Winter, historically a largely un-monitored season, is influential and changing. There is evidence of the importance of under-ice phytoplankton in temperate lakes, but it is currently unknown if high winter phytoplankton biomass translates to high productivity and what influence it has on year-round lake metabolism. Winters are getting shorter, but our ability to forecast change is hindered by our limited understanding of under-ice processes. Here, we compare under-ice and open-water rates of areal gross production (AGP) and areal respiration (AR) from 3 Canadian reservoirs and one large lake using oxygen (O2) δ18O-O2 models and fluorometry. During the open-water season, AGP was 5× greater than under-ice rates, with AR rates 8× higher than measured during winter. Open-water samples indicated autotrophy (P:R= 1.10) with heterotrophy dominant under ice (P:R= 0.67). Consistent with current assumptions, the cold under-ice environment is associated with low primary productivity. Our results challenge the assumption that mean water column irradiance is lowest during the winter in dimictic water bodies; we find similar light conditions during the open-water season. Winter mean light is regulated by snow thickness; upon manual snow removal, we observe a 67 % increase in under-ice mean water column irradiance. The first-ever under-ice application of the δ18O2-method indicated that AGP responded to improvements in light. This study reveals further insights into the importance of under-ice metabolism on year-round processes in a changing climate. 

Lake Simcoe has undergone eutrophication and hypoxia since the 1960s. Climate change, leading to enhanced summer thermal stratification, has been identified as a key stressor. In this study, we modeled the impacts of climate change on hydrodynamics and biogeochemistry in Lake Simcoe by applying a 1-dimensional (vertical) model forced with A2 and B1 scenario outputs from a global climate model over 2000-2100. The model was calibrated in 2008 and validated in 2009, with maximum root mean square error (RMSE) of modelled temperature between 1.5 and 3.0 degrees C and dissolved oxygen RMSE between 0.5 and 2.5 mg L-1. Phytoplankton chlorophyll a was simulated with RMSE between 1.25 mu g L-1 (large diatoms) and similar to 0.5 mu g L-1 (other groups). Interannual variability in spring water temperature and length of stratification were related to changes in the North Atlantic and Artic Oscillation indices, respectively. Under A2 and B1 forcing, the duration of stratification will increase by 45 and 38 days in summer between spring and fall turnover, respectively. The extended stratified period leads to a reduction in hypolimnetic dissolved oxygen from 3-7 to <3 mg L-1, thereby reducing the quality of cold-water fish habitat and increasing internal phosphorus loading from the benthos. These internal loads, combined with increased water temperatures, lead to increased cyanobacteria concentrations, beginning around 2070.

Multi-wavelength Chl a fluorometers are increasingly applied to assess phytoplankton photosynthetic capacity and composition, but their usefulness is limited by uncertainties in fluorescence excitation spectra (FES). We investigated this issue using the Phyto-PAM fluorometer to evaluate the effects of innate and irradiance-dependent variations in background ( F ) and variable ( F v ) FES on analysis of three pigment groups (cyanobacteria, chlorophytes and chromophytes). The effects on group-specific estimates of minimum fluorescence ( F 0 ), a proxy for biomass, and F v / F m , the quantum yield of photochemistry, presented some challenges to the interpretation of group-specific results. F 0 estimates usually had a 5–15% margin of error, even when measuring highly uneven mixtures, and applying imperfectly matched calibration FES or stressing samples with photosynthetically active and ultraviolet radiation; errors in F v / F m were commonly < 15%. Despite such relatively good accuracy, estimates for F 0 and, especially, F v / F m are unreliable for groups at low relative abundance, and results can sometimes be reported for groups not actually present. We report margins of error for different levels of relative abundance to inform interpretation of measurements from natural communities and show that F and F v spectra for some taxa can differ in ways that produce severe errors in F 0 and F v / F m estimates if used uncritically.

