The quality of surface water on Sanibel Island, Florida, 1976-77

Abstract

The quality of surface water in parts of the interior of Sanibel Island has been periodically degraded by high concentrations of salt or macronutrients and by low concentrations of dissolved oxygen. In 1976 the chloride concentration of surface water ranged from about 500 milligrams per liter to almost that of seawater, 19,000 milligrams per liter. The highest salinities were during the dry season of 1976 in the Sanibel River near the Tarpon Bay control structure and are attributed to leakage of saline water past the structure. The highest concentrations of macronutrients occurred during the dry season in the eastern reach of the Sanibel River, where concentrations generally exceeded 4.0 milligrams per liter total nitrogen and 0.9 milligrams per liter total phosphorus. Organic nitrogen accounted for about 95 percent of the total nitrogen, and ammonia accounted for most of the remainder. Concentrations of ammonia exceeded 0.1 milligrams per liter in 31 percent of the wet-season samples and 14 percent of the dry-season samples. Nighttime concentrations of ammonia exceeded those in daytime by several orders of magnitude; concentration of dissolved oxygen fluctuated inversely to that of ammonia. The concentration of dissolved oxygen and the magnitude of the day-night fluctuation decreased with depth. In 1976 the median concentration of dissolved oxygen near the surface was 7.5 milligrams per liter in the dry season and 3.5 milligrams per liter in the wet season. Concentrations were lowest in the wet season along an eastern reach of the Sanibel River and in several nearby ponds and canals where near-anaerobic conditions prevailed. The high concentrations of macronutrients and the low concentrations of dissolved oxygen at some surface-water sites are attributed, in part, to urban runoff and sewage effluent that flow directly into the surface water or seep through the shallow water-table aquifer into surface-water bodies.