The Carbonate System in Florida Bay

 

Frank J. Millero

Rosenstiel School of Marine and Atmospheric Science

University of Miami

Miami, FL 33149

Tel: 305 361 4707

Fax: 305 361 4144

Email: fmillero@rsmas.miami.edu

 

            For the last three years, we have been studying the carbonate system in Florida Bay.  These studies include measurements of pH, total alkalinity (TA), total inorganic carbon dioxide (TCO₂) and the partial pressure of CO₂ (pCO₂) of waters in the bay and surrounding waters.  The results of this study have shown that carbonate is added to the waters flowing into the bay through Taylor Slough along with phosphate.  The increased carbonate concentration results from the low pH of the waters in the Mangrove fringe due to the oxidation of organic carbon.  For the western basin in the summer, our measurements indicate that CaCO₃ is precipitated from the high salinity waters.  Measurements of pCO₂ in the Fall show a “pull down” of 40 matm due to primary production (relative to the atmosphere).  We are proposing to continue these measurements and supplement these studies by examining the composition of the sediments and pore waters.  We hope to provide data that can be used to elucidate the conversion of inorganic carbon to organic carbon in the bay, and the areas of precipitation and dissolution of CaCO₃.  Studies of the composition of the sediments will include nutrients (PO₄), carbonate parameters (pH, TA, TCO₂), and trace metals (Fe, Mn, Cu, Hg, etc.).  Laboratory measurements with sediments will be carried out to examine the rates of precipitation of CaCO₃ in mixtures with Gulf Stream seawater.

            All of our fieldwork will be coordinated with Dr. Peter Ortner (AOML/NOAA) and Dr. Thomas Lee (RSMAS/UM).  Over the last three years, Dr. Ortner has provided us with samples collected in the bay at 20 sites.  Drs. Ortner and Lee have also deployed our instruments (nutrient and CO₂) on their cruises in the waters adjacent to the bay.  We plan to continue this relationship in the future along with other scientists, Dr. Larry Brand (RSMAS/UM) and Dr. David Rudnick (South Florida Water Management District) whom have provided samples to our group.

 

The role of suspended CaCO₃ in the phosphate cycle in Florida Bay

 

Measurements on Florida Bay sediments produced results between those for calcite and aragonite (Figure 1).   The adsorption in the sediments appears to be related to the amount of Mg²⁺ in the lattice (Figure 2).  The sediment with High Magnesium Calcite (HMC) has a higher adsorption than pure calcite and the sediments with Low Magnesium Calcite (LMC).  These results can be attributed to the stronger association of phosphate with Mg²⁺ sites relative to Ca²⁺ sites in the lattice (opposite to the interactions in the solution phase). 

 

 

Figure 1.  Adsorption of PO4 on CaCO3               and Florida Bay sediments.       Sediments.	Figure 2.  Equilibrium values of PO4 on                  CaCO3 and Florida Bay

 

 

The seasonal variation of the carbonate system in Florida Bay

 

A survey of the carbonate parameters; pH, total alkalinity (TA), and total inorganic carbon dioxide (TCO₂) were measured every two months from May 1997 to January 2000. A number of cruises in waters surrounding the bay, including the Florida Straits and Gulf of Mexico, have examined the partial pressure of carbon dioxide (pCO₂) in addition to the other carbonate parameters. Calculations of the partial pressure of carbon dioxide (pCO₂) and the saturation state of CaCO₃ (W) have been made from the TA and TCO₂ measurements. These measurements have been used to fully characterize and compare the carbonate system in Florida Bay with the surrounding waters.  The results were found to correlate with the salinity, which varies from 17 in the rainy season (March) to 42 in the dry season (July).  The values of pH and pCO₂ have been fitted to linear functions of salinity.  The pH was low and the pCO₂ was high for the fresh water input from the mangrove fringe due to the photochemical and biological oxidation of the organic material.  The TA and TCO₂ of the input fresh waters were much higher than seawater due to the low pH and low saturation state of these waters.  The pH was high and the pCO₂ was low in November in regions where the Chlorophyll is high due to biological production.  During the summer when the salinity is the highest, the normalized values of TA and TCO₂ in Florida Bay waters were lower than open-ocean seawater.  The TA, TCO₂, and saturation states of aragonite (W_Arg) and calcite (W_Cal) were found to be linear functions of salinity.  These results demonstrate that the values of TA and TCO₂ in the bay are strongly influenced by the TA (3080 mmol kg^-1) and TCO₂ (3040 mmol kg^-1) of the input of fresh waters.  The low values of W for the freshwater end member are caused by the low pH and indicate that these waters are close to equilibrium with aragonite.  In the summer, the NTA of the bay waters (2000 mmol kg^-1) are below the levels of the outside seawaters (2400 mmol kg^-1).  This is due to the inorganic precipitation of CaCO₃ due to the mixing of sediments with the input ocean waters or biological loss by macroalgae.

