patterns of P vs N deposition to the lake surface and their influence on primary productivity and algal species composition.     

Phosphorus Inputs to Lake Simcoe from 1990 to 2003: Declines in Tributary Loads and Observations on Lake Water Quality

Total phosphorus (TP) inputs to Lake Simcoe have led to hypolimnetic dissolved oxygen (DO) depletion and loss of cold water fish habitat. Since 1990, efforts have been made to reduce the total TP input to the lake below a defined target of 75 t/year, which was predicted to lead to reductions in spring TP concentration and improvements in end-of-summer hypolimnetic DO concentrations. The total TP load to the lake during the most recent period of record (1998/99-2003/04) ranged from 53 to 76 t/yr and averaged 67 t/yr, compared to an average of 114 t/yr estimated between 1990/91 and 1997/98 (range 85-157 t/yr). Reductions in TP loads from the catchment via tributary discharge (∼26 t) accounted for the majority of the decrease in total load between the two time periods. Total P concentrations decreased significantly in four out of six long-term monitored tributaries; however, concentrations in all six tributaries remain above the level recommended to avoid nuisance plant growth (30 μg/L). Although TP loads to the lake are currently below the target 75 t/yr, excessive growths of filamentous algae and macrophytes continue to be a problem in the nearshore zone. End-of-summer minimum hypolimnetic DO concentrations (average 4.3 mg/L, 1998/99-2003/04) remain substantially below the level (7 mg/L) that is considered protective of lake trout. Efforts to reduce TP loads to the lake therefore need to continue.     

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.     

Urban total phosphorus loads to the St. Clair-Detroit River System

The St. Clair-Detroit River System watershed is a large, binational watershed draining into the connecting channel between lakes Huron and Erie. In addition to extensive agricultural lands, it contains large urban areas that discharge phosphorus from point source facilities, runoff of impervious surfaces, and overflows of combined sewers. To help guide actions to reduce phosphorus input to Lake Erie, we analyzed the spatial and temporal dynamics of loads from the three largest urban areas in the watershed (southeast Michigan; Windsor, Ontario; and London, Ontario), and used a previously calibrated storm water management model (SWMM) to explore options for reducing loads around metro Detroit. Point sources in these three urban areas contribute, on average, 81% of the total urban load and 19% of the Detroit River’s total phosphorus (TP) load to Lake Erie, while combined sewer overflows and runoff both contribute about 10% each to the urban load and about 2.5% each to the Detroit River’s load to Lake Erie. Most of the urban load (56%) comes from a single point source, the wastewater treatment facility in Detroit; however, TP loads from that facility have decreased by about 51% since 2008 due to improvements in wastewater treatment. Model simulations suggest that increasing pervious land area or implementing green infrastructure could help reduce combined sewer overflows in certain upper portions of the metro Detroit sewer system, but reductions were much less expressed for wet-weather discharge from the system.     

The composition of particulate phosphorus: A case study of the Grand River,Canada

We examined the fractions of particulate phosphorus (PP) in the lower reaches of the Grand River, Canada, to test the hypothesis that the river is a source of both particulate-bound orthophosphate and labile species of PP. At the mouth of the Grand River, the proportion of particulate organic P (POP) was, on average, 57.7% of total PP, which was significantly higher than the proportion of particulate inorganic P (PIP) in PP. Analysis with ³¹P nuclear magnetic resonance (NMR) spectroscopy showed that the proportion of P species other than orthophosphate in the NaOH- EDTA extract was 1.75 times greater than that of orthophosphate. Labile P species (e.g. nucleotides and pyrophosphate) were present in the NMR spectrum; whereas, refractory organic P (myo-inositol hexakisphosphate) was absent. These results suggest that during winter and spring, the Grand River supplies primarily bioavailable phosphorus species in organic forms to Lake Erie, rather than inorganic orthophosphate. These results suggest that labile organic P is contained in PP rather than alkaline extractable inorganic P. Future studies should examine POP species in other rivers of the Lake Erie watershed.     

