The Effect of Water Velocity on Periphyton Taxonomic Composition of
Experimental Mesocosms Receiving Water from Stormwater Treatment Areas
Barbara A. Hiaasen
, John C. Volin and Steve P. Simmons
Florida Atlantic University, Davie, FL
Extensive anthropogenic alterations of the natural hydrology and water quality of central and southern Florida began more than a century ago, continuing most intensively within the past fifty years. The resulting hydrologic changes and increased nutrient loading, in particular phosphorus, have been associated with many observed ecological changes in the Everglades ecosystem including changes in periphyton community composition. The abundant calcareous
(calcium-precipitating) periphyton mats typically dominated by blue-green algae and diatoms have been replaced in many areas by assemblages dominated by eutrophic cyanobacteria and filamentous green algae. This is of concern to restoration ecologists because of the essential role that the historic oligotrophic periphyton assemblages play in ecosystem processes. Numerous studies have shown that the structure and composition of periphyton communities in freshwater systems are extremely sensitive to phosphorus availability and enrichment as well as changes in water chemistry, temperature, concentrations of other nutrients and ions, substrate, hydroperiod, water depth, and water velocity.
The Everglades periphyton community has been well studied with regard to most of these variables however, research examining the relationship of water velocity to taxonomic composition and P uptake in the Everglades is lacking.
The Comprehensive Everglades Ecosystem Restoration Plan includes the construction of over 16,000 ha of wetlands called Storm Water Treatment Areas
(STAs). The STAs are designed to filter nutrient-loaded urban and agricultural effluent before it is discharged into natural areas. Several experiments are currently underway to determine the effects of different hydraulic loading rates, hydraulic retention times and water depths on STA performance. However, the effects of different velocities on phosphorus uptake and algal taxonomic composition in this system have not been evaluated.
Studies dating back to the 1960’s suggest that the rate of phosphorus uptake by freshwater algae is influenced by the rate of metabolic activity of a particular algal species and that species thus exhibit a differential uptake response based on current velocity. This species specific difference in nutrient uptake documented in other freshwater systems should hold true for the Everglades periphyton communities as well and should be reflected in taxonomic differences observed at different flow rates. The objective of this research is to investigate the effects of
water velocity on the taxonomic composition of periphyton communities colonizing artificial freshwater mesocosms constructed within STA-1W. Several metrics including periphyton relative abundance and generic diversity, water column total phosphorus, pH and temperature, and dry weight biomass are being quantified during the course of the research.
Twenty-four experimental Plexiglas boxes measuring 1.22 m x 2.44 m x 0.31 m were installed at the outlet end of STA-1W. A pump station delivered water from the outlet canal of the STA to the study site. A total of 6 mesocosm units of equal size were created by connecting a set of 4 boxes together to form one flowthrough treatment unit. These six units were comprised of 3 replications of a slow velocity treatment design and 3 replications of a fast velocity treatment design.
This design, along with controlled water inlet volume, ensured an order of magnitude difference in channel velocity between the fast (approximately 2.0 cm s
) and slow (0.22 cm s
) treatments. These rates are achievable in an artificial treatment wetland such as an STA. The hydraulic loading rate, hydraulic retention time and colonizing surface were equal between treatments and replications, with the water flow rate being the single treatment variable.
A set of Plexiglas periphytometers was installed in each of the twenty-four treatment boxes on March 27, 2002. Samplers were located within boxes in such a way as to capture inlet, outlet and mid-treatment path taxonomic composition. A series of 6 sequential collections of the samplers was conducted over a 12-week period. A comparative analysis of generic taxonomic differences is being performed on composite sub-samples from both within and between fast and slow treatments, comprising a total of 12 sub-samples from each of 5 collection periods for a total of 60 samples. Species richness, relative abundance and taxonomic composition of sampled periphyton assemblages is being assessed by identifying and enumerating both diatoms and soft-bodied algae separately and computing metrics of interest using the natural unit or valve counts from a known volume of sub-sample. Microscopic analysis at 1000x magnification is being conducted using a compound light microscope with a blue filter, bright field illumination, epi-fluorescence and DIC (Nomarski) optics. Elucidation of algal indicator taxa for each velocity treatment will be attempted after statistical analysis. Preliminary analyses have documented approximately 70 genera in the fast treatment and 75 genera in the slow treatment with an approximate similarity of 85 percent.
Concurrent sampling by co-researchers included collection of water and algal samples for analysis of TP, SRP, TKN, alkaline phosphatase, and dry weight
The documented relationship between the ecology of the Everglades periphyton communities and P enrichment suggests that periphyton may provide a reliable measure of the phosphorus bioavailability and assimilative capacity of this wetland ecosystem. Studying the effects of water velocity on periphyton characteristics such as species abundance and composition, biomass, and P uptake should provide data that can be used to optimize STA design and establish greater nutrient removal efficiency of these treatment technologies. This optimization will contribute to achieving final post-treatment discharge concentrations of watercolumn total phosphorus that meet restoration goals and compliance with the
Everglades Forever Act of 1994.
Contact: Barbara A. Hiaasen, Florida Atlantic University Environmental
Sciences, 2912 College Avenue, Davie, FL, 33314, Phone: 954-236-1157, Fax:
954-236-1099, E-mail: [email protected]
, Water Quality