Paul R. Carlson, Jr., Ph.D
Florida Department of Environmental Protection
100 Eighth Ave. SE, St. Petersburg FL. 33701
Phone 727.896.8626 x 4104
Email: carlson_pr@epic7.dep.state.fl.us

Laura A. Yarbro, Ph.D.
Florida Department of Environmental Protection
100 Eighth Ave. SE, St. Petersburg FL. 33701
Phone 727.896.8626 x 4209
Email: yarbro_l@epic7.dep.state.fl.us

INTRODUCTION - To determine the role of sediment nutrients in Florida Bay phytoplankton blooms and the overall nutrient budget of Florida Bay, we are measuring spatial and temporal patterns of sediment nutrient inventories and fluxes of nutrients from sediments to the water column. Our project has three objectives: (1) Determine spatial and seasonal variations in benthic nutrient inventories and flux rates; (2) Quantify role of benthic microalgae in mediating fluxes of N, P and Si from sediments to water column; and (3) Determine the role of P sorption and desorption to iron and calcium minerals in Florida Bay sediments to the benthic fluxes of P.

WORK PLAN - This project is being carried out as a collaborative effort between Drs. Carlson and Yarbro at FMRI and Drs. Alina Szmant and Larry Brand at the Rosensteil School of Marine and Atmospheric Sciences. FMRI and RSMAS staff conducted collaborative spatial surveys of dissolved and solid-phase forms of nitrogen, phosphorus, iron, silica, sulfide, and benthic chlorophyll at 24 sites across Florida Bay in summer (July 1997) and again in winter (December 1997-January 1998). FMRI staff are now doing more detailed measurements of nutrient distribution and fluxes at five sites across the Bay. At these sites, we are making quarterly measurements of community oxygen metabolism and benthic nutrient flux rates as well as vertical profiles of sediment porewater nutrients, pH, sulfide, and C02. We are also determining the distribution of phosphorus among organic and inorganic sediment pools. RSMAS staff are performing stable nitrogen isotope experiments at two of the benthic flux sites to estimate rates of nitrogen remineralization, nitrification, and denitrification. Drs. Szmant and Brand have prepared a separate report describing their work, so this report will focus on the sediment phosphorus fractionation, porewater nutrient profiles, and benthic flux measurements being performed by FMRI investigators.

Seasonal and Spatial Variation in Nutrient Inventories- In July 1997, we collected four cores at each of our 24 sites and sectioned them into top, middle, and bottom segments. Porewater was collected from each segment by centrifugation and analyzed for phosphate, dissolved organic carbon, phosphorus, and nitrogen, nitrate+nitrite, ammonia, sulfide, and pH. Bulk sediment was dried for porosity, total nitrogen, and total phosphorus analyses. Separate cores were collected at each site for benthic algal pigment analysis. Winter samples were collected at the same sites in December 1997 and January 1998.

Data from the July sampling were presented at the Florida Bay Science Conference held in Miami, May 12-14, 1998, and copies of data graphics from that presentation are included in the second section of this report as well as in the body of this section. Sediment porosity (Figure 1) was found to be high (>0.85) in the western Bay and declined eastward to values cat 0.6 in the eastern half of the Bay. In small areas where mud has accumulated near Shell Creek and Swash Keys, porosity values in the eastern Bay reach 0.7. Organic matter content of surface sediments (Figure 2) is also high in the western Bay and generally lower in the eastern Bay. Surficial sediment pH exhibits a southwest-to-northeast gradient within Florida Bay (Figure 3) with values as low as 6.9 in the western Bay. Values increase in a northeastward direction to 7.7-7.8 near the mouth of Little Madeira Bay and Trout Cove.

Sediment iron concentrations (Figure 4) exhibit a slightly different pattern with high values occurring in the western Bay and along the northern shore of the Bay, indicating that surface water runoff also delivers iron to Florida Bay. Sediment total phosphorus is generally high in the western Bay and declines eastward, although the same isolated mud pockets in the eastern Bay which have slightly higher porosities than surrounding areas also have higher total phosphorus concentrations than areas in the east-central Bay (Figure 5).

