Thermal habitat characteristics for warmwater fishes in coastal embayments of Lake Ontario

Along 40 km of the Lake Ontario shoreline near Toronto, Ontario, small coastal embayments (4.38-848 × 10³ m²) have been constructed or modified by lake-infilling to restore warmwater fish habitat. We describe how the thermal regimes of these embayments differ from those of small inland lakes, how embayment bathymetry alters the degree of exchange with Lake Ontario, and predict how embayment thermal regimes affect warmwater fish growth. The accumulated growing-degree days, calculated from a 14 °C baseline (∑ GDD₁₄), of seven South-Central Ontario inland lakes and an embayment disconnected from Lake Ontario ranged from 844 to 1157 GDD. Compared to the inland lakes and disconnected embayment, the coastal embayments connected to Lake Ontario achieved fewer ∑ GDD₁₄ and had a greater range, 390-1047, reflecting differences in their degree of exchange with and the cooling effect of Lake Ontario. The thermal regime of coastal embayments differs most from inland lakes during May-late-July. During early summer, mean embayment depth explains over 50% of the variability in ∑ GDD₁₄, with deeper embayments cooling more from lake exchange than shallow embayments. After mid-summer, the cooling embayments are warmed by exchanges with Lake Ontario. This late-summer warming is insufficient to compensate for the cooling effect of the Lake earlier in the summer. Bioenergetic simulations of bluegill, Lepomis macrochirus, growth indicate that most of these embayments are too cool for adequate summer growth of YOY sunfish. Nevertheless, small coastal embayments that are shallow with suitable controls on exchange with the Lake can provide acceptable warmwater fish habitat.     

Determinants of temperature in small coastal embayments of Lake Ontario

Along 25 km of the Lake Ontario shoreline near Toronto, Ontario, small coastal embayments (0.4–32 ha) have been constructed or modified by lake-infilling to restore warmwater fish habitat. The variation in thermal habitat quality for warmwater fishes among these embayments is very high; temperatures range from those found within a small pond to those of much cooler Lake Ontario. Since meteorological conditions and surface heat fluxes are almost identical, the temperature variation among embayments must be caused by differences in bathymetry or exchange with Lake Ontario. However, a previous study on these embayments found paradoxically that temperatures were not strongly associated with channel size or embayment bathymetry. This paper resolves the paradox by showing that flushing times for almost all of the constructed embayments were less than 1 day, and often less than 12 h. With so little time to warm within the embayments, water temperatures of almost all embayments remained very close to the temperatures of the adjacent lake waters. The coldest embayments connected directly to open Lake Ontario and warmer embayments connected to Lake Ontario through other embayments or protected harbors, where the inflowing water from Lake Ontario had already substantially warmed. To allow embayments along the exposed shoreline of Toronto to reach acceptable temperatures for warmwater fish, we use heat budgets to calculate that average summer flushing times must be increased from their current length of 1.5 to 5.5 h to approximately 30 h. Such changes could be achieved through large reductions in the channel cross section.     

Diatoms abound in ice-covered Lake Erie: An investigation of offshore winter limnology in Lake Erie over the period 2007 to 2010

The limnology of offshore Lake Erie during periods of extensive (> 70%) ice cover was examined from ship borne sampling efforts in 2007 to 2010, inclusive. Dense and discrete accumulations of the centric filamentous diatom Aulacoseria islandica (> 10 μg Chl-a/L) were located in the isothermal (< 1 °C) water column directly below the ice and only detectable in the ship wake; viable phytoplankton were also observed within ice. Evidence from these surveys supports the notions that winter blooms of diatoms occur annually prior to the onset of ice cover and that the phytoplankton from these blooms are maintained in the surface waters of Lake Erie and reduce silicate concentrations in the lake prior to spring. The mechanisms by which high phytoplankton biomass rise at this time of year requires further investigation, but these winter blooms probably have consequences for summer hypoxia and how the lake responds to climate change.     

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.     

