Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/29/1998
Publication Date: N/A
Interpretive Summary: Total phosphorus (P) concentrations of lake water in Lake Okeechobee, Florida are not correlated to the amount of phosphorus entering from outside sources. The objectives of this study were to determine how much phosphorus cycling is occurring in lake (i.e. - movement of P from sediment to water) and to determine what controls this P cycling. Phosphorus release was measured from sediments taken from the lake at eight sites which represented the main sediment types, and at four different times during one year. Recycling of phosphorus from sediments was found to be as important as external sources during normal conditions and much higher during periods when the lake is anaerobic (without oxygen).
Technical Abstract: Although total P has been shown to be increasing in the waters of Lake Okeechobee, Florida, the concentrations have not been correlated to external loads. The objectives of this study were to determine: (I) the P flux from various sediment types within Lake Okeechobee, (ii) the factors which control the direction and magnitude of the flux, and (iii) the amount tof P associated with various inorganic P phases within the sediment. Phosphorus flux was measured from intact sediment cores taken from eight sites in Lake Okeechobee which represented major sediment types and major inflows of the lake at four time periods in 1989-1990. At the same location-times, dissolved reactive phosphorus (DRP) in porewater was determined using porewater equilibrators and/or sediment cores. Results indicated that P flux from sediments was very sensitive to changes in O2 status of the overlying water, with anaerobic conditions resulting in high rates of P release. Although there were steep porewater DRP gradients in sediments (varying from 0.1 mg P L**-1 at the sediment/water interface to over 1 mg P L**-1 at lower depths) P flux was not regulated by such gradients. The lack of dependence of P flux on DRP gradients found in this study is indicative of the role redox reactions (involving Fe) have with respect to P chemistry in the top few cm of the sediment. Internal P loads (i.e., flux from bottom sediments) were found to be approximately equivalent to external P loads (approximately 1 mg P m**-2 d**-1). These internal loads could increase dramatically if the surface oxidized zone in the sediments becomes reduced.