Temporal and spatial trends of organochlorines and mercury in fishes from the St. Clair River/Lake St. Clair corridor,Canada

The temporal and spatial relationships of a suite of organochlorine contaminants and mercury were examined in various fish species of the St. Clair River/Lake St. Clair corridor, Canada, in order to evaluate the effectiveness of remediation efforts and to assess the risk to human and wildlife fish consumers. In Lake St. Clair, fish tissue concentrations of mercury, polychlorinated biphenyls (PCBs), octachlorostyrene (OCS), hexachlorobenzene (HCB), and dichlorodiphenyltrichloroethane (DDT) decreased consistently from the 1970s until the 1980s and 1990s, after which the rate of contaminant decline slowed or concentrations stabilized. This trend was consistent in up to 13 species (both young-of-the-year and adult fishes) comprising different trophic positions and dietary habits, suggesting that the changes were reflective of ambient conditions rather than food web processes. Elevated concentrations of mercury, PCBs, OCS, HCB, and DDT were detected in St. Clair River young-of-the-year spottail shiner compared with fish from Lake Huron, indicating that non-atmospheric inputs of these chemicals, likely originating from sediment, remain in the St. Clair River. Current concentrations of mercury and PCBs, and mercury, PCBs, and DDT remain of concern to human and wildlife fish consumers, respectively. Given that contaminant decreases have generally stabilized in fish, we suggest that further natural recovery of contaminants in St. Clair corridor fishes will be slow since contaminants will likely continue to be influenced by sediment levels.     

Relationships between wind-driven and hydraulic flow in Lake St. Clair and the St. Clair River Delta

A three-dimensional hydrodynamic forecasting model of the Great Lakes Huron-Erie Corridor is used to investigate mixing and the relationship between hydraulic and wind-induced currents in a shallow lake system in which lake inflows come through several channels of a river delta. The hydrodynamics in Lake St. Clair and the channels of the St. Clair River Delta are evaluated for (1) a one-year simulation from 1985 including water age calculation, (2) 8 different wind direction scenarios, and (3) a storm event. Observations and model simulations show distinct regions in the lake in which currents are forced by either hydraulic flow from the river system or from wind stress over the lake. However, during severe storm events, these regions are found to shift or even disappear due to changes in the delta channel inputs into the lake. These changes underscore the need for realistic, unsteady river flow boundary conditions at interfaces between a shallow lake and river delta. Steady inflow conditions will not allow for potential shifting of these current zones, and will also fail to resolve flow retardation or reversals during storm events.     

Temporal trends in near-shore sediment contaminant concentrations in the St. Clair River and potential long-term implications for fish tissue concentrations

Near-shore surface sediment of the St. Clair River (Ontario) extending along 9 km of waterfront downstream of local industries were contaminated with mercury (Hg), hexachlorobenzene, hexachlorobutadiene and octachlorostyrene based on data collected in 2006 and 2008. However, unlike Hg, concentrations of organic compounds were low at most stations suggesting improvements in sediment quality since 2001 (post sediment remediation projects). Concentrations of Hg were greater than Provincial Sediment Quality Guidelines and ranged as high as 41 μg/g in the surficial 0–5 cm sediment. Measured (2001–2004 data) and estimated (2006 data) concentrations of methyl Hg in invertebrates were greater than concentrations in invertebrates collected from upstream reference stations. Predicted methyl Hg concentrations in walleye using food chain multipliers, benthic invertebrate Hg tissue concentrations and current sediment concentrations exceeded human health consumption guidelines and were consistent with measured sport fish data supporting assumptions about Hg biomagnification. Historical data showed that for fish collected from the lower St. Clair River and Lake St. Clair, Hg concentrations have declined since 1978, but have remained stable since the mid 1980s, consistent with the sediment Hg data. The consistency in the sediment and fish datasets and the absence of a known significant local point source of Hg suggests that the sediment may be an important source of Hg to the St. Clair River food-chain.     

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.     

