The scientists couldn’t see their feet in the meter-deep water of the Caloosahatchee River last week — tannins from freshwater runoff had turned the river a deep reddish brown.
But Mark Thompson and RIck Bartleson of the Sanibel-Captiva Conservation Foundation Marine Laboratory had a job to do: Go underwater with masks and snorkels and document the density of shoal grass (often known by its genus name, Halodule) on the river bottom near Iona.
In a study dating back to 2004, marine lab researchers are looking at the effects of high freshwater flows on the river’s seagrass species (shoal grass, turtle grass and manatee grass).
During the dry season, little rain falls, so little fresh water is added to the flow of the Caloosahatchee; during the wet season, rain falling on the Caloosahatchee watershed between the mouth of the river and Lake Okeechobee runs into the river, lowering salinity the estuary.
As Okeechobee fills during the rainy season, water managers release fresh water down the river to prevent flooding in the communities surrounding the lake.
If freshwater runoff and releases make the river too fresh, seagrasses can die, and if high flows turn water dark, seagrasses can die from lack of sunlight.
“Our main point is, OK, we can have large freshwater discharges this time of year, so does that have a detrimental impact on seagrass habitat?” SCCF research assistant Thompson said. “We plot seagrass density in the early summer, then see where we end up after freshwater discharges. Are there long-term effects over the course of five or ten years due to freshwater releases?”
Seagrasses are important for many reasons, including:
- They provide shelter for many fish and invertebrate species.
- They are food for wildlife, including manatees, aquatic birds and sea turtles.
- They improve water quality by trapping sediments.
SCCF monitors seagrasses at six sites from Iona to Tarpon Bay and two control sites near Demere Key in Pine Island Sound that are not significantly influenced by the Caloosahatchee.
At each site, researchers run a 100-meter transect parallel to the shore and a 100-meter transect perpendicular to the shore.
They drop a 1-square-meter quadrat (four-sided frame) along each transect at five random spots and document seagrass inside the quadrats to determine density.
This summer, releases from Lake Okeechobee have been low, but heavy rains in September caused runoff that lowered salinity and turned the water dark.
“Today we’re finding moderate coverage and low density,” Thompson said. “The height of the grass looks good: Height is a metric for how healthy the plants are. We’ve probably lost a little density from when we were out earlier in the summer, but it doesn’t look dramatic like in 2013.”
In the summer of 2013, record wet-season rain resulted in large volumes of freshwater from runoff and Okeechobee releases flowing down the Caloosahatchee — flows at the W.P. Franklin Lock reached 10,000 cubic feet per second; when flows are greater than 2,800 cfs, salinity drops to the point of harming saltwater organisms, including seagrasses.
“At the upper two sites, Halodule, which is all that’s there, was pretty much wiped out,” Thompson said. “The density was next to zero. We found a few shoots, and they didn’t even look alive.”
At downstream sites, densities of turtle and manatee grass decreased, but shoal grass increased because it took over where the other species had disappeared.
Despite 2013 seagrass losses in the river, densities were back up at the beginning of this summer because freshwater flows the past two years have been low.
“That’s something we’re looking at: How resilient are these seagrasses?” Thompson said. “If we have a year like 2013 and then years like 2014 and 2015 after it, they’ll come back. But what if you have four years like 2014? We don’t know. We haven’t had that yet.”
Ultimately, SCCF seagrass data could help water managers make decisions about discharges from Lake Okeechobee, research scientist Rick Bartleson said.
“There’s a lot of variability in the data, even things like manatees eating the seagrass,” he said. “One year’s data doesn’t mean anything. We need a lot of data to filter out the noise.”
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