Plankton Metabolic Balance and its Controlling Factors in the Coastal Zone of the Laurentian Great Lakes

Plankton metabolic balance (PMBm) of the surface mixed layer was calculated as the ratio of areal rates of gross photosynthesis (AGP) to community respiration (AR), and estimated for four Laurentian Great Lakes coastal sites of contrasting physical, optical and nutrient regime: western Lake Ontario, Hamilton Harbour, Georgian Bay and Woods Bay. The applied methods were the oxygen light-and-dark bottle and 14C bottle methods as well as the oxygen stable isotope method (18O method). PMBm was net autotrophic in most of the cases (73% of the observations). Within- and inter-system variations in PMBm were heavily dependent on both a ratio of light-saturated photosynthesis to community respiration (Pmax/R) and a ratio of euphotic to mixing depths (Zeu/Zm). While short-term within-system variations in PMBm were driven by the interplay of chlorophyll a (Chl a), total phosphorus (TP) and Zeu/Zm ratio, its inter-lake long-term variability had a different behaviour. Average ratios of AGP/AR were dependent only on DOC or single physical parameters such as Zeu or Zm, while PMBm determined as the ratio between average AGP and AR was controlled by the joint effect of DOC, TP and Chl a. DOC affected average AGP/AR ratios primarily via its control over fluctuations of the physical environment and had a depressing effect on AGP rates but did not control rates of AR. Independent measurements of volumetric rates of photosynthesis (P) and community respiration (R) were made by 18O method adjusted for wind-driven gas exchange and compared against estimates from bottle estimates. The 18O method in Lake Ontario gave internally inconsistent results (e.g. negative absolute rates of P and R) and poor agreement with independent estimates of P, R and P/R despite superficially plausible estimates for P/R. The low productivity of Lake Ontario and frequent disturbances of water column masked the biological signal in both DO abundance and its isotopic signature, and thus invalidated the assumptions of steady state conditions. However, in Hamilton Harbour and some other relatively sheltered sites that were sampled occasionally, 18O method predicted absolute rates of P that were well correlated well with bottle estimates. Isotope model estimates for R and P/R in the harbour were not well correlated with bottle estimates but were of comparable magnitude on average, and differences were explicable in terms of physical forces and the different time scales of response for the two methods. The Hamilton Harbour hypolimnion presented an anomalous behavior in oxygen stable isotopes (18O depletion) where seasonal development of DO depletion was not accompanied by the progressive isotope enrichment expected from respiratory fractionation. The Lake Ontario and harbour hypolimnion results both appear to show that simple steady state models that assume literature values for fractionation processes and ignore physical dynamics are of limited applicability to lakes

A phosphorus mass-balance model for the Lake St. Clair-Lake Erie system: How important is in-lake phosphorus loading?

Management of eutrophication in lakes usually focuses on reducing external phosphorus (P) loads. However, several in-lake mechanisms can add significant amounts of new and recycled P to the water column, hence, contributing to eutrophication and altering a lake’s response to changes in its external P loading. For large lakes, these in-lake mechanisms remain poorly characterized, although they can potentially have a major impact on the effectiveness of nutrient control measures within the watershed. Direct measurements of in-lake P inputs may not only be difficult to obtain, but their scaling up in time and space is also fraught with uncertainties, especially in large lakes. Mass balance modeling provides a conceptually simple theoretical framework to estimate the magnitude of in-lake P loads when estimates of external loads and export fluxes of P can be estimated. Furthermore, when integrated over a sufficiently long time span, the various P inputs and outputs can be assumed to be in balance (steady state condition). We built a post-2000 (2003 through 2016) steady-state model of the net annual total P (TP) budget for the Lake St. Clair–Lake Erie system. The budget shows that the net TP output from the lake system substantially exceeds the sum of all external TP inputs. To balance the budget, in-lake processes must add 3783 metric tons per year (MTA) to the water column, or about one third of all externally derived TP inputs combined. For Lake Erie alone, the in-lake generated load is 3563 MTA, or about one half of all the external P inputs from the lake’s watershed. We further estimate the in-lake P loading fluxes for the individual basins along the Lake St. Clair–Lake Erie system and discuss the applicability of the TP mass balance approach to inform decisions about nutrient control measures.This work is part of the Lake Futures project within the Global Water Futures (GWF) program and supported financially by the Canada First Research Excellence Fund (CFREF)

Nutrient Loss Rates in Relation to Transport Time Scales in a Large Shallow Lake (Lake St. Clair, USA—Canada): Insights From a Three‐Dimensional Model