Modelling of nearshore microbial water quality at confluence of a local tributary in Lake St. Clair

Microbial water quality, measured as Escherichia coli (E. coli) concentration, at beaches along the southern shore of Lake St. Clair in Canada, often exceeds public safety guidelines. Belle River, located near a public beach and a drinking water intake, is one of the several smaller tributaries of the lake whose contribution to nearshore microbial water quality is currently unknown. A flexible mesh 3D coupled TUFLOW-FV and Aquatic Ecodynamic (AED2+) model was used to simulate the hydrodynamics and microbial water quality in Lake St. Clair. A higher resolution nested model was developed within the lake-wide TUFLOW-FV model for better spatial and temporal resolution in the local region surrounding Belle River. Regular and up to a factor of four difference in predicted E. coli concentrations were observed with the nested and lake-wide models at the public beach next to Belle River, whereas the difference was marginal at the drinking water intake about a kilometre away from the shore. While the E. coli loading to Lake St. Clair from Belle River is considered negligible, >90% of the predicted daily E. coli concentration at the beach and > 50 % at the water intake were attributed to Belle River from amongst all watershed sources to Lake St. Clair considered in the model. The model results also show that the construction of a new 150 m jetty in 2018, replacing the older 25 m jetty separating Belle River from the public beach, is expected to significantly reduce E. coli concentrations observed at the beach.     

Application of distance sampling techniques for diving ducks on Lake St. Clair and western Lake Erie

Lake St. Clair and western Lake Erie are important migration staging areas for diving ducks including canvasbacks (Aythya valisineria), redheads (Aythya americana), and lesser and greater scaup (Aythya affinis and Aythya marila). Starting in 1983, the Michigan Department of Natural Resources (MDNR) attempted to census diving ducks on the United States portion of Lake St. Clair throughout autumn migration; however, in 2010 the MDNR expanded the traditionally surveyed area to include all of Lake St. Clair and a portion of western Lake Erie. The idea of achieving a census over the expanded study area was unrealistic, and instead distance sampling techniques were adopted in an effort to generate statistically valid estimates of detection probabilities and abundances for diving ducks during spring and autumn migration. We found distance sampling techniques to be a viable option for estimating diving duck abundance as long as flock size is accounted for as a covariate affecting the detection function. Diving ducks were generally more abundant on our study area during autumn migration with a mean of 306,327 ducks/survey (SE = 40,729) compared to an average spring abundance of 91,053 ducks/survey (SE = 19,175). Peak abundance occurred on 20 November 2012 with an estimated 596,335 diving ducks on Lake St. Clair and western Lake Erie. Ultimately, our methodology could be used to establish long-term, standardized data collection techniques and applied to conservation planning for waterfowl in the Great Lakes region.     

St. Clair-Detroit River system: Phosphorus mass balance and implications for Lake Erie load reduction,monitoring, and climate change

To support the 2012 Great Lakes Water Quality Agreement on reducing Lake Erie's phosphorus inputs, we integrated US and Canadian data to update and extend total phosphorus (TP) loads into and out of the St. Clair-Detroit River System for 1998–2016. The most significant changes were decreased loads from Lake Huron caused by mussel-induced oligotrophication of the lake, and decreased loads from upgraded Great Lakes Water Authority sewage treatment facilities in Detroit. By comparing Lake St. Clair inputs and outputs, we demonstrated that on average the lake retains 20% of its TP inputs. We also identified for the first time that loads from resuspended Lake Huron sediment were likely not always detected in US and Canadian monitoring programs due to mismatches in sampling and resuspension event frequencies, substantially underestimating the load. This additional load increased over time due to climate-induced decreases in Lake Huron ice cover and increases in winter storm frequencies. Given this more complete load inventory, we estimated that to reach a 40% reduction in the Detroit River TP load to Lake Erie, accounting for the missed load, point and non-point sources other than that coming from Lake Huron and the atmosphere would have to be reduced by at least 50%. We also discuss the implications of discontinuous monitoring efforts.     

Spatial Distributions and Temporal Trends in Sediment Contamination in Lake St. Clair

