Regime shift theory holds that shallow lakes are widely susceptible to bistability, whereby nutrient fluctuations or other external drivers can lead to sudden transitions between clear-water macrophyte dominance and turbid phytoplankton dominance. Each of these regimes can be stabilized by self-reinforcing feedback mechanisms, making lake restoration efforts challenging. Such feedback mechanisms often include the functional role of benthic primary producers (submerged aquatic vegetation and/or benthic algae) which are capable of reducing both biotic and abiotic turbidity in the water column. We here focus on a large (385 km²), shallow (Zmax = 4.3 m), and turbid (mean ZSecchi = ~0.25 m) lake, Utah Lake (UT), for which there are historical records of clear-water indicator macrophyte species presence. Although the lake is currently eutrophic (mean annual total phosphorus = 80 μg/L) and features recurrent harmful algal blooms, its water clarity is heavily influenced by wind-driven sediment resuspension and bioturbation from invasive carp, raising concerns that nutrient reductions may only minimally increase water clarity. In order to inform future management and restoration strategies for this lake, we modeled light requirements for benthic primary production, using monitoring data to provide realistic lake-specific restoration targets.
Results/Conclusions
We determined that a clear-water stable state dominated by benthic primary production would require a water clarity increase of at least 0.9 m (considering 2018 water levels and clarity). However, an analysis of monitoring data indicated that such an increase is not attainable via reductions in chlorophyll a concentrations alone, given wind-driven sediment resuspension. Performing the same analysis on shallow embayments, we found that a water clarity increase of just 0.3 m would provide sufficient light to establish localized self-reinforcing clear-water states. Monitoring data indicated that this increase in water clarity could be attained by reducing water column chlorophyll a concentrations to ~18 μg L-1 (from 2018 mean lake-wide values, ~40 μg L-1). Joining limnological theory and monitoring data, our analysis indicates that external nutrient loading reductions may be effective at establishing self-reinforcing clear-water conditions in the shallow bays of Utah Lake, but that additional remediation may be required for lake-wide macrophyte restoration.