A three‐dimensional hydrodynamic‐ecological model is applied to Lake Erie to predict the response of dissolved oxygen (DO) to independent changes in air temperature, wind speeds and total phosphorus (TP) loading. Warmer temperatures and lower wind speeds increased the size and duration of hypoxic and anoxic regions by lengthening the stratified period. Decreased wind speed increased hypolimnion thickness while decreasing its temperature and DO consumption rate. Decreased TP loading improved DO conditions with a reduction of 75% effectively abolishing hypoxia. Anoxia was more sensitive to air temperature, wind, and nutrient changes than was hypoxia. New metrics that capture the spatial and temporal dimensions of low DO conditions were more sensitive than the commonly cited maximum areas of hypoxia or anoxia. Over most of the relevant range of forcing factors, the simple and first‐order effect of a 1°C temperature change was equivalent to a 10–14% change in TP loads, while a 1% change in wind speed was equivalent to a 2–3% change in TP loads. Reduced ice cover in warmer climates will likely increase air temperature effects even further.

Rivers often transport phytoplankton to coastal embayments and introduce nutrients that can enrich coastal plankton communities. We investigated the effects of the Nottawasaga River on the nearshore (i.e. within 500 mu m of shore) phytoplankton composition along a 10-km transect of Nottawasaga Bay, Lake Huron in 2015 and 2016. Imaging flow cytometry was used to identify and enumerate algal taxa, which were resolved at sizes larger than small nanoplankton (i.e. >5 mu m). Multivariate analysis (perMANOVA and redundancy analysis) and a dilution model were used to examine how nutrients and the transport of algal taxa affected community composition in the bay. Sampling stations with different percentages of river water had significantly different phytoplankton communities. Phytoplankton community composition was also strongly associated with nutrients, including total phosphorus, which also varied with the percentage of river water. The majority of the 51 phytoplankton taxa identified in 2016 had numerical abundances in the bay that could be explained simply by the dilution of incoming river water. Phytoplankton transported from the river had a higher proportion of edible-sized cells (<30 mu m), particularly in summer when colonial cyanobacteria were numerically dominant in the bay. Six taxa were more abundant than expected from the dilution of river water and included some cyanobacteria with late summer maxima. Five of the taxa that were transported from the river were less abundant than expected in the bay. Whereas impacts of fertilisation due to the characteristically higher nutrient concentration in the river are to be expected, the strong and highly correlated effects of transport within the narrow coastal band of this study largely concealed any distinct fertilisation effects. Riverine inputs may strongly influence the nearshore assemblage of phytoplankton in oligotrophic embayments in large lakes, creating hotspots for productivity, species turnover, and trophic dynamics.

The effects of acute solar radiation stress on photosynthetic efficiency in freshwater unialgal cultures representing three phytoplankton pigment groups were measured by pulse amplitude modulated fluorometry (Walz Phyto-PAM) and compared to previous observations on field populations. Ultraviolet radiation (UVR) (UV-B and UV-A) induced a loss of photochemical quantum efficiency (Fv/Fm) in all 13 taxa examined in culture, while effects of photosynthetically active radiation (PAR) were smaller and often insignificant. Cyanobacteria were the most sensitive to PAR and UVR stress, chlorophytes the least and chromophytes intermediate but variable. The kinetics of maximal (Fm) and minimal (F0) fluorescence responses suggested uncoupling of antenna pigments from reaction centers (decreased Fm) persistent after dark adaptation was a common response, in particular for chromophytes, while the extent of impairment from damaged reaction centers (increased F0) was more variable. Changes in Fv/Fm with irradiance exposure were well described by the Kok model of photoinhibition and indicated that damage, rather than recovery, processes were predictive of acute cumulative inhibition. Field populations of cyanobacteria and chromophytes tended to greater tolerance and lower damage rates than laboratory strains. The results for cultures under standardized conditions supported field results in showing cyanobacteria more sensitive to acute UVR exposure than eukaryotic algae, and thus lacking any innate resistance of photosystem II to sunlight stress that might help explain their success in surface bloom formation.