The sediments of Lake St. Clair were surveyed in 2001 for a range of compound classes including metals (such as total mercury and lead), polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins and dibenzofurans, organochlorine pesticides, polycyclic aromatic hydrocarbons, and short- and medium- chain chlorinated paraffins, in order to evaluate the spatial distribution and temporal trends of contamination. Concentrations of contaminants were generally low compared to the lower Great Lakes (Erie and Ontario), and were typically below the Canadian Sediment Quality Probable Effect Level (PEL) guidelines. The only exceptions were for mercury and DDE, where concentrations exceeded their respective PEL at one of the thirty-four sites sampled. With the exception of mercury, it was difficult to interpret spatial trends in contaminant concentrations due to these low levels, although relatively elevated concentrations of several contaminants were found in L’Anse Creuse Bay and at the outflow of the Thames River. In the case of mercury, historically-contaminated sediments in the St. Clair River associated with chlor-alkali production appeared to contribute to loadings to Lake St. Clair. There have been substantial reductions in sediment contamination in Lake St. Clair over the past three decades, as determined through sediment core profiles as well as through comparison of current data to those from historical surveys conducted in the early 1970s. These results indicate that management actions to reduce contaminant loadings to Lake St. Clair have been generally successful.     

A one hundred year review of the socioeconomic and ecological systems of Lake St. Clair,North America

There is a growing concern about continued impairment of aquatic ecosystems resulting from increasing population size, land use, climate change, and the feedbacks that may harm human well-being. We describe a 100 year multi-disciplinary overview of changes in Lake St. Clair, North America to identify knowledge gaps and needs to build the foundation for creating coupled human and natural system models. Our historical analysis indicates that the socioeconomic dynamics are inextricably linked to the urban dynamics of the Detroit metropolitan area. Environmental degradation and human health issues led to the adoption of relevant policies, including construction of wastewater treatment facilities by the 1960s. Climate trends during the 100-year period indicate a wetter region, which is influencing lake levels. Since the mid-1980s and 90s invasive zebra and quagga mussels (Dreissena polymorpha and Dreissena rostriformis bugenis) have significantly altered the ecological structure and function of the lake. Waterborne illnesses due to contaminated drinking water were once an issue but current human health risks have shifted to contaminated recreational waters and coastal pollution. Key research needs for building coupled models include geo-referencing socioeconomic and ecological data to accurately represent the processes occurring within the political and watershed boundaries; assessing ecosystem services for human well-being; and developing research hypotheses and management options regarding interactions among land use, people and the lake. Lake St. Clair has gone through extensive changes, both socioeconomically and ecologically over the last 100 years and we suggest that it serves as a useful case study for the larger Great Lakes region.     

Characteristics of a refuge for native freshwater mussels (Bivalvia: Unionidae) in Lake St. Clair

The Lake St. Clair delta (∼ 100 km²) provides an important refuge for native freshwater mussels (Unionidae) wherein 22 of the ∼ 35 historical species co-occur with invasive dreissenids. A total of 1875 live unionids representing 22 species were found during snorkeling surveys of 32 shallow (∼ 1 m) sites throughout the delta. Richness and density of unionids and zebra mussel infestation rates varied among sites from 3 to 13 unionid species, 0.02 to 0.12 unionids/m², and < 1 to 35 zebra mussels/unionid, respectively. Zebra mussel infestation of unionids in the delta appears to be mitigated by dominant offshore currents, which limit densities of zebra mussel veligers in nearshore compared to offshore waters (13,600 vs. 28,000/m³, respectively). Glycogen concentrations in the tissues of a common and widespread species in the delta (Lampsilis siliquoidea) suggest that zebra mussels may be adversely affecting physiological condition of unionids in a portion of the Lake St. Clair delta. Physiological condition and community structure of unionids within the delta may also be influenced by differences in food quantity and quality resulting from the uneven distribution of water flowing from the St. Clair River. The delta likely supports the largest living unionid community in the lower Great Lakes and includes several species that have been listed as Endangered or Threatened in Canada and/or the state of Michigan, making it an important refuge for the conservation of native unionids.     

Lake sturgeon (Acipenser fulvescens) spawn in the St. Marys River Rapids,Michigan

The St. Marys River connects Lake Superior to Lake Huron, comprising the international border between Michigan, United States, and Ontario, Canada. This Great Lakes connecting channel naturally encompasses various habitats including lakes, wetlands, islands, tributaries, side channels, and main channels. The St. Marys River Rapids are shallow rock areas with high flow velocities (>1 m/s) in the upper river adjacent to the navigation locks and electric power generating stations, while the Little Rapids are shallow, recently restored rocky areas with lower velocities located about 7 km downstream. The St. Marys River Rapids provide important spawning habitat for several native and introduced fishes, but spawning by lake sturgeon (Acipenser fulvescens) was not previously documented. We sampled for lake sturgeon eggs and larvae in both locations during June and July 2018–2019 using weekly benthic egg mat lifts and overnight D-frame larval fish drift nets. Viable lake sturgeon eggs (11 in 2018, 45 in 2019) were collected in the tailrace of a hydroelectric power facility adjacent to the St. Marys River Rapids. Larval lake sturgeon (21 in 2018, 1 in 2019) were collected in the same area as the eggs. Neither lake sturgeon eggs nor larvae were collected at Little Rapids in either year. Our results are the first documentation of successful lake sturgeon spawning and larval drift in the upper St. Marys River. While our observations showed spawning in a human-made tailrace area, the fate of larvae produced here is unknown and warrants further research.     