The joint effects of riverine,thermal, and wind forcing on a temperate fjord lake: Quesnel Lake,Canada

Field data obtained in 2003–2004 are used to describe the influence of atmospheric and riverine forcing on the thermal history of fjord-type lakes using Quesnel Lake, British Columbia, Canada as an example. Typical of fjord-type lakes, Quesnel Lake is narrow, long (> 100 km total thalweg), multi-armed (three arms of comparable size), deep (maximum depth > 500 m), and has multiple basins separated by sills. The lake's annual thermal history responds to the joint forcing of surface heat exchange, river inputs, and wind stress. The lake's annual heat budget is dominated by surface radiative fluxes, and riverine input of heat is insignificant in this medium residence time lake (10 years). Despite being insignificant to the annual heat budget, the three major rivers that feed the three arms of the lake contribute to the overall lake circulation pattern. Since these rivers have differing salinities, it is possible to identify, using temperature/salinity correlation diagrams, patterns of riverine circulation interflowing in the lake. Data from a 1-year thermistor-chain record suggests exchange between surface and intermediate waters occurs twice annually, but deep-waters below 150–250 m are mainly renewed during autumn, by strong and episodic atmospheric forcing. This is because wind-forced turbulent diffusion and gravitational convection triggered by wind forced displacement of isotherms (initiating thermobaric instability) are required for full overturn and deep-water renewal.     

Does Manganese Influence Phosphorus Cycling under Suboxic Lake Water Conditions?

The comparative roles of iron and manganese in internal phosphorus loading were examined in a managed lake. Sediments and the water column of Irondequoit Bay, an embayment along Lake Ontario's southern shore, were sampled monthly during summer thermal stratification. Total phosphorus, total iron, and total manganese concentrations in the sediment averaged 1.389 ± 0.150 g/kg dry wt, 24.415 ± 0.760 g/kg dry wt, and 1.727 ± 0.053 g/kg dry wt, respectively. Elevated total phosphorus (maximum = 0.915 mg P/L) and soluble reactive phosphorus (maximum = 0.749 mg P/L) concentrations were observed in the hypolimnion. Sequential extraction of phosphorus fractions from the top 25 cm of deepwater sediment revealed that approximately 25% of phosphorus was stored in a redox-sensitive form, most likely associated with iron and manganese oxyhydroxides. Typically, phosphorus released from sediments is associated with iron, not manganese. However, iron and manganese profiles from the water column indicated that manganese from the sediment was cycling with phosphorus into the overlying waters, while iron did not demonstrate evidence of cycling. Although reductive dissolution of iron likely occurs in the sediment, iron was retained in the sediment and kept out of the water column by the maintenance of low concentrations of dissolved oxygen in the hypolimnion.     

Great Lakes ice classification using satellite C-band SAR multi-polarization data

The objective of this study is to advance development of algorithms to classify and map ice cover on the Laurentian Great Lakes using satellite C-band synthetic aperture radar (SAR) multi-polarization data. During the 1997 winter season, shipborne polarimetric backscatter measurements of Great Lakes ice types, using the Jet Propulsion Laboratory C-band scatterometer, were acquired together with surface-based ice physical characterization measurements and environmental parameters, concurrently with European Remote Sensing Satellite 2 (ERS-2) and RADARSAT-1 SAR data. This fully polarimetric dataset, composed of over 20 variations of different ice types measured at incidence angles from 0° to 60° for all polarizations, was processed and fully calibrated to obtain radar backscatter, establishing a library of signatures for different ice types. Computer analyses of calibrated ERS-2 and RADARSAT ScanSAR images of Great Lakes ice cover using the library in a supervised classification technique indicate that different ice types in the ice cover can be identified and mapped, but that wind speed and direction can cause misclassification of open water as ice based on single frequency, single polarization data. Using RADARSAT-2 quad-pol and ENVISAT ASAR dual-pol data obtained for Lake Superior during the 2009 and 2011 winter seasons, algorithms were developed for small incidence angle (< 35°) and large incidence angle (> 35°) SAR images and applied to map ice and open water. Ice types were subsequently classified using the library of backscatter signatures. Ice-type maps provide important input for environmental management, ice-breaking operations, ice forecasting and modeling, and climate change studies.     

