Using algae as environmental impact indicators in urban freshwater ponds
Urban freshwater ponds are bodies of water that sustain a diversity of higher plants, vertebrates, invertebrates, amphibians and algae. The importance of ponds in the enhancement of diversity in urban habitats is little understood due to a lack of available knowledge of the processes and interactions that operate within the biotic components of these small aquatic ecosystems. The objectives of this study were to explore the use of algae as indicators of the biotic and abiotic challenges that occur in Sustainable Urban Drainage Systems (SUDS) and to investigate the relationships between algal population dynamics, and stress physiology and water chemistry and quality. An output of this study is the recommendation of management strategies which support the sustainable enrichment of diversity in SUDS. A longer-term consideration is to explore the potential for developing algae as potential phytoremediators in SUDS ponds. Three ponds were studied in Duloch Park, Dunfermline, Fife. Water quality and algal distribution studies indicated that excessive nutrient loads from surface water runoff, caused the ponds to become eutrophic during most of the course of the study. The ponds were particularly impacted by periodic inputs of suspended solids, road salts, and runoff from construction sites and soil erosion incidents. Cladophora glomerata (filamentous blanket weed) was the major algal species present in the ponds and was thus selected as the key indicator organism throughout the study. Due to the short pond retention times, planktonic microalgal populations were extremely low despite adequate nutrient supplies being available. Chlorophyll a/b ratios for C. glomerata fell below the normal chlorophyll ratio of 2.6:1 indicating excessive exposure to high irradiance. A relationship may also exist between the decline in C. glomerata abundance and heavy metal accumulation in the sediments, competition effects due to excessive plant growth of Phragmites australis and duckweed and pond turbidity. At the biochemical level the investigation of a total antioxidant assay (based on the chemical reagent ABTS) was developed for C. glomerata to assess sub-cellular stress responses as markers of environmental change. Fluctuations in total antioxidant activity were assigned to combinations of biotic and abiotic stress, life cycle changes and turbidity impacts within the ponds. More detailed studies of individual antioxidants (superoxide dismutase, catalase, peroxidase, glutathione reductase and glutathione-stransferase) demonstrated that C. glomerata had increased SOD activities. Enzymes associated with the removal of toxic H2O2 were detected in samples from specific locations and time frames. Depletion in protective non-protein and protein bound SH groups in certain pond samples indicated a significant level of oxidative stress possibly caused by xenobiotics. High levels of glutathione reductase activity were detected in all C. glomerata samples suggesting efficient enzyme recycling. It is proposed that this helps protect algal cell membranes from further oxidative damage. As sampling of algae from ponds is restricted to seasonal growth patterns and availability of algal mats an in vitro assay was devised to detect oxidative markers of stress ( OH) in vitro. This was developed using the microalga Euglena gracilis and comprised a non-destructive gas chromatographic technique. Experiments were constructed to simulate xenobiotic stresses in vivo. Cells exposed to high iron and salt concentrations at levels similar to those detected in the ponds, demonstrated high antioxidant activities, suggesting that algae and specifically E. gracilis may be a suitable candidate for phytoremediation programmes in SUDS. The study concludes by presenting an integrated scheme demonstrating the importance of understanding SUDS biotic components at environmental, physiological and biochemical levels. Using this scheme recommendations for improving pond management strategies are made with a view to maximising the biological potential, diversity and sustainability of SUDS ponds in the Scottish urban landscape.
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