Cladophora (green algae) and dreissenid mussels over a nutrient loading gradient on the north shore of Lake Ontario

Benthic algae, much of it the green alga Cladophora, blanket the nearshore lakebed of the north shore of Lake Ontario. Nearshore field studies in Toronto, Ajax, Oshawa and Cobourg in 2012 and 2013, and Toronto in 2015 examined the distribution of Cladophora over a nutrient gradient on the north shore of the lake. Concentrations of total phosphorus, dissolved phosphorus and total inorganic nitrogen in the water column decreased from west to east over the gradient corresponding with decreasing watershed development and size. However, high surface cover and nuisance levels of Cladophora biomass were found across the gradient, including the least developed study area where total phosphorus concentrations were similar to those in the open lake. The abundance of Dreissena, high in all areas, was measured concurrently with Cladophora biomass and correlated positively at depths of 6 to 10 m. External loading of phosphorus provides a basis for abatement of Cladophora; however, the influences of enrichment along the shoreline, internal loading at the lakebed and lake trophic status in sustaining growth remain obscure. Nuisance levels of Cladophora are not isolated to locations experiencing elevated external loading and should be viewed in the context of interacting area-specific and lake-wide nutrient supplies.     

The Great Lakes Cladophora Model: Development,testing, and application to Lake Michigan

A recent review of the Great Lakes Water Quality Agreement has concluded that while controls on phosphorus inputs to Lake Michigan achieved the desired effect in offshore waters, the nearshore region continues to suffer from elevated phosphorus levels. Failure to achieve trophic state goals in the nearshore is manifested in nuisance growth of Cladophora and attendant impacts on property owners, utilities, and the public health and welfare. This study focuses on a site in Lake Michigan near Milwaukee, Wisconsin, where nuisance growth of Cladophora and associated beach fouling occur regularly. A mechanistic model simulating Cladophora growth, suitable for guiding nutrient management in the Great Lakes nearshore, is presented. The model represents an update of the Canale and Auer framework, reflecting current understandings of Cladophora ecology and offering a user-friendly interface making the software more widely available to decision makers. This Great Lakes Cladophora Model (GLCM) is first validated for the Auer/Canale data set collected in 1979 at a site on Lake Huron and then for a data set developed in 2006 for a site on Lake Michigan. Model performance under the strikingly different forcing conditions (depth, light, phosphorus levels) characteristic of these two sites affirms the widespread applicability of the tool. The GLCM is then extended to examine the impacts of ecosystem perturbation (dreissenid colonization) on Cladophora growth and to future approaches to monitoring and management.     

Cladophora,mass transport,and the nearshore phosphorus shunt

The nearshore phosphorus shunt hypothesis and the potential for mussels to excrete phosphorus sufficient to meet the growth requirements of Cladophora are now well accepted by scientists studying Great Lakes biogeochemistry. The response of algal growth to near bottom water column phosphorus concentrations and the interplay between excretion and mass transport in yielding those concentrations have, however, not been elucidated. Here we present soluble reactive phosphorus profiles from the near bottom environment of Lake Michigan at a site near Good Harbor Bay, Michigan, where both mussels and Cladophora were present. Soluble reactive phosphorus was observed to accumulate under quiescent conditions, establishing a concentration boundary layer (CBL), 5–15 cm thick, with near bottom concentrations on the order of 2–8 μg P/L. A one-dimensional model was applied to determine mass transport conditions mediating the transition from CBL formation to CBL destruction. Significant wave height (SWH) was used as an indicator of mass transport intensity, and it was determined that the formation/destruction transition occurred at a SWH of 0.2 m at the 8-m study site depth. The Great Lakes Cladophora Model was applied to determine the time intervals required to saturate (1 day with the CBL present) and deplete (14 days with the CBL absent) algal internal P stores. A review of SWH conditions at the study site indicated that a CBL would be expected to form at a frequency sufficient to support the phosphorus nutrition of Cladophora over the entire May to August interval.     

