CHNEP breaks down what macroalgae is all about

July 30, 2021
By Staff Report

A three-day workshop was recently held locally to discuss a growing problem in Florida, excessive macroalgae. The workshop focused on the estuaries of the four National Estuary Program entities in Florida (including the Coastal & Heartland National Estuary Partnership, Indian River Lagoon National Estuary Program, Sarasota Bay Estuary Program, and Tampa Bay Estuary Program). 

Over three days, participants learned about what macroalgae is, where it is becoming overabundant, what contributes to it occurring, and what its impact is on aquatic life. There was also discussion on gaps in scientific knowledge and what research is needed to fill those gaps. Macroalgae are easily seen aquatic plants, unlike smaller organisms which appear to just tint the water like red tide or cyanobacteria blue-green algae.

Here are three main groups: red (ex. Gracilaria), green (ex. Caulerpa – pictured above, Ulva), and brown (ex. Sargassum). When macroalgae becomes overly abundant, it can negatively affect fisheries, recreation, and important aquatic habitats.

Many aspects about the macroalgae blooms occurring in our region have not been well studied. What is known is that excessive nutrients can increase macroalgae growth, which can then reduce the available light needed for seagrass.

Recently, the eastern portion of Charlotte Harbor experienced a significant seagrass loss of approximately 50 percent, in areas where macroalgae have been observed to be proliferating.

Unfortunately, there are similar occurrences all across Florida. For example, in the last seven years, Sarasota Bay has seen higher nitrogen levels than the previous 15 years, which has resulted in seagrass declines and macroalgae increases. By sharing such information at this workshop, it was clear to participants that this is becoming a serious statewide issue.

When water quality declines to the point that it is no longer safe for its intended use, typically a total maximum daily load, or TMDL, pollution limit

is established to lower pollution levels. Many TMDLs in Florida have been established to control nutrient pollution, a contributing factor to excessive

macroalgae growth. In the Indian River Lagoon and Kings Bay, the nutrient TMDLs are being met but macroalgae blooms are still occurring. Workshop participants discussed whether those limits may need to be adjusted to further reduce nutients to lessen excessive macroalgae growth.

The macroalgae can also uptake nutrient pollution from the water column, reducing measurable amounts in samples, leading to better water quality readings. However, those nutrients are re-released when the algae later dies off, leaving that nutrient pollution still in the system to cause future problems.

Macroalgae can also affect the whole aquatic food chain, both through what feeds on it directly and how it affects those that feed on what it is displacing – seagrass. Research has shown that macroalgae, especially those in tropical and reef areas, can produce toxins that can act as an herbivore deterrence. Aquatic herbivore animals such as manatees are left without enough of their traditional food source, seagrass. Meanwhile, there is anecdotal evidence that after a macroalgae bloom, there is an explosion of “sea hares” or sea slugs (pictured below) that feed upon the algae. The change in aquatic plants changes the animals in that system as well.

The CHNEP is working to support macroalgae monitoring efforts occurring within the CHNEP area. Eyes on Seagrass is a University of Florida – IFAS citizen science project comprised of volunteers conducting surveys in April and July. The program focuses on gathering seagrass and macroalgae data, to determine the amount and types that are present.

CHNEP interns are assisting in sample collection, and the CHNEP offered a training event for those interested in volunteering.

Additionally, the Southwest Florida Water Management District has conducted aerial seagrass mapping since 1988. They have been able to see the successional variation of rooted macroalgae from the aerial imagery and have documented patterns.

From the limited data collected to date, it appears that the macroalgae is most abundant in many Gulf estuaries in the summertime, whereas in Charlotte Harbor and Pine Island Sound, it is in the winter.

The CHNEP is in the process of collecting all publicly accessible harmful algae data to create an interactive mapper that will allow natural resource managers and the public to view all of that data with other information, such as seagrass and water quality data.

By putting it all together, we hope to learn more about related interactions. Although these monitoring efforts and a few others are occurring, added macroalgae monitoring is needed to address filling knowledge gaps in understanding the scope and implications of this growing problem.

Conditions are likely to become even more favorable to macroalgae growth with climate change. Increasing water temperatures and more potential

runoff and spills from more intense storms and heavy rainfall events could wash even more nutrient pollution into waterways.

While macroalgae has been around for millions of years, its widespread overabundance is relatively recent and there are improvements to water management policies and infrastructure that can reduce some of the contributing factors to its excessive growth.