The insecticides in use on Sanibel appear to be highly toxic and therefore potentially damaging to wildlife. For example, recent unpublished tests appear to show the principal insecticide, fenthion, to be toxic to aquatic species in concentrations of fractions of parts per billion. Thus, the fenthion may evaporate while the oil falls and itself becomes the effective anti-mosquito toxin. Water quality in the Sanibel River and other parts of the interior drainage system varies considerably. Heavy nutrient influx, stagnation, and deposition of organic detritus all degrade the general quality of the water.
Salinity varies seasonally with recharge and discharge.
In most parts of the surface-water system, chlorides decrease during the wet season as a result of flushing and dilution by rainfall. In some parts of the Sanibel River, high-chloride water at the bottom is trapped because of bottom irregularities. The trapped water decreases in chlorides only by dilution, which occurs during periods of high water. Chlorides increase during the dry season because evaporation and evapotranspiration losses cause concentration of dissolved solids. The vegetative zonation of the interior wetland is determined by a combination of water level and salt tolerance.