            A close examination of the CO₂ parameters throughout the year show that the lowest pCO₂ occurs in the western part of the Bay in July in a region that also has the highest levels of Chlorophyll (Figure 3 and Figure 4).  This could be the result of the pulldown of CO₂ during primary production in the bay.  The TA and TCO₂ levels do not appear to be affected by this production possibly due to the carbonate buffering capacity.

 

 

Figure 3.  pCO2 (matm) contours in Florida Bay

Figure 4.  Chlorophyll (mg L-1) contours in Florida Bay

 

 

Contour plots of TA for three months are shown in Figures 5-7.  The TA and TCO₂ of the input waters are much higher than seawater due to the high pH and low aragonite and calcite saturation states of these waters.  Their low pH results in the high values of TA in

 

 

 

Figure 5.  Total alkalinity (mmol kg-1) contours in Florida Bay – November 1997

Figure 6.  Total alkalinity (mmol kg-1) contours in Florida Bay – March 1998

Figure 7.  Total alkalinity (mmol kg-1) contours in Florida Bay – July 1998

 

 

these waters due to the dissolution of CaCO₃.  During the summer when the salinity is the highest, the values of TA and TCO₂ are lower (TA = 2100 and TCO₂ = 1900 mmol kg^-1) than the seawater values (TA = 2400 and TCO₂ = 2100 mmol kg^-1 when S = 35).  This is due to the precipitation of CaCO₃ in these waters.  This precipitation can be caused by inorganic precipitation due to the mixing of sediments with the high salinity waters or the formation of biogenic CaCO₃ by macroalgae.

The general features of the carbonate system in the bay are similar to studies made on estuarine waters entering from the western part of the everglades.  For example, measurements made in the Shark River yield high values of TA (3500 mmol kg^-1), TCO₂ (3000 mmol kg^-1), and pCO₂ (1000 matm) and low values of pH (7.9).  As stated earlier, we feel that these large increases of CO₂ are related to photochemical and biological oxidation.  Further studies are needed to elucidate these effects and quantify the CO₂ flux to the atmosphere in this coastal zone.

            The high values of TA for the input waters along the mangrove fringe are due to their low pH, which results in the dissolution of CaCO₃.   Preliminary studies completed in February 2000 examined carbonate and nutrient parameters for samples collected by Dr. David Rudnick in the everglades, the mangrove fringe and the bay.  Stations of particular interest occur along a transect across the mangrove fringe near the outflow of Taylor Slough.  Changes in salinity, phosphate concentration, total alkalinity and pH occur as the freshwater input travels across the mangrove fringe, Figure 8.  The low pH of the freshwater end member, that is responsible for the high total alkalinity values, also leads to an increase in phosphate concentration along the mangrove fringe.  Increase phosphate concentrations are due to the desorption of phosphate as the calcium carbonate mineral is dissolved.  Calcium carbonate has a high affinity for phosphate adsorption; conversely during desorption the phosphate can be very easily released from calcium carbonate back to the solution.

 

 

Figure 8.  Measurement along transect across mangrove fringe.