Modeling Dissolved Oxygen in a Dredged Lake Erie Tributary

A two-dimensional numerical model was developed to study dissolved oxygen (DO) kinetics in a dredged Lake Erie tributary. The model design was aimed to specifically address the fact that many tributaries to the Great Lakes are dredged periodically for navigation, and that resultant changes in morphology and hydraulics can have significant impacts on DO. Due to the greater depths caused by dredging, river velocities slow considerably and vertical mixing is not as effective, leading to thermal stratification and potential short-circuiting of warmer upstream flow. The model solves the two-dimensional (laterally averaged) hydrodynamic and mass balance equations to simulate transport and transformation relevant to dissolved oxygen using an alternating direction, implicit finite difference method. Effects of oxygen-demanding pollutants from municipal and industrial discharges and also from nonpoint sources are included. A model application was developed for the Black River (Ohio), a tributary of Lake Erie. The river is dredged periodically, becomes stratified during the low flow summer months, and is affected by changing lake levels associated with seiching in Lake Erie. After calibration and confirmation, the model was used as a diagnostic tool to understand the impact of various loading sources on low DO levels observed along the bottom of the river. It is shown that sediment oxygen demand (SOD), combined with the river hydraulics, is the primary cause for low DO levels in the Black River.     

Sediment Oxygen Demand in the Central Basin of Lake Erie

Three separate procedures were used to estimate the sediment oxygen demand (SOD) in the central basin of Lake Erie and were compared with other estimates determined previously and with historical data. First, whole core incubations involved sealing sediment cores at 12°C to ensure no interaction between the overlying water and the atmosphere and monitoring continuously to define the linear disappearance of oxygen. Second, sediment plugs were placed inside flow-through reactors and the influent and effluent concentrations were monitored to obtain steady-state reaction rates. Third, an extensive data set for the central basin of Lake Erie was compiled for input into the diagenetic BRNS model, and the SOD was calculated assuming all primary redox reactions, but no secondary reactions. All three procedures produced estimates of SOD that were in reasonable agreement with each other. Whole core incubations yield an average SOD of 7.40 × 10⁻¹² moles/cm²/sec, the flow-through experiments had an average SOD of 4.04 × 10⁻¹² moles/cm²/sec, and the BRNS model predicts an SOD of 7.87 × 10⁻¹² moles/cm²/sec over the top 10 cm of sediment and appears to be calibrated reasonably well to the conditions of the central basin of Lake Erie. These values compare reasonably well with the 8.29 × 10⁻¹² moles/cm²/sec obtained from diffusion modeling of oxygen profiles (Matisoff and Neeson 2005). In contrast, values reported from the 1960s to 1980s ranged from 10.5–32.1 × 10⁻¹² moles/cm²/sec suggesting that the SOD of the central basin has decreased over the last 35 years, presumably, in response to the decrease in phosphorus loadings to Lake Erie. However, since hypoxia in the hypolimnion persists these results suggest that improvement in hypolimnetic oxygen concentrations may lag decreases in loadings or that the hypolimnion in the central basin of Lake Erie is simply too thin to avoid summer hypoxia during most years.     

Oxygen Depletion in Central and Eastern Lake Erie: Relationship with Bacteria,Chlorophyll, POC,and Morphometry

Hypolimnion oxygen depletion in central and eastern Lake Erie was related to bacteria, particulate organic carbon, and chlorophyll concentrations during 1979. The central basin had the higher oxygen depletion rate and this was associated with more microbiota and particulate organic carbon. After compensation for temperature differences, the rate of oxygen depletion per unit bacteria, chlorophyll, and particulate organic carbon was found to be similar in each basin. These observations are consistent with the hypothesis that lake morphometry affects oxygen depletion through the control of hypolimnion particle concentrations. The implication that most of the hypolimnion oxygen metabolism occurs in the water column means that the role of sediments as a site of oxygen consumption should be reassessed.     