A comparative examination of recent changes in nutrients and lower food web structure in Lake Michigan and Lake Huron

The lower food webs of Lake Huron and Lake Michigan have experienced similar reductions in the spring phytoplankton bloom and summer populations of Diporeia and cladocerans since the early 2000s. At the same time phosphorus concentrations have decreased and water clarity and silica concentrations have increased. Key periods of change, identified by using a method based on sequential t-tests, were 2003–2005 (Huron) and 2004–2006 (Michigan). Estimated filtration capacity suggests that dreissenid grazing would have been insufficient to directly impact phytoplankton in the deeper waters of either lake by this time (mid 2000s). Despite some evidence of decreased chlorophyll:TP ratios, consistent with grazing limitation of phytoplankton, the main impact of dreissenids on the offshore waters was probably remote, e.g., through interception of nutrients by nearshore populations. A mass balance model indicates that decreased phosphorus loading could not account for observed in-lake phosphorus declines. However, model-inferred internal phosphorus dynamics were strongly correlated between the lakes, with periods of increased internal loading in the 1990s, and increased phosphorus loss starting in 2000 in Lake Michigan and 2003 in Lake Huron, prior to dreissenid expansion into deep water of both lakes. This suggests a limited role for deep populations of dreissenids in the initial phosphorus declines in the lakes, and also suggests a role for meteorological influence on phosphorus dynamics. The high synchrony in lower trophic level changes between Lake Michigan and Lake Huron suggests that both lakes should be considered when investigating underlying causal factors of these changes.     

Great Lakes Cladophora in the 21st century: same algae—different ecosystem

Nuisance growth of the attached, green alga Cladophora was considered to have been abated by phosphorus management programs mandated under the Great Lakes Water Quality Agreement. The apparent resurgence of nuisance growth in Lakes Erie, Michigan and Ontario has been linked conceptually to ecosystem alterations engineered by invasive dreissenid mussels (Dreissena polymorpha and Dreissenabugensis). Here, we apply contemporary modeling tools and historical water quality data sets in quantifying the impact of long-term changes in phosphorus loading and dreissenid-mediated changes in water clarity on the distribution and production of Cladophora. It is concluded that reductions in phosphorus loading in the pre-dreissenid period achieved the desired effect, as model simulations were consistent with the biomass declines reported from the early 1970s to the early 1980s. These declines were, however, largely offset by dreissenid-driven changes in water clarity that extended the depth of colonization by Cladophora, increasing total production. We were not able to isolate and quantify the significance of dreissenid mediation of phosphorus cycling using the historical database. Phosphorus management remains the appropriate mechanism for reducing nuisance levels of Cladophora growth. The development of action plans will require an improved understanding of nearshore phosphorus dynamics such as might be obtained through regular monitoring of soluble reactive phosphorus levels, internal phosphorus content and Cladophora biomass in impacted nearshore regions of the Great Lakes.     

Phosphorus Uptake Dynamics as Related to Mathematical Modeling of Cladophora at a Site on Lake Huron

Cladophora is a significant symptom of eutrophication in Lakes Erie and Ontario and is a local problem associated with nutrient perturbations in Lakes Huron, Michigan, and Superior. This paper presents results of measurements of phosphorus uptake rates as a function of internal phosphorus levels by Cladophora growing near Harbor Beach, Michigan. Cladophora collected near the Harbor Beach wastewater treatment plant had high levels of internal phosphorus and low (or even negative) phosphorus uptake rates. Cladophora distant from the wastewater treatment plant had low internal phosphorus levels and rapid phosphorus uptake rates. The experimental results are discussed in terms of quantitative kinetic formulations which may be incorporated into mathematical models useful for predicting the response of Cladophora to alternative management and control strategies. Preliminary model simulations of Cladophora biomass, internal phosphorus, and external phosphorus are qualitatively similar to observed field data.     

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.     