Zebra mussels (Dreissena polymorpha) alter the transport dynamics and fate of particulate matter in aquatic systems by intercepting, retaining, and recycling suspended materials. This study examines the effect of particle processing by dressenids on the nature (grain size distribution, settling velocity, porosity  and density) and transport properties (critical shear stress for erosion, erosion rates, and bed stability) of suspended particulate matter in lakes.

Multi-wavelength fluorometers, such as the bbe FluoroProbe (FP), measure excitation spectra of chlorophyll a (Chl-a) fluorescence to infer the abundance and composition of phytoplankton communities as well as the concentration of chromophoric dissolved organic matter (CDOM). Experiments were conducted on laboratory cultures and on natural communities of freshwater phytoplankton to determine how the response of phytoplankton to high irradiance might affect fluorometric estimates of community composition and concentrations of Chl-a and CDOM. Cultures of a representative cyanobacterium, bacillariophyte, synurophyte, cryptophyte, and chlorophyte revealed changes in Chl-a excitation spectra as irradiance was increased to saturating levels and non-photochemical quenching (NPQ) increased. The degree of change and resulting classification error varied among taxa, being strong for the synurophyte and cryptophyte but minimal for the cyanobacterium. Acute-exposure experiments on phytoplankton communities of varying taxonomic composition from five lakes yielded variable results on apparent community composition. There was a consistent decrease in CDOM estimates, whereas Chl-a estimates were generally increased. Subsequent exposure to low PAR relaxed NPQ and tended to reverse the effects of high irradiance on composition, total Chl-a, and CDOM estimates. Relaxation experiments on near-surface communities in a sixth, large lake, Georgian Bay, showed that total Chl-a estimates increased by 44% on average when dark treatments were used to relax NPQ, though, in contrast to the findings from the small lakes, there was little effect on CDOM estimates. We observed a statistically-significant, negative linear relationship between the photon flux density of in situ irradiance and the accuracy of taxonomic assignment by FP in Georgian Bay. Not discounting the correlations between light intensity and the accuracy of the FP that were observed in this study, we conclude that the applicability of the reference spectra to the system under investigation is a more important consideration than variability in natural irradiance conditions.

Summary The effects of UV‐B, UV‐A and PAR on the photochemistry of phytoplankton from Hamilton Harbour, Lake Ontario, were measured to assess how well a multi‐wavelength Pulse Amplitude Modulated fluorometer (Phyto‐PAM) could discriminate among different pigment groups and to test whether bloom‐forming cyanobacteria in this embayment have comparatively high resistance to sunlight stress. Estimates of abundance for the three groups (blues, greens and browns) identified by the Phyto‐PAM generally agreed with microscope counts, but the maximum quantum efficiency of photochemistry (Fv:Fm) was usually quantified only for the dominant group. In acute exposure experiments, the average inhibition of Fv:Fm by PAR, UV‐A and UV‐B was <10, 30 and 60% respectively. More inhibition was observed in the cyanobacteria compared to the eukaryotic phytoplankton, due to higher rates of photosystem damage rather than lower rates of recovery based on the Kok model for photoinhibition. Both Fv:Fm and photosynthetic carbon incorporation showed similar patterns of inhibition. Based on Fv:Fm our results showed no evidence that cyanobacteria are more resistant to UVR stress compared to other groups, however, their success as bloom‐forming species suggests they must have other mechanisms to tolerate if not thrive under high irradiance conditions. These results demonstrate both the utility and limitations of the Phyto‐PAM for the assessment of group‐specific abundance and physiological responses in a natural community: general seasonal and inter‐group variation can be captured, but improved resolution of less abundant groups would enhance its application.