Seasonal Variation of Nutrient Limitation in Western Lake Erie

Monthly nutrient enrichment experiments were performed from April through October, 1983, to identify growth limiting nutrients of natural phytoplankton assemblages collected from a station in the Pigeon Bay waters off the north shore of Lake Erie's western basin. Data from these experiments suggest that silica was the major limiting nutrient in April, phosphorus was the major limiting nutrient in May-September, and trace metals were limiting in October. These results show that nutrient enrichment experiments should be performed often enough to account for seasonal changes in the physicochemical environment as well as seasonal succession in phytoplankton. The conclusion that phosphorus was the major limiting nutrient in summer is different from the conclusions of the late 1960s and early 1970s that nitrogen was the major limiting nutrient in the summer in the western basin and suggests a possible shift from nitrogen to phosphorus limitation. Pigeon Bay nutrient trend data show that summer nitrate:soluble reactive phosphorus ratios have increased from approximately 4 in the late 1960s to over 40 in the late 1970s and early 1980s, corroborating that a shift has probably occurred. This shift in nutrient limitation is probably due to reduced bioavailable phosphorus loadings and increased nitrate loadings to the western basin. The southern portion of the western basin is physicochemically different from the northern portion and may not have responded in the same manner.     

Oxygen use by nitrification in the hypolimnion and sediments of Lake Erie

Nitrification is an oxygen consumptive process, consuming 2 mol of oxygen per mol of ammonium oxidized. Hypolimnion and sediment samples were collected during the summers of 2008–2010 in Lake Erie to determine the total oxygen consumption and oxygen consumption from nitrification by blocking nitrification with selective inhibitors. Oxygen consumption by nitrification in the hypolimnion was 3.7 ± 2.9 (mean ± 1 SD) μmol O₂/L/d, with nitrification accounting for 32.6 ± 22.1% of the total oxygen consumption. Nitrification in the hypolimnion contributed more to oxygen consumption in the eastern sites than western sites and was lowest in September. The nitrification rate did not correlate with environmental factors such as oxygen, nitrate or ammonium, or nitrifier numbers. Oxygen consumption by nitrification in sediment slurries was 7.1 ± 5.8 μmol O₂/g/d, with nitrification accounting for 27.0 ± 19.2% of the total oxygen consumption with the lowest rates in July and the lowest percentages in June. Oxygen consumption by nitrification in intact sediment cores was 682 ± 61.1 μmol O₂/m/d with nitrification accounting for 30.4 ± 10.7% of the total oxygen consumption. Nitrification rates in intact cores were generally highest in September. The proportion of oxygen consumed by nitrification corresponds closely with what would be predicted from complete oxidation of a Redfield molecule (23%). While nitrification is unlikely to be the dominant oxygen consumptive process, the rates observed in Lake Erie were sufficient to theoretically deplete a large portion of the hypolimnetic oxygen pool during the stratified period.     

Evidence for internal phosphorus loading,hypoxia and effects on phytoplankton in partially polymictic Lake Simcoe,Ontario

Hypoxia and cyanobacterial blooms were extensive in Lake Simcoe during the 1980s and are still a problem to a lesser degree despite extensive nutrient load reduction from the catchment basin. The continuing signs of productivity indicate a potential internal phosphorus (P) source. Internal P load, as a redox-dependent release from bottom sediments, is hard to determine in a large, relatively shallow and partially unstratified lake such as Lake Simcoe. Of the lake's three major basins, only Kempenfelt Bay stratifies long enough to develop hypoxia in the stagnant summer hypolimnion. The following indications of sediment P release are available from historic data: 1) hypolimnetic hypoxia still occurs in Kempenfelt Bay although the hypoxic factor (number of days that an area equal to the bay's surface area is overlain by water of ≤ 2 mg/L dissolved oxygen, DO) has decreased substantially and significantly from 15.8 d/yr (1980–1994) to 4.0 d/yr (1995–2011); 2) hypoxic factors for other lake sections and at different DO levels also indicate widespread hypoxia; 3) concentrations of redox dependent metals, Fe and Mn, increase with depth; and 4) euphotic zone P and chlorophyll concentrations increase and water clarity decreases during fall turnover. Cyanobacterial blooms appear to occur in response to internal load as supported by occasional cyanobacteria counts. These indicators provide evidence that internal loading is likely occurring and affecting the water quality in Lake Simcoe. We expect that further monitoring, specific for internal load, will corroborate these results.     