Characteristics of double-crested cormorant colonies in the U.S. Great Lakes island landscape

The Great Lakes form the largest freshwater island system in the world and provide breeding habitat for a large proportion of the continental population of double-crested cormorants (Phalacrocorax auritus). Here, cormorants have a high profile due to conflicts with humans; by 2007, most active (64%) breeding sites in U.S. waters were managed. This study used data from the U.S. Great Lakes Colonial Waterbird Database and The Nature Conservancy's Great Lakes Island GIS database to identify important features of breeding sites in the U.S. Great Lakes and broaden understanding of cormorant presence at the island-landscape scale. Islands 0.5–10 ha were used more frequently than expected, and most sites had remoteness values of ≤ 3 km. Colony size was positively correlated with years occupied and large colonies (> 1000 pairs) developed primarily (95%) on island sites > 1.0 ha. Sites supporting large colonies were more remote than those supporting smaller colonies. Presence of other colonial waterbird species, especially Herring Gulls (Larus argentatus), also characterized cormorant sites. Islands used by cormorants comprised a small proportion (n = 90, 3%) of the U.S. Great Lakes island resource, and < 1% of the total island area. Certain characteristics of breeding sites (e.g., small islands, proximity to mainland) may increase negative attitudes about cormorants. To understand cormorant impacts to island resources (e.g., vegetation; other colonial waterbird species), we suggest cormorant presence in the Great Lakes be considered in the broader context of island science, conservation and known threats, and at a landscape scale.     

Satellite SAR Remote Sensing of Great Lakes Ice Cover,Part 2. Ice Classification and Mapping

During the 1997 winter season, shipborne polarimetric backscatter measurements of Great Lakes (freshwater) ice types using the Jet Propulsion Laboratory C-band scatterometer, together with surface-based ice physical characterization measurements and environmental parameters, were acquired concurrently with Earth Resource Satellite 2 (ERS-2) and RADARSAT Synthetic Aperture Radar (SAR) data. This polarimetric data set, composed of over 20 variations of different ice types measured at incident angles from 0° to 60° for all polarizations, was processed to radar cross-section to establish a library of signatures (look-up table) for different ice types. The library is used in the computer classification of calibrated satellite SAR data. Computer analysis of ERS-2 and RADARSAT ScanSAR images of Great Lakes ice cover using a supervised classification technique indicates that different ice types in the ice cover can be identified and mapped, and that wind speed and direction can have an influence on the classification of water as ice based on single frequency, single polarization data. Once satellite SAR data are classified into ice types, the ice map provides important and necessary input for environmental protection and management, ice control and ice breaking operations, and ice forecasting and modeling efforts.     

Thermal and hydrologic suitability of Lake Erie and its major tributaries for spawning of Asian carps

Bighead carp Hypophthalmichthys nobilis, silver carp H. molitrix, and grass carp Ctenopharyngodon idella (hereafter Asian carps) have expanded throughout the Mississippi River basin and threaten to invade Lakes Michigan and Erie. Adult bighead carp and grass carp have been captured in Lake Erie, but self-sustaining populations probably do not exist. We examined thermal conditions within Lake Erie to determine if Asian carps would mature, and to estimate time of year when fish would reach spawning condition. We also examined whether thermal and hydrologic conditions in the largest tributaries to western and central Lake Erie were suitable for spawning of Asian carps. We used length of undammed river, predicted summer temperatures, and predicted water velocity during flood events to determine whether sufficient lengths of river are available for spawning of Asian carps. Most rivers we examined have at least 100 km of passable river and summer temperatures suitable (> 21 C) for rapid incubation of eggs of Asian carps. Predicted water velocity and temperature were sufficient to ensure that incubating eggs, which drift in the water column, would hatch before reaching Lake Erie for most flood events in most rivers if spawned far enough upstream. The Maumee, Sandusky, and Grand Rivers were predicted to be the most likely to support spawning of Asian carps. The Black, Huron, Portage, and Vermilion Rivers were predicted to be less suitable. The weight of the evidence suggests that the largest western and central Lake Erie tributaries are thermally and hydrologically suitable to support spawning of Asian carps.     