Response of wetland vegetation to the post-1986 decrease in Lake St. Clair water levels: Seed-bank emergence and beginnings of the Phragmites australis invasion

Water-level fluctuations are critical for maintaining the diversity and resultant habitat value of wetland plant communities in the Laurentian Great Lakes. However, activation of the seed bank can also provide an opportunity for invasive species to displace native species, as occurred when common reed, Phragmites australis, expanded across many wetlands after lake levels receded following highs in 1997. Timing of the invasion process is not clear, however, as Phragmites propagules had to be present to exploit the exposed soils. A data set from Dickinson Island on the St. Clair River delta collected in 1988–1991, 1996 during a previous lake-level decline was analyzed to document prior Phragmites growth, as well as overall seed-bank response. Above-ground biomass was determined for all plants each year in randomly placed quadrats in a 5-ha area exposed when lake levels decreased by 0.65 m from 1986 to 1988. A total of 38 taxa were identified in 1988, but the number decreased, along with biomass of many species, as canopy-dominating Typha angustifolia and Phragmites increased in later years. Although Phragmites did not expand greatly until after the decline from the 1997 high, it likely inoculated the area with viable seed during the previous low. Because post-1997 lake levels were lower than those post-1986, they exposed a greater area for Phragmites colonization from seed; lake levels also remained low for a longer time. Differences in bathymetry below the 1986 and 1997 lake-level elevations likely played a role in greater post-1997 spatial expansion of Phragmites at other sites in the Great Lakes also. The next high lake level will likely be required to displace Phragmites, but the effect will be temporary.     

Influence of water temperature on rainbow smelt spawning and early life history dynamics in St. Martin Bay,Lake Huron

Rainbow smelt are an important prey species for native and introduced salmonines in the Great Lakes. In Lake Huron, rainbow smelt populations are characterized by variable recruitment and year-class strength. To understand the influence of water temperature on reproduction, growth, and survival during larval-fish stages, we sampled spawning tributaries and larval-fish habitats during 2008 and 2009 in St. Martin Bay, Lake Huron. Spawning by rainbow smelt occurred primarily when stream temperatures were between 3 and 10 °C, which resulted in a 7–10-day spawning period during 2008, and a 15–20-day spawning period during 2009. Regardless of these differences in spawning temperatures and duration, peak larval-fish densities during 2008 were double those observed during 2009. Length–frequency analysis of larval-fish populations during both years revealed stream-hatched fish during May and a later emergence of larval rainbow smelt during summer, presumably originating from lake spawning. Warmer bay water temperatures led to earlier emergence of lake-spawned rainbow smelt larvae during 2009. Stream-hatched fish larvae experienced large-scale mortality during May 2008 resulting in a bay population consisting primarily of lake-spawned rainbow smelt larvae, but during 2009 both stream- and lake-hatched cohorts experienced higher survival concomitant with significantly higher mean population growth rates. Higher larval-fish growth rates during 2009 appeared to be density-dependent and facilitated by warmer water temperatures during late June and cooler water temperatures during July. Temperature-mediated differences in annual growth rates and irregular contributions from stream- and lake-hatched fish larvae are important factors affecting survival and abundance of young-of-the-year rainbow smelt in Lake Huron.     

