Evidence of Lake Whitefish Spawning in the Detroit River: Implications for Habitat and Population Recovery

Historic reports imply that the lower Detroit River was once a prolific spawning area for lake whitefish (Coregonus clupeaformis) prior to the construction of the Livingstone shipping channel in 1911. Large numbers of lake whitefish migrated into the river in fall where they spawned on expansive limestone bedrock and gravel bars. Lake whitefish were harvested in the river during this time by commercial fisheries and for fish culture operations. The last reported landing of lake whitefish from the Detroit River was in 1925. Loss of suitable spawning habitat during the construction of the shipping channels as well as the effects of over-fishing, sea lamprey (Petromyzon marinus) predation, loss of riparian wetlands, and other perturbations to riverine habitat are associated with the disappearance of lake whitefish spawning runs. Because lake whitefish are recovering in Lake Erie with substantial spawning occurring in the western basin, we suspected they may once again be using the Detroit River to spawn. We sampled in the Detroit River for lake whitefish adults and eggs in late fall of 2005 and for lake whitefish eggs and fish larvae in 2006 to assess the extent of reproduction in the river. A spawning-ready male lake whitefish was collected in gillnets and several dozen viable lake whitefish eggs were collected with a pump in the Detroit River in November and December 2005. No lake whitefish eggs were found at lower river sites in March of 2006, but viable lake whitefish eggs were found at Belle Isle in the upper river in early April. Several hundred lake whitefish larvae were collected in the river during March through early May 2006. Peak larval densities (30 fish/1,000 m³ of water) were observed during the week of 3 April. Because high numbers of lake whitefish larvae were collected from mid-and downstream sample sites in the river, we believe that production of lake whitefish in the Detroit River may be a substantial contribution to the lake whitefish population in Lake Erie.     

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.     

Detroit River load estimation; the need for a new monitoring approach

The Great Lakes Water Quality Agreement (GLWQA) established new Lake Erie phosphorus loading targets, including a 40% total phosphorus load reduction to its western and central basins. The Detroit and Maumee rivers’ loads are roughly equal and contribute about 90% of the load to the western basin and 54% to the whole lake. They are key drivers of central basin hypoxia and western basin algal production. So, accurate estimates of the Detroit River load are important. Direct measurement of that load near its mouth is difficult due to requiring real-time knowledge of flows around islands and the influence of Lake Erie’s seiches. Consequently, most estimates sum the loads to the St. Clair/Detroit River system. But this approach is complicated by uncertainties in the Lake Huron load and load retention in Lake St. Clair. Routine GLWQA reassessments will confirm or adjust over time the goals, loading targets, and approaches based on evolving information. So, there is a need to improve monitoring approaches that ensure accurate Detroit River loads. New approaches should take into account both the characteristics of this dynamic connecting channel and the uses of monitoring results: 1) determining the Detroit River loads to drive models, develop mass balances, set load reduction targets, and track progress; and 2) assessing the sources and processing of the loads to help guide reduction strategies. Herein, we review temporal and spatial variability in the St. Clair/Detroit River system, and suggest adjustments to monitoring that address those variabilities and both uses.     

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.     

Post-tournament dispersal of smallmouth bass in western Lake Erie

Catch-and-release tournaments for smallmouth bass Micropterus dolomieu transport fish from many capture locations to a centralized release site. In the Great Lakes, these fish may be transported long distances (i.e., >100 km) and subsequently become concentrated at a release site, possibly negatively affecting subpopulations at popular fishing sites if fish remain concentrated in the area. To assess timing of post-tournament dispersal, 23 smallmouth bass were implanted with acoustic transmitters and released into Sandusky Bay (Lake Erie) after a large tournament (50 boats) in September 2018. Tagged fish were tracked via a grid of acoustic receivers. All surviving fish (n = 19) left the immediate vicinity of the release site (<500 m) after an average (±SE) of 13.2 (±2.4) d, with many (58%) returning to the main lake (19.7 d ± 3.4). Eight (42%) of the surviving fish left the release site but remained in the bay after 50 d. Based on the results from this study, long-term (>1 month) concentration of smallmouth bass at release sites is unlikely to be a problem for Lake Erie bass tournaments, but temporary (up to 1 month) accumulations of fish are possible. A fraction of tournament-released smallmouth bass resided in Sandusky Bay for extended periods (>2 months), which could be considered functionally harvested from main lake subpopulations. Currently, the frequency of bass tournaments on Lake Erie is uncertain, and better documentation of tournament frequency and scale is necessary to assess potential negative impacts on bass populations.     

