Sediment Chemistry

Phosphorus in Florida Bay Sediments

Florida Bay Project Profile - 160

---

Questions: How do levels of sediment phosphorus in Florida Bay change throughout the year? What role might sediment phosphorus play in fueling fall and winter microalgal blooms in the Bay? What is the relationship between sediment phosphorus and sulfide and iron concentrations?

Background/Project: The release of phosphorus from bay sediments into the water column might fuel microalgal (phytoplankton) blooms which occur each fall and winter in Florida Bay. Sulfide and iron dynamics also might play a role in stimulating blooms by controlling phosphorus release from the sediments. When iron present in bay sediments becomes bound to sulfides and not to phosphorus, sediment levels of phosphorus increase which may lead to increased phosphorus in the water column. Previous studies have shown that bay sediments do have high porewater (the water in the spaces between sediments) sulfide concentrations. This project examined the potential role of sediment phosphorus in the initiation and maintenance of fall and winter microalgal blooms in Florida Bay. This was accomplished by obtaining information on the seasonal variation of sediment phosphorus and the linkage to sediment iron and sulfide dynamics. In 1996 and 1997, porewater sulfide concentrations, and phosphorus and iron fractions were measured from 634 cores collected at 24 sites throughout the bay.

Findings: To more closely examine the seasonal variation, the Bay was divided into six geographic zones within which the data were pooled. Significant seasonal variation in inorganic phosphorus, was observed in all six zones. Organic phosphorus showed significant seasonal variation in the northwest, north central and south central zones. This pattern corresponds to patterns observed in microalgal blooms which often begin in the north central zone and often spread southward into the south-central zone. In addition, the highest concentrations of many phosphorus fractions occurred in the fall and winter, coinciding with the development of the blooms. Concentrations of phosphorus were greatest in the sediments of the western Bay and declined eastward. This fact suggested that the Gulf is a possible source of phosphorus. In addition, the seasonal variation exhibited by sulfides and phosphorus indicated that the dynamics of sulfide and iron may play a role in the release of phosphorus from Bay sediments which, in turn, affect microalgal populations. And finally, the researchers found that bay sediments contained much more iron than expected from carbonate sediments and that most of sediment phosphorus is bound to iron and calcium.

Status: The descriptive phase of this project is complete. Measurements of nutrient fluxes from the sediments to overlying water are now being made.

Funding Source: NOAA National Oceanic Service; Florida Marine Research Institute

Restoration Impacts: Understanding the processes that control phosphorus levels in sediments and the water column is important for predicting the factors that control microalgal blooms and water quality in Florida Bay.

Related Profiles: • FBPP 34 • FBPP 39 • FBPP 155