Gertrud K. Nürnberg

Trophic State of Clear and Colored, Soft- and Hardwater Lakes with Special Consideration of Nutrients, Anoxia, Phytoplankton and Fish

ABSTRACT Idiosyncrasies, erroneous assumptions and gaps are still hampering lake evaluations, despite several decades of defining the trophic state of lakes. I reevaluated nutrient and algal biomass limits that group lakes into oligo-, meso-, eu- and hyper eutrophic lakes at transition concentrations of 10, 30 and 100 μg · L−1 average total phosphorus concentration of the summer epilimnion. I investigated the direct and indirect effects of general lake characteristics like morphometry and geochemistry (water hardness and color) on lake trophic state and formulated models to deal with their significant influences. Nutrient and biomass concentrations as well as quantitative measures of anoxia were used to determine the trophic state. I demonstrated that the number of coldwater fish species may be used to classify the lower trophic levels in geographically constraint areas. Limits and models were established with data from well-studied lakes and verified with literature data from worldwide lakes. Trophic sta...

Assessing internal phosphorus load – Problems to be solved

Abstract Internal loading as phosphorus (P) released from anoxic sediment surfaces often represents the main summer P load to lakes and reservoirs and can have an immense effect on their water quality. Many difficulties in internal load assessment exist, however, including ignoring internal load altogether, ambiguity about the origin of sediment released P and inexact definitions. Most of these problems are due to the difficulty in distinguishing internal from external P sources, which is particularly challenging in polymictic lakes. To prevent misconceptions and facilitate its evaluation, internal load in stratified and polymictic lakes should be expressed in a similar way to external loads: as annual, gross and areal load of total phosphorus (TP). Possible approaches to internal load quantification are: in situ determination from hypolimnetic P increases, mass balance approaches, and estimates from anoxic active area and P release. Further suggestions to facilitate the study of internal loading include: (a) the differentiation between polymictic and stratified lakes, sections of lakes, and time periods when evaluating indicators and impact of internal load; (b) the separation of internal load (upward flux) from sedimentation (downward flux) of external and internal loads, and (c) the consideration of the downward flux of both external (Lext, mg/m2/yr) and internal (Lint, mg/m2/yr) loads by a retention model (Rsed) when predicting lake TP averages in a mass balance model of the form (qs = annual areal water load in m/yr):

Modeling the effect of development on internal phosphorus load in nutrient-poor lakes

[1]xa0A steady state lake phosphorus (P) mass balance model was used to predict the equilibrium P concentration (annual, volume-weighted average) of the lake water from natural and anthropogenic, external and internal P inputs. Internal P load was modeled as the product of sediment release rates and anoxic factors. Both these components were predicted from lake P concentration computed from external load to create a link between external and internal load components. Such estimates allow the modeling and setting of objectives of several hundred lakes on the Canadian Shield. In particular, estimates of predevelopment lake P concentration made by removing all anthropogenic inputs were compared with postdevelopment conditions in which additional loading was added to the model. This was accomplished by determining how much development would increase external as well as internal phosphorus load and ultimately annual average lake P concentrations. By comparing proposed lake development scenarios with existing or predevelopment scenarios, it can be determined whether water quality objectives will be exceeded.

An Artificially Induced Planktothrix rubescens Surface Bloom in a Small Kettle Lake in Southern Ontario Compared to Blooms World-wide

ABSTRACT To combat hypolimnetic anoxia and sediment phosphorus release in a small, mesotrophic kettle lake on the Oakridge Moraine north of the Metropolitan Toronto, Southern Ontario, oxygenation and aeration was applied to the hypolimnion alternately during the summer of 1998 until mid-November and then to the entire water column until the end of December. This treatment coincided with the proliferation of a toxic strain of the purple cyanobacterium, Planktothrix rubescens, from almost undetectable values to bloom conditions under ice in the following winter and spring. Although small numbers of P. rubescens have been detected during several years before the treatment, prolonged artificial mixing in the fall and winter of 1998 distributed numerous filaments throughout the water column and to the surface when light was suitably low for these algae to survive and grow. Algae were supported by simultaneous entrainment and mixing of nutrients from the enriched bottom water. Such blooms of P. rubescens and related bluegreens have been found in many lakes with comparable characteristics and during similar episodes like those of the study lake. Lakes were typically stratified, mesotrophic hardwater lakes, with phosphorus levels that have recently been increasing to levels above 20 μg L−1. Blooms occurred during periods of low light and enhanced mixing, in several cases after treating the lake with whole-lake aeration and mixing. Recommendations to prevent such blooms in Lake Wilcox are (1) the discontinuation of artificial mixing during periods of natural destratification in the fall and winter, (2) the prevention of further eutrophication, and (3) the installation of an in-lake treatment, such as hypolimnetic withdrawal, to decrease internal phosphorus loading from anoxic sediment surfaces.