Contrasting sources and mobility of trace metals in recent sediments of western Lake Erie

Concentrations of the major and trace metals varied considerably in the western basin of Lake Erie, ranging from 0.9 to 25.3?mg/g for aluminum, from 2.9 to 36.5?mg/g for iron, from 6.4 to 74.8?mg/g for calcium, from 1.2 to 13.5?μg/g for cobalt, from 2.8 to 61.6?μg/g for copper, from 2.7 to 83.0?μg/g for lead, from 0.1 to 2.9?μg/g for cadmium, and from 7.1 to 127.3?μg/g for strontium. Distinct patterns of sediment metal variability allowed the identification of two major fluvial sources and some active in-lake biogeochemical processes. The inputs of Sr were largely from the Maumee River, the inputs of Cu, Pb, Cd, and Co were dominated by the Detroit River, and the inputs of Fe and Al were roughly evenly from the two rivers. The removal of Sr and Ca from the water column was mainly through coprecipitation with calcite. In contrast, the transfer of Cu, Pb, Cd, and Co was largely attributed to the removal of fine sediment particles from the Detroit River mouth and adjacent nearshore areas and the deposition of the metals scavenged by settling organic materials in the basin's central deeper areas. The mobility of the trace metals was different during the in-lake mass transfer, with Co being the most mobile and Cd being the least mobile. Furthermore, the trace metal mobility differences have decreased significantly during the past half-century due to a substantial increase in organic matter from eutrophication in the basin.     

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.     

Variability of sedimentary phosphorus fractions in the western and Sandusky basins of Lake Erie

Surface sediments and three sediment cores from the western basin and one sediment core from the Sandusky basin were analyzed to document spatial and temporal changes in five phosphorus fractions and total phosphorus (TP). The areal distributions of the bioavailable fractions NaCl-Pi, NaBD-Pi, and NaOH-Pi and the refractory organic fraction Res-P were broadly consistent and contrasted with those of the detrital fraction HCl-Pi which showed that high concentrations occurred mostly in high-energy littoral zones and low concentrations largely in profundal depositional areas. The contrasting distributions were induced by interactions among tributary inputs, wave action, circulation, and biogeochemical cycling and transfer in the basin. As revealed by the Sandusky basin sediment record, the detrital fraction HCl-Pi was dominant (70% of TP) during European settlement and decreased rapidly by 28.0% in the early 1910s due largely to impoundments of the Maumee and Sandusky Rivers. While HCl-Pi has ever since remained relatively constant, NaCl-Pi, NaBD-Pi, and NaOH-Pi increased significantly between 1950 and 1970 in the two basins. However, the post-regulation sediment records differed considerably among these coring sites. There was a marked increase of TP in two cores, corresponding to recent return of eutrophication and massive harmful algal blooms but contrasting with a relatively constant, low loading into the lake. This signified the role of internal loading as derived partialy from legacy pollution. Furthermore, NaCl-Pi has increased progressively throughout all the records. We conclude that the increased levels of NaCl-Pi in surface sediments may have altered the internal loading and contributed to the resurgences of harmful algal blooms in Lake Erie.     

Observations of Sediment Transport in Lake Erie during the Winter of 2004–2005

Time series measurements of current velocity, wave action, and water transparency were made at two sites—one in 24 m of water and the other in 53 m—in Lake Erie during the fall and winter of 2004–2005. The observations at the shallow site show that bottom resuspension occurred several times during the deployment. Although local resuspension did not occur at the deeper station, several advection episodes were observed. The storms during the observation period were not unusually large, so the processes observed are probably typical of those that occur on a yearly basis. The observations agree reasonably well with previous estimates for both the bottom shear stress during storms, and for the critical shear stress needed to resuspend bottom sediment, but previous estimates of the particle settling velocity are probably too low, while previous estimates of the sediment entrainment rate are too high. The results show that bottom material in the central basin is reworked numerous times before it is finally buried. Deposition in the eastern basin is a more continuous process, but the events observed were not sufficient to match the long-term accumulation rate, so deposition at this site is probably also due in part to larger, more infrequent storms.     