A modeling study to determine the contribution of interbasin versus intrabasin phosphorus loads on the southwestern nearshore of Lake Ontario

Elevated phosphorus and nuisance algae such as Cladophora have been persistent environmental concerns in the coastal areas of Lake Ontario. Phosphorus is regarded as one of the drivers of nearshore Cladophora and the most likely mitigation that can be used to control levels of this nuisance algae in the lakes. The Niagara River, carrying the Lake Erie interbasin load, is the major contributor of the overall phosphorus load to Lake Ontario. Due to circulation patterns in the lake, this contribution is especially significant in the southwestern nearshore areas. Here we apply a mathematical model to provide insight into the relative contribution of the Niagara River versus loadings from local rivers (intrabasin loads) on the nearshore phosphorus concentrations in this region. We performed numerical experiments to determine to what extent the Niagara, Genesee and smaller local rivers impact the nearshore (<20 m depth) phosphorus concentrations. Our model results show that the Niagara River dominates the nearshore region between its discharge location and the Genesee River’s mouth, but the Genesee River strongly impacts the nearby Ontario Beach region in the very nearshore (<5 m depth). Smaller rivers have some impact close to their discharge locations. However, uncertainty with the Niagara River phosphorus load is the limiting factor in making any credible nearshore phosphorus predictions. Model accuracy is also impacted by insufficient short time scale phosphorus loads for all of the rivers, the dynamic nature of the lake circulation in shallow nearshore areas, and the simplified assumptions of the model.     

Ecological Studies and Mathematical Modeling of Cladophora in Lake Huron: 3. The Dependence of Growth Rates on Internal Phosphorus Pool Size

The relationship between growth rate and internal phosphorus pool size was examined using field populations of Cladophora glomerata from Lake Huron. Algal samples, representing a range of internal phosphorus concentrations, were harvested from the lake and used for laboratory measurements of growth. Rates of net photosynthesis and respiration were measured under controlled conditions of light and temperature using a dissolved oxygen (light/dark bottle) technique. The net specific growth rate and respiration rate were calculated from photosynthesis and respiration measurements using a fixed stoichiometric relationship and the measured carbon content of the algae. The maximum rates for net specific growth rate, gross specific growth rate, and specific respiration rate were 0.77, 1.08, and 0.44 day^?1, respectively. Management decisions may be importantly influenced by the relationships derived from these data. A non-linear response by growth to reductions in phosphorus loading is suggested from the results of these experiments. The internal nutrient status of algal populations (e.g. Cladophora) must be considered in predictions of the impact of phosphorus management strategies on aquatic systems.     

The Wall of Green: The Status of Cladophora glomerata on the Northern Shores of Lake Erie's Eastern Basin, 1995–2002

The biomass, areal coverage, algal bed characteristics, and tissue phosphorus concentrations of Cladophora glomerata were measured at 24 near shore rocky sites along the northern shoreline of Lake Erie's eastern basin between 1995–2002. Midsummer areal coverage at shallow depths (≤ 5 m) ranged from 4–100%, with a median value of 96%. Cladophora biomass began accumulating at most sites during early May, and achieved maximum values by mid-July. Peak seasonal biomass ranged from < 1 to 940 g/m² dry mass (DM), with a median value of 171 g/m² DM. Nearshore water concentrations of total phosphorus (TP) were lower than during pre-phosphorus abatement years. However, Cladophora biomass levels were similar to reported values in those years. The midsummer “die off” occurred shortly after the biomass peak, when water temperatures neared 22.5°C. Areal coverage declined after die-off to < 10%, mean filament lengths declined from 33 cm to < 1 cm, and mean biomass declined to < 1 g DM/m². Tissue phosphorus varied seasonally, with initial high values in early May (0.15 to 0.27% DM; median 0.23% DM) to midsummer seasonal low values during peak biomass (0.03 to 0.23% DM; median 0.06% DM). Cladophora biomass is sensitive to changes in phosphorus and light availability, and reductions in biomass previously achieved through phosphorus control may now be reversed because of increased water transparency and phosphorus availability to the benthos following establishment of dreissenids.     

