SOFIA - Diurnal Variation in Rates of Calcification and Carbonate Sediment Dissolution in Florida Bay

>Results

C_net on banks ranged from 0.010 to 2.206 g CaCO₃ m^-2 24 h^-1, with an average of 0.643 g CaCO₃ m^-2 24 h^-1 (Table 2). Subaerial exposure of Nine Mile Bank during low tide prevented collection of continuous 24-h data sets at this location. Results for Nine Mile Bank listed in Table 2 represent approximately 13.5 h of data collection during day and night combined from two measurement attempts on October 2, 1998 and March 12, 1999. The average rate from this data set was extrapolated to 24 h. Highest rates of bank calcification (2.206 g CaCO₃ m^-2 24 h^-1) occurred on Nine Mile Bank located in the western region of Florida Bay. The lowest bank calcification rates occurred on Barnes Key Bank and Russell Bank (0.086 and 0.010 g CaCO₃ m^-2 24 h^-1, respectively) located in central Florida Bay.

Seasonal measurements were made on Russell Bank during winter (dry, cool) and summer (warm, wet) months from March 1999 to September 2000. The average C_net on Russell Bank for all measurements is 0.392 g CaCO₃ m^-2 24 h^-1. Calcification rates on Russell Bank showed some seasonality with highest rates occurring during winter (March), and lowest rates occurring during summer (September; Table 2). The lowest calcification rates observed for banks were associated with hypersalinity events that we observed in central Florida Bay during September 1999 and July 2000. The hypersalinity event in September 1999 was concurrent with a high turbidity event. Hypersalinity events have been documented in Florida Bay during previous years (Fourqurean and Robblee 1999).

Rates of C_net for intermediate density Thalassia beds ranged from 1.697 g CaCO₃ m^-2 24 h^-1 to net dissolution of carbonate sediment (indicated by negative values) of -3.734 g CaCO₃ m^-2 24 h^-1 (Table 3). Buchanon Keys basin intermediate Thalassia beds (located in western Florida Bay) showed an average C_net of 0.340 g CaCO₃ m^-2 24 h^-1, while Manatee Keys intermediate Thalassia beds (located in central Florida Bay) showed an average net dissolution rate of -0.510 g CaCO₃ m^-2 24 h^-1. The overall average for all intermediate Thalassia beds showed net dissolution of -0.230 g CaCO₃ m^-2 24 h^-1. The highest rate of dissolution observed for all data sets (-3.734 g CaCO₃ m^-2 24 h^-1) occurred in intermediate Thalassia beds near Manatee Keys and was associated with the September 1999 hypersalinity and high turbidity event that occurred in central Florida Bay.

Sparse Thalassia beds located near Captain Key and Manatee Keys, and mud bottom sites near Manatee Keys showed average rates of net carbonate sediment dissolution of -0.310 and -0.220 g CaCO₃ m^-2 24 h^-1, respectively (Table 3). C_net for sparse Thalassia ranged from 0.497 g CaCO₃ m^-2 24 h^-1 to dissolution of -0.899 g CaCO₃ m^-2 24 h^-1, while mud bottom rates ranged from 0.417 to dissolution of -0.979 g CaCO₃ m^-2 24 h^-1. While several attempts were made to collect hard bottom data in Buchanon Keys basin, only one complete 24-h data set was acquired in March 2000 due to difficulties in maintaining anchorage of the incubation chamber's support vessel on the hard substrate. Buchanon Keys basin hard bottom showed the highest C_net measured for basins of 2.127 g CaCO₃ m^-2 24 h^-1.

Twenty-two of 27 data sets showed diurnal cycles of calcification with the highest rates of net calcification (or lowest rates of net dissolution) occurring during daylight (C_day ranged from 2.900 to -0.410 g CaCO₃ m^-2 d^-1) and lowest rates of calcification (or highest rates of net dissolution) occurring during the night (C_night ranged from 0.210 to -1.900 g CaCO₃ m^-2 night^-1; Table 2 and Table 3). Of the five complete bank calcification data sets collected, two (from Russell Bank) showed net calcification during both day and night (Fig. 2, Table 2). Average C_day, C_night, and C_net for banks was 0.432 g CaCO₃ m^-2 d^-1, 0.211 g CaCO₃ m^-2 night^-1, and 0.643 g CaCO₃ m^-2 24 h^-1, respectively. Net carbonate sediment production (C_net) was observed at all bank locations.

Diurnal cycles of calcification were observed for all substrate types from each location within the basins. Figure 3 shows examples of diurnal trends in calcification rates and the corresponding TA measurements inside the incubation chamber from which they were calculated. Net carbonate sediment dissolution was observed during the night for all basin sites except for hard bottom sites located in Buchanon Keys basin (Fig. 3, Table 3). Net carbonate sediment dissolution (C_net) was observed at 56% of all intermediate Thalassia sites, 75% of all sparse Thalassia sites, and 60% of all mud bottom sites.

