Allen's Blog

October 30, 2019

Harvesting Hydrilla in Florida Lakes

A Long-Term Strategy for Large-Scale Removal of Legacy Phosphorus

The Blue-Green Algae Task Force (TASK FORCE) was created in 2019 by Florida's Governor Ron DeSantis in response to increased public concern over recent explosive blooms of Cyanobacteria, also known as blue-green algae, within Lake Okeechobee, the Caloosahatchee River, and the St. Lucie River. And these are not the only water resources impacted by such blooms. Lake Apopka, a 33,000 acre lake in Central Florida, and the headwaters of the Oklawaha River, has been in a continuous Cyanobacteria bloom for over fifty years, and flow from the lake has degraded water quality within downstream lakes and rivers including Lake Dora, Lake Eustis, Lake Griffin, and ultimately the St. Johns River and its associated lakes. In addition, Florida's springs have also suffered from explosion of the Cyanobacteria species Lyngbya sp. which has displaced much of the native eel grass lawns and their associated  ecology.  

In the  TASK FORCE Consensus Report of October 11, 2019 it is clearly noted that the intent of the TASK FORCE  is to serve as an "advisory body------to aid the Department of Environmental Protection in fulfilling its mission to protect, conserve and manage the state’s natural resources and enforce its environmental laws. The Task Force, through its discussion and deliberations, provides guidance and specific, science-based recommendations with the goal of expediting improvements and restoration of Florida’s water bodies that have been adversely affected by blue-green algae blooms."

(left) Blue Green Algae bloom in South Florida 2016     

(center) Typical healthy lawn of eel grass in a Florida Spring         

(right) Invasion of an eel grass lawn by the Blue-Green Algae Lyngbya (now Microseira) wollei in a Florida Spring

photos by UF/IFAS Center of Aquatic and Invasive Plants

UNDERSTANDING LEGACY NUTRIENTS

Included in the Task Force Consensus report is a comment related to the need to "expedite legacy nutrient removal." 

"Legacy nutrients, as indicated previously, are a concern in the South Florida landscape, and the task force recommends that their contribution to loading figure prominently in the Lake Okeechobee, Caloosahatchee and St. Lucie River and Estuary BMAPs (Basin Management Action Plan). The task (force) further recommends that projects with the demonstrated potential to expedite legacy nutrient removal merit special attention and be designated as priority projects."

The entire consensus report may be seen by clicking the button provided below this text.

In a recent PASOP blog on the PAY-FOR-PERFORMANCE strategy (see button above), legacy nutrients were identified as excess nutrient loads which remain in the sediments of a drainage basin (including lake sediments), and which can become available under the right environmental conditions. The TASK FORCE identified legacy nutrients as "nitrogen and phosphorus sequestered in soils, groundwater and sediments, (which can) contribute also to excessive nutrient loading of surface waters throughout the state." While the scientific community has been aware for decades of legacy nutrients and the problems they present, it was the recent Cyanobacteria blooms that forced serious Institutional and Political consideration of their potential deleterious impact. In a 2015 report by the University of Florida Water Institute [1] it was noted that over 110,000 metric tons of biologically available legacy phosphorus is held within the sediments of the Lake Okeechobee Basin. If  this legacy phosphorus were to be effectively removed, about 2,000 tons would need to be removed each year for fifty years. Like any challenge this  may be seen by some as a insurmountable, while others might see it as a great opportunity. I prefer the latter perspective. So how can we make chasing this legacy phosphorus an opportunity without sounding like someone who believes in Unicorns and magic dust?  

(a)

(b)

(c)

(d)

(a) Harvesting and loading transport barge--Hydrilla West Lake Tohopekaliga Florida 2019 (photo by Texas Aquatic Harvesting 2019) 

(b) Barge Transport of Hydrilla to Land Base Depository (photo by Texas Aquatic Harvesting 2019)   

(c) Chopping of Aquatic Plants (Water Hyacinth) (photo by HydroMentia, Inc.)   

(d) Windrow Composting of harvested Aquatic Plants (photo by HydroMentia, Inc.) 

So let's apply some critical thinking to this challenge. The exotic and invasive aquatic plant known as Hydrilla is a submerged plant that for a number of reasons has the ability to outpace native submerged plants such as Southern Naiad and Coontail. In fact its growth is so explosive that it can rather quickly cover the surface of a water body ("topped out"),  as noted in the picture below. When Hydrilla tops out it renders the surface impassable to most boat traffic, and can deleteriously impact water quality by impeding oxygen exchange with the atmosphere, and by sloghing large quantities of necrotic tissue.  None of this is desirable, and it is not difficult to understand why there is a desire to eliminate Hydrilla, or at least control it so that it does not "top out" as shown in ths picture. 

Hydrilla "Topped out" on Florida's Withlacoochee River.