Lake responses to long-term hypolimnetic withdrawal treatments

Abstract Hypolimnetic withdrawal is an in-lake restoration technique based on the selective discharge of bottom water to enhance the removal of nutrients and electro-chemically reduced substances that build up when the hypolimnion becomes anoxic. Comparison of water quality variables before and during treatment in about 40 European and 8 North American lakes indicates that hypolimnetic withdrawal is an efficient restoration technique in stratified lakes. Water quality improvement was apparent in decreased summer average epilimnetic phosphorus (P) and chlorophyll concentrations, increased Secchi disk transparency, and decreased hypolimnetic phosphorus concentration and anoxia. In particular, summer average phosphorus decreases were significantly correlated with annual water volumes and P masses withdrawn per lake area, indicating the importance of hydrology and timing of the treatment. Observations as well as models revealed that avoiding extreme temperature changes in the water column is critical for a successful application of the technique. The removal of colder bottom water may increase bottom water temperatures, which not only increases sediment release rates and sediment oxygen demand but, more important, may lead to thermal instability, resulting in enhanced entrainment of nutrient-rich hypolimnetic water and increased surface eutrophication. Hypolimnetic withdrawal also improved water quality in man-made lakes with bottom outlets unless too much withdrawal led to thermal instability. It failed to have a positive effect in a shallow oligomictic lake, probably because nutrient export was not much increased. A recognized disadvantage of hypolimnetic withdrawal is its impact on downstream waters, including eutrophication, temperature increase, oxygen depletion, and odor development. In the experiences evaluated, treatment of the withdrawal water ranged from no treatment in older remote applications in the European Alps, to passive treatment in wetlands and settling ponds, and modern waste water technologies in more recent applications. Overall, hypolimnetic withdrawal is an effective low-cost restoration technique to combat and potentially reverse eutrophication in stratified lakes and reservoirs.

The Phosphorus-Chlorophyll Relationship in Lakes: Potential Influences of Color and Mixing Regime

ABSTRACT We used summer epilimnetic means from a large dataset (369 lakes from North America, Europe, Asia, and New Zealand) to examine whether color or mixing regime significantly influence the log-log relationship between chlorophyll a (Chl a) and total phosphorus (TP) in lakes. We found no significant difference in regression models for clear (color < 10 Pt units) vs. humic lakes (color > 20 Pt units), even when data were screened so that both types of lakes were represented by the same range of TP. Likewise, there was no significant difference in regression models for mixed vs. thermally stratified lakes. Knowing that a particular lake is clear vs. humic, or mixed vs. stratified, therefore is not helpful in developing a predictive model of Chl a from TP. However, when we simultaneously considered water color and mixing regime, a potentially useful feature was identified. Mixed humic lakes have a somewhat lower (significant at p = 0.10) ratio of Chl a / TP than mixed clear water lakes, whereas no such difference exists between humic and clear water lakes that are thermally stratified. Likewise, when we plotted the Chl a / TP ratio vs. color, there was a negative slope for mixed lakes, but no significant relationship for stratified lakes. We suggest that in stratified humic lakes, which are often sheltered deep systems with a high Osgood Ratio, phytoplankton can compensate for reduced underwater irradiance by migrating toward the water surface, whereas in mixed humic lakes, that response is not possible. Lake managers dealing with mixed humic lakes might expect a lower yield of Chl a / TP than in mixed clear water lakes. Further research is needed to test this hypothesis. In any case, caution is warranted when applying such results to lake management, because given the numerous effects of high P inputs (e.g., changes in benthic invertebrate communities, biological oxygen demand, and sediment metabolism) it is unwise to focus solely on the Chl a response to protect water quality.