Re-interpreting Great Lakes shorelines as components of wave-influenced deltas: An example from the Portage River delta (Lake Erie)

Deltas deliver both sediment (bedload sands and suspended load muds) and water to a coastal environment. In the past, deltaic models have emphasized the sandy bedload component constructing a depositional feature that protrudes from the coastline. In contrast, wave-influenced deltas form where river discharge effectively blocks the prevailing longshore drift. The resulting delta is asymmetric, with an extensive strandplain of multiple beach ridges updrift, and fewer beach ridges with wetlands, ponds, and subsidiary bay-head deltas downdrift. In Lake Erie, an analysis of 28 vibracores from the Portage River delta demonstrates significant updrift-downdrift sedimentological differences. Updrift of the delta consists of > 3 m thick gravel-rich sands overlying glaciolacustrine sediment. The deposits are organized into coarsening-upward, progradational shoreline sequences showing facies transitions from lower shoreface to upper shoreface to beachface to backbeach. A 1939 aerial photograph suggests > 15 prograding shoreline sequences were accreted during present lake levels (highstand systems tract), resulting in re-attachment of a bedrock high (Catawba Island) to the mainland. Downdrift of the delta consists of a one progradational shoreline sequence < 1 m thick that overlies peats and glaciolacustrine sediment. The peats have ¹⁴C ages between 1616 and 2025 cal YBP, and are interpreted as wetlands that formed during an earlier phase of rising lake levels (lowstand and transgressive systems tracts). The overlying beach ridge was accreted during present lake levels (highstand system tract). The coastal features in this portion of Lake Erie are best understood as components of an evolving wave-influenced delta, the first recognized in the Great Lakes.     

Philopatry and vagrancy of white bass (Morone chrysops) spawning in the Sandusky River: Evidence of metapopulation structure in western Lake Erie using otolith chemistry

Although natal homing and philopatry are well studied in anadromous salmon, few studies have investigated philopatric behavior in large, freshwater systems. In western Lake Erie, white bass (Morone chrysops) undergo seasonal spawning migrations from the open-water regions of Lake Erie to nearshore reef complexes and tributaries. The three primary spawning locations in Lake Erie are within 80 km of each other and are not separated by physical barriers. We used naturally occurring differences in otolith strontium concentrations among major spawning locations to address philopatry and vagrancy to the Sandusky River spawning location. Most individuals spawning in the Sandusky River were natal to this river (73%). No statistically significant differences in the extent of homing by sex or age of spawning were found, although a potential pattern of decreased homing with increased age of fish was observed. Given the proportion of vagrant individuals we found spawning in the Sandusky River (27%), it is unlikely that Lake Erie white bass spawning populations are genetically distinct. Furthermore, the white bass population in Lake Erie appears to be structured as a metapopulation, with non-philopatric individuals serving as a link between spawning populations.     

An analysis of MODIS-derived algal and mineral turbidity in Lake Erie

Satellite-derived estimates of chlorophyll concentrations based on colour ratio algorithms traditionally fail in turbid waters such as those found in Lake Erie, resulting in chlorophyll concentrations often orders of magnitude in error and spatial distributions mirroring that of known suspended sediment distributions. Methods are presented here that were used to simultaneously extract algal and mineral suspended particulate matter for Lake Erie from the red and near-infrared bands of NASA's MODIS-Aqua sensor. Results produced spatially and temporally distinct seasonal cycles in agreement with bio-geo-physical processes on the lake. Derived imagery was used to monitor seasonal cycles of both algal and mineral particulate matter on the lake and determine areas of persistently elevated concentrations that may highlight regions of potential water quality concern.     

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.     

On hypoxia and fish kills along the north shore of Lake Erie

Hypoxia is a common feature in the offshore central basin of Lake Erie. In the late summer of 2012, a strong wind-induced upwelling event transported oxygen depleted water to the nearshore zones of northern Lake Erie. Wind speed, duration and direction relative to the shoreline of individual wind events determined the extent of nearshore zone affected by the hypoxic waters. The upwelling event resulted in adverse water quality along some stretches of the northern shoreline of Lake Erie with persistent anoxia, which was mainly responsible for the mortality of fish.     

Heavy metals in sediments and uptake by burrowing mayflies in western Lake Erie basin

