Orange County NC Website
Browse       Search
=39 <br /> • <br /> • r <br /> • <br /> X100 ' ` • a'�•+' • • • s • •• ` <br /> w. • • • • r • •�i'� ••w» s r • •�' ••`• r1 • LAKELIN <br /> 0. S6 Uppe <br /> © N •• a,r • �. �`• •• • • ��• r • Lowe <br /> so <br /> ir <br /> • • • i •• • •• ' • • <br /> • •�` ` • a `4` J. • •%•••• a • <br /> 0.0 0.5 1.0 0 1 2 0.0 0-i 0.2 0.3 <br /> Total Inorganic Nitrogen(mglL) Total Nitrogen(mg1L) Total Phosphorus(mg1L) <br /> Figure 10.Comparison of Observed Chlorophyll-a Concentrations in Falls Lake to Nutrient Concentrations in Falls Lake(1984 to 2020) <br /> Excludes outliers that are greater than five standard deviations from the mean. <br /> DWR assesses the presence of different algal species in Falls Lake by counting and identifying algal <br /> cells in a water sample. This information is used to estimate the amount of space in the water that <br /> algae occupy. This estimate is called biovolume, and DWR provides these estimates monthly at <br /> three locations in Falls Lake. As chlorophyll-a is an indicator of algae growth, these two measures <br /> are expected to increase and decrease together. However, when the biovolume estimates in Falls <br /> Lake are compared to the chlorophyll-a concentrations, they do not always track together. <br /> Depending on environmental stressors and dominant algae groups, chlorophyll-a can reach high <br /> concentrations even when total biovolume is low (Figure 11). This complicates simulation of <br /> chlorophyll-a because the models are designed to predict more chlorophyll-a when more algae are <br /> present. Shifts in algal groups and complex algal metabolic and species variation relationships <br /> complicate the water quality assessment of Falls Lake. <br /> The lake models can simulate three algal <br /> groups, typically modeled as green algae, . Station NEW19P Data-Chlomphylla Standard-DWR Biowlume Threshold <br /> 30,000 - <br /> diatoms, and blue green algae. However, _ • <br /> 25,000 — <br /> evaluation of DWR algal group data shows E 20,000 <br /> that significant concentrations in chlorophyll-a 0 15,000 •-- <br /> were sometimes caused by species not © 10,000 <br /> categorized as one of these three groups. The m 5,000 <br /> DWR data also show that green algae are a a 20 40 &e B0 100 126 <br /> very small component of the algae in Falls Chlorophyll-a(Ugiu <br /> Lake. The alternate species of algae were Figure 11.Scatter plot of algal biovolume(the amount of space <br /> simulated as a group called "green/other occupied by algae in a water sample)and chlorophyll-a at a station <br /> algae." below Highway 50 At this station,nearly 80 percent of the <br /> exceedances of the chlorophyll-a standard occur when the <br /> biovolume is less than the threshold for a bloom. <br /> Evaluation of Lake Model Scenarios <br /> Following calibration to observed lake water quality data, WARMF Lake and EFDC were used to <br /> predict changes in lake water quality resulting from changes in the watershed or lake operations. <br /> One of the scenarios evaluated was a land conversion to "all forest" and wetlands with removal of <br /> watershed-scale human inputs (rates of atmospheric deposition were not changed for this scenario). <br /> This scenario simulates all lands as forests but leaves wetlands in place. It also removes point <br /> sources,fertilizer application, and onsite wastewater treatment systems. This scenario establishes <br /> the lowest potential loading to Falls Lake and resulting lake water quality. Even this hypothetical "all <br /> 20 <br />