The Great Lakes Fishery Commission sponsored a 2-day workshop that sought to enhance the ability of Great Lakes agencies to understand, predict, and ideally manage fisheries production in the face of changes in natural and anthropogenic forcings (e.g., climate, invasive species, and nutrients). The workshop brought together 18 marine and freshwater researchers with collective expertise in aquatic ecology, physical oceanography, limnology, climate modeling, and ecosystem modeling, and two individuals with fisheries management expertise. We report on the outcome of a writing exercise undertaken as part of this workshop that challenged each participant to identify three needs, which if addressed, could most improve the ability of Great Lakes agencies to manage their fisheries in the face of ecosystem change. Participant responses fell into two categories. The first identified gaps in ecological understanding, including how physical and biological processes can regulate early life growth and survival, how life-history strategies vary across species and within populations, and how anthropogenic stressors (e.g., nutrient runoff, climate change) can interact to influence fish populations. The second category pointed to the need for improved approaches to research (e.g., meta-analytic, comparative, spatial translation) and management (e.g., mechanistic management models, consideration of multi-stock management), and also identified the need for improved predictive models of the physical environment and associated ecosystem monitoring programs. While some progress has been made toward addressing these needs, we believe that a continued focus will be necessary to enable optimal fisheries management responses to forthcoming ecosystem change.

The use of spectral fluorometers for assessing phytoplankton concentrations and taxonomic composition in aquatic environments is increasingly common. However, the accuracy of such assessments suffers because the necessary norm spectra (spectral fingerprints) are derived using selected taxa and laboratory conditions that may not adequately represent the taxa and environmental conditions in the study area. Ordination analysis of raw fluorescence data has been proposed as a better means of interpreting spectral fluorescence data. We applied nonmetric multidimensional scaling and cluster analysis to raw in situ fluorescence data from Sturgeon Bay, a small, mesotrophic embayment of Georgian Bay (Lake Huron) to obtain system-specific norm spectra for the bbe FluoroProbe. The revised spectra gave improved estimates of phytoplankton taxonomy (root mean square error of 10% versus 14%) and of dissolved organic carbon and chlorophyll a concentrations. While promising, this method should be further explored in other systems with different and (or) weaker gradients in phytoplankton biomass and taxonomic composition.

Unattended sensor networks are a cost-effective strategy to enhance the resolution of environmental datasets and are required to understand how large aquatic ecosystems respond to complex stressors (e.g., climate change). We made unattended and continuous measurements of the quantum yield of photosystem II ([Formula: see text]) photochemistry in the surface mixed layer of Lake Erie during three lake-wide cruises to observe how phytoplankton physiology varied across nutrient and taxonomic gradients. Three prominent diel [Formula: see text] patterns were noted. The diel maximum consistently occurred at sunrise or sunset, nocturnal measurements were consistently lower than diel maxima, and daytime values were strongly diminished by nonphotochemical quenching. The diurnal pattern was modeled as a function of irradiance to a mean accuracy of 0.03 to 0.04. Contrary to previously published reports in Lake Erie, [Formula: see text] was largely insensitive to indices of nutrient deficiency through space and time. This finding was consistent with much recent literature about [Formula: see text] and suggests that Lake Erie phytoplankton, like many others, can tune their photosynthetic machinery to maintain relatively high efficiency of photochemistry in photosystem II even when deficient in phosphorus or nitrogen.

A three-dimensional hydrodynamic model (ELCOM) coupled with a biological model (CAEDYM) was calibrated with field data from Lake Simcoe (2008) and used to examine the expected impact of dreissenid mussels on the distribution of phytoplankton and nutrients, mussel energetic, and the interactions with hydrodynamic conditions. In accordance with the near-shore shunt hypothesis and the actual distribution of mussels, the model predicted a considerable impact of mussels on phytoplankton in nearshore areas (<15 m depth; −38 % phytoplankton). Horizontal advection could account for the lower (−15 % phytoplankton) offshore impact despite the absence of mussels within this region of the lake. These predictions were similar to observed long-term changes in phytoplankton biomass after the dreissenid mussel invasion. Simulated decreases in the initial mussel biomass by 50 % led to increased phytoplankton nearshore, increased mussel growth and decreased phosphorus excretion relative to ingestion. In contrast, increases in initial mussel biomass by 50 % resulted in minor changes in phytoplankton, since near-bottom depletion of phytoplankton caused growth and energy-limitation of mussels. The model, which was the first to capture metabolically-driven variations of mussel-mediated nutrient cycling, predicted a partial uncoupling of carbon and phosphorus cycling by mussels, driven by varying controls on mussel growth. A 60 % increase in the intensity of three storms during the summer had little net-effect on phytoplankton biomass, but reduced energy-limitation of mussels and increased mussel growth due to increased supply of phytoplankton. Our model demonstrates that near-bottom depletion of phytoplankton is an important process limiting the impact of mussels, and indicates that it affects the mussels’ cycling of energy and nutrients. It further suggests that stronger storm events could lead to further increase of flux of organic matter from the pelagic zone to the benthos, further altering energy pathways in the lake.

