Harmful Algal Blooms | Virginia DEQ

Algae are common single-celled or colonial organisms found in freshwater and marine habitats throughout the world. Like plants, they are photosynthetic, and form the base of aquatic food webs and produce a significant portion of earth’s atmospheric oxygen. Algae can occasionally form blooms, of which a small number may be harmful. An algal bloom occurs when environmental conditions (including light, temperature, and nutrient content) favor rapid growth of one or a few types of algae such that they discolor the water or mats and scums appear.

Of the 30,000 to 1 million plus species of algae estimated to exist, a small percentage have the potential to cause detrimental impacts to aquatic ecosystems due to their abundance, size, or production of toxins. These impacts range from benign to harmful: discolored water, biofouling, reduced light penetration and consequent shading of aquatic grasses and corals, strong odors, bad tasting drinking water, reduced dissolved oxygen, fish kills and animal mortalities, and in some cases production of biotoxins that can be of public health concern. Harmful algal blooms, or HABs, include algae that can be toxic to humans. In fresh waters, cyanobacteria (blue-green algae) are the primary algae of concern, because they can produce toxins that can be ingested. This may occur either through the consumption of contaminated drinking water or through accidental ingestion during swimming and other recreation.

Potentially harmful algae are a small subset of the total algal community, but can quickly become dominant in aquatic systems with ecological imbalances, such as when excessive nutrients become available or other human-based impacts have created a favorable environment for harmful algae dominance. HAB events pose a challenge and potential financial burden for those who rely on Virginia’s waterways for recreation, vacation rentals, fishing charters, seafood production, and other water tourism-related activities.

In part due to greater awareness of HABs and their potential impacts, the reporting and identification of potential HABs has increased in recent years. In 2021, the General Assembly directed DEQ to convene a workgroup, in conjunction with the Virginia Department of Health (VDH) and the Virginia Department of Agriculture and Consumer Services, to conduct research and report findings and recommendations related to: (i) the location, frequency, and severity of harmful algae blooms in Virginia waters; (ii) the factors that lead to the formation and occurrence of harmful algae blooms; and, (iii) plans and strategies for state agencies to lead or support appropriate mitigation efforts. The workgroup reported its findings in Sept. 2021.

In 2022, the General Assembly appropriated $3.5 million to DEQ to study HABs in Lake Anna and the Shenandoah River basin. These studies are being conducted with VDH, which has issued swimming advisories for Lake Anna and algal mat alerts for sections of the Shenandoah River basin. The studies funded by the General Assembly will focus on the environmental factors that cause these HABs and management approaches that could be employed to prevent and mitigate their impacts.

The Shenandoah River basin and Lake Anna are different and unique aquatic systems (free-flowing stream vs a constructed impoundment) and have different HABs that trigger advisories and alerts from VDH. DEQ intends to manage each of these HAB studies as separate and distinct projects while recognizing the need for consistency in the overall approaches and methodologies of the studies both for their current application as well as transferability of the information for future HAB events in Virginia.

Study goals

The key goals of the Lake Anna HAB study will be pursued in two phases, while the Shenandoah HAB study goals are identified in phase 1. The key goals of each phase are described below.

Phase I (Data collection, evaluation and source identification): 2 years

Gather necessary information to identify and understand the processes leading to HAB initiation, persistence and decline, to include the physical, chemical, biological, geomorphological, bathymetric, and meteorological conditions.
Identify primary sources of pollutant inputs contributing to the formation of HABs to include distinction between internal and external loadings to the system.
Identify technologies to help predict or provide early detection of a HAB event (remote sensing, in-situ monitoring).

Phase II (Identification of management strategies and practices): 1 year

Identify management approaches to prevent and/or mitigate HABs and associated cost estimates, considering short term and long term strategies and practices.
Develop watershed models, as appropriate, that can inform watershed management planning.
Engage with partners and stakeholders to identify past, existing, and potential future programs and practices that could inform watershed management planning.
Where applicable, develop an approach which satisfies [EPA and WQMIRA] requirements to address impaired waters, such as a watershed plan.
Engage with stakeholders and project partners to communicate progress and leverage efforts of other projects toward these goals.

Lake Anna

Lake Anna is a reservoir formed by the impoundment of the North Anna River in central Virginia to provide cooling water for the North Anna Power Station – a nuclear power plant operated by Dominion Power. The lake, and surrounding communities, are a popular vacation destination for recreation including boating, watersports, and fishing. Portions of Lake Anna, notably the riverine and portions of the transitional areas of the lake, have had consistent elevated cell counts of potential toxin-producing cyanobacteria species starting in the mid-summer each year since 2018. The cell counts have exceeded thresholds established by VDH, triggering the issuance of swimming advisories for portions of the lake. DEQ identified a 303(d) recreation use impairment for the upper portion of Lake Anna in the 2022 Integrated Report due to the HAB advisories.

Lake Anna

Map of Lake Anna. HABs have been reported in the portion of the lake shown in green. Publicly accessible waters are within the black border.

Shenandoah River basin

Filamentous algae attached to the stream bottom has been a concern in the Shenandoah River basin for over a decade. Concerns have largely centered on the negative impact that filamentous algal blooms have on stream aesthetics and recreational activities such as swimming, wading, kayaking, and fishing.

In July and August 2021, DEQ investigated multiple reports of potential HABs. Subsequent laboratory analysis indicated that algal toxins were present within algal mats in multiple locations along the North Fork Shenandoah River. While toxins and potentially harmful algal cells were also detected in the water at some sites, concentrations were at low levels and well below VDH advisory thresholds for recreational use. However, due to the presence of cyanobacteria in algal mats detected at sites along the river, VDH issued and subsequently extended a recreational advisory for the North Fork Shenandoah River to ensure the public was aware of the presence of these algal mats and to ensure that people, in addition to their pets and livestock, could take caution to avoid contact with visible algal mats and scum. The advisory covered approximately 52.5 miles of the North Fork Shenandoah River.

In 2022, in response to a recurrence of the benthic mats in the North Fork Shenandoah in the Town of Strasburg, VDH issued an algal mat alert for an 11.5-mile stretch of the river and cautioned the public to avoid algal mats observed along the shoreline or stream bottom.

Shen River

Map of the Shenandoah River basin. Stream segments highlighted in orange identify areas where VDH has issued alerts for potentially harmful algal mats.

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