|Elizabeth Tyner, Department of Marine Science, University of South Florida, St. Petersburg, FL
Norman J. Blake, Department of Marine Science, University of South Florida, St. Petersburg, FL
Larry J. Doyle, Department of Marine Science, University of South Florida, St. Petersburg, FL
The central west Florida continental shelf provides a unique illustration of a pristine, carbonate sand sheet isolated from clastic input. The amount of carbonate sediment produced is integral in determining sedimentation rates and sources of deposits along Florida's coastal zone area. Production rates also can provide information for interpretation of the fossil record. However, there exists no accurate measure of the sedimentation rate or carbonate production rate on the shelf. The eastern Gulf of Mexico in our study area between Tarpon Springs and Ft. Myers is considered to be relatively stable. There are no outstanding topographic features yet some distinct hydro-biological zones are identified. This area enjoys a high species diversity, higher number of species and species abundance as well as a higher biomass than other areas within the Gulf of Mexico. The modern sediment found in this carbonate environment is of biogenic origin. Therefore, it is important to determine the carbonate production rate and rate of carbonate accumulation on the shelf. Quantifying the carbonate input and output to the system is an excellent tool for enumerating the processes controlling the carbonate content of the sediment. The carbonate production rate is expressed as the measure of the mass of CaCO3 in grams in grams per square meter per year. In this study, gross production rate for calcium carbonate is calculated, not the net accumulation rate.
To determine the rate of carbonate produced on the shelf, data was used from two extensive Bureau of Land Management Studies completed in 1974 and 1975-1976. These cruises performed many water column and benthic studies. Our data primarily comes from the boxcore samples taken from series of lease areas in the Mississippi, Alabama, Florida Outer Continental Shelf (MAFLA) region. We confined our study to the area between Tarpon Springs and Ft. Myers. This data set has been supplemented with a Mineral Management Service study conducted between 1992 and 1994 in sites off Egmont Key, Sarasota, and Manasota Key. The sites have an average depth range of 36 m to 63 m. Two of the lease areas are centered around the Florida Middle Grounds, an anomaly for the central west Florida shelf. All of this research was compiled by leading specialists but without a uniform presentation format. This has created some difficulty with interpretation as there is no consistency in the results between taxa or between studies. The nature of non-linear biological systems also creates problems with the accuracy of the computations. Although the sedimentation and accumulation rates are thought to be constant within the region, temporal changes are occurring in the system as pulses from variations in seasonality, spatial distribution, mechanical events like storms and in year classes for the organisms, to name a few. There are also gaps in the data and missing information can not be filled in from the now twenty-year old study. However, these limitations do not preclude the determination of a valuable carbonate production rate.
Of the several transfer mechanisms for carbonate in the system, biological, mechanical and chemical, biological production and biological removal by mechanical and chemical methods are the dominant contributors to the system. Mechanical transfer from riverine input, aerial transfer, shoreline and sea-floor erosion, suspended load removal and bottom load transport by ocean currents can all provide some carbonate exchange on the shelf. Yet most of the transfer or relocation is relatively insignificant or within the system and carbonates tend to be less portable than other constituents. Exceptions include Loop Current effects and storm events. Chemical transfer at these depths, shallower than the CCD and ACD, primarily occurs at the sediment-surface seawater interface. Therefore, we concentrated our study upon the production f carbonates by biological organisms.
The majority of the carbonate producing organisms are benthic and have relatively long life spans increasing the accuracy of the production calculations. The different carbonate producing taxa are similar to those in other regions like non-reef dominated carbonate environments and coral reef environments. However, the production rates and species composition vary by geographical area creating a unique system The large trend is for a decrease in production with depth due to a resultant decrease in light and primary production. The carbonate producers in this region include: molluscs, micromolluscs, foraminifera, echinoderms, calcareous sponges, coral, calcareous algae, serpulid worms, arthropods, nannobacteria, and fish otoliths. Each taxa provides a carbonate contribution to the system. The carbonates are not broken down between calcite, aragonite of high-magnesium calcite. Fish otoliths as carbonate producers are considered negligible as are the arthropods with the exception of the Cirripedia, barnacles. At this time, we have the serpulid species of the area identified yet not quantified. The carbonate-precipitating nannobacteria may have a significant contribution although the extent is unknown.
Macromolluscs comprise an average of 3.75% of the biomass in the offshore sites and 49% at the coastline. This biomass contributes a range of .12228 (g CaCO 3 /m 2 )yr -1 to 2.21 (g CaCO 3 /m 2 )yr -1 . These have been calculated from biomass numbers and abundance values. The potential production in a reefal community for macromolluscs is approximately 104 (g CaCO 3 /m 2 )yr -1 with most reef systems producing an average of 103 (g CaCO 3 /m 2 )yr -1 . The west Florida shelf is thought to produce nearly 25% of the carbonate of a reef. Therefore, these values for the macromolluscs seem quite low. Work is progressing to develop a formula unique for this assemblage of organisms with a more accurate growth rate and turnover rate.
Foraminifera produce a significant amount of carbonate in this environment. A range of 14 to 57 different live species were identified at each station which represent only 16% average of the forams collected within the lease area. An average of 86% of these are carbonate producing. Their abundance ranges from 7.5 x 104/m 2 to values of 1.5588 x 107 specimens/m 2 . Carbonate production ranges from 6.375 x 101 (g CaCO 3 /m 2 )yr -1 to a high of 1.325 x 104 (g CaCO 3 /m 2 )yr -1 at the Florida Middle Grounds with an average of 2.752 x 103 (g CaCO 3 /m 2 )yr -1 . Very few micromolluscs were found alive, with live to dead ratios ranging from 0:1594 to 1:34. This also requires further investigation for the lack of living specimens when there are so many dead specimens identifiable from the samples. The carbonate production rate appears to be a contribution of approximately 101 (g CaCO 3 /m 2 )yr -1 .
Echinoderms have a more uneven distribution and ranged from 2.19% to 58.62% of the biomass. This provides a contribution of between .40 and 1.63 (g CaCO 3 /m 2 )yr -1 . The species of coral and coralline algae identified are consistent with those found in coral reefs yet production should be between 10% and 25% of that found in shallower more tropical regions. Literature comparisons reveal a production rate of 1 (g CaCO 3 /m 2 )/yr -1 .
The summation of these production ratios gives a rate on the order of 103 (g CaCO 3 /m 2 )yr -1 with the dominant factor the foraminiferal contribution. However, comparison with the sediment composition in this area reveals the dominant contributor to be of molluscan origin and foram sediment only a tenth of the molluscs. At the Florida Middle Grounds, molluscs comprise 36.07% of the modern sediment and forams only 4.1%. In the area surrounding the Middle Grounds, molluscs dominate with 47% of the sediment contribution compared with only 10% for the forams. Consequently work is now ongoing to determine the reason for this disparity. Is the foram death assemblage being transported cross-shelf to the slope or is it more easily recycled into the system? Perhaps the values for the live molluscan population are low or the production rate is not accurate for this area. Current analysis also focuses on the trends noted within the system either by latitude, longitude, depth water column parameters or other significant variables. Work is continuing to develop a more accurate formula for calculation of carbonate production in this area using average size, mass and turnover rates for species within the lease areas on the shelf. Other evaluation that would prove important in determining the production rates would be the age of the sediment and a comparison with the fossil record.