Primary production by phytoplankton helps fuel hypolimnetic oxygen depletion as well as food web productivity, and is thought to have been affected by dreissenid mussel colonization in Lake Simcoe. Measured by short-term 14C uptake, areal and volumetric rates of photosynthetic carbon uptake (production) were lower in nearshore (<15m) than offshore sites during the study period (Aug. 2010–Aug. 2011). Chlorophyll a (Chl a) concentrations were also lower at nearshore sites, except in summer (June–Aug.). Production increased from May to an annual maximum in September (350mgCm−2d−1 nearshore and 650mgCm−2d−1 offshore) and remained high through November; its annual minimum (<10mgCm−2d−1) was in February. A secondary spring maximum occurred in April in the offshore (450mgCm−2d−1) but not the nearshore (30mgCm−2d−1). Chl a and production of net phytoplankton (>20μm) was greatest from August through November at offshore sites (median contribution >35% of total) but was much less at nearshore sites (median contribution <10%). Production and Chl a were dominated by nanoplankton (2–20μm), with median contributions >40% in most seasons. The lower production at nearshore than offshore sites supports previous inferences that dreissenid mussels have caused differential loss of planktonic primary production, especially of net phytoplankton, in nearshore waters. The results from Lake Simcoe are consistent with evidence from other lakes that dreissenids can have major impacts on seasonal blooms of diatoms as the mussels continue to spread within and among North American lakes.

The ratio of gross photosynthesis to community respiration (P:R) is a vital characteristic of ecosystem function but is poorly defined in large lakes because traditional methods are impractical for measuring P and R on appropriate time and space scales. The new steady state in situ 18O method has a great power to assess metabolic patterns in large lakes. However it could be confounded by seasonal and/or episodic temperature changes, which can violate the steady-state assumptions by altering the saturation level of dissolved O2 without actual gain or loss of O2 when air–water gas exchange is too slow to compensate for the altered solubility. We estimated P, R and P:R at three coastal sites of varied depth, productivity and physical exposure in the Laurentian Great Lakes by both the 18O method and experimental incubations. At two shallow sites with low and moderate exposure to open lake forces, the 18O and incubation methods returned similar estimates for P, R and P:R. The deepest site experienced large and frequent upwelling episodes and the 18O method was unreliable, with P and R estimates usually negative. We define an O2 saturation ratio (γ) that determines the applicability of the 18O method to any given system. Estimates obtained using observed O2 saturation above this threshold are invalid. The γ ratio increases with lake productivity. Higher productivity and/or shallower lakes appear good candidates for use of the 18O method, but applications in deeper, physically-driven lakes will likely need explicit attention to temperature-driven non-steady state effects.

