Lake O Releases Impact Seagrass Health
Seagrasses are ecologically important and sensitive to changes in salinity, water clarity, and water quality, making them an excellent indicator species for evaluating environmental conditions on short and long timescales. Changes in seagrass growth, abundance, species composition, and coverage can help us monitor changes in water quality. Seagrass requires abundant sunlight and clean water to thrive, and it is a vital part of the ecosystem. Seagrass — encompassing a variety of submerged plants stabilizes sediment, improves water quality, provides critical habitat for several organisms, and acts as a nursery for juvenile species of recreational and commercial fishes. Seagrasses are a vital part of the food web, consumed by manatees, sea turtles, birds, gastropods, and fish.
There are several species of seagrass in the Caloosahatchee watershed with varying salinity tolerances. The start of the estuary (the W.P. Franklin Lock and Dam, known as S-79) with the lowest salinity is historically home to the freshwater plant known as tape grass (Vallisneria americana) which cannot tolerate salinities above 15, with declining growth starting at salinities greater than around 3 to 9. The salinity tolerances of seagrasses vary between species. Of the several species in Pine Island Sound, San Carlos Bay, and Matlacha Pass — turtle grass (Thalassia testudinum) — is most sensitive to the effects of low salinity, while shoal grass (Halodule wrightii) can tolerate lower and more variable salinities.
The management of freshwater releases from Lake Okeechobee plays a critical role in establishing a heathy salinity gradient from freshwater at the head of the Caloosahatchee estuary (S-79) to marine water in the Gulf of Mexico. This gradient promotes a greater diversity and abundance of seagrass, such as shoal and turtle grass, as well as submerged aquatic vegetation like tape grass.
One of the most important metrics used by stakeholders, policy makers, and water managers is the RECOVER (REstoration COordination & VERification) salinity envelope which recommends an optimal 14-day average flow from S-79 between 750–2,100 cubic feet per second (cfs). This flow rate yields optimal salinity for the three indicator species chosen in the study: tape grass, shoal grass, and oysters. Based on the models in RECOVER, when flow is in the optimal range, the salinity in the upper portions of the estuary is good for tape grass (<10), and in the lower part of the estuary (near the mouth of the Caloosahatchee), salinities are in a good range for shoal grass (15 – 45).
The RECOVER flow targets are based solely on salinity ranges, but other environmental factors affect the growth of seagrass as well. Low water clarity can be caused by a combination of turbidity from wind and wave action, chlorophyll from phytoplankton, and color from organic substances, such as tannins. High flows from the lake and the watershed are rich in nutrients which can promote the growth of phytoplankton. High flows from the Caloosahatchee watershed are high in tannins, making the water brown as demonstrated in the image taken from Lighthouse Beach Park on Nov. 22, the day after intense rainfall and above optimal 14-day average flows with 50 percent of the flow for the week coming from watershed. On Dec. 1, about 35 percent of the watershed’s flow was coming through S-79. The decline in precipitation and reduction of flows from the watershed resulted in flows in the RECOVER optimal range for salinity and reduced tannins, resulting in clear water that increases sunlight availability for seagrass.