Needed: Early-term adjustments for Lake Erie phosphorus target loads to address western basin cyanobacterial blooms

For Lake Erie, it is already time to revise the phosphorus target loads set to address the problem of cyanobacterial blooms in the Western Basin. Current targets were proposed by the Annex 4 task group in 2015, adopted by U.S. and Canadian governments in 2016, and set as objectives of domestic action plans in 2017. These targets, applicable to all spring discharges below the 90th percentile, set a maximum load for both total phosphorus (TP) and dissolved reactive phosphorus (DRP) equivalent to 60% of their 2008 spring loads. This essentially mandates 40% reductions in both particulate phosphorus (PP) and DRP loading relative to 2008 loads. These targets do not explicitly incorporate the difference in bioavailability between DRP (~100% bioavailable) and PP (~25% bioavailable). From 2008 to 2017, DRP comprised 24% of the spring TP load and over half (~56%) of the total bioavailable phosphorus (TBAP) load, while PP comprised 76% of the TP load but only ~44% of the TBAP load. Subsequent deposition of PP in the estuarine and nearshore zones further reduces its significance in bloom development. By ignoring differences in bioavailability, the current targets provide no guidance for choosing among practices based on their relative effectiveness in reducing DRP or PP and their combined reductions in TBAP loading. Current targets place more emphasis on PP than needed to efficiently reach targeted cyanobacterial bloom reductions. To clarify appropriate management approaches and lead to greater success in reducing cyanobacterial blooms, target loads should be based on TBAP.     

Relationships Between Bacterial Populations and Oxygen Levels in the Central Basin of Lake Erie

Studies were made to ascertain the relationships between bacteria and particulate organic carbon, particulate nitrogen, and the depletion of oxygen from the hypolimnetic waters of the central basin of Lake Erie. Heterotrophic bacterial populations, dissolved oxygen, particulate organic carbon, and total particulate nitrogen were determined during May, August, and September. Data indicate 1) a relationship between heterotroph populations and dissolved oxygen; 2) epilimnion oxygen concentrations of 9 mg/L were associated with bacterial levels of ?4 × 10³ colonies per mL; 3) bacterial densities started to increase from the thermocline downward; 4) hypolimnion oxygen levels of ?l mg/L were associated with bacterial densities of approximately three times the epilimnion values; 5) bacterial oxygen uptake rate in the hypolimnion varied from 1.5 to 7.4 × 10^?10 mg/L/hr; 6) particulate organic carbon and bacteria showed similar distribution patterns in the water column; 7) particulate nitrogen remained unchanged at 0.1 mg/L in the water column showing no relationship to the other parameters measured.     

Burrowing mayfly (Ephemeroptera: Ephemeridae: Hexagenia spp.) bioturbation and bioirrigation: A source of internal phosphorus loading in Lake Erie

Traditional lake eutrophication models predict lower phosphorus concentrations with decreased external loads. However, in lakes where decreased external phosphorus loads are accompanied by increasing phosphorus concentrations, a seeming “trophic paradox” exists. Western Lake Erie is an example of such a paradox. Internal phosphorus loads may help explain this paradox. We examined bioturbation and bioirrigation created from burrowing mayfly, Hexagenia spp., as a possible source of internal phosphorus loading. Phosphorus concentrations of experimental microcosms containing lake sediments, filtered lake water, and nymphs (417/m²) collected from western Lake Erie were compared to control microcosms containing sediments and lake water over a 7-day period. Phosphorus concentrations in microcosms containing Hexagenia were significantly greater than microcosms without nymphs. Further, we estimate the soluble reactive phosphorus flux from the sediments due to Hexagenia is 1.03 mg/m²/day. Thus, Hexagenia are a source of internal phosphorus loading. High densities of Hexagenia nymphs in western Lake Erie may help explain the “trophic paradox.” Furthermore, Hexagenia may be a neglected source of internal phosphorus loading in any lake in which they are abundant. Future studies of phosphorus dynamics in lakes with Hexagenia must account for the ability of these organisms to increase lake internal phosphorus loading.     