Assessing and addressing the re-eutrophication of Lake Erie: Central basin hypoxia

Relieving phosphorus loading is a key management tool for controlling Lake Erie eutrophication. During the 1960s and 1970s, increased phosphorus inputs degraded water quality and reduced central basin hypolimnetic oxygen levels which, in turn, eliminated thermal habitat vital to cold-water organisms and contributed to the extirpation of important benthic macroinvertebrate prey species for fishes. In response to load reductions initiated in 1972, Lake Erie responded quickly with reduced water-column phosphorus concentrations, phytoplankton biomass, and bottom-water hypoxia (dissolved oxygen < 2 mg/l). Since the mid-1990s, cyanobacteria blooms increased and extensive hypoxia and benthic algae returned. We synthesize recent research leading to guidance for addressing this re-eutrophication, with particular emphasis on central basin hypoxia. We document recent trends in key eutrophication-related properties, assess their likely ecological impacts, and develop load response curves to guide revised hypoxia-based loading targets called for in the 2012 Great Lakes Water Quality Agreement. Reducing central basin hypoxic area to levels observed in the early 1990s (ca. 2000 km²) requires cutting total phosphorus loads by 46% from the 2003–2011 average or reducing dissolved reactive phosphorus loads by 78% from the 2005–2011 average. Reductions to these levels are also protective of fish habitat. We provide potential approaches for achieving those new loading targets, and suggest that recent load reduction recommendations focused on western basin cyanobacteria blooms may not be sufficient to reduce central basin hypoxia to 2000 km².     

Modeling phosphorus reduction strategies from the international St. Clair-Detroit River system watershed

Nutrient loading from nonpoint sources has degraded water quality in large water bodies globally. The water quality of Lake Erie, the most productive of the Laurentian Great Lakes bordering the United States and Canada, is influenced by phosphorus loads from the Detroit River that drains an almost 19,000 km² international watershed. We used the Soil and Water Assessment Tool (SWAT) to evaluate a range of management practices to potentially reduce total phosphorus (TP) and dissolved reactive phosphorus (DRP) loads. Scenarios included both single practices and bundles of multiple practices. Single practice scenarios included fertilizer rate reduction (Rate) and sub-surface placement (PL), filter strips (FL), grassed waterways, cover crops (CC), wetlands (WT), controlled drainage, and changes in tillage practices. Bundle scenarios included combinations of Rate, PL, FL, CC, and WT with three adoption strategies: application on all applicable areas, on 55% of randomly selected applicable areas, and on 55% of high phosphorus yielding applicable areas. Results showed that among the single practice scenarios, FL, WT, PL, CC, and Rate performed well in reducing both TP and DRP loss from agricultural dominated sub-watersheds. Over all, the CC, FL, WT bundle performed best, followed by the CC, PL, WT bundle, reducing the load up to 80% and 70%, respectively, with 100% implementation. However, targeting high phosphorus yielding areas performed nearly as well as 100% implementation. Results from this work suggest that there are potential pathways for phosphorus load reduction, but extensive implementation of multiple practices is required.     

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.     

Phosphorus retention and transformation in a dammed reservoir of the Thames River,Ontario: Impacts on phosphorus load and speciation

Extensive efforts are underway to reduce phosphorus (P) export from the Lake Erie watershed. On the Canadian side, the Thames River is the largest tributary source of P to Lake Erie’s western basin. However, the role of dams in retaining and modifying riverine P loading to the lake has not been comprehensively evaluated. We assessed whether Fanshawe Reservoir, the largest dam reservoir on the Thames River, acts as a source or sink of P, using year-round discharge and water chemistry data collected in 2018 and 2019. We also determined how in-reservoir processes alter P speciation by comparing the dissolved reactive P to total P ratio (DRP:TP) in upstream and downstream loads. Annually, Fanshawe Reservoir was a net sink for P, retaining 25% (36 tonnes) and 47% (91 tonnes) of TP in 2018 and 2019, respectively. Seasonally, the reservoir oscillated between a source and sink of P. Net P release occurred during the spring of 2018 and the summers of 2018 and 2019, driven by internal P loading and hypolimnetic discharge from the dam. The reservoir did not exert a strong influence on DRP:TP annually, but ratio increases occurred during both summers, concurrent with water column stratification. Our analysis demonstrates that Fanshawe Reservoir is not only an important P sink on the Thames River, but also modulates the timing and speciation of P loads. We therefore propose that the potential of using existing dam reservoirs to attenuate downstream P loads should be more thoroughly explored alongside source based P mitigation strategies.     

