Eutrophic watersheds are classified as having high productivity, due to the ample amounts of nutrients they receive. They characteristically have murky waters that are low in dissolved oxygen, and extremely high in algae. A very accurate way measure this unusually high productivity is to determine the concentrations of chlorophyll in the watershed algae. Chlorophyll is the main component of energy fixation in photosynthesis, and provides an accurate assessment of algal biomass, as well as nutrient availability.
The purpose of this study was to first develop a technique for measuring algal chlorophyll concentration in Lake Matoaka, which is located in Williamsburg, VA, and has 5 surrounding runoff streams (College Creek, Crim Dell, Strawberry, Pogonia, and Berkeley). The major objective of this study, however, was to establish any spatial or temporal patterns associated with chlorophyll and oxygen throughout the lake. To determine chlorophyll concentration, filtered lake water samples were treated in an acetone solution for 24 hours. Spectrophotometry was used to measure the absorbance of each sample solution prior and after acidification. These absorbance levels are related to overall concentration.
Chlorophyll was found to be highest in concentration at the deepest, coldest parts of Lake Matoaka; interestingly enough, the deepest and coldest parts also had the lowest levels of dissolved oxygen. This is likely because decomposition, which uses up oxygen, is occurring at a much higher rate than oxygen-producing photosynthesis. The bottom of the lake is decomposing much faster than the alga is photosynthesizing. A spatial pattern was also observed; peaks in average chlorophyll concentration were found at sampling locations near the mouth of the adjoining disturbed streams producing the most runoff. Relatively pristine streams were associated with much lower concentrations of chlorophyll. Interestingly enough, this same pattern was seen for conductivity. This indicates that high levels of nutrients are being flushed into the lake from the more developed adjacent streams, and thus causing increases in algal growth.
For additional documentation Lindsay Schwarting provided a PowerPoint Presentation entitled "Patterns of Eutrophication in Lake Matoaka" provided here in PDF form.