TRANSPORT AND DISTRIBUTION OF NUTRIENTS IN THE LOXAHATCHEE RIVER ESTUARY,SOUTHEASTERN FLORIDA, 1979–811

ABSTRACT: Concentrations of total nitrogen, total phosphorus, and total organic carbon in the Loxahatchee River estuary decreased with increasing salinity in a manner indicating that mixing and dilution of freshwater by seawater was the primary process controlling the down-stream concentrations of nutrients. Most of the nutrients in the surface freshwater inflows entered the estuary from five major tributaries; however, about 10 percent of the total nitrogen and 32 percent of the total phosphorus were from urban stormwater runoff. The input of nutrients was highly seasonal and storm related. During a 61-day period of above average rainfall that included Tropical Storm Dennis, the major tributaries discharged 2.7 metric tons of total phosphorus, 75 metric tons of total nitrogen, and 1,000 metric tons of organic carbon to the estuary. This period accounted for more than half of the total nutrient load from the major tributaries during the 1981 water year (October 1, 1980, through September 30, 1981). Inorganic phosphorus and nitrogen increased relative to total phosphorus and nitrogen during storm runoff. Nutrient yield from the basin, expressed as grams per square meter of basin area, was relatively low. However, because the basin area (544 square kilometers) is large compared with the volume of the estuary, the basin might be expected to contribute significantly to estuarine enrichment were it not for tidal flushing. Approximately 60 percent of the total volume of the estuary is flushed on each tide. Because the estuary is well flushed, it probably has a large tolerance for nutrient loading.