A nutrient mass balance and a three‐dimensional, coupled hydrodynamic‐ecological model, calibrated and validated for Lake St. Clair with observations from 2009 and 2010, were integrated to estimate monthly lake‐scale nutrient loss rates, and to calculate 3 monthly transport time scales: flushing time, water age, and water residence time. While nutrient loss rates had statistically significant relationships with all transport time scale measures, water age had the strongest explanatory power, with water age and nutrient loss rates both smaller in spring and fall and larger in summer. We show that Lake St. Clair is seasonally divided into two discrete regions of contrasting water age and productivity. The north‐western region is dominated by oligotrophic waters from the St. Clair River, and south‐eastern region is dominated by the nutrient enriched, more productive waters from the Thames‐Sydenham River complex. The spatial and temporal variations in local transport scales and nutrient loss rates, coupled with strong seasonal variations in discharge and nutrient loads from the major tributaries, suggest the need for different load reduction strategies for different tributaries.Key PointsWe applied a three‐dimensional ecosystem model to simulate physical, chemical, and biological dynamics in a large shallow lakeWe found that spatially dependent water residence time represents lake flushing better than traditional flushing timeWater age influences the spatial and temporal distribution of nutrient retention, primary production, and algal biomass distributionPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/145320/1/wrcr23330.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145320/2/wrcr23330_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145320/3/wrcr23330-sup-0001-2017WR021876-s01.pd

Temporal and spatial dynamics of large lake hypoxia: Integrating statistical and three‐dimensional dynamic models to enhance lake management criteria

Hypoxia or low bottom water dissolved oxygen (DO) is a world‐wide problem of management concern requiring an understanding and ability to monitor and predict its spatial and temporal dynamics. However, this is often made difficult in large lakes and coastal oceans because of limited spatial and temporal coverage of field observations. We used a calibrated and validated three‐dimensional ecological model of Lake Erie to extend a statistical relationship between hypoxic extent and bottom water DO concentrations to explore implications of the broader temporal and spatial development and dissipation of hypoxia. We provide the first numerical demonstration that hypoxia initiates in the nearshore, not the deep portion of the basin, and that the threshold used to define hypoxia matters in both spatial and temporal dynamics and in its sensitivity to climate. We show that existing monitoring programs likely underestimate both maximum hypoxic extent and the importance of low oxygen in the nearshore, discuss implications for ecosystem and drinking water protection, and recommend how these results could be used to efficiently and economically extend monitoring programs.Key Points:We modeled seasonal and spatial dynamics of Lake Erie hypoxiaWe showed hypoxia starts nearshore and can persist after traditional monitoring programs endWe recommend monitoring adjustments and explore impacts of different hypoxia definitionsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/133547/1/wrcr22074.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/133547/2/wrcr22074-sup-0001-2015WR018170-s01.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/133547/3/wrcr22074_am.pd

Scientists’ Warning to Humanity: Rapid degradation of the world\u27s large lakes

Large lakes of the world are habitats for diverse species, including endemic taxa, and are valuable resources that provide humanity with many ecosystem services. They are also sentinels of global and local change, and recent studies in limnology and paleolimnology have demonstrated disturbing evidence of their collective degradation in terms of depletion of resources (water and food), rapid warming and loss of ice, destruction of habitats and ecosystems, loss of species, and accelerating pollution. Large lakes are particularly exposed to anthropogenic and climatic stressors. The Second Warning to Humanity provides a framework to assess the dangers now threatening the world\u27s large lake ecosystems and to evaluate pathways of sustainable development that are more respectful of their ongoing provision of services. Here we review current and emerging threats to the large lakes of the world, including iconic examples of lake management failures and successes, from which we identify priorities and approaches for future conservation efforts. The review underscores the extent of lake resource degradation, which is a result of cumulative perturbation through time by long-term human impacts combined with other emerging stressors. Decades of degradation of large lakes have resulted in major challenges for restoration and management and a legacy of ecological and economic costs for future generations. Large lakes will require more intense conservation efforts in a warmer, increasingly populated world to achieve sustainable, high-quality waters. This Warning to Humanity is also an opportunity to highlight the value of a long-term lake observatory network to monitor and report on environmental changes in large lake ecosystems

On the Role of a Large Shallow Lake (Lake St. Clair, USA‐Canada) in Modulating Phosphorus Loads to Lake Erie

It is often assumed that large shallow water bodies are net sediment nondepositional annually and that if they have nutrient loads from multiple sources, those loads are quickly homogenized before exiting the water bodies. Where this is not the case, it impacts understanding and predicting consequences of nutrient load reductions, both for the water body and for those downstream of it. We applied a three‐dimensional ecological model to a large shallow lake, Lake St. Clair (US/Canada), to quantify the total and dissolved reactive phosphorus (TP and DRP) transport and retention, and construct tributary‐specific relationships between phosphorus load to the lake and the amount of phosphorus that leaves the lake for the three major tributaries. Lake St. Clair is situated between the St. Clair and Detroit rivers, the latter enters Lake Erie. Efforts to reduce Lake Erie’s re‐eutrophication requires an understanding of nutrient transport and retention in each of its subwatersheds including those that feed indirectly via Lake St. Clair. We found that over the simulation period, the lake retained a significant portion of TP (17%) and DRP (35%) load and that TP and DRP retention was spatially variable and largely controlled by a combination of lake depth, resuspension, and plankton uptake. Compared to the Clinton and Sydenham rivers, the Thames River contributed a larger proportion of its load to the lake’s outflow. However, because the lake’s load is dominated by the St. Clair River, 40% reductions of nutrients from those subwatersheds will result in less than a 5% reduction in the load to Lake Erie.Key PointsA large shallow lake with a 9 day water retention time still retains 17% of its total phosphorus and 35% of its dissolved phosphorus inputsTributary loads are not well‐mixed within the lake, leading to spatial‐temporal differences in phosphorus retentionWhile wind‐induced resuspension drives interannual variability in phosphors retention, depths greater than 5 m are net depositionalPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/153723/1/wrcr24289.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153723/2/wrcr24289_am.pd

