MONITORING ACCRETION AND SUBSIDENCE OF ORGANIC SOILS OF THE EVERGLADES and RESTORATION REPORT CARD
Barry Glaz (Chair), USDA-ARS; William Boyd and Greg Hendricks, USDA-NRCS - Wellington; Ken Liudahl USDA-NRCS - Lake Worth; Chris Deren and George Snyder, EREC; Jim Schortemeyer, Florida Game and Freshwater Fish Commission; and Pat Gleason.
Under the natural hydrology of the Everglades, the balance between aerobic and anaerobic conditions permitted the slow accretion of organic soils in various regions. This soil accretion was due to the decomposition of dead plant material, primarily roots (Gleason and Stone, 1994) mostly by aerobic microorganisms. Anaerobic microorganisms were probably capable of some decomposition of simple C and H compounds present. As soils slowly drained during the dry season, aerobic microorganisms had sufficient time to decompose materials of various complexity into soil like material, but only sufficient time to completely oxidize some, not all, of this material. As a result of complete oxidation, the C and H originally tied up in insoluble portions of the organic matter were released as CO2 and H2O. In balance, because of their mostly anaerobic status, slightly more organic matter accumulated than oxidized and thus soils accreted, or increased in depth and mass under natural conditions.
After changes to the natural system and resultant changes in relative aerobic and anaerobic status due to changed hydroperiods, rates of soil formation changed. In the Everglades Agricultural Area (EAA), and at times in the Water Conservation Areas (WCAs), reduced hydroperiods allowed aerobic microbial activity to increase sufficiently so that more organic soil oxidized than accumulated. This resulted in soil subsidence, or loss of soil depth and mass due to the greater loss of C and H than that accumulated from partially decomposed organic matter. Fires in the WCAs have also caused substantial soil subsidence. Although the word subsidence primarily describes a loss of depth, with their loss of depth due to oxidation, these soils also lose mass. Subsidence can also occur due to compaction or loss in buoyancy due to drying, in which case a loss of mass does not occur. Rates of accretion and primary plant species accreted were not the same for all organic soils in the Everglades.
Monitoring the status of organic soils of the Everglades can serve as a primary tool in carrying out the adaptive management strategies that ecologists propose for restoring the Everglades. Our only practical means of controlling the oxidation that is primarily responsible for soil subsidence is water management. Therefore, we can assume that as we more closely approach natural accretion rates, we would also more closely approach the natural, predrained hydroperiods characteristic of the the Everglades. Monitoring status of organic soils should therefore be an integral part of analyzing the accuracy of any model used to guide hydrological restoration.
Soils have been surveyed for portions of Monroe, Dade, Collier, Broward, and Palm Beach Counties (References 15-19). These surveys are located in the attached map. No soil surveys are available for areas such as the Loxahatchee National Wildlife Refuge, WCAs, Everglades National Park, and Big Cypress Basin. Soil series discussed below (in italics) such as the Loxahatchee, Everglades, Grady, Okeechobee, and Brighton peats are not classified in current soil surveys of the Everglades. Complete soil surveys and data inventories throughout the Everglades are needed. With this product would come some of the information necessary to predict natural accretion rates, accretion or subsidence rates under the managed system, and changes in these rates due to restoration activities.
Jones (1948) and Gleason and Stone (1994) classified the organic soils of the Everglades. Fifty-eight percent or 451,150 hectares of the total 778,628 hectares identified as organic soil in the Everglades are primarily composed of sawgrass (Cladium jamaicense Crantz) remains. Most of these were classified by Jones as Everglades peat. In the EAA, these soils have been classified as Terra Ceia, Pahokee, Lauderhill, and Dania mucks (McCollum et al., 1976); the major distinguishing character being depth to bedrock going in order from Terra Ceia as the deepest to Dania as the most shallow. A small portion (9,000 hectares) of the Everglades peats described by Jones (1948) are the low pH Brighton peats.
Jones (1948) and Gleason and Stone (1994) identified 296,000 hectares of Loxahatchee peats . The primary plant species comprising the Loxahatchee peat was the white water lily (Nymphaea ). Custard apple (Annona ) was the primary plant species of 13,000 hectares of Okeechobee muck as identified by Jones (1948). The present survey for Palm Beach County (United States Department of Agriculture, 1978) identifies this soil as Torry muck. Sawgrass, willow (Salix spp.), and elderberry (Sambucus canadensis L.) were the primary plant species of the 10,600 hectares of Okeelanta peaty muck identified by Jones (1948) and Gleason and Stone (1994). The Palm Beach County soil survey (United States Department of Agriculture, 1978) concurs with this identification. Bay, myrtle, and ferns were the primary species forming the 7,800 hectares of Gandy peat identified by Jones (1948) and Gleason and Stone (1994). Gleason and Stone (1994), page 171, show some of these soils on a map of southern Florida.
