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Measurement of Recovery in Lakes Following Phosphate Mining


Surface mining of phosphate-bearing rock in central Florida annually affects approximately 6,000 acres. Reclamation of these mined lands is directed by the Florida Department of Natural Resources under Chapter 16C-16 Mine Reclamation Rules of the Florida Administrative Code. Rules direct the reclamation of lakes, wetlands, and uplands. The reclamation of lakes is intended to provide ecosystems that will support fish and wildlife and that function in the same manner as natural or unmined lakes.

The research summarized in this report, “Measurement of Recovery in Lakes Following Phosphate Mining,” was sponsored by the Florida Institute of Phosphate Research Applied Research Program. The purpose of the project was to apply a simple, direct method to the assessment of recovery of lakes reclaimed after mining. The specific objectives of the project were to (1) determine colonization rates and equilibrium species numbers for microbial communities developing on artificial substrates in natural, reclaimed, and unreclaimed lakes; (2) determine the effect of physical and chemical factors on microbial colonization rates and equilibrium species numbers in lakes; and (3) estimate the rate of recovery of lakes after mining.

A broad range of lake ages was selected for study. Several physical and chemical parameters were measured in water samples taken concurrently with biological collections in each lake studied. These analyses determined levels of nutrients and trace metals as well as commonly measured parameters. Polyurethane foam artificial substrates were placed in each lake and collected on an expanding time scale over a two-week period. The number and kind of protozoan species colonizing these substrates were determined. Relative abundances of species were estimated.

During the first sampling effort in March 1984, 10 lakes were studied. Reclaimed lakes ranged in age from less than 1 year to 7 years while unreclaimed lakes studied were over 60 years old. Two natural lakes were also studied. Chemical differences among lakes were very apparent. Potassium, hardness, alkalinity, conductivity, and total organic carbon separated lake types. However, chemical differences among lakes did not correlate with distributions of protozoan species in lakes.

Species distributions appeared related to phosphate and aluminum concentrations. No clear pattern distinguishing reclaimed lakes from natural or unreclaimed lakes was apparent. Examination of the colonization process, however, showed that protozoan colonization was markedly reduced in very young lakes (<1 year old).

During the second sampling effort, 21 lakes were studied. These lakes included all the lakes studied in the first phase of the project and 11 additional lakes generally less than 4 years old. This added group of lakes included sampling of water in two active mine pits and two newly reclaimed (ca. 6 month old) lakes. Similar to results of the first sampling effort, lakes could be distinguished by composite factors based on conductivity, calcium, alkalinity, pH, nutrients, and aluminum.

Selenium and aluminum levels were high, and pH’s were low in the newly reclaimed lakes. Natural and unreclaimed lakes and active mine sites could be distinguished by their protozoan communities. Examination of the photosynthetic portion of these communities further strengthened the differentiation. Reclaimed lakes spanned the range of variability for other lakes. Examination of protozoan species by functional group revealed differences between newly reclaimed lakes and other lakes. Protozoan colonization of artificial substrates was severely depressed in newly reclaimed lakes as compared with other systems studied, including active mine sites.

A variety of analyses, both chemical and biological, showed that newly reclaimed lakes (<6 months old) were very different from other lakes studied. Aluminum and selenium levels were generally high in these lakes, and pH’s were generally low. Waters were typically turbid. Microbial communities were different in terms of colonization dynamics and functional group composition. Species numbers were generally low. Comparison of these lakes to older lakes showed that lakes greater than one year of age were very similar to all ages of reclaimed and unreclaimed lakes and that older reclaimed lakes spanned the variability present in the natural and unreclaimed lakes studied. These studies suggest that recovery of the microbial component of lakes after reclamation is rapid and effective. Protozoan species represent intermediate steps between bacterial degraders and higher trophic levels. They have a variety of functions and, to some extent, represent the many functional groups found in the larger ecosystem. These species are generally sensitive to perturbations including toxic and nutrient inputs.

This investigation demonstrates that recovery can be documented by examining a portion of the microbial biota of reclaimed systems. It has not examined the relationship between community structure and system function. Further studies examining the processing of carbon and inorganic nutrients will be needed to determine if reclaimed ecosystems have functional attributes comparable to natural or unmined lake ecosystems in the same geographic area. Examination of the structure and dynamics of a complex subset of reclaimed ecosystems demonstrates the ability of the biota to integrate a number of important factors in community patterns. These factors have been identified to some extent by statistical analysis of physico-chemical factors but are limited by the complexity and adequacy of water sampling.