First record of migrating silver American eels (Anguilla rostrata) in the St. Lawrence Estuary originating from a stocking program

In fall 2009, six migrating silver American eels (Anguilla rostrata) originating from a stocking program were caught in the brackish waters of the St. Lawrence Estuary. These maturing eels were all females with gonads developed at a similar stage as other migrating eels in the estuary. Fluorescent oxytetracycline marks observed on the otoliths allowed us to assert without any doubt that they came from glass eels caught in Nova Scotia and stocked 4 years earlier in the Richelieu River, 500 km upstream from the recapture location. Their length varied between 570 and 668 mm, which is within the size range of naturally recruited female silver eels in Nova Scotia, while silver eel are most generally longer than 80 cm in the St. Lawrence estuary. Their growth rate was also exceptionally fast. This direct observation is the first evidence that American eels stocked as glass eels can migrate seaward at least as far as the estuary in synchrony with naturally recruited female silver eels en route to their spawning grounds in the Sargasso Sea.     

Environmental impacts of desalination on the ecology of Lake Urmia

Lake Urmia, the second largest hypersaline lake by area in the world, has fluctuated in salinity over time, but in recent years, it has reached a maximum of 340 g/L. The lake is the main habitat for the endemic Iranian brine shrimp, Artemia urmiana, and is a protected aquatic environment. Efforts have been made by the Iranian government to enhance the diversity of its wildlife. One approach has been to look for a method to reduce the salt content of the lake. We investigate the feasibility of this by first considering the water chemistry of Lake Urmia and then the various technologies used to extract salt from marine and brackish waters. Average concentrations of Na, Mg, K, Ca, Cl, SO₄, and HCO₃ were 125 g/L, 11.3 g/L, 2.63 g/L, 0.55 g/L, 216 g/L, 22.4 g/L, and 1.38 g/L, respectively, and cations and anions were balanced, However, Lake Urmia waters have a ‘very high’ salinity hazard and a high sodium adsorption ratio (SAR). Moreover, the saturation index (SI) for each of the major cations was greater than zero, indicating that the water in Lake Urmia is supersaturated, and precipitation is likely. The extraction of available salts from the lake for the use in petrochemical industries is economically feasible. However, technologies based on removing salts by distillation or reverse osmosis and then using this fresh water to dilute lake salinity are problematic. A better strategy would be better to allow more fresh water to reach the lake rather than creating fresh water through reverse osmosis and distillations processes. While concerns have been raised about the salinity tolerance of A.urmiana, it has successfully tolerated various salinity ranges from 166 to 340 g/L, and so the species is not threatened, unless the lake desiccates. Because the lake is saturated with salts, it seems unlikely that salinity could increase much higher.     

Distribution and Fate of Escherichia coli in Lake Michigan Following Contamination with Urban Stormwater and Combined Sewer Overflows

Escherichia coli distribution and persistence in nearshore Lake Michigan were assessed following heavy rains and sanitary sewer overflow (SSO) and combined sewer overflow (CSO) events over a 5-year period, including an 18-day period following 25.4 cm of rainfall in which intensive studies were conducted following multiple CSO and SSO events. E. coli levels in the Milwaukee estuary and harbor following SSO and CSO events ranged from 10⁴ to nearly 10⁵ CFU/100 mL, which were significantly higher (p ≤ 0.05) than levels following rainfall alone. Sites outside of the breakwall but within the contamination plume (e.g., within 2 km of the harbor) were an order of magnitude lower. Locations 2–5 km from the harbor ranged from below detection limits, of < 1 to 5 CFU/100 mL. E. coli levels corrected for dilution based on specific conductivity measurements were lower than what would be expected for loss due to dilution alone, suggesting a combination of die-off and dilution, were responsible for the rapid disappearance of these organisms outside of the harbor. E. coli and fecal coliforms measured concurrently demonstrated that fecal coliforms could be recovered longer than E. coli in the open waters of the lake. E. coli isolated directly from sewage treatment plant influent were found to have a marked increase in antibiotic resistance traits for ten antibiotics commonly used in the human population compared with isolates from two animal sources of fecal pollution. However, E. coli obtained from sewage impacted water (n = 2,513) and from stormwater impacted water (n = 1,465) collected the previous year when there were no sewage overflows, were found to have no significant difference (p < 0.05) in the frequency of resistance when comparing the two conditions. E. coli survival characteristics and population dynamics are most likely influenced by multiple factors in complex systems such as the watershed/estuarine/lake environments of the Great Lakes.     