Benthic Nutrient Fluxes- Based on our spatial surveys, five sites across Florida Bay were selected for intensive measurements of benthic nutrient fluxes and for studies of phosphorus fractionation within the sediment solid phase: Swash Key; Calusa Key, Rankin Lake, Johnson Key Basin, and Rabbit Key Basin. At each site, equilibrium-dialysis samplers (peepers) are deployed and allowed to equilibrate with sediment porewaters for 30 days. After equilibration, the peepers are recovered, and potential diffusive fluxes of nutrients, sulfide, and total C02 are estimated from vertical profiles of porewater constituents. Benthic nutrient fluxes and community oxygen metabolism are measured at the same sites using four clear, cylindrical acrylic chambers. Chambers are deployed at mid-day and are sampled through the night and into the following morning. Water is circulated within chambers by small (350-500 gph) bilge pumps. Chambers are sampled all night, and the sampling interval is determined by monitoring chamber oxygen concentrations. When oxygen concentrations fall to 1.5 ppm, a bubble of oxygen gas is injected into the chambers. Parameters measured during flux experiments are chamber and bottom water temperature, conductivity, dissolved oxygen, and pH. Nutrients include PO₄, TDP (DOP by difference), NH₄, N02+N03, TN, silicate, and DOC. After chamber measurements are complete, sediment cores are taken from chamber locations for measurement of benthic microalgal chlorophyll. Seagrass shoots, if present in chambers, are harvested from 20 cm x 20 cm quadrats, counted, and measured. Four replicate cores from each site are sequentially extracted with magnesium chloride, dithionite/bicarbonate buffer, and hydrochloric acid to measure exchangeable, iron-bound, and calcium-bound phosphorus, respectively.

Data Included in This Report- Porewater profiles of total carbon dioxide, sulfide, and pH for fall 1997, March 1998, May 1998, August 1998, and November 1998 are included in the third section of this report( click here). Porewater TCO2 concentrations generally range from 2 mM at the sediment surface to nearly 20 mM at depths of 25 cm, particularly in summer and fall. Porewater sulfide concentrations greater than 2 mM occurred at all sites except Calusa Key in August. pH values typically decline with depth from values greater than 8 at the sediment surface to asymptotic values between 7.0 and 7.5 at 10 cm.

Scatter plots of porewater TCO2 (Figure 6) and sulfide concentrations(Figure 7) have very high coefficients of determination (r2 values) and stoichiometric ratios of approximately 5 moles CO2 per 1 mole sulfide. Most diagenetic models predict approximately equal molar quantities of CO2 and sulfide produced by organic matter oxidation by sulfate reduction, so the "missing" sulfide either diffuses out of the sediments, is oxidized in place, or accumulates in solid-phase minerals.

Scatter plots of the relationship between TCO2 and pH are best fitted by a rectangular hyperbola. The data indicate significant fluctuations in pH (7.5-8.5) in the upper 510 cm of sediments. Below the 10 cm depth, sediment pH values range between 7.0 and 7.5. The excursions of pH in the surficial sediments might have sign)ficant effects on movement of phosphorus from sediments to the overlying and will be investigated further.

OVERALL PROJECT STATUS- is summarized in Table 1, below. At this time, we have conducted four complete benthic flux samplings in fall 1997, spring 1998, summer 1998, and fall 1998. We have completed analyses of peeper constituents, and we are verifying those data now. Analyses of chamber nutrient samples and oxygen data are being performed. Additional chamber flux measurements are planned for spring, summer, and (possibly) fall 1998, if it is possible to extend the project through December 1999. A formal request will be submitted shortly. We are currently discussing data needs with water quality modelers, and we will provide information to them as quickly as possible.