Resources for Mitigating HABs

Introduction to Selecting a Mitigation Strategy

algae bloom

Numerous methods for managing and mitigating algae and CyanoHAB blooms in lakes and reservoirs are included in the following flowchart. small version of the flowchart image The flowchart highlights important elements and decision points leading to the selection of mitigation methods relevant to the characteristics of specific lakes. The flowchart is a simplification but clearly illustrates the complexity of the problems associated with making effective mitigation decisions. Frequently, multiple mitigation techniques and repeated applications are required to ensure and maintain success.

The flowchart is divided into three primary categories:

  1. Bloom prevention,
  2. Bloom reduction and prevention of future blooms, and
  3. Bloom reduction with no prevention of future blooms.

Cautions for Chemical Use

small version of the Algaecide  flowchart Flowchart category (3) typically relies on the application of algaecides to quickly stop or reduce a specific nuisance bloom. Permits are required for algaecide use – see presentation and weed control permit webpage. Algaecides are sometimes critically needed in a timely fashion to control specific problem blooms, e.g., in drinking water supplies. Algaecides do nothing to address root causes or prevent future CyanoHAB events and if improperly used can seriously affect non-target species and food web, as illustrated on the following flowchart.

A particularly concerning impact to non-target species of algaecides applied to a massive bloom is the potential for rapid die-off of the bloom to cause depletion of dissolved oxygen and consequently a massive fish kill. Whether a fish die-off occurs is related to the size of the algal bloom, the rapidity of the algal bloom decline, the depth and volume of water in the lake, and other factors. Chemical treatments can be very effective at controlling the size of algal blooms and should be carried out at an early stage in the bloom—before algal biomass reaches a critical level—to avoid rapid oxygen depletion and fish kills in treated lakes. (Fish kills can also happen due to natural die-off of blooms that cause rapid oxygen depletion.) This requires an appropriate and routine monitoring program (see Resources section below) that can detect a bloom early in its development. It also requires a system in place and ready to implement to respond to the detection.

Algaecides may also result in the release of toxins contained in cells into the water, making them more problematic to remove during drinking water treatment, and having potential adverse ecological impacts.


Moreover, the release of toxins may also impact cultural uses. Tribes and Tribal communities who engage in traditional water uses such as gathering, ceremonies, navigation, subsistence consumption and other intrinsic uses of waters within their ancestral territories are often bound to both location and timeframe for these activities, as practiced by their families and ancestors before them.

Prior to any algaecide treatments on a waterbody, waterbody managers should engage local Tribes and their Regional Water Board to determine the best course of action to protect Tribal cultural uses and mitigate for any increased lysing of cyanobacteria cells that would increase exposure to cyanotoxins during Tribal cultural uses. Herbicide and algaecide treatments should always avoid locations and times that could create impacts on Tribal cultural uses and users.

Flowchart categories (1) and (2) may rely on the application of other chemicals. For example, aluminum sulfate (alum) is used to coagulate and flocculate phosphorus to sequester it in bottom sediments. If lake water buffering capacity is insufficient, then chemical buffering agents must be used to avoid drops in pH that can be toxic to fish.

urban waterbody

Addressing the Root Cause

Flowchart categories (1) and (2) are more focused on addressing the root causes of the blooms through nutrient management using a wide variety of approaches. Both categories have many management options in common. The primary driver of CyanoHAB blooms are nutrients, with phosphorus being widely recognized as the most important nutrient, but certainly not the only one.

In-depth Understanding of Nutrient Sources and Dynamics

It is critical to understand the sources and dynamics of nutrients in problem lakes prior to selecting mitigation strategies, see Nutrients Dynamics Document. Selection of mitigation measures should be based on at least one year of monitoring data and a basic understanding of the characteristics unique to your lake, including its basic hydrology. Although the same processes are at work in most lakes and reservoirs, the relative importance of individual processes are dependent upon many factors including lake and watershed morphometry, land use, hydrodynamics, nutrient sources, and many other factors. Each lake is unique with its own “personality” and history. The task is to understand it sufficiently to make informed management decisions.

Resources for Lake Managers and the General Public

The following resources are organized to systematically direct lake managers and the general public to information useful for understanding applicable limnology principles, management techniques, mitigation strategies, permitting requirements, and to pertinent professional societies.

Mitigation Subcommittee Resources