Brian J. Huser

Geoengineering in lakes: welcome attraction or fatal distraction?

Abstract The use of geoengineering techniques for phosphorus management offers the promise of greater and quicker chemical and ecological recovery. It can be attractive when used with other restoration measures but should not be considered a panacea. The range of materials being proposed for use as well as the in-lake processes targeted for manipulation continues to grow. With increasing political imperatives to meet regulatory goals for water quality, we recommend a coordinated approach to the scientific understanding, costs, and integration of geoengineering with other approaches to lake management.

Longevity and effectiveness of aluminum addition to reduce sediment phosphorus release and restore lake water quality.

114 lakes treated with aluminum (Al) salts to reduce internal phosphorus (P) loading were analyzed to identify factors driving longevity of post-treatment water quality improvements. Lakes varied greatly in morphology, applied Al dose, and other factors that may have affected overall treatment effectiveness. Treatment longevity based on declines in epilimnetic total P (TP) concentration averaged 11 years for all lakes (range of 0-45 years). When longevity estimates were used for lakes with improved conditions through the end of measurements, average longevity increased to 15 years. Significant differences in treatment longevity between deeper, stratified lakes (mean 21 years) and shallow, polymictic lakes (mean 5.7 years) were detected, indicating factors related to lake morphology are important for treatment success. A decision tree developed using a partition model suggested Al dose, Osgood index (OI, a morphological index), and watershed to lake area ratio (related to hydraulic residence time, WA:LA) were the most important variables determining treatment longevity. Multiple linear regression showed that Al dose, WA:LA, and OI explained 47, 32 and 3% respectively of the variation in treatment longevity. Other variables (too data limited to include in the analysis) also appeared to be of importance, including sediment P content to Al dose ratios and the presence of benthic feeding fish in shallow, polymictic lakes.

Geo-Engineering in Lakes: A Crisis of Confidence?

The effective management of lakes suffering from eutrophication is confounded by a mosaic of interactions and feedbacks that are difficult to manipulate. For example, in lake processes can delay the relinquishment of legacy phosphorus (P) manifested within bed sediments for decades, even after effective catchment management. This recovery time is often deemed unacceptable and researchers have explored many in-lake management measures designed to “speed-up” recovery. The manipulation of biogeochemical processes (commonly targeting P) using materials to achieve a desired chemical and/or ecological response has been termed geo-engineering in lakes, and is becoming a commonly considered eutrophication management tool (Figure 1). Although this approach has been employed for many years it remains contentious largely due to variable results reported in the literature. This uncertainty risks ineffective management based on poorly designed or inappropriate applications. To address this, it is important that current levels of confidence in the approach be effectively communicated and that methods of increasing confidence are clearly demonstrated. We draw here on experiences of researchers and water managers at a global scale to demonstrate recent advances and consensus on recommendations (numbered below) for best practice. This information, although vital to underpinning successful management, has not been available in the peer reviewed literature.

A simple model for predicting aluminum bound phosphorus formation and internal loading reduction in lakes after aluminum addition to lake sediment.

The conversion of mobile phosphorus (P) to aluminum bound P (Al-P) after addition of Al to over 300 sub-samples from 35 sediment cores collected from 20 lakes in the upper Midwest, United States was investigated in this study. Consistent relationships between mobile P reduction and Al-P formation were detected across a broad range of mobile sediment P contents (0.04-2.8 g P m(-2) cm(-1) or 0.083-2.8 mg P g(-1)DW) and lake types. The conversion of mobile P to Al-P was dependent on the initial mobile sediment P content and the amount of Al added to the sediment. An empirical model was then developed to predict the formation of Al-P based on the amount of Al added relative to the initial mass of mobile P in the sediment. The results were compared to sediment collected from an Al treated lake and good agreement was found between the model and in-situ changes to sediment P fractions caused by Al treatment. The model developed in this study, unlike previous models with extreme, singular endpoints, allows for a continuum of estimates for mobile P conversion to Al-P, along with efficiency of P binding by Al, as Al dose varies. Model results can be used in conjunction with mobile sediment P based predictions for internal P loading to calculate an Al dose required to meet internal phosphorus loading goals for lake management and restoration without the need for expensive, time consuming Al additions to sediment.