Ecological Studies and Mathematical Modeling of Cladophora in Lake Huron: 5. Model Development and Calibration

The development of a mathematical model for calculating the spatial distribution and temporal variation in Cladophora biomass and selected forms of phosphorus at a site on Lake Huron is described. The model is intended for use in evaluating the utility of various phosphorus management strategies in reducing the nuisance growth of Cladophora in the Great Lakes. The model is composed of a transport component which accounts for bulk transfer and exchange of phosphorus within the system and a kinetic component which describes chemical and biological reactions. The kinetic component includes the effects of light, temperature, and internal phosphorus levels on Cladophora growth rate, aspects of phosphorus uptake, and the impact of sloughing and self-shading on standing crop. The numerical value of coefficients associated with kinetic equations have been independently determined through laboratory studies.The model has been calibrated to a data set collected in the vicinity of the Harbor Beach, Michigan, wastewater treatment plant. A satisfactory fit was achieved for model calculations and observed levels of Cladophora biomass, internal phosphorus, and soluble reactive phosphorus. The model has few degrees of freedom because the coefficient values were fixed from field and laboratory measurements. A standing crop of Cladophora biomass of 300 gDW/m² with an internal phosphorus level of 0.35%P was characteristic of stations in the vicinity of the nutrient source. Soluble reactive phosphorus levels fluctuated widely due to mixing of the effluent plume with offshore waters, but averaged approximately 30 μgP/L. A distinct spatial trend was observed with Cladophora biomass, internal phosphorus, and soluble reactive phosphorus decreasing with distance from the point source of nutrients. The opportunity for model verification by perturbation through phosphorus removal is discussed.     

Environmental Factors Controlling Physiological Changes in Cladophora in Lake Erie

Several physiological characteristics of Cladophora glomerata from eastern Lake Erie were monitored frequently during the summer of 1977. Soluble reactive phosphorus and nitrate-nitrogen in the lake water were also measured.Cladophora biomass generally increased throughout June, although there were periodic declines. In mid-July, biomass sharply decreased to a low level. Chlorophyll content tended to be high during periods of increasing biomass but was lower after each biomass decline. Changes in biomass did not correlate with changes in either phosphate-phosphorus or nitrate-nitrogen in the lake water. Cellular nitrogen levels fluctuated independent of lake water nitrate-nitrogen concentrations. Cellular nitrogen levels remained above the critical concentration, 1.1%-N. Luxury phosphorus levels were observed to increase following pulses of soluble orthophosphate in the lake water. Measurements of cellular total phosphorus indicate that levels remain well above the critical concentration of 0.06%-P. It is concluded that neither nitrogen nor phosphorus is limiting the growth of Cladophora at the sites sampled. The mid-summer die-off does not appear to be due to a simple nutrient deficiency.     

Factors affecting Cladophora growth in the eastern basin of Lake Erie: Analysis of a monitoring dataset (2012–2019)

Cladophora is a naturally occurring benthic alga in the Great Lakes which can reach nuisance levels in the nearshore, leading to beach closures and other impacts. A monitoring program was initiated in 2012 in the eastern basin of Lake Erie to identify ecological factors driving its growth. Inflows from the Grand River, the largest river to the north shore, were generally positively associated with phosphorus concentrations in the nearshore and negatively associated with light reaching the lakebed. At the depths sampled (3 m–18 m), Cladophora was strongly influenced by light availability, and due to shading by the Grand River plume, an overall negative association was found between Cladophora biomass and phosphorus inputs. Phosphorus limitation was only observed at shallow sites farthest from the Grand River. Positive associations between dreissenid mussel coverage and both Cladophora biomass and tissue phosphorus suggest that nutrient cycling by dreissenids supports Cladophora growth. Our results indicate that i.) the Grand River has a strong influence on nearshore nutrient levels and water clarity; and ii.) Cladophora is limited by both phosphorus and light to varying degrees within the study area, although light appears to be the dominant factor, at least at these depths, years, and locations. The implication that phosphorus reductions could lead to increased Cladophora biomass by improving light conditions will need to be considered carefully against the known historical success of controlling nuisance algae through nutrient management.     