Of the 22 data sets exhibiting diurnal calcification trends, 18 data sets (81.8%) showed net calcification during the day and net dissolution of carbonate sediments during the night, while 4 data sets (18.2%) showed net dissolution during both day and night with the lowest rates of dissolution occurring during the day and the highest dissolution rates occurring during the night. Three of the four data sets showing low dissolution during the day and higher dissolution rates during the night occurred on sparse Thalassia beds near Captain Key (March 21, 2001) and Manatee Keys (March 26 and 27, 2001) in central Florida Bay, and the remaining data set was collected on an intermediate Thalassia bed in Buchanon Keys basin (March 14, 1999). Four data sets (Manatee Keys intermediate Thalassia on September 17, 1999, Captains Key sparse Thalassia on March 20, 2001, and Manatee Key mud bottom on March 26 and 27, 2003) showed higher rates of dissolution during the day and lower rates of dissolution during the night. Seven out of eight of the data sets showing dissolution both during day and night were acquired on days of complete cloud cover. The remaining data set was collected on Manatee Keys intermediate Thalassia on September 17, 1999, during the hypersalinity event. Insufficient data were collected from Nine Mile Bank to establish diurnal cycles in rates of calcification.

During 8 incubation chamber deployments (3 on Manatee Keys intermediate Thalassia, 1 on Buchanon Keys basin intermediate Thalassia, and 4 on Manatee Keys mud bottom site), ambient TA was measured concurrently with incubation chamber measurements made every 4 h. These measurements were made to determine whether changes in incubation chamber TA (

TA) reflect changes in the ambient environment. Ambient and chamber

TA for Manatee Keys intermediate Thalassia on March 24, 2000, was 0.071 and 0.066 mmol kg^-1, respectively (Fig. 4). Buchanon Keys basin intermediate Thalassia (March 28, 2000) showed an incubation chamber

TA of 0.190 mmol kg^-1 and a

TA in ambient water of 0.136 mmol kg^-1. The

TA for chamber and ambient water from Manatee Keys mud bottom site (March 26, 2003) was 0.044 and 0.056 mmol kg^-1, respectively. Ambient TA measurements reflect diurnal cycles observed in incubation chamber measurements during calm sea states when wind-driven currents and mixing by waves are at a minimum. TA decreases during daylight as calcification occurs and increases during the night when carbonate sediment dissolves. The average diurnal change in incubation chamber and ambient TA for the 8 concurrent data sets was 0.085 and 0.065 mmol kg^-1, respectively. The average

TA for all incubation chamber deployments and for ambient water on mud banks was 0.093 and 0.083 mmol kg^-1, respectively. The agreement between chamber and ambient TA was less evident when ambient water was well mixed by strong currents and waves. Ambient TA on banks and in basins ranged from 2.192 to 2.837 and 2.277 to 2.711 mmol kg^-1, respectively, while incubation chamber TA ranged from 2.192 to 3.019 mmol kg^-1 for all data sets. Ambient pH on banks and in basins ranged from 7.99 to 8.92 and 7.97 to 8.59, respectively, while pH in the incubation chamber ranged from 7.70 to 8.77 during all field expeditions. Comparison of diurnal trends in TA to DO and pH indicated that

TA was inversely related to diurnal changes in pH and DO (Fig. 5). TA increased while DO and pH decreased during the night, and TA decreased while DO and pH increased during the day.


Fig. 4. (left) Comparison of diurnal variation in total alkalinity inside of the incubation chamber and outside of the chamber in ambient water for representative basin sites including intermediate Thalassia seagrass beds near Manatee Keys and Buchanon Keys, and mud bottom near Manatee Keys. [larger image]	Fig. 5. (right) Diurnal trends in dissolved oxygen (DO), pH, and total alkalinity (TA) for representative basin and mud bank study sites. [larger image]

CORRELATIONS WITH TEMPERATURE AND SALINITY

Seasonal calcification data sets collected for mud banks and Manatee Keys intermediate Thalassia beds were used to correlate net calcification rates to salinity and temperature (Fig. 6). Mud bank calcification rates showed a negative correlation with both increasing salinity and temperature, with correlation coefficients (r²) of 0.78 and 0.64, respectively. A moderate negative correlation (r² of 0.41) existed between Manatee Keys intermediate Thalassia calcification rates and salinity, and very little correlation (r² of 0.07) existed between calcification rates and temperature.

graphs showing relation between net calcification and salinity, and net calcification and temperature for seasonal mud bank measurements on Russell Bank and Barnes Key Bank and for basin measurements near Manatee Keys

Fig. 6. Relation between net calcification (C_net) and salinity, and C_net and temperature for seasonal mud bank measurements on Russell Bank and Barnes Key Bank and for basin measurements near Manatee Keys. Positive values indicate net carbonate sediment production. Negative values indicate net carbonate sediment dissolution. R is the correlation coefficient. The linear regression equation is in the form y = b + mx. [larger image]

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