The use of a mass balance phosphorus budget for informing nutrient management in shallow coastal lakes

Eutrophication has degraded ecosystem, cultural and recreational values in Lake Forsyth, a small, shallow, coastal lake in New Zealand. To inform catchment management decisions designed to prevent algal blooms and improve water quality, a sub-catchment scale, mass-balance approach to understanding the behaviour of the critical nutrient, phosphorous (P), has been taken. To determine a P budget for the lake, and identify key P reservoirs, hydrological inflows and outflows were measured over a 15 month period. These were combined with total (TP) and dissolved reactive P (DRP) concentrations in these flows, to determine the external load of P transported to the lake. Lake water was also analysed for TP and DRP concentrations, and chemical extractions were used to determine the mass and mobility of P in the lake sediments. Biomass surveys and chemical digestions were used to quantify the mass of P contained in lake macrophytes. Changes in the lake water P reservoirs were then used to assess the contribution of external P loading relative to fluxes of P from the sediment to the lake water column (internal P loading). More than 7000 kg P per year was delivered to the lake, 68% of which came from a single sub-catchment. P associated with suspended particulate material accounted for 80% of the external P load transported into the lake and 61% of the load delivered over the study period was transported during a single flood event. A reduction of 53% in the external P load is necessary to achieve a recommended areal loading guideline. As the lake has no permanent outflow, this external load and the low flushing rates have created a large legacy reservoir of P in the lake sediments with 70% of external P loads retained in the lake. It is the release of P from these lake sediments rather than fluctuations in external loading that control P concentrations in the lake water column during the blooms of nitrogen-fixing cyanobacteria. The results indicate the importance of targeting both external and internal loading processes in the catchment. The sub-catchment scale, mass-balance approach to determining a P budget and quantifying P reservoirs enable critical source areas for external P loads to be identified, and the potential efficacy of targeted interventions to reduce P sources and minimise P transport, such as wetlands and sediment retention basins, to be assessed.     

Models for identifying significant environmental factors associated with cyanobacterial bloom occurrence and for predicting cyanobacterial blooms

Compared with microscopic indices such as biomass, inverted satellite images can reflect cyanobacterial blooms from a macroscopic perspective, can provide planar information for blooms, and can more definitely reflect the occurrence of visible cyanobacterial blooms. We therefore adopted inverted images (from MODIS imagery) to judge whether cyanobacterial blooms had occurred in a water area at a given time. We constructed two probit models for identifying significant environmental factors related to cyanobacterial bloom occurrence and for short-term forecasts of bloom occurrence. The models used the index of cyanobacterial bloom occurrence as the dependent variable and the predicted variable, respectively, and used three categories (water quality, hydrology, and weather) of monitoring variables as the independent variables (or predictive variables). We used the Hill Dagong water area of Lake Tai in China as a case study of the new methods. The results produced by the identification model are consistent with the general conclusions in this research field indicating the validity of the model. The mean relative error of the forecast model is 13.5%, which is close to or lower than that of two previous models. Compared with the previous models, our forecast model also has advantages in terms of spatial and temporal precision. The new models have both practical applicability and the ability to be generalized and can, therefore, be easily adapted for the prevention, control, and prediction of cyanobacterial blooms in other bodies of water.     

Summer phytoplankton nutrient limitation in Maumee Bay of Lake Erie during high-flow and low-flow years

Algal production in Maumee Bay in western Lake Erie is highly affected by inputs of nitrogen (N) and phosphorus (P) from the Maumee River, which drains predominantly agricultural lands, leading to the formation of cyanobacterial blooms. In a 3-year study, precipitation and discharge ranged from relatively low (2012) to relatively high (2011) with corresponding changes in the size of the cyanobacterial bloom. This study aimed to quantify the relation between river discharge and algal nutrient limitation in Maumee Bay. During the summer growing seasons, 20 nutrient enrichment bioassays were performed to determine which nutrient (P or N) might limit phytoplankton growth; and ambient N and P concentrations were monitored. The bioassays suggested that phytoplankton growth shifted from P-limited to N-limited during summer of the low and intermediate discharge years (2012 and 2010, respectively), whereas during the high discharge year (2011) phytoplankton were nutrient-replete before becoming N-limited. Phosphorus-replete growth during the high discharge year likely was due to high P loads from the river and dissolved P concentrations greater than 1 μmol/L. Symptoms of N-limited growth occurred during August and September in all three years and during July of 2012 when NO₃⁻ plus NH₄⁺ concentration was less than 7.29 μmol/L suggesting low or no correspondence between N-limitation and size of the cyanobacterial bloom. Occurrence of a relatively small cyanobacterial bloom in 2012 following the record-breaking bloom in 2011 suggests the possibility of fast-reversal of eutrophication in Maumee Bay if P loading from the watershed could be decreased.