Agricultural conservation practices could help offset climate change impacts on cyanobacterial harmful algal blooms in Lake Erie

Harmful algal blooms (HABs) are a recurring problem in many temperate large lake and coastal marine ecosystems, caused mainly by anthropogenic eutrophication. Implementation of agricultural conservation practices (ACPs) offers a means to reduce non-point source nutrient runoff and mitigate HABs. However, the effectiveness of ACPs in a changing climate remains uncertain. We used an integrated biophysical modeling approach to predict how Lake Erie cyanobacterial HAB severity (bloom biomass) may change under several climate and ACP implementation scenarios, using western Lake Erie and its largely agricultural watershed as our study system. An ensemble of general circulation model projections was used to drive spatially explicit land use and hydrology models of the Maumee River watershed, the output of which informed a predictive model of Lake Erie HAB severity. Results show that, in the absence of changes in ACPs, the frequency of severe HABs is projected to increase during coming decades, owing to increased inputs of nutrients from the watershed. These anticipated increases are due to increased total precipitation and more frequent higher-magnitude rainfall events. While further implementation of ACPs appears capable of reducing severe HAB events, widespread implementation would be necessary to reduce HAB severity below current management targets. This study highlights how continued climate change will only exacerbate the need for land management practices that can reduce nutrient runoff in agriculturally dominated ecosystems, such as Lake Erie. It also shows how interdisciplinary, biophysical modeling approaches can help identify strategies to mitigate HABs in the face of anthropogenic stressors.     

A simple 1-dimensional,climate based dissolved oxygen model for the central basin of Lake Erie

A linked 1-dimensional thermal-dissolved oxygen model was developed and applied in the central basin of Lake Erie. The model was used to quantify the relative contribution of meteorological forcings versus the decomposition of hypolimnetic organic carbon on dissolved oxygen. The model computes daily vertical profiles of temperature, mixing, and dissolved oxygen for the period 1987–2005. Model calibration resulted in good agreement with observations of the thermal structure and oxygen concentrations throughout the period of study. The only calibration parameter, water column oxygen demand (WCOD), varied significantly across years. No significant relationships were found between these rates and the thermal properties; however, there was a significant correlation with soluble reactive phosphorus loading. These results indicate that climate variability alone, expressed as changes in thermal structure, does not account for the inter-annual variation in hypoxia. Rather, variation in the production of organic matter is a dominant driver, and this appears to have been responsive to changes in phosphorus loads.     

Calcium Carbonate in Postglacial Lake Erie Sediment

Geotechnical, geochemical, electron-microscopic and biostratigraphic investigation of a 16.8–m long core of a postglacial lacustrine mud collected from the central Lake Erie basin revealed zones containing up to 30% calcite. These zones were associated with a very fine sediment, 80% of which was finer than 4 y.m. Shear strength values of the sediment ranged from 2 kN/m² near the lake bottom to about 9 kN/m² close to the Pleistocene boundary. The concentration of CaO was positively correlated with the concentration of inorganic C and Sr. The concentration of SiO₂, A1₂O₃, Fe₂O₃, K₂O, and Rb gradually decreased with depth. The major crystalline phases were illite (44 wt %), chlorite (10 -13 wt %), calcite (6 - 30 wt %), quartz (6 - 20 wt %), albite (6 - 9 wt %), K-feldspar (2 wt %), and dolomite (2 wt %). The shells of mollusc species present in the lower portion of the postglacial sedimentary column indicated that the carbonate-high sediments were deposited in warmer water than the carbonate-low sediments above and between them.     

Seasonal Phosphate Demand for Lake Erie Plankton

From April to mid-October 1979, in the eastern and central basins of Lake Erie, the pattern for phosphate demand was assessed by four independent methods: conventional monitoring, measurements of polyphosphate, phosphate turnover time, and a newly developed phosphate deficiency index. Thermal stratification and P limitation occurred faster in the central than in the eastern basin and the plankton remained P limited throughout most of the stratified season. This condition relaxed as the thermocline deepened in the central basin, exposing the epilimnion to a greater area of sediments. In September, when mixing of the entire water column occurred, the plankton were no longer P limited. In contrast, as the thermocline deepened in the eastern basin, metalimnetic water was entrained but the plankton remained P limited until mid-October. These observations support the view that a reduction of phosphorus in Lake Erie would probably affect the phytoplankton biomass during the periods of P limitation.     

Modelling Ice Dissipation in Eastern Lake Erie

A previously developed mathematical model, which considered both the heat exchange processes at the air-ice interface and the ice transport rate via the Niagara River Was used to simulate the ice dissipation process in eastern Lake Erie. The model, which assumed the ice thickness to be constant over the lake, has been further developed to take into account variable ice thickness. Buffalo meteorological data were used for the computation of the net heat exchange at the air-ice interface. Results of the model application illustrate the relative importance ofin-lake ice melt to ice transport via the Niagara River during the dissipation period. Model results were found to give reasonably good agreement with the limited ice information available for the 1976 dissipation period.     