Documenting Complex Surface Temperature Patterns from Advanced Very High Resolution Radiometer (AVHRR) Imagery of Saginaw Bay,Lake Huron

Interannual differences in nutrient concentrations, phytoplankton densities, and the dispersal of zooplankton and fish life history stages (e.g., planktonic larvae, resting eggs) in embayments often are contingent upon bay and coastal water exchange and movements. However, predicting water mass exchange is difficult in that many factors influence the circulation of waters within bays. The central theme of this contribution is that circulation phenomena derived from simple models can be identified and classified using satellite-obtained lake surface temperature maps, and the incidence of events related to gross temperature regime (season) and winds (shorter-term perturbations). To illustrate this potential, the high sampling frequency of AVHRR imagery was used to examine seasonal surface temperature patterns between Saginaw Bay and Lake Huron over a 3-year period. Linear regressions of CoastWatch IMGMAP and OCNMAP daytime sea surface temperature (SST) algorithms against shipboard bulk temperatures were highly significant, with r² values of 0.98 and 0.94. Synoptic reconnaissance of lake surface temperatures from AVHRR verified many previously known general seasonal events, yet provided much better spatial coverage. The thermal bar persisted in the bay for approximately 6 weeks from late April until mid-June. The combination of shallow depths and impounded river discharges caused inner bay waters to warm more rapidly than outer bay and open lake waters. A thermal gradient of 6 to 10°C persisted between the inner bay and Lake Huron waters throughout the summer. The persistence of major spatial thermal gradients restricted mixing of inner bay waters with the outer bay and open lake, whereas inner and outer bay temperatures converged during fall months, increasing the likelihood of mixing. However, frequent wind-induced effects can cause circulation reversals and move surface waters in quite complex patterns across the bay. AVHRR image classification confirms the sensitivity of inner bay waters to wind stress-related circulation, aiding interpretations of historical data sets.     

Modification of temperature behaviour through regulation of a British river system

The effects of flow regulation on temperature behaviour in the River Exe, Devon, U.K. have been studied, and attention is given to the local impact of impoundment and its downstream persistence. Temperature levels and seasonal thermal regime are considered as well as diel temperature fluctuations and temperatures during extreme weather conditions. Variations in temperature during individual reservoir releases have also been monitored. The impact of impoundment has been to make the stream environment immediately below the dam more homothermous so that temperatures rearely rise above 17·5°C or fall below 2·0°C, and monthyl average diel ranges are < 3 and < 1°C in summer and winter months respectively. In contrast to other reservoirs in Britain and abroad, this effect has been largely caused by increased groundwater flow downstream from the reservoir following impoundment. Regulation has also affected downstream temperature behaviour and has moderated the thermal regime of the Exe mainstream. This influence is generally restricted to a distance of up to 20 km from the dam, but in conditions of hot weather and low flows it may extend to almost 40 km from the impoundment.     

Movement patterns of smallmouth and largemouth bass in and around a Lake Michigan harbor: The importance of water temperature

Smallmouth bass Micropterus dolomieu and largemouth bass M. salmoides in southwestern Lake Michigan use shallow, warm harbors for spawning during spring. After the reproductive period ends in early summer, catch rates from standardized sampling of smallmouth bass in harbors decrease. Fishes are presumed to use the main lake during summer but little is known about how black basses use main-lake habitat in Lake Michigan or mechanisms driving this transition. We tracked smallmouth bass (N = 26) and largemouth bass (N = 8) using radio and acoustic telemetry during 2005–2006 near North Point Marina, IL. A temperature difference persisted between inside and outside the harbor for much of May–October where harbor temperatures were generally warmer than those outside the harbor. Smallmouth bass responded to water temperature changes, inhabiting the harbor until temperatures outside the harbor approached 18.5 °C, at which time they left the harbor. Frequent temperature fluctuations of > 3 °C occurred outside the harbor within 24-hour periods. Sudden reductions in water temperature were associated with smallmouth bass temporarily returning to the harbor until the temperature outside the harbor again approached 18.5 °C. As water cooled during fall, smallmouth bass again returned to the harbor. Largemouth bass exhibited comparatively restricted movements during this time and rarely ranged outside the harbor. Thus, home range estimates for smallmouth bass (142.6 ha) were an order of magnitude greater than those of largemouth bass (12.9 ha). Both water temperature and species played a role in determining the degree of movement outside the harbor but both black basses used the harbor as a thermal refuge.     

