Analysis and Discussion
Analysis and Discussion
The primary difference the 2 sites selected was the presence, or absence, of trees. Site A had a closed and well established riparian zone that provided shade and habitat, where Site B had no trees in the riparian zone, only bull rush and weed. The presence of over hanging trees is important as it provides an important food source, shade to regulate water temperature and habitat in which aquatic organisms can hunt and hide. (WetLink 2009) Another important function of overhanging trees is providing shade that limits the light intensity and availability in the creek. Previous research has found that reducing light intensity reduces the growth of "nuisance plants and algae including toxic blue greed algae". (River Landscapes, 2006) When the light intensity increases, it can promote excessive aquatic plant growth. This can lead to a build-up of sediment, wider and shallower creek bed and increased bank erosion. Excessive plant growth also results in depleted oxygen levels as plants die and decompose, which, in turn, reduces the water quality. The human impact on the Bulimba Creek catchments in this respect has been extensive. For almost 200 years the Bulimba catchments has been cleared of trees for purposes such as farming, industry, and timber-getting and in recent decades urbanization and aesthetics. Today only small areas of bushland remain. (Brisbane City Council) The following table shows the canopy overhang for Site A and B.
Without the overhanging riparian zone the light intensity at Site B was over 15 times higher in the morning and over 12 times higher in the afternoon than Site A. The effects of this increased light intensity were evident with the macrophyte growth at site B being significantly more than that of site A. Measurement of light intensity at site B were completely accurate due to cloud cover, which reduces the light intensity, at the time of data collection.
An established riparian zone will act a buffer removing fertilizer runoff which can cause excessive macrophyte growth. As most Australian rivers and soils have low nitrogen and phosphate levels native plants have adapted to grow under these conditions. Introduced plants, however, thrive on higher levels of nitrogen and phosphate, leading to increased and excessive macrophyte growth in water that has high concentrations of these nutrients. (Walls Creek Catchment Crawl, 2006) At site A there was no excessive macrophyte growth suggesting that the open riparian zone does not prevent nutrient runoff from the surrounding residential area.
Riparian vegetation also plays an important role in the banks and substation creating many habitats for aquatic organisms. Large Woody Debris (LWD) created by branches falling into the water column vary water current and creating habitat for many aquatic organisms. The roots of trees in an established riparian zone will bind the soil together, preventing...