Variability in phosphorus binding by aluminum in alum treated lakes explained by lake morphology and aluminum dose

Sediment cores from six aluminum sulfate treated lakes in Minneapolis, MN were analyzed to determine the effectiveness of phosphorus (P) binding by aluminum (Al). Two of the study lakes are polymictic and the remaining four are dimictic. Above background concentrations of Al and Al-bound-P (P(Al)) were detected in all six lakes at varying sediment depths. In contrast to previous studies, however, the binding relationship between Al and P was not consistent between lakes and substantial variation was also detected within each sediment profile. Average lake sediment Al:P(Al) ratios ranged from 5.6 to 15 (molar) with higher ratios, or less efficient P binding, generally being detected in deep, dimictic lakes with high sediment Al content due to treatment. Multiple linear regression was used to explain the variability among average Al:P(Al) ratios detected in each core and a lake morphometry index (Al Depth Index, core collection depth divided by the square root of lake area) along with Al dose described most of the variation (92%). Even though P bound to the added Al appears to be permanently removed from the internal P cycle in each lake (as evidenced by burial with new sediment), the differences in binding efficiency may indicate lower P inactivation, on a per unit Al basis, when elevated amounts of Al are added to the sediment, especially in deeper areas of lakes where sediment focusing may cause elevated Al accumulation to occur.

Phosphorus inactivation by aluminum in Lakes Gårdsjön and Härsvatten sediment during the industrial acidification period in Sweden

Acidification of lakes exposed to acid deposition is generally accompanied by a severe decrease in production (oligotrophication). In this study, we examined sediment from Lakes Gardsjon and Harsvatten, Sweden, to determine whether sediment phosphorus (P) retention increased during the years corresponding to lake acidification. Sediment from both lakes had increases in aluminum (Al) in the upper 10 cm, and dating of Lake Gardsjon sediment revealed that the Al increase occurred from 1950 to 2001 in this lake. The increase in Al input caused an increase inAl-bound P (Al–P) formation and overall sediment total phosphorus retention during the same period. Lake Gardsjon received an additional 12.9 g·m–2 of Al, above preacidification background levels, that bound 1.1 g·m–2 of P and removed it from the in-lake P cycle from 1950 to 2001. A substantial portion (up to 76%) of the total external P load eventually was converted to Al–P and buried in the sediment over this period. The increase in sediment P burial due to increased formation of Al–P in systems similar to Lake Gardsjon may have detrimental effects on nutrient cycling, and as a result, on productivity within the lake, leading to acido-oligotrophication.

Effects of alum treatment on water quality and sediment in the Minneapolis Chain of Lakes, Minnesota, USA

Abstract The effects of aluminum sulfate (alum) treatment on water quality in 4 lakes of the Minneapolis Chain of Lakes (MN, USA) were examined. Lakes Harriet and Calhoun (treated in 2001) and Cedar Lake and Lake of the Isles (treated in 1996) all showed initial water quality improvement based on surface water total phosphorus (TP), chlorophyll a (Chl-a), and Secchi disk depth. Three lakes (Harriet, Calhoun, and Cedar) were at or below historical estimates of growing season average TP after treatment and showed continued improvement in surface water quality through 2005. Lake of the Isles, which received the lowest alum dose, returned to pretreatment conditions after 6 years. Estimates of sediment phosphorus (P) release rates, however, indicated that alum treatment still limited internal P release in all 4 lakes. Although the alum application to Lake Harriet was a littoral-only treatment, water quality improved in this lake as well. The aluminum hydroxide floc drifted to the deeper part of the lake, reducing internal P release from deeper sediments by 85% in the 2 years following treatment, leading to unexpected improvements in surface water Chl-a and TP concentrations.