Spatiotemporal patterns of water quality in Lake Ontario and their implications for nuisance growth of Cladophora

This study, motivated by a resurgence in Cladophora, investigates changes in the nutrient environment in the littoral zone of Lake Ontario. We measured nutrient concentrations from 2004 to 2008 at two littoral zone (2–12 m) sites on the north shore of Lake Ontario where Cladophora has experienced a resurgence and compared concentrations with data collected in the late 1970s. Spring total phosphorus (TP) and soluble reactive P (SRP) concentrations have significantly declined at these two sites. Furthermore, P loading from the major tributaries to our study sites declined between 1964 and 2008. Upwelling events were not detectably associated with increases in P concentrations at our sites. We conclude that a recent upsurge in nuisance Cladophora, at least at these sites, cannot be explained by deteriorating littoral zone water quality in terms of P concentrations or by changes in catchment loading. For additional context, we also examined trends in coastal (14–20 m) and offshore (> 50 m) nutrients using Environment Canada epilimnetic surveillance data, 1975–2008. Significant declines in TP and SRP concentrations have occurred in north coast waters, concurrent with declines in the offshore. However, nutrient concentrations, notably spring SRP, have not decreased among south coast stations, potentially reflecting greater coastal entrapment of catchment-derived waters. We infer that EC-monitored north coast stations reflect integrated interannual water quality, while south coast stations are more strongly influenced by catchment loading. The effects of higher nutrient concentrations along the south coast, which co-occur with lower water transparency, on benthic algal growth have yet to be determined.     

Seasonal patterns for Secchi depth,chlorophyll a,total phosphorus,and nutrient limitation differ between nearshore and offshore in Lake Michigan

Data on Secchi depth, chlorophyll a, total phosphorus (TP), and nutrient status of phytoplankton were collected at five nearshore sites (11–17 m deep) and two offshore sites (>100 m) between the Grand River and Muskegon River outflows during March-December 2014–2018 to describe seasonal patterns and to compare the two depth regions in southeastern Lake Michigan. In contrast to the offshore, where spring chlorophyll a and TP concentrations declined dramatically following the dreissenid mussel expansion, the nearshore region of southeastern Lake Michigan was still characterized by low Secchi depth and elevated chlorophyll a and TP in the spring. During May, median Secchi depth was 5 times higher in the offshore than the nearshore, whereas chlorophyll a and TP were over 9 and 3 times higher in the nearshore, respectively. Even though spring chlorophyll a and TP have declined substantially at some of the nearshore sites compared to 1996, particularly the sites closest to tributary outflows, the overall yield of chlorophyll a per unit TP did not change over time in the nearshore. There were indications of P-deficiency in the nearshore in 2014–2018, but P-deficiency was even more severe in the offshore during the spring where yield of chlorophyll a per unit TP was also lower than in the nearshore. Although dreissenid mussels can be abundant in the nearshore, their populations are patchy and inputs from tributaries provide conditions that apparently dampen any potential filtering impacts of mussels in the nearshore compared to the offshore, especially during the spring.     

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.     

Monitoring Cladophora Growth Conditions and the Effect of Phosphorus Additions at a Shoreline Site in Northeastern Lake Erie

A study to measure environmental conditions, Cladophora standing crop, internal nutrient levels, and the effect of the addition of phosphorus to Cladophora growth at a single location on the north shore in the eastern basin of Lake Erie is described. In 1979, the mean standing crop for depths 0.5–3 m was 431 gDW/m² as measured at the time of maximum standing crop in early July. Thereafter, the alga was sloughed and carried ashore causing a rapid decline in standing crop. These events coincided with the attainment of lake temperatures exceeding 20° C. Total phosphorus concentrations averaged about 18 μg P/L while soluble reactive phosphorus levels remained near the limit of detection. Stoichiometric ratios of nitrate nitrogen to soluble reactive phosphorus approximated 150:1, suggestive of phosphorus limited conditions. Internal phosphorus and nitrogen levels averaged about 0.06% and 1.80%, respectively. In 1980, phosphorus (0.34 kg/day) was discharged at the 0.5 m depth commencing July 19. No response was noted until the water temperature dropped below 20° C in September when a rapid regrowth occurred, apparently in response to the nutrient addition. It is concluded that Cladophora grows in response to available phosphorus in the eastern basin of Lake Erie and that limitation of this nutrient may be expected to reduce Cladophora growth.