Redefining algal bloom management pathways in North Carolina

Algal blooms are a common occurrence in North Carolina lakes and reservoirs, and recent data suggests they are occurring more frequently. Due to environmental, health, and economic impacts of these occurrences, the North Carolina Department of Environmental Quality (NCDEQ) is invested in understanding the trends in drivers of harmful algal blooms. This project analyzes Ambient Lake Monitoring Data sourced from the Division of Water Resources within NCDEQ to assess potential strategies for both identifying harmful algal blooms and managing the overall occurrences of algal blooms for North Carolina lakes and reservoirs. Our objectives were to: 1) define reference conditions in lakes and reservoirs in North Carolina, 2) predict waterbodies most at risk of algal blooms based on physical and spatial factors, 3) analyze and make recommendations for phytoplankton thresholds, 4) determine temporal trends in blooms, and 5) compare North Carolina water quality criteria the with nationwide policy landscape.

Reference conditions were estimated by taking the median value of monthly 25th percentiles from the most sampled months (May – September) for chemical and biological data. Relative to other ecoregions, the Piedmont’s reference values were consistently high for chemical and biological data; further, the reference conditions for phytoplankton metrics in the Piedmont often exceed current bloom and dominance criteria set by NCDEQ, suggesting it is typical for the Piedmont to experience algal blooms even under the best circumstances. Principal component and redundancy analyses were also performed to assess the relationships among phytoplankton and environmental variables. Results show cyanobacteria vary differently than most other algal groups and are highly associated with chlorophyll-a and nutrients.

In order to determine the physical and spatial drivers that make waterbodies vulnerable to algal blooms, we created linear models to predict algal bloom indicators from physical and land use characteristics. Our results suggest that the most vulnerable systems are small reservoirs with dense stream networks that are surrounded by land uses producing high nutrient loads. These findings highlight potential avenues for improved bloom management, such as updating inconsistent bloom criteria and increasing monitoring of at-risk waterbodies. Future studies may incorporate additional physical and spatial characteristics such as precipitation, finer resolution watershed data, and more detailed data about reservoirs including residence time, reservoir order, and reservoir age.

Bloom criteria were assessed by calculating the percent of samples which would be classified as potentially harmful algal blooms based on NCDEQ criteria. Unit density and biovolume metrics were found to classify samples across the state inconsistently, with 53.4% and 14.6% in exceedance, respectively. Additionally, sample data were compared to World Health Organization (WHO) guidelines for safe recreational waters using the metric of cyanobacteria cell density. Based on WHO guidelines, 78.3% of samples would be considered at least a moderate risk to human health and 54.6% would be considered a high risk. These results suggest NCDEQ may benefit from updating current bloom criteria to classify samples more consistently; furthermore, the inclusion of cyanobacteria cell density as a metric may allow NCDEQ to have assessments more comparable to international guidelines.

Algal temporal trends were analyzed by running a mixed model with year as a fixed effect and waterbody as a random effect to account for temporal inconsistency in monitoring efforts. Chlorophyll-a was found to increase by 1.4 µg/L per year in May and decrease by 1.3 µg/L per year in September. Cyanobacterial relative abundance was found to increase by 1% per year in May and August, and 2% per year in June and September. Our results indicate a seasonal shift in algal blooms, occurring earlier in the year, and an increase in cyanobacteria dominance, which could lead to an increase in toxins being produced in waterbodies.

Administrative codes for 47 states were assessed for nutrient criteria standards and associated sub-sectioning trends (site-specific, statewide, ecoregional, hydromorphic, designated use) and water quality parameters (chlorophyll-a, total nitrogen, total phosphorus, clarity). Nationally, it is common for states to subsection nutrient criteria in multiple ways to apply criteria to narrowed conditions. Most states have multiple water quality parameters used within criteria. These trends were further broken down by North Carolina ecoregions to find the ranges in nutrient criteria values used in bordering states that share ecoregions in common with North Carolina. North Carolina’s existing nutrient criteria for chlorophyll-a fall approximately midway within these ecoregional ranges, and as NCDEQ moves forward with revising nutrient criteria for lakes and reservoirs, these ecoregional ranges can be used as a reference. NCDEQ is revising nutrient criteria for lakes and reservoirs at a site-specific scale presently, and while this may be the most efficient method of setting the criteria, other options of sub-sectioning could be useful including sub-sectioning based on ecoregion or physical characteristics. Further modeling of chlorophyll-a by ecoregion or physical attribute are needed to assess these options.

Based on our results, potential management improvements that NCDEQ could implement include changing water monitoring schedules, updating bloom criteria, and prioritizing lake monitoring based on certain physical and spatial characteristics. Additionally, NCDEQ may benefit from further investigating what drivers contribute to increasing algal blooms and cyanobacteria dominance, and how those drivers concurrently change with a changing climate.