Characterizations of the light-scattering attributes of mineral particles in Lake Ontario and the effects of whiting

Light-scattering attributes of minerogenic particles from the upper waters of Lake Ontario, collected lake-wide from pelagic waters in late 2007 summer and early 2008 spring cruises and over the summer interval at a near-shore site in 2008, were characterized by scanning electron microscopy interfaced with automated image and X-ray analyses (SAX). SAX results were used to estimate minerogenic scattering and backscattering coefficients (b_m and b_b,m) through Mie theory. Two minerogenic particle regimes with respect to light scattering were resolved: (1) clay mineral dominance and (2) dominance by ‘whiting’ (CaCO₃ precipitate) in late summer in portions of the lake. Clay minerals made noteworthy and important contributions to overall particulate scattering and backscattering coefficients (b_p and b_bp, respectively) in spring and early summer. Dramatic increases in values of b_p, and particularly b_bp, as well as decreases in Secchi disk depth (SD), were observed during whiting from the associated large increases in b_m and b_b,m. Features of these events were the primary drivers of the spatial patterns in late summer and temporal differences observed for scattering and SD. Particles in the size range of 1-10 μm were responsible for minerogenic scattering during stratification, but those with sizes > 10 μm made noteworthy contributions at certain sites during spring turnover. The credibility of the SAX-Mie estimates of b_m and b_b,m was supported by the extent of optical closure obtained with paired bulk measurements of overall b_p and b_bp (2007 summer cruise), and independent estimates of organic particulate scattering and backscattering through empirical bio-optical models.     

The thermal variability of the waters of Fathom Five National Marine Park,Lake Huron

Measurements of the thermal stratification at 3 locations within Fathom Five National Marine Park in Lake Huron, Ontario during the summers of 2006 and 2007 found large oscillations in the position of the thermocline. These oscillations led to considerable variability in the temperature at a given depth, with frequent changes in temperature at a rate of 5 °C per hour, and brief periods where temperatures changed at the rate of 10 °C per hour. The thermal stress due to such fast rates of temperature change has been previously implicated in negative effects on many aquatic organisms. The thermocline was observed to move by as much as 20 m vertically, and had dominant periods of oscillation of 12, 17 and 24 h. The strongest temperature variability occurs in the depth range of 10–20 m, which accounts for 20% of the total lakebed area within Fathom Five. The temperature variability was lowest in deep regions well below the thermocline and at a sheltered area behind a reef. This variability was a ubiquitous feature of the water column of Fathom Five during the summer stratification, and the impact of these frequent short-term thermal fluctuations on benthic and fish habitat is discussed in this note.     

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.     

Evidence of persistent,recurring summertime hypoxia in Green Bay,Lake Michigan

Six years (2009–2015) of temperature and dissolved oxygen profile data show hypoxic conditions are common in the bottom waters of southern Green Bay, Lake Michigan during the summer. Depleted oxygen concentrations (<5?mg?L^?1) affect nearly 70% of the 38 stations sampled representing an area of ~500–600?km². Stratification typically lasts 2+?months, from late June to early September, and some stations exhibit bottom water hypoxia (<2?mg?L^?1) at a frequency of nearly 25% when sampled during this period. A monitoring program initiated in 1986 by the Green Bay Metropolitan Sewerage District has provided a 23?year, recreational season record (May–September) of continuous (15?min interval) in situ bottom water oxygen and temperature measurements at the Entrance Light station of the Green Bay navigational channel. The duration of the hypoxic season ranges from 2?weeks to over 3?months at this shallow 7?m offshore site. This variability likely results from a combination of thermal stratification, oxygen consumption in deeper waters of the bay, and physical forcing mechanisms that drive cool, oxygen depleted, bottom waters on a southerly trajectory across this sensor. These data suggest the duration of hypoxic conditions may have increased during the stratified season in recent years. Hypoxia in the bay would also appear to be sensitive to relatively small changes in these forces, particularly changes in organic carbon loading and the duration of stratification.