Degree Granting Department
Biology (Integrative Biology)
Thomas L. Crisman
benthic invertebrates, lake age, lake morphometry, macrophytes, water quality
Created lakes are an end product of phosphate mining in Florida. Beginning in 1975, Florida reclamation rules set criteria for phosphate created lakes aiming to approximate the structure of natural lakes, and emphasizing extensive littoral zones with both emergent and submersed vegetation.
Lake development relative to lake age and design were examined for 22 phosphate lakes representing a 40-year age trajectory, utilizing water quality and benthic invertebrate communities from littoral and deep-water locations (Chapter 1). The relative importance of morphometry and lake age in the development of littoral plant communities was examined using multiple vegetation surveys throughout the year, in a separate investigation of 39 littoral locations from 13 lakes reflecting age and morphometry gradients (Chapter 2). The same littoral locations were used to investigate benthic community composition in relation to emergent and submersed plant habitats (depth of 0.5 m and 1.5 m respectively), water quality, sediment characteristics, morphometry and lake age (Chapter 3).
Phosphate lakes >5.5m deep are warm monomictic water bodies, stratifying from April to October/November, and developing thermoclines at approximately 4 m. Shallow lakes (<5.5 m deep) are polymictic. Phosphate lakes become eutrophic or hypertrophic immediately after creation and remain so through time. Unlike reservoirs, they do not go through a trophic surge, which is an initial period of great productivity that declines as sediment nutrient storage is exhausted. Among the 22 sampled lakes, epilimnetic physical and chemical water quality variables (Secchi depth, chlorophyll a, turbidity, pH) did not differ along the age trajectory. Only specific conductance declined significantly, 10-20 years after lake creation, most probably due to bank stabilization. All water quality variables were within the range of natural lakes in the region, and were most similar to urban rather than suburban or rural natural lakes.
Benthic invertebrate abundance (total, Chironomidae, Oligochaeta, invertebrates other than Chironomidae and Oligochaeta) and taxa richness displayed great variability with lake age and did not differ among decadal lake groups at any depths examined (1 m, deepest point). Invertebrate abundance did not differ between littoral and deep habitats, except for lakes >10 years old, where littoral abundance was greater than deep-water abundace, suggesting faster colonization of the littoral zone. Littoral taxa richness was greater than richness in deep-water, which was attributed to taxa other than Chironomidae and Oligochaeta. Deep-water and most littoral habitats were dominated by Chironomidae, whereas Oligochaeta were occasionally dominant in the littoral zone. Dissolved oxygen was the variable explaining most variance of both littoral abundance and richness.
Littoral macrophyte communities in phosphate lakes differed from natural Florida lakes. Although most phosphate lakes developed emergent littoral zones, submersed vegetation was absent, with few exceptions of the non-indigenous Hydrilla verticillata. Plants clustered in groups reflecting their hydrological designations. Frequently encountered plants classified either in the obligate wetland plant group, characterized by Typha, and inhabiting mostly intermediate-age lakes or the facultative wetland plant group, characterized by Panicum repens. Most plants in the latter group were bimodally distributed in youngest and oldest lakes, whereas few plants were ubiquitous. Littoral plant community composition was determined by both lake morphology and age, but the relative influence of these two components was not clear. Littoral plant composition was a good predictor of lake development.
Morphometric (slope) and sediment characteristics (organic content, particle size) did not differ between the two littoral depths examined (0.5 m and 1.5 m). Slope correlated moderately but significantly with lake age, and organic content was low in all samples.
Total littoral benthic invertebrate abundance and its major component Chironomidae, started from low values at newly created lakes and remained low or increased unpredictably with time. Total, Chironomidae, Oligochaeta and Gastropoda abundances did not differ between the depths of 0.5 m and 1.5 m, whereas taxa richness and the abundance of invertebrates other than Chironomidae, Oligochaeta, and Gastropoda was greater at the shallower depth (0.5 m).The relative effect of lake morphology versus age on benthic invertebrate composition was not clear. Total and Chironomidae abundance increased with lake age at 0.5 m, and bank slope at 1.5 m. Both littoral invertebrate abundance and richness in phosphate lakes were lower than in a natural, mesotrophic, macrophyte-dominated lake in the region, characterized by great Oligochaeta densities.
Scholar Commons Citation
Mitraki, Chrysoula, "Ontogeny and Littoral Structure of Lakes Created on Phosphate Mined Lands of Central Florida" (2012). USF Tampa Graduate Theses and Dissertations.