Contaminant concentrations in bald eagles nesting on Lake Superior,the upper Mississippi River,and the St. Croix River

We measured concentrations of DDE, total PCBs, and mercury in bald eagle (Haliaeetus leucocephalus) nestlings at three locations in the upper Midwest: Lake Superior, the upper Mississippi River, and the St. Croix River, 2006–2008. We also analyzed trends in concentrations of these contaminants for eagles on the southern shore of Lake Superior, from 1989 to 2008, using the current and previously published data. Concentrations of DDE in nestling blood plasma samples were greatest on Lake Superior (geometric mean: 16.2 μg/kg, n = 29), whereas concentrations of total PCBs were highest in Mississippi River samples (88.6 μg/kg, n = 51). Mercury concentrations were highest along the upper St. Croix River (6.81 μg/g wet weight in feathers, n = 19). For Lake Superior, DDE concentrations declined significantly in nestling blood plasma samples from 1989 to 2008, an average of 3.0% annually. Similarly, total PCBs in Lake Superior eaglets decreased 4.0% annually from 1989 to 2008, and mercury concentrations in nestling feathers from Lake Superior nests also decreased significantly from 1991 to 2008, 2.4% per year. With the possible exception of mercury on the upper St. Croix River, mean concentrations in 2006–2008 of all three compounds were below levels associated with significant impairment of reproduction for all sites, and reproductive rates at all three sites averaged > 1.2 young per occupied territory, which is greater than the rate indicative of a healthy population.     

黄河高含沙水流的高效输沙特性形成机理(黄河下游河道存在巨大的输沙潜力)

利用黄河主要干支流渭河、北洛河、黄河下游及三门峡水库大量实测资料,结合高含沙水流流变特性分析得出:河道中的高含沙水流的阻力与低含沙水流相同,均可用曼宁公式进行阻力计算;由于黄河泥沙组成较细,d50=0.03~0.10mm,随着含沙量的增加颗粒的沉速大幅度降低,泥沙在垂线上分布变得更加均匀,当含沙量大于200kg/m3以后发生改变;从泥沙存在对水流结构,流速在垂线上的分布特性上分析,含沙量200kg/m3左右时输送最困难,并得到河道实测资料的证实。由于粘性的增大,粗颗粒泥沙在河道中也能顺利输送,洪水期实测含沙量可达800~900kg/m3,表现出高含沙水流的高效输沙特性;黄河下游艾山以下河段实测洪水的最大含沙量为200kg/m3,在流量2 000~3 000m3/s时,高含沙洪水均可顺利输送。利用河道输送高含沙水流入海的主要障碍是改造宽浅游荡河段为窄深稳定的河槽。     

Assessment of fish mercury levels in the upper St. Lawrence River,Canada

The stretch of the St. Lawrence River near Cornwall, Ontario, Canada, between the Moses–Saunders power dam to the eastern outlet of Lake St. Francis in Québec, is currently listed as an Area of Concern (AOC), and has restrictive fish consumption advisories in place, largely due to high mercury levels. This study examined long-term temporal trends of mercury concentrations in northern pike, smallmouth bass, walleye and yellow perch from the St. Lawrence River, including the Cornwall AOC. In addition, differences in fish mercury concentrations among river sections were compared for each species using historical (1975–1979) and recent (2000–2011) data. Statistically significant declines in mercury concentrations were observed in all river sections for yellow perch (~ 40%), as well as in northern pike and walleye in at least one river section. For the river section encompassing the AOC, recent mercury concentrations are 33–59% lower than historical mercury concentrations for all four species. Further, a comparison of recent mercury concentrations between the AOC river section and other Ontario lakes/rivers suggests that AOC mercury values are within the 75th percentile of values for other Ontario water bodies in three of the four species considered. However, current fish mercury values for the AOC river section remain higher than one or more upstream river sections. These results indicate that recovery is still ongoing in this AOC.     

Genetic origins and diversity of lake sturgeon in the St. Louis River Estuary

Lake sturgeon Acipenser fulvescens were extirpated from the St. Louis River Estuary (SLRE) by the early 1900’s due to overfishing and habitat degradation. A restoration stocking program began in 1983, and continued almost annually until 2000. Lake sturgeon stocked into the SLRE were primarily obtained from the Wolf River (Lake Winnebago) genetic stock (n = 861,000) but some sturgeon were obtained from the Sturgeon River (Lake Superior) genetic stock (n = 61,380). Recently, spawning and natural recruitment has been documented near the Fond du Lac Dam, the upstream limit for lake sturgeon migrating from Lake Superior. However, the genetic origin of lake sturgeon spawning in the SLRE was unknown. Our objectives were to determine (1) the genetic origins and (2) genetic diversity of lake sturgeon spawning in the SLRE. Using both GENECLASS2 and ONCOR, a majority (79–81%) of lake sturgeon captured in the SLRE during spawning (2016–2018) assigned to the Wolf River genetic stock (Lake Winnebago) with greater than 80% probability using established microsatellites and a standardized genetic baseline. Other genetic stocks present (≥1%) included the Pic and Goulais rivers and possibly the Black Sturgeon River (identified using GENECLASS2, but not ONCOR); no fish assigned to the Sturgeon River using either method. Genetic diversity metrics showed that the SLRE lake sturgeon population was similar to other Lake Superior lake sturgeon populations. Overall, the SLRE Sturgeon population appears headed towards recovery. Adaptive management practices currently being employed should be continued to help guide further recovery of this population.     

