Dynamic Internal Controls on Eutrophication in Lake Elsinore; the Role of ‘Bottom-Up’ and ‘Top-Down’ Limitation on Eutrophication in a Managed Lake System
The Santa Ana Water Regional Water Quality Control Board (Regional Board) adopted TMDLs for nutrients and related impairments in Lake Elsinore and Canyon Lake in 2004. Soon after TMDL approval, responsible parties named in the Nutrient TMDLs began the process to form the Lake Elsinore/Canyon Lake Task Force (“Task Force”) to implement the TMDLs collaboratively where appropriate. In June 2015, the Task Force petitioned the Regional Board to reopen and revise the TMDLs based on the wealth of new information developed since TMDL adoption. A key element of the TMDL reconsideration was to account for long term dynamics effecting internal nutrient loads and eutrophication in Lake Elsinore.
In typical freshwater lakes, limitation of algal productivity is determined by the bioavailable nutrient pool within the photic zone. The predominant source of nutrients originating within lakes is from settling of internally (phytoplankton decay) or externally (mostly from eroded soils) loads of nutrient rich organic material, followed by processes of sediment diagenesis and diffusive flux back to the water column. This ‘bottom-up’ conceptual model does not account for reductions of algal concentrations by zooplankton grazing, a ‘top-down’ means by which aquatic ecosystems limit the magnitude and duration of algae blooms in typical freshwater lakes.
For Lake Elsinore, the Task Force hypothesized that grazing by zooplankton plays a significant role in controlling algal growth. Maintaining inflows of water to offset evaporative losses is difficult during extended droughts and with upstream retention of runoff within Canyon Lake in moderate hydrologic years. The impact of extended droughts that historically lead to lakebed desiccation is a complete reset of the aquatic ecosystem. Prior to desiccation, water quality is degraded by evapo-concentration of nutrients and other salts in the water column. As the lake volume slowly declines to zero, the concentrations of ammonia and TDS reach extremely high values that far exceed acute toxicity thresholds for aquatic organisms. Evapo-concentration of salts is a naturally occurring process that frequently exceeds thresholds for a healthy biological community. In many years. High TDS stressors have mostly eliminated populations of large bodied cladocerans such as Daphnia spp, which are much more efficient algae grazers than other more salt tolerant groupings, including copepods and rotifers. Without top-down control by this food web, hypereutrophic conditions are intensified. A low proportion of Daphnia within the zooplankton community was measured in 2015 and was consistent with findings by other from 2003-04 and 2009-10 when cladocerans comprised approximately < 0.6% of the community.
Water quality data compiled from monitoring programs over the past 15 years showed a statistically significant difference in algae, as measured by chlorophyll-a concentration, in years when the average total dissolved solids was less or greater than the Basin Plan objective of 2000 mg/L for Lake Elsinore. The TMDL revision also involves long term (99-year) lake water quality simulations that will assess these impacts for a reference watershed scenario involving pre-development levels of external nutrient loads to the lake.