Heavy rains causing problems on the Caloosahatchee River

A casual observer wouldn't notice, but runoff from recent rains has lowered salinity in

The Caloosahatchee is a tidal river; salt water moves upstream and downstream with the tide, and aquatic plants and animals have adapted to the changes in salinity.

But when the watershed gets too little rain, salinity upstream increases for extended periods of time, and freshwater organisms can be harmed; when the watershed gets a lot of rain, salinity drops, and salt-tolerant species downstream can be harmed.

Current low salinity levels are harming benthic (bottom-dwelling) organisms, said Rick Bartleson, a Sanibel-Captiva Conservation Foundation research scientist.

"The benthic critters in the lower part of the estuary are more attuned to salt water," he said. "They don't survive large pulses of fresh water. Shoal grass and turtle grass in the lower estuary are also a concern."

Bartleson has found dead Carolina marsh clams along the east riverbank south of the Midpoint Bridge; that localized die-off might be due to hypoxia (low oxygen levels), caused when fresh water, which is less dense than salt water, overlies salt water and prevents oxygen from getting to the bottom.

To protect the aging Herbert Hoover Dike, which surrounds Lake Okeechobee, water managers want to keep lake levels below 15.5 feet; when levels approach that threshold, the U.S. Army Corps of Engineers can release down the Caloosahatchee and St. Lucie rivers.

Large volumes of fresh water from runoff and releases can carry excess nutrients downstream, triggering algal blooms that smother seagrasses and cause fish kills. At this point, the Corps isn't releasing water, so freshwater issues are due to runoff.