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U.S. Department of the Interior
Geochemical Productivity Monitoring in Florida Bay
Calcification, photosynthesis, and respiration were measured during winter (March) and summer (September) of 1999 on Russell Bank using an upstream/downstream sampling strategy. Spatial geochemical parameters were determined by establishing upstream and downstream sampling sites along 200-400m transects across Russell Bank (latitude 25o 04.052' N, longitude 80o 37.655' W to latitude 25o 03.874' N, longitude 80o 37.613' W). Current directions and velocities were determined by deploying Sontek current meters on the bank (Figure 9).
Calcification, photosynthesis, and respiration were calculated from total alkalinity, pH, dissolved oxygen, air:sea CO2 and O2 gas fluxes, salinity, temperature, and wind measurements. Water samples were collected and analyzed for total alkalinity at upstream and downstream sites approximately every 4 hours throughout 24-hour time intervals. Total alkalinity was used to calculate net calcification using the alkalinity anomaly technique of Smith and Key (1975). Total carbon was calculated using carbonate system equations from Millero (1979). Air-sea CO2 fluxes were measured directly at each station inside of a floating bell (Sugiura et al., 1963; Frankignoulle and Distéche (1984); Frankignoulle, 1988; Gattuso et al., 1993; Kayanne et al., 1995) using the procedure and calculations of Frankignoulle (1988). Air-sea O2 fluxes were determined by measuring atmospheric and water pO2 and calculating fluxes as described in Wanninkhof (1992). Differences in oxygen and carbon metabolism between upstream and downstream stations were corrected for O2 and CO2 exchange with the atmosphere as described in (Gattuso et al., 1993). Productivity and metabolic rates per unit area were calculated using the difference in concentration between upstream and downstream stations (Table 1), the volume of water transported along a transect, and the transect area such that the change in concentration of a parameter (C m-2 s-1) = (C m-3 x m3hr-1)/m2 (Barnes and Devereux, 1984).
Productivity data from 4 hour sample intervals was used to calculate net daily production rates for mud banks. Net daily production rates were used to derive average hourly rates of calcification, photosynthesis and respiration. Preliminary results indicate net carbonate sediment production during winter (0.02 g CaCO3/m2/hr) and net sediment dissolution during summer (-4.0 x 10-3 g CaCO3/m2/hr) (Figure 10). Negative respiration values are indicative of carbon fixation and suggest that negative photosynthesis values (based on oxygen) result perhaps from oxygen consumption through chemical oxidative processes (e.g. sulfide oxidation, etc.). Additional nutrient measurements will be made in future experiments to quantify these processes. Geochemical changes resulting from water column (as opposed to benthic) processes on Russell Bank were examined in March by incubating water from the bank top, in situ, but isolated from the bottom, using a bucket. Initial results show that temporal geochemical shifts in the water column resulting from planktonic components are negligible compared to those resulting from benthic processes measured using the SHARQ. Comparison of changes in calcification, photosynthesis, and respiration from the 4-hour "bucket" incubation period to the same 4-hour time period for March measurements in Russell Bank Basin using the SHARQ (shown below) indicate that carbonate sediment and photosynthetic production by the benthos is three orders of magnitude greater than for plankton:
Additional productivity measurements in basins and on banks are required to gain a more complete characterization of trends in carbonate sediment productivity. However, an initial comparison of these preliminary data to rates of carbonate sediment accumulation calculated from standing crop and turnover methods, sediment thickness and age, and dated cores reveals some interesting results. If, for Russell Bank, we apply the winter rate of sediment production over 6 months in a year, and the summer rate of dissolution over the other 6 months, net annual sediment production can be estimated at approximately 85 g CaCO3/m2/year. This corresponds to an average accumulation rate of approximately 7.4 cm/1000 years of calcium carbonate sediment produced on the bank top. Dated cores from Russell Bank represent sediment that has been produced on the bank and remained there, and sediment that has been transported to the bank from elsewhere in the Bay. These cores indicate sediment accumulation rates of 0.33 to 1.91 cm/year or 330 to 1,910 cm/1000 years (Holmes et al., in review). This supports findings by Halley et al. (1999) and Prager and Halley (1999) that newly formed sediment is a minor contribution to banks and much of bank sediments have been transported to their present location. Bosence (1989) estimates carbonate sediment production equivalent to approximately 88cm/1000 years. However, Walter and Burton (1990) suggest that much of the sediment produced is redissolved. Our results showing net dissolution of sediments in basins and periods of dissolution on banks support high carbonate sediment dissolution rates in the Bay. Lastly, it is interesting to note that bank sediment accumulation rates calculated in this study (7.4 cm/1000 years) correspond well with sediment accumulation rates of Stockman et al. (1967) who estimated 8.0 cm/1000 years based on sediment thickness and age.
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U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 04 September, 2013 @ 02:03 PM (KP)