Quantification of Oxygen Depletion in Lakes and Reservoirs with the Hypoxic Factor

ABSTRACT The amount of dissolved oxygen (DO) in lake water has biological and chemical implications. There are no easy ways to quantify exeedences of important thresholds of DO in lakes, except the complete lack of DO, quantified as an annual or seasonal anoxic factor (Nürnberg 1995a, b). Here an analogous concept, the “Hypoxic Factor” (HF) is introduced. According to specific water quality standards, certain levels of oxygen depletion can be quantified, e.g., DO concentrations below 5 mg L−1 or 6.5 mg L−1. The HF is computed from the duration and extent of hypoxia by using DO profiles and hypsographic data. It is expressed in such a way that a value of 365 d yr−1 would mean the whole lake has a DO concentration below the specified level at all times. The application to a TMDL project, where DO standards are to be employed in a large Snake River reservoir, demonstrates the potential usefulness of HF. This quantification of hypoxia allows the testing of hypotheses, e.g., the dependency of hypoxia on stage height, flow and nutrients. Significant relationships were found between annual expressions of hypoxia and hydrological variables (seasonal flows, water retention time) and a climatic index.

Trophic state decrease after lanthanum-modified bentonite (Phoslock) application to a hyper-eutrophic polymictic urban lake frequented by Canada geese (Branta canadensis)

Abstract Urban lakes are important assets to highly populated regions; however, extensive usage and other influences degrade their water quality, which then requires rehabilitation and maintenance. Hyper-eutrophic Swan Lake, Greater Toronto, Canada (5.5 ha, 4.4 m maximum depth) was a gravel pit that became degraded by elevated total phosphorus (TP) concentrations, mostly from internal P sources. Because Swan Lake is a terminal lake with limited flushing and small external load, a phosphate adsorbing and sediment capping agent, lanthanum-modified bentonite (Phoslock), was applied in spring 2013 to intercept the internal load. Average TP concentration decreased from 0.247 to 0.099 mg/L in the first and 0.060 mg/L in the second post-treatment year. A TP mass balance model adequately predicted post-treatment annual average TP concentration by not including the pre-treatment internal load estimate of 650 to 1100 mg/m2/yr. Phytoplankton biomass decreased only in the second post-treatment year, when Secchi transparency (highly correlated with chlorophyll concentration) increased to a growing season average of 1.4 m (range 0.7–2.7) compared to 0.5 m (0.37–0.63) before treatment. We explain the lack of response in the first treatment year with a relatively late application (29 Apr–1 May 2013), when P released from the winter bottom sediments had already been taken up by phytoplankton. Recently, a growing population of waterfowl (mostly Canada goose, Branta canadensis) were the highest contributors of nutrients (75%), as indicated by a mass balance based on literature-derived goose P export and biweekly bird census. We recommend waterfowl management or repeated treatment to further improve water quality.

Attempted management of cyanobacteria by Phoslock (lanthanum-modified clay) in Canadian lakes: water quality results and predictions

ABSTRACT Nürnberg GK. 2017. Attempted management of cyanobacteria by Phoslock (lanthanum-modified clay) in Canadian lakes: water quality results and predictions. Lake Reserv Manage. 33:163–170. When internal phosphorus (P) loading from the bottom sediments outweighs external P inputs to lakes, lake water quality and cyanobacteria blooms will not respond to external measures alone and therefore require an in-lake restoration treatment. Lake characteristics and governmental regulations do not permit a random choice of such methods. A treatment developed by an Australian research institute (CSIRO) more than 15 years ago has recently been introduced to Canada and is licensed in Ontario. The lake water treatment consists of the application of a phosphate adsorbing and sediment capping agent called Phoslock in North America, a clay (bentonite) that has been systematically amended by the phosphate adsorbing element lanthanum. When applied, Phoslock sinks to the lake bottom where it intercepts the upward flux of internal load from sediment P release. Although the number of monitored Canadian applications is still small, suggestions to optimize restoration effects can be made, including the avoidance of high flushing during application, system isolation, and timing to coincide with high levels of inorganic P in the lake water. Initial results in at least 2 cases (Swan Lake, City of Markham, Greater Toronto Area; and Henderson Lake, Lethbridge, Alberta) were promising despite the unexpected external P input from waterfowl and possibly other external sources. Other applications are planned in a Quebec lake and are being discussed in several other provinces.