Geochemical Availability of Some Trace and Major Elements in Surficial Sediments of the Detroit River and Western Lake Erie

Surficial bottom sediment from twenty locations in the Detroit River and western Lake Erie has been analyzed for potentially available Ba, Be, Cd, Cr, Co, Cu, Pb, Mn, Mo, Ni, P, V, and Zn. The highest concentrations of all but one of these elements were found at a station at the river-lake interface very close to a dumping ground. The environmental mobility of Cd, Zn, Co, Pb, Cr, Ni, and P is controlled by hydrous iron oxides, whereas V and Mo is controlled by aluminosilicates. The iron oxide phase exhibits a very high sorption capacity for phosphorus (molar adsorption coefficient = .361) which is attributed to the high loadings of Fe and P at the confluence of the Rouge and Detroit rivers as well as continuous inputs along the length of the Detroit River. It is suggested that phosphorus controls instituted in the 1970s will probably result in the iron oxide phase having greater sorption capacity for toxic metals because of the decreasing competition from phosphorus for binding sites.     

Unionid mussels from nearshore zones of Lake Erie

Concern exists that the introduction of dreissenid mussels following long-term effects of pollution may have completely eliminated native mussel species from Lake Erie. Natural seiche events were used to facilitate surveys for live unionids on five occasions in the western basin of Lake Erie and Sandusky Bay between 2007 and 2009, and beach and estuary surveys were conducted at numerous additional sites between 2004 and 2009. Sixteen unionid species were found living in or near Lake Erie, including six sites in the nearshore zone of the lake. Each community consisted of live individuals from two to eight species, and evidence included live and/or fresh dead material from several state listed species at multiple sites. Where estimated, the mean overall density was low at 0.09 unionids/m², although similar to other known unionid refuges in the lower Great Lakes. While the ephemeral nature of seiche events makes them a limited survey tool, their application combined with increasing numbers of fresh shells washing ashore over the past few years indicates that unionids are extant in the western basin of Lake Erie, and may further suggest that conditions may be improving for native mussel species.     

Lyngbya wollei in western Lake Erie

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

Lake Erie Nutrient Loads, 1970–1980

The Buffalo District, Corps of Engineers’ Lake Erie Wastewater Management Study and Heidelberg College's Water Quality Laboratory supported a tributary water quality monitoring program from 1974 to 1980 of the major United States tributaries to Lake Erie. This program was designed to measure nutrient loads by monitoring concentration changes occurring in association with increased streamflow. Soluble orthophosphate loads, chloride loads, and silica loads decreased from 1970 to 1980. Nitrogen species were highly variable and increased over the period. Total phosphorus loads to Lake Erie have decreased during the period as a result of phosphorus removal at wastewater treatment plants. The effect of the phosphorus reductions can be seen in the lake concentrations and were predicted by a three-basin phosphorus budget model developed in the early 1970s. The results show that phosphorus removal programs are having the predicted effect on Lake Erie water quality.     

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.     

Phytoplankton growth dynamics in offshore Lake Erie during mid-winter

The phytoplankton community in offshore Lake Erie in mid-winter was active but little net growth was occurring which suggests that high reported accumulations of phytoplankton in this lake in February are likely the product of previous bloom conditions. We measured phytoplankton dynamics as size-specific growth and loss rates of phytoplankton using dilution assays and antibiotic assays in ice-covered offshore waters of Lake Erie during the mid-winter period in 2008, 2009, and 2010. Total chlorophyll-a specific rates (average ± standard deviation) measured using dilution assays for growth ([0.72 ± 0.35/d]) and loss ([0.98 ± 0.36/d]) were closely matched. Growth and loss rates of picocyanobacteria determined using an antibiotic technique ranged from − 0.10 to 1.22/d and − 0.11 to − 2.35/d, respectively. The results indicate a trend of higher grazing rate than growth rate but that this difference is not significantly different from zero, suggesting a state of phytoplankton population size equilibrium at this time of year in the waters sampled.