Pulse amplitude-modulated (PAM) fluorometry was used to obtain rapid light curves (RLCs) for phytoplankton from small Canadian lakes of varying water clarity, including metalimnetic communities from two clear lakes. RLCs were measured before and after exposure to a number of experimental spectra containing ultraviolet radiation (UVR) and photosynthetically active radiation (PAR). Photochemical (qP) and non-photochemical (NPQ) quenching during RLCs were positively and significantly correlated with recent in situ light history, but the maximum quantum efficiency of Photosystem II (alpha) and maximum relative rate of electron transport (rETR(max)) were not. rETR(max) and alpha were diminished by experimental exposures to UVR and/or high PAR, but significantly less so in phytoplankton from brighter environments. UVR exposures diminished the inducible (photoprotective) NPQ of most epilimnetic phytoplankton. Both the inducible and total (photoprotective + photoinhibitory) NPQ of metalimnetic phytoplankton were stimulated by spectral exposures. Our results are the first obtained from natural communities of freshwater phytoplankton to show that aspects of PSII photophysiology (as inferred from RLCs) vary according to in situ light history.

Marine fisheries recruitment research has emphasized approaches that explore physical–biological interactions during early life stages (ELS). Herein, we review evidence that such approaches would benefit our understanding of fish recruitment in large freshwater lakes, which exhibit similar physical processes and contain fishes with comparable life-history characteristics as marine ecosystems. A review of the primary literature (1965–2008) for freshwater and marine ecosystems revealed that coupled biophysical research on fish ELS (i) has benefited our ability to understand and predict fish recruitment in marine ecosystems; (ii) has been virtually absent from small lake ecosystems but has been growing in the Laurentian Great Lakes; and (iii) has shown that similar to marine ecosystems, physical processes can control fish recruitment in large lakes through direct and indirect pathways, often involving interactions between biological processes and physicochemical conditions. In addition to identifying specific research gaps and opportunities, this perspective points to the need for increased research on physical–biological coupling in large lake ecosystems, as well as the continued erosion of barriers between marine and freshwater fisheries recruitment science.

Dreissenid mussels have been hypothesized to cause selective decreases of phytoplankton in nearshore areas (nearshore shunt hypothesis) as well as the near-complete loss of the offshore phytoplankton spring bloom in some Laurentian Great Lakes. To evaluate whether mussels can reasonably be expected to mediate such changes, we extended the three-dimensional hydrodynamic-ecological model (ELCOM-CAEDYM) to include mussels as a state variable and applied it to Lake Erie (USA-Canada). Mussel-mediated decreases in mean phytoplankton biomass were highly sensitive to the assigned mussel population size in each basin. In the relatively deep east basin, mussels were predicted to decrease phytoplankton in both nearshore and offshore zones, even during periods of thermal stratification but especially during the spring phytoplankton maximum. Spatially, impacts were associated with mussel distributions but could be strong even in areas without high mussel biomass, consistent with advection from areas of higher mussel biomass. The results supported the nearshore shunt hypothesis that mussel impacts on phytoplankton should be greater in nearshore than offshore waters and also supported suggestions about the emerging importance of deep water offshore mussels. The results of this study provide an important insight into ecological role of mussels in lowering plankton productivity in some world's largest lakes.

A three dimensional hydrodynamic model, the Estuary and Lake Computer Model (ELCOM), validated by field data collected in 2008, is used to investigate the thermal structure response in Lake Erie to changes in air temperature and wind speed. We define spatially and temporally varying regions for the epilimnion, thermocline, and hypolimnion. Increasing the air temperature warms up the epilimnion but has little effect on the hypolimnion. The stratification forms earlier and breaks down later. The thermocline is raised modestly in the warmer air temperature scenario. Stronger winds cool the epilimnion slightly, but warm up the hypolimnion with much larger temperature changes. The stratification duration is shortened, and the thermocline depth is noticeably deepened. Due to the large differences in depths and layer thicknesses of the three basins, the responses to changes in meteorological forcing vary among the basin. Exploiting the power of the three dimensional model to provide a more authentic characterization of thermal stratification in large lakes, it is shown that patterns inferred from simple isotherm dynamics when studying the stratification period as typically done with one dimensional models are not always accurate. The present results for Lake Erie show the potential for complicated and interactive effects of climate forcing on important biogeochemical processes, especially hypolimnetic oxygen depletion.