Stay in the know about wildlife, water quality, and ecosystems on Sanibel and Captiva Islands and in Southwest Florida

Recovery Will Take Time for Sanibel’s Freshwater Habitats

February 7, 2023

Since Hurricane Ian rearranged Sanibel habitats, the SCCF Marine Laboratory has been monitoring about 30 lakes every other month as well as evaluating water quality in the Sanibel Slough. Prior to Ian, the lakes on Sanibel were populated primarily by freshwater wetland plants and fish.  Most or all of the freshwater fish in the lakes died after the storm surge — they could not handle the rapid increase in salinity that the storm surge brought.

“Salinity or salt content is now one of the most important indicators of recovery of our freshwater plant and animal communities,” said SCCF Research Associate Mark Thompson. “When we look at the rate of salinity decrease since the storm surge, we estimate it may be at least next summer until salinities are low enough that native freshwater fish could survive again.”

For six years prior to Ian, the lab monitored about 80 lakes with funding from the City of Sanibel’s Communities for Clean Water Program, providing substantial baseline data.  

The 30 lakes currently being sampled had a salinity range of 0-3 PSU (practical salinity units) before Ian. In October, the salinity range of those lakes was 20-33 PSU. For reference, the salinity of the Gulf of Mexico is 30-35 PSU, while freshwater is 0-1.5 PSU. 

“As we get more rain the salinity will gradually be reduced in all the lakes on Sanibel,” said Thompson. “However, we are currently in the dry season and freshwater inputs to lakes and groundwater are reduced during this period — change will likely be slow over the dry season.”

Some residents have emailed SCCF asking if restocking neighborhood lakes is an option.

“It would not be possible to re-stock our lakes currently with native freshwater fish, because they are intolerant of current salinities,” said Thompson. “When we looked at the rate of salinity decrease since the storm surge, we estimate it may be at least next summer until salinities are low enough that native freshwater fish could survive again.”  

However, he also cautions that it may be longer.

“Even by next summer salinities may not be acceptable to native freshwater fish due to salt in the groundwater which flows into the lakes and latent salt in the soils which rainwater flows through and over,” he said.

Some fish can survive in a wider range of salinities such as juvenile snook (Centropomus undecimalis), mullet (Mugil cephalus), and some killifish. Lakes will naturally restock with these fish and other salinity-tolerant species over the coming months. 

With natural forces in play, the lakes will not be devoid of fish. Birds, overland hydraulic connections during storms, king tides, and any small tornadoes move fish around. Before Ian, the primary fish biomass in Sanibel lakes — like most of South Florida — was Mayan cichlids (Cichlasoma urophthalma) and blue tilapia (Oreochromis aureus). Both are non-native invasive freshwater fish. 

“Mayan cichlids are likely to repopulate lakes soon because they are more tolerant of moderate salinity than our native sunfish and bass,” said Thompson. “They are also more tolerant of low dissolved oxygen levels which occur when lots of organic matter is washed into a lake and begins decomposing.” 

Restocking waterbodies with non-native fish such as Mayan cichlids or tilapia is illegal. They outcompete native fish in these habitats. 

While Sanibel’s lakes are transitioning from saltwater wetlands to freshwater ponds they will go through many changes. The microbe and plankton populations will continuously change as water quality changes. 

“You will have blooms of algae that you have not seen before. There is always an algae that will grow in almost any condition,” he said. “The algae that grows best for the conditions you have at the moment will dominate.”

In all salinity conditions, reducing the “food” which goes into a waterbody reduces the intensity of algae blooms. Food for algae comes by the way of nutrients flowing into the lakes such as fertilizer and septic/sewer system discharges. 

“You can reduce the amount that runs into your lake by planting more and more vegetation all around your neighborhood and allowing your lake border vegetation to grow wild as far onto your property as you can,” said Thompson. “The better you can simulate a wild natural ecosystem on the property around your lake, the less algae bloom-forming food will runoff into your lake.”

Unfortunately, much of the lands around lakes has been de-vegetated post-Ian and the storm surge may have washed additional “algae food’ into waterbodies. Overall, Thompson says it will be a challenging road to lake recovery. 

“When the lakes finally become primarily freshwater again and the conditions are more stable, algae blooms may become more intense with low dissolved oxygen than previous to Ian due to vegetation loss in the watershed and due to the inputs from sewer overflows, and everything that was in our homes,” he explains

The salinity of some of Sanibel’s lakes is not declining yet, staying near sea-water strength. These lakes are generally the deeper lakes found on western Sanibel and north of Periwinkle and San Cap Road. The lakes were originally excavated deep into the surface water table, and they interact with groundwater continuously. 

The storm surge has turned groundwater from fresh to salt in many areas and these lakes will not have a reduction in salt until the groundwater is recharged with freshwater. Freshwater recharge happens much more slowly on western Sanibel and north San Cap Road. 

“With sea level rise and vegetation loss through development, the challenge for creating ‘naturally aesthetic’ barrier island artificial lakes will become more difficult,” said Thompson. “We will experience more tidal connections with interior island freshwater systems. Many of Sanibel’s wetland areas will gradually become estuarine — a mixture of salt and fresh.” 

Commercial lake management efforts create artificially aesthetic lakes by using chemicals such as copper (toxic) to kill phytoplankton and effectively poison the lake ecosystem. Copper stays around forever – it is a toxic heavy metal. 

“We advocate against adding most chemicals to lakes to control algae. Although, hydrogen peroxide is an exception,” he said. “Native vegetation is the single best, natural way to improve waterbody characteristics and promote a natural community.”

Some lakes with good water quality prior to Ian may experience major deterioration due to extreme nutrient-laden inputs. 


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