Persistent and widespread long-term phosphorus declines in Boreal lakes in Sweden

We present an analysis of long-term (1988-2013; 26years) total phosphorus (TP) concentration trends in 81 Swedish boreal lakes subject to minimal anthropogenic disturbance. Near universal increases in dissolved organic carbon (DOC) concentrations and a widespread but hitherto unexplained decline in TP were observed. Over 50% of the lakes (n=42) had significant declining TP trends over the past quarter century (Sens slope=2.5%y-1). These declines were linked to catchment processes related to changes in climate, recovery from acidification, and catchment soil properties, but were unrelated to trends in P deposition. Increasing DOC concentrations appear to be masking in-lake TP declines. When the effect of increasing DOC was removed, the small number of positive TP trends (N=5) turned negative and the average decline in TP increased to 3.9%y-1. The greatest relative TP declines occurred in already nutrient poor, oligotrophic systems and TP concentrations have reached the analytical detection limit (1μgL-1) in some lakes. In addition, ongoing oligotrophication may be exacerbated by increased reliance on renewable energy from forest biomass and hydropower. It is a cause of significant concern that potential impairments to lake ecosystem functioning associated with oligotrophication are not well handled by a management paradigm focused exclusively on the negative consequences of increasing phosphorus concentrations.

Aluminum application to restore water quality in eutrophic lakes: maximizing binding efficiency between aluminum and phosphorus

ABSTRACT Huser BJ. 2017. Aluminum application to restore water quality in eutrophic lakes: maximizing binding efficiency between aluminum and phosphorus. Lake Reserv Manage. 33:143–151. Aluminum (Al), typically added to lakes to reduce internal cycling of legacy phosphorus (P) in sediment, was added to the littoral zone of Lake Harriet (Minnesota) to inhibit the uptake of sediment P by buoyancy regulating species of plankton. Analytical results from sediment collected over an 11-year period showed that the added Al (32 g/m2) started to move outside the treatment zone almost immediately after treatment, with <5% remaining within the treatment area after 6 months. Although the original treatment design failed, the application method was, unexpectedly, a success with respect to binding efficiency between Al and P in the sediment. As the Al drifted to deeper areas of the lake, internal P release in non-treated, deeper areas of the lake declined and the binding ratio between Al and Al-bound P decreased, reaching 2.1 (molar) in profundal sediments 10 years after treatment. The increased contact with available (mobile) sediment P increased binding efficiency, resulting in a 163–581% increase of P bound per unit Al compared to previous whole-lake aluminum treatments. The binding efficiency exceeded expectations showing that, in addition to the amount of Al added, treatment location and subsequent translocation of the Al floc can substantially affect binding efficiency and treatment effectiveness.

Ecological instability in lakes: a predictable condition?

Bryan M. Spears,*,† Laurence Carvalho,† Martyn N. Futter,‡ Linda May,† Stephen J. Thackeray, Rita Adrian, David G. Angeler,‡ Sarah J. Burthe,† Tom A. Davidson, Francis Daunt,† Alena S. Gsell, Dag O. Hessen, Heather Moorhouse, Brian Huser,‡ Stephen C. Ives,† Annette B. G. Janssen, Eleanor B. Mackay, Martin Søndergaard, and Erik Jeppesen †Centre for Ecology & Hydrology, Penicuik, Midlothian, EH26 0QB, U.K. ‡Swedish University of Agricultural Sciences, Department of Aquatic Sciences and Assessment, Box 7050, 750 07 Uppsala, Sweden Lake Ecosystems Group, Centre for Ecology & Hydrology, Lancaster Environment Centre, Library Avenue, Bailrigg, Lancaster, LA1 4AP, U.K. Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Müggelseedamm 301, D-12587 Berlin, Germany Department of Bioscience Lake Ecology, Aarhus University, Vejlsøvej 25, Building B2.22, 8600 Silkeborg, Denmark Department of Aquatic Ecology, NIOO-KNAW, Droevendaalsesteeg 10, 6708 PB, Wageningen, The Netherlands University of Oslo, Department of Biosciences, Box 1066 Blindern, 0316 Oslo, Norway Centre for Ecology & Hydrology, Maclean Building, Benson Lane, Crowmarsh Gifford, Wallingford, Oxfordshire, OX10 8BB, U.K.