On the Role of a Large Shallow Lake (Lake St. Clair, USA‐Canada) in Modulating Phosphorus Loads to Lake Erie

It is often assumed that large shallow water bodies are net sediment nondepositional annually and that if they have nutrient loads from multiple sources, those loads are quickly homogenized before exiting the water bodies. Where this is not the case, it impacts understanding and predicting consequences of nutrient load reductions, both for the water body and for those downstream of it. We applied a three‐dimensional ecological model to a large shallow lake, Lake St. Clair (US/Canada), to quantify the total and dissolved reactive phosphorus (TP and DRP) transport and retention, and construct tributary‐specific relationships between phosphorus load to the lake and the amount of phosphorus that leaves the lake for the three major tributaries. Lake St. Clair is situated between the St. Clair and Detroit rivers, the latter enters Lake Erie. Efforts to reduce Lake Erie’s re‐eutrophication requires an understanding of nutrient transport and retention in each of its subwatersheds including those that feed indirectly via Lake St. Clair. We found that over the simulation period, the lake retained a significant portion of TP (17%) and DRP (35%) load and that TP and DRP retention was spatially variable and largely controlled by a combination of lake depth, resuspension, and plankton uptake. Compared to the Clinton and Sydenham rivers, the Thames River contributed a larger proportion of its load to the lake’s outflow. However, because the lake’s load is dominated by the St. Clair River, 40% reductions of nutrients from those subwatersheds will result in less than a 5% reduction in the load to Lake Erie.Key PointsA large shallow lake with a 9 day water retention time still retains 17% of its total phosphorus and 35% of its dissolved phosphorus inputsTributary loads are not well‐mixed within the lake, leading to spatial‐temporal differences in phosphorus retentionWhile wind‐induced resuspension drives interannual variability in phosphors retention, depths greater than 5 m are net depositionalPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/153723/1/wrcr24289.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153723/2/wrcr24289_am.pd

Silica retention in the Iron Gate I reservoir on the Danube River: the role of side bays as nutrient sinks

There are longstanding concerns about the environmental impacts of super-dams such as Iron Gate I, the Danube River's largest hydropower scheme. Iron Gate I is suspected of trapping up to 80% (∼590 000 tons per year) of dissolved silica in the form of sedimenting diatom frustules and 30 000 000 tons per year of suspended solids. This study, however, indicates that (i) conditions are unfavorable for primary production in Iron Gate I except for the small quiescent center of Orsova Bay, and the diatom production is much too low for the suspected silica uptake; (ii) Orsova Bay is the most important sediment trap as resuspension does not occur, with ∼1% (82 000 tons per year) suspended solids retention, and (iii) also the only significant silica trap, with ∼0.2% (1000 tons per year) retention. It is most conservatively estimated that no more than 5% of dissolved silica can be retained by the Iron Gate I reservoir, and therefore the earlier estimate of the huge retention can definitely be ruled out

Scientists’ Warning to Humanity : Rapid degradation of the world’s large lakes

Large lakes of the world are habitats for diverse species, including endemic taxa, and are valuable resources that provide humanity with many ecosystem services. They are also sentinels of global and local change, and recent studies in limnology and paleolimnology have demonstrated disturbing evidence of their collective degradation in terms of depletion of resources (water and food), rapid warming and loss of ice, destruction of habitats and ecosystems, loss of species, and accelerating pollution. Large lakes are particularly exposed to anthropogenic and climatic stressors. The Second Warning to Humanity provides a framework to assess the dangers now threatening the world’s large lake ecosystems and to evaluate pathways of sustainable development that are more respectful of their ongoing provision of services. Here we review current and emerging threats to the large lakes of the world, including iconic examples of lake management failures and successes, from which we identify priorities and approaches for future conservation efforts. The review underscores the extent of lake resource degradation, which is a result of cumulative perturbation through time by long-term human impacts combined with other emerging stressors. Decades of degradation of large lakes have resulted in major challenges for restoration and management and a legacy of ecological and economic costs for future generations. Large lakes will require more intense conservation efforts in a warmer, increasingly populated world to achieve sustainable, high-quality waters. This Warning to Humanity is also an opportunity to highlight the value of a long-term lake observatory network to monitor and report on environmental changes in large lake ecosystems.publishe