During the past two decades, burrowing Hexagenia mayflies have returned to the western basin of Lake Erie. Because of their importance as a prey resource for higher trophic levels and their extensive residence time in potentially contaminated sediment, Hexagenia may be a source of heavy metal transfer. To better understand the distribution and transfer of heavy metals in sediment and mayflies, sediment and mayfly nymphs were collected from 24 locations across the western basin of Lake Erie in May 2007. Following USEPA protocols, samples were analyzed for 16 elements using ICP-OES or ICP-MS. Metal concentrations in the sediments exceeded the Threshold Effect Level for at least one metal at all sample sites. Sediment heavy metal distribution profiles indicate metal concentrations are correlated with organic matter content, and the highest heavy metal concentrations were found in the central deeper region of the western basin where organic content in the sediments was greatest. Hexagenia were distributed throughout the western basin, with greatest density (1350/m²) within the Detroit River plume. The Cd and Zn levels in mayflies were on average approximately 4 and 2 times greater, respectively, than sediment levels, and the Cd concentrations in the sediments exceeded the Threshold Effect Level at 27 of 28 sites and exceeded the Probable Effect Level at 9 of 28 sites. Spatial representation of heavy metal concentrations in mayflies exhibited a similar pattern to the spatial distribution of heavy metals and organic matter in the sediments with higher concentrations of metals found in mayflies residing in the central deeper region of the western basin.     

A Bayesian assessment of the PCB temporal trends in Lake Erie fish communities

The temporal trends of polychlorinated biphenyls (PCBs) in Lake Erie fish were evaluated using 30 years of fish contaminant data (1977–2007). The first step of our statistical analysis was based on simple exponential decay models parameterized with Bayesian inference techniques to assess the declining rates in four intensively sampled fish species, i.e., walleye (Stizostedion vitreum), coho salmon (Oncorhynchus kisutch), rainbow trout (Oncorhynchus mykiss) and white bass (Morone chrysops). Because the exponential model postulates monotonic decrease of the PCB levels, we included first- or second-order random error terms in our statistical formulations to accommodate non-monotonic patterns in the dataset studied. Generally, our results suggest that the PCBs have been decreasing over the last 30 years with relatively weak rates that vary among the different fish species examined. Yet, our analysis with the exponential decay model also identified an increasing trend in the PCB concentrations of walleye skinless–boneless filet data, which is manifested after the mid-90s. In the second step, we used dynamic linear modeling (DLM) analysis to account for the fact that the fish length covaries with the PCB concentrations and that different sized fish may have been sampled over time. Our DLM analysis suggests that the previously reported trend of the walleye filet data is actually an artifact associated with the bias of the fish sampling practices followed. The coho salmon and rainbow trout PCB concentrations have been decreasing steadily during the study period but the associated rates were relatively weak. Finally, the PCB trends in white bass appear to have been stabilized over that last decade, although the robustness of this result remains to be confirmed due to the temporal inconsistencies of the information used. We conclude by emphasizing the importance of explicitly accounting for the different covariates (e.g., length, age, lipid content) that can potentially hamper the detection of the actual temporal trends of fish contaminants.     

基于投影寻踪的湖泊生态健康评价

提出基于投影寻踪的模糊模式识别模型进行湖泊生态健康评价,介绍其主要的4个步骤:1筛选能够反映湖泊健康状况的指标体系;2建立评价标准;3用投影寻踪法确定各指标权重;4采用模糊模式识别模型评价湖泊健康等级,将此模型应用于洪泽湖生态健康评价。结果表明:洪泽湖健康状态处于微病态状态,TN、TP含量过高是影响洪泽湖健康的主要原因;基于投影寻踪的模糊模式识别模型的评价结果符合湖泊实际生态健康状况;模型可推广应用于其他生态系统评价。     

Freshwater mussel community response to warm water discharge in western Lake Erie

Native unionid mussels are endangered in the Laurentian Great Lakes due to habitat degradation and biofouling by invasive dreissenids. However, a robust community was discovered living within the thermal discharge of a power plant at Oregon, Ohio, on the south shore of Lake Erie. Our study compared this community to nearby communities outside the thermal plume, and examined habitat characteristics that may affect unionids. Unionids were sampled from the exposed lake bed at three sites during a seiche in 2011: (1) within the thermal plume, (2) at Bayshore Park (2.0 km east of the plant), and (3) at the University of Toledo's Lake Erie Center (4.0 km east). In 2010, sediment samples were collected along a 2 km transect extending east from the plant discharge roughly parallel to the south shore of Lake Erie. Results indicated that the community within the thermal plume had higher densities, higher diversity (H′), more small individuals but overall larger sizes than communities outside the plume. Both the rate and intensity of fouling by dreissenids were lower within the plume. Dry mass of coarse surface sediment and sediment organic matter content were negatively related to distance from the plant (R² = 0.497, and 0.479, respectively). An unexpected discovery was that the bulk of the coarse sediment was comprised of shell material from Asian clams and dreissenid mussels, suggesting a contribution of these exotic species to sediment accumulation. In total, our results suggest that several habitat characteristics close to the power plant are favorable to unionids.     

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.     

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.