In the natural, predrained EAA, McDowell et al. (1969) estimated that formation of organic sc began about 4,400 years ago. They estimated that it took from 500 to 1,000 years to form the first 7.6 cm of a marry-muck on top of the bedrock. After this beginning, they estimated that soils accreted at the much faster rate of about 7.3 cm per 100 years for about 2,300 years. Then for about 1,200 years, peats accreted more rapidly, resulting in a 4,400-year average of 8.4 cm per 100 years. Factoring out initial rates of subsidence due to compaction and loss of buoyancy, most of the organic soils of the EAA have been subsiding at rates ranging from 2.5 to 3 cm per year since drainage, which began in earnest in the early 1900's (Stephens and Johnson, 1951; Stephens and Speir, 1969; Johnson, 1974; Stephens et al., 1984; and Smith, 1990). In a study of five locations, Shih et al. (1979) found 40 year subsidence rates at each location varied from 1.5 cm to 3.1 cm per year.
Direct measurements of soil elevation over 15 survey lines in the northern EAA were taken at approximately five-year intervals from the mid-1930's through 1978. Some lines date back to 1913. The elevation measurements provided direct measures of long-term subsidence rates at specific locations in the EAA. Shih et al. (1979), cited above, used these survey lines. Regular surveys of these transect lines should be immediately resumed. Also, similar lines should be installed in other areas of the Everglades.
Craft and Richardson (1993A) predicted peat accretion rates for the period 1964-1989 at several WCA locations. They reported that peat accretion rates varied by location in WCAs with distance from source of nutrient enrichment being the primary cause of differences, that is, the closer to a nutrient enrichment source, the higher the rate of peat accretion. In WCA 3A the mean rate was 25 cm per 100 years, but it ranged from 20 to 32 cm per 100 years. WCA 2A rates varied from 16 to 40 cm per 100 years. In another report, Craft and Richardson (1993B) measured rates of peat accretion at 18 WCA locations and reported a range of accretion from 3 to 66 cm per 100 years. We were unable to find a source that documents the natural peat accretion rates in the WCAs.
Natural peat accretion in the WCAs was at times interrupted by natural fires and or drought (Davis, 1946), but has probably been interrupted more often due to these causes since the WCAs were formed. In the current managed system, frequent peat fires have occurred east of WCA 3 adjacent to U.S. 27. Severe peat fires have also occurred immediately north of the WCAs on the Holey Land, Brown's Farm, and Rotenberger Wildlife Management Areas. From 1970-80, several peat fires occurred in WCA 3. Peat losses averaged 7.6 cm (Schortemeyer, 1980). Tree islands in the WCAs are more susceptible to peat fires. Some tree islands have lost from 30 to 60 cm of soil due to peat fires. While these peat fires in the WCAs are most common during dry cycles, they also occur during periods of heavy rainfall.
No input was provided for this section.
An overriding concern was that care must be taken in understanding the goal for long-term accretion of organic soils of the Everglades. We would not expect that had the system remained unchanged by humans that accretion would have continued indefinitely. Rather, due to limits of amount of water available, an equilibrium would have been expected where for long durations, perhaps decades, natural subsidence would have equaled natural accretion. Under this equilibrium, fires may have become more frequent. Part of the process for developing a report card should be to determine depths at which this equilibrium would have occurred. Although we are certain in the EAA and STAs that we are far away from approaching such an equilibrium, we are not certain about the WCAs. Therefore, particularly for the WCAs, until these equilibrium levels are determined, assigning grades will be nebulous.
Two other concerns were that only approximate information is available on natural accretion rates. We were only aware of one radiocarbon study (McDowell, et al., 1969) and this was only of two profiles about 100 m apart at the Everglades Research and Education Center of the University of Florida in Belle Glade. More thorough sampling of all organic soils is needed to determine natural accretion rates throughout the Everglades. Also, accretion in and of itself is not the only concern. In addition, the characteristics of what is accreting must be similar to the characteristics of what would have accreted naturally.
A major comment raised during the workshop convened by the University of Miami and the Everglades Partnership was that a wealth of radiocarbon studies have been performed. It became apparent during this discussion that scientists working on organic and marl soils throughout the Everglades need a better understanding of the work of their peers. To achieve this, it was proposed that a conference be organized for scientists working on organic and marl soils in the Everglades.
The following table lists the criteria and grades assigned to each from which a mean score for each major region (EAA, STAB, WCAs) is determined. Each item was judged from 1 to 5 or not applicable (NA). A score of 1 meant that essentially no progress was made and a score of 5 meant that everything identified is being done. The table reflects these scores. This judges how well we are achieving identifiable tasks aimed at restoring natural accretion rates. This table was not discussed specifically at the workshop in Miami. However, based on the lack of communication that was identified among scientists, after an organic soils' conference, several of these grades may change.
|Subsidence or |
|Adaptation of NRCS |
|Knowledge of natural |
|Knowledge of historic |
|Rounded mean Score||3||4||2|
Some of the discussion that helped us assign these numbers follows.
