Submerged aquatic vegetation, or SAV, is an essential component of estuarine ecosystems. The Saint John River and the Indian River Lagoon just to the south are examples of what can happen when the delicate balance of environmental factors is affected.
The most common after-sales service in this area is Vallisneria americana, commonly called eelgrass but also called tadpole or water celery. But it doesn’t look anything like celery to me. V. americana is a freshwater species that can tolerate some salt. Fresh water being salt free and full strength seawater being 35 parts per thousand, duct tape can live in salinities ranging from its preferred fresh water to around 18 parts per thousand for short periods of time.
The exact limit of salt tolerance is unclear and usually depends on the duration and intensity of the plants’ exposure to salt water as well as other environmental factors.
Like any plant, submerged aquatic vegetation needs sunlight, lots of sunlight, to photosynthesize and thrive. And with the dark tannin-rich waters of the St. Johns, that means plants can only grow in shallow water. Below about 3 feet, there is insufficient sunlight to support plant life. Anything that decreases the amount of light, such as sediment runoff that clouds the water and increases turbidity, also reduces the amount of plant life.
Excess nutrients entering the water can trigger algal blooms which further reduce the amount of sunlight available to vegetation. Sometimes these algal blooms can themselves be toxic. This is currently happening in Tampa Bay. The spill of thousands of gallons of phosphate-laden water during the Piney Point incident in March resulted in a devastating algal bloom and fish deaths in the Tampa / Sarasota area today.
Something similar to Piney Point has happened in the Indian River Lagoon in recent years. Water rich in nutrients gradually poured into the lagoon. This increases the amount of algae in the water and decreases the sunlight available to aquatic vegetation, resulting in a net loss of SAV to support animals like our beloved charismatic megafauna, the manatee.
In recent decades, manatees have returned from their extinction path to one where their numbers were steadily increasing. There had been concerns about having enough food for the growing manatee population, but until recently this had not been a major concern. However, the evidence seems to indicate that manatees die famine, especially in the Indian River Lagoon.
Submerged aquatic vegetation has been significantly reduced in the lower Saint John River basin as salinity has gradually increased over the past decades. Hurricane Irma also had a dramatic impact on the amount of grass in the river.
The good news is that vegetation is making a gradual comeback. A loose group of volunteers monitor progress. There is therefore still room to hope that part of the after-sales service will return. And while the loss of SAV has not been identified as a current threat to manatees in the Saint John River, the possibility exists.
Glad you asked River Life
What are some of your favorite memories from your career as a marine scientist?
Thanks for asking. I had to take a break and think about everything I had done in my career. At the top of the list is the number of incredibly fascinating people I have had the pleasure of meeting and learning from their knowledge and understanding. I’ve been reminded over and over again how kind and generous scientists are with sharing what they know.
Second, the number of places I have been able to travel and experience, from diving 1,500 feet deep in the Sea-Link submersible in the Bahamas, to the many trips to the Galapagos, China, Japan, Australia and diving in the Great Barrier Reef. . Marine science has given me countless opportunities to explore and learn about the world’s oceans. I have been truly blessed throughout my career.
River Life takes place on the first Tuesday of every month in The Times-Union. Email Quinton White, Executive Director of the University of Jacksonville’s Marine Science Research Institute, with questions about our waterways at [email protected] To learn more about the MSRI, visit ju.edu/msri.