Abstract
Lanthanum (La) modified bentonite is being increasingly used as a geo-engineering tool for the control of phosphorus (P) release from lake bed sediments to overlying waters. However, little is known about its effectiveness in controlling P across a wide range of lake conditions or of its potential to promote rapid ecological recovery. We combined data from 18 treated lakes to examine the lake population responses in the 24 months following La-bentonite application (range of La-bentonite loads: 1.4–6.7 tonnes ha−1) in concentrations of surface water total phosphorus (TP; data available from 15 lakes), soluble reactive phosphorus (SRP; 14 lakes), and chlorophyll a (15 lakes), and in Secchi disk depths (15 lakes), aquatic macrophyte species numbers (6 lakes) and aquatic macrophyte maximum colonisation depths (4 lakes) across the treated lakes. Data availability varied across the lakes and variables, and in general monitoring was more frequent closer to the application dates. Median annual TP concentrations decreased significantly across the lakes, following the La-bentonite applications (from 0.08 mg L−1 in the 24 months pre-application to 0.03 mg L−1 in the 24 months post-application), particularly in autumn (0.08 mg L−1 to 0.03 mg L−1) and winter (0.08 mg L−1 to 0.02 mg L−1). Significant decreases in SRP concentrations over annual (0.019 mg L−1 to 0.005 mg L−1), summer (0.018 mg L−1 to 0.004 mg L−1), autumn (0.019 mg L−1 to 0.005 mg L−1) and winter (0.033 mg L−1 to 0.005 mg L−1) periods were also reported. P concentrations following La-bentonite application varied across the lakes and were correlated positively with dissolved organic carbon concentrations. Relatively weak, but significant responses were reported for summer chlorophyll a concentrations and Secchi disk depths following La-bentonite applications, the 75th percentile values decreasing from 119 μg L−1 to 74 μg L−1 and increasing from 398 cm to 506 cm, respectively. Aquatic macrophyte species numbers and maximum colonisation depths increased following La-bentonite application from a median of 5.5 species to 7.0 species and a median of 1.8 m to 2.5 m, respectively. The aquatic macrophyte responses varied significantly between lakes. La-bentonite application resulted in a general improvement in water quality leading to an improvement in the aquatic macrophyte community within 24 months. However, because, the responses were highly site-specific, we stress the need for comprehensive pre- and post-application assessments of processes driving ecological structure and function in candidate lakes to inform future use of this and similar products.
Highlights
Nutrient (i.e. mainly phosphorus (P) and nitrogen (N)) pollution has resulted in widespread degradation of ecological structure and function of freshwater lakes at a global scale (Smith, 2003)
Limited and water clarity constrained by phytoplankton we would have expected to see a strong correlation between total phosphorus (TP) and chlorophyll a concentration, and a negative correlation between these two variables with Secchi disk depth, prior to application
Chlorophyll a concentrations (n 1⁄4 15 lakes) decreased and Secchi disk depth (n 1⁄4 15 lakes) increased and these responses were most pronounced in summer
Summary
Nutrient (i.e. mainly phosphorus (P) and nitrogen (N)) pollution has resulted in widespread degradation of ecological structure and function of freshwater lakes at a global scale (Smith, 2003). To address this issue, environmental policies have been implemented to reduce nutrient loads to lakes. After P loading from the catchment is reduced, lake recovery can take several decades (Jeppesen et al, 2005; Sharpley et al, 2013) This is because P, accumulated in lake bed sediments when catchment inputs were high, continues to be released during the recovery process (“internal loading”), maintaining poor water quality conditions (Søndergaard et al, 2012; Spears et al, 2012). The effective control of internal loading may accelerate ecological recovery once external inputs have been reduced (Mehner et al, 2008)
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