Since 1980, some agronomic research has been accomplished, primarily by University of Florida scientists, that has helped us understand the processes causing subsidence and has identified agronomic options to reduce its rate. USDA-NRCS has recently initiated a project, whose primary purpose is to assist EAA growers implement best management practices (BMPs) that will reduce the phosphorus content of water that leaves their farms. However, NRCS is also including reduction of subsidence rates in the implementation strategies. One EAA company and USDA- ARS formally agreed to begin initial experiments aimed at developing a sugarcane- based agriculture that would tolerate more water and thus help control soil subsidence. While a major step, the plans only included initial research, and no actual research has begun. South Florida Water Management District scientists continued studies of management strategies in STAs that will reduce phosphorus content of water leaving the STAs by relying upon soil accretion. However, these studies aim to maximize the STAs for phosphorus reduction which may lead to strategies that maximize soil accretion. This may not be positive for all aspects of restoration if these managed soil accretion rates exceed rates that occurred naturally and if the accreted soil is different in composition and properties from what accreted naturally.
In addition to scoring task accomplishment, the committee also felt it was important to assign a general overall score that would reflect our opinion on the issue of restoring natural accretion rates for each of the three regions. Based on higher agricultural water tables, increases in rice acreage, and more acknowledgement from growers that subsidence is a critical issue, we assigned a score of 2 for the EAA. The STAs were judged as a little farther along than the EAA so a score of 3 was given. Since we were unaware of important historical and current information (such as natural and current rates of accretion, and soil survey) we did not assign an overall grade to the WCAs.
1. Craft, C.B. and C.J. Richardson. 1993A. Peat accretion and N, P, and organic C accumulation in nutrient-enriched and unenriched Everglades peatlands. Ecological Applications 3 :446-458.
2. Craft, C.B. and C.J. Richardson. 1993B. Peat accretion and phosphorus accumulation along a eutrophication gradient in the northern Everglades. Biogeochemistry 22:133-156.
3. Davis, J.H. 1946. The peat deposits of Florida. Fla. Geol. Surv. Bull., No. 30.
4. Gleason, Patrick J. and Peter Stone. 1994. Age, origin, and landscape evolution of the Everglades peatland. In Davis, S.M. and J.C. Ogden (Eds.) Everglades: The Ecosystem and its Restoration. St. Lucie Press, Delray Beach, FL.
5. Jones, L.A. 1948. Soils, geology, and water control in the Everglades region, Bull. 442, University of Florida Agricultural Experiment Station, Gainesville.
6. Johnson, L. 1974. Beyond the Fourth Generation, University Presses of Florida, Gainesville, 230 pp.
7. McCollum, S.H., V.W. Carlisle, and B.G. Volk. 1976. Historical and current classification of organic soils in the Florida Everglades. Soil and Crop Science Society of Florida Proceedings 35:173-177.
8. McDoweII, L.L., J.C. Stephens, and E.M. Stewart. 1969. Radiocarbon chronology of the Florida Everglades peat. Soil Science Society of America Proceedings 33:743-745.
9. Schortemeyer, J.L. 1980. An Evaluation of Water Management Practices for Optimum Wildlife Benefits in Conservation Area 3a. Florida Game and Fresh Water Fish Commission. Ft. Lauderdale, FL. 74 pp.
10. Shih, S.F., E.H. Stewart, L.H. Allen, Jr., and J.E. Hilliard. 1979. Variability of depth to bedrock in Everglades organic soil. Soil and Crop Science Society of Florida Proc. 38:66-71.
11. Smith, G. 1990. The Everglades agricultural-area revisited. Citrus Vegetable Mag., 53(9):40-42.
12. Stephens, J.C., L.H. Allen, Jr., and E.C. Chen. 1984. Organic soil subsidence. Geological Society of America, Reviews in Engineering Geology 6:107-122.
13. Stephens, J.C. and L. Johnson. 1951. Subsidence of organic soils in the upper Everglades region of Florida. Soil Sci. Soc. Fla. Proc., 11:191-237.
14. Stephens, J.C. and W.H. Speir. 1969. Subsidence of organic soils in the U.S.A. Int. Assoc. Sci. Hydrol., Publ. No. 89, Tokyo, pp. 523-534.
15. United States Department of Agriculture Soil Conservation Service in cooperation with University of Florida, Institute of Food and Agricultural Sciences, Agricultural Experiment Stations, Soil Science Department. 1978. Soil Survey of Palm Beach County Area, Florida.
16. United States Department of Agriculture Soil Conservation Service in cooperation with University of Florida, Institute of Food and Agricultural Sciences, Agricultural Experiment Stations, Soil Science Department. 1984. Soil Survey of Broward County, Eastern Part, Florida.
17. United States Department of Agriculture Soil Conservation Service in cooperation with University of Florida, Institute of Food and Agricultural Sciences, Agricultural Experiment Stations, Soil Science Department. 1995. Soil Survey of Monroe County, Keys Area, Florida.
18. United States Department of Agriculture Soil Conservation Service in cooperation with University of Florida, Institute of Food and Agricultural Sciences, Agricultural Experiment Stations, Soil Science Department. 1996. Soil Survey of Dade County Area, Florida.
19. United States Department of Agriculture Soil Conservation Service in cooperation with University of Florida, Institute of Food and Agricultural Sciences, Agricultural Experiment Stations, Soil Science Department. In preparation. Soil Survey of Collier County, Florida.