Establishment patterns of non-native fishes: Lessons from the Duluth-Superior harbor and lower St. Louis River, an invasion-prone Great Lakes coastal ecosystem

We conducted a 2-year, multi-gear survey in the lower St. Louis River, which includes the Duluth-Superior harbor, an international shipping port and non-native species invasion “hotspot.” Our objectives were to quantify the contribution of non-native species to the overall fish assemblage and assess their spatial distribution and abundance. We captured 10 non-native fishes; none, however, were first detections. Non-native fishes composed roughly one quarter of the total species richness, were found in 84% of samples, and composed 15% of the total abundance. The spatial distribution and abundance of non-native fishes was dependent on gear selection (and thereby habitat selection), sampling location, and abundance measure (individuals or biomass). We used a recently published non-native species establishment framework to integrate catch data from the different gears. Viewed in the context of this framework, we identified only two non-native fishes, Eurasian ruffe (Gymnocephalus cernuus) and round goby (Neogobius melanostomus), as both widespread and abundant, whereas we identified three as localized and rare. Moreover, the time since first detection of non-native fishes was not predictive of their frequency of occurrence across the study area, underscoring the importance of environmental and biological factors in controlling fish establishment success. Although non-native fishes constitute a considerable portion of the trawl catch in the river channel, from a multi-gear system-wide perspective, native fishes collectively make up the majority of the fish biomass and abundance in the lower St. Louis River.     

长江河口南槽近期滩槽水沙输移特性分析

为了深刻认识近期长江河口演变趋势,依据2005,2006年和2010年长江口南槽滩槽洪枯季实测水文泥沙数据,分析了主槽与浅滩的水动力条件及悬沙浓度的时空分布特征。利用机制分解法,研究了河口滩槽间泥沙输移及交换机制。研究结果表明,长江口南槽主要以往复流为主,落潮流速大于涨潮流速。随着水深变浅,水流流速过程线存在差异,使不同区域的水流挟沙能力和输沙过程不同。输沙以平流项输沙通量为主,但潮泵作用在南槽地区泥沙输移中也占有重要地位,形成"主槽出沙、浅水进沙"的输沙模式,促使南汇边滩前缘缓慢淤涨。     

Proliferation of phytoplankton along a 500 km transect of the St. Lawrence River from its outflow at Lake Ontario

Observation of phytoplankton and water chemistry along the main channel of the St. Lawrence River was made at a high spatial resolution (every 12 km) in order to infer the factors that influence development of this phytoplankton community. The phytoplankton community in the main channel was collected over a 10-d period (mid July 2018) from the headwaters to near the beginning of the St. Lawrence River estuary. Total phosphorus concentration in river water increased with distance downstream (154–2,750 nM) and phytoplankton biomass (1.4–10.5 µg chl-a/L) was strongly correlated (r = 0.84, 46 d.f.) to the concentration of total phosphorus. Diatoms, chrysophytes and dinoflagellates dominated the phytoplankton community at the outlet of Lake Ontario and total chlorophyll-a concentrations increased three-fold with 500 km transit downstream from Lake Ontario; phycocyanin-rich Cyanobacteria showed the greatest proportional increase (227%). Total P concentrations observed in 1997 along this transect were identical to those observed in 2018; however, chl-a concentrations were much lower in 1997, a finding attributed to a greater filter feeding benthic organism impact on the standing crop of phytoplankton. Observations support the hypothesis that the phytoplankton community composition in this large river is strongly influenced by the headwater characteristics (Lake Ontario) and gradually influenced by entrainment of nutrient-rich tributary waters.