Scientific Investigations Report 2007–5125
By Denise M. Argue, Richard G. Kiah, Jane F. Denny, Jeffrey R. Deacon, William W. Danforth, Craig M. Johnston, and Amy P. Smagula
U.S. Geological Survey Scientific Investigations Report 2007–5125
Geophysical, water, and sediment surveys were done to characterize the effects of surficial geology, water and sediment chemistry, and surficial-sediment composition on the distribution of variable leaf water-milfoil in Moultonborough Bay, Lake Winnipesaukee, New Hampshire. Geophysical surveys were conducted in a 180-square-kilometer area, and water-quality and sediment samples were collected from 24 sites in the survey area during July 2005.
Swath-bathymetric data revealed that Moultonborough Bay ranged in depth from less than 1 meter (m) to about 15 m and contained three embayments. Seismic-reflection profiles revealed erosion of the underlying bedrock and subsequent deposition of glaciolacustrine and Holocene lacustrine sediments within the survey area. Sediment thickness ranged from 5 m along the shoreward margins to more than 15 m in the embayments. Data from sidescan sonar, surficial-sediment samples, bottom photographs, and video revealed three distinct lake-floor environments: rocky nearshore, mixed nearshore, and muddy basin. Rocky nearshore environments were found in shallow water (less than 5 m deep) and contained sediments ranging from coarse silt to very coarse sand. Mixed nearshore environments also were found in shallow water and contained sediments ranging from silt to coarse sand with different densities of aquatic vegetation. Muddy basin environments contained the finest-grained sediments, ranging from fine to medium silt, and were in the deepest waters of the bay.
Acoustic Ground Discrimination Systems (AGDS) survey data revealed that 86 percent of the littoral zone (the area along the margins of the bay and islands that extends from 0 to 4.3 m in water depth) contained submerged aquatic vegetation (SAV) in varying densities: approximately 36 percent contained SAV bottom cover of 25 percent or less, 43 percent contained SAV bottom cover of more than 25 and less than 75 percent, and approximately 7 percent contained SAV bottom cover of more than 75 percent. SAV included variable leaf water-milfoil, native milfoil, bassweed, pipewort, and other species, which were predominantly found near shoreward margins and at depths ranging from less than 1 to 4 m.
AGDS data were used in a Geographic Information System to generate an interpolated map that distinguished variable leaf water-milfoil from other SAV. Furthermore, these data were used to isolate areas susceptible to variable leaf water-milfoil growth. Approximately 21 percent of the littoral zone contained dense beds (more than 59 percent bottom cover) of variable leaf water-milfoil, and an additional 44 percent was determined to be susceptible to variable leaf water-milfoil infestation.
Depths differed significantly between sites with variable leaf water-milfoil and sites with other SAV (p = 0.04). Variable leaf water-milfoil was found at depths that ranged from 1 to 4 m, and other SAV had a depth range of 1 to 2 m. Although variable leaf water-milfoil was observed at greater depths than other SAV, it was not observed below the photic zone.
Analysis of constituent concentrations from the water column, interstitial pore water, and sediment showed little correlation with the presence of variable leaf water-milfoil, with two exceptions. Iron concentrations were significantly lower at variable leaf water-milfoil sites than at other sampling sites (p = 0.04). Similarly, the percentage of total organic carbon also was significantly lower at the variable leaf water-milfoil sites than at other sampling sites (p = 0.04).
Surficial-sediment-grain size had the greatest correlation to the presence of variable leaf water-milfoil. Variable leaf water-milfoil was predominantly growing in areas of coarse sand (median grain-size 0.62 millimeters). Surficial-sediment-grain size was also correlated with total ammonia plus organic nitrogen (Rho = 0.47; p = 0.02) and with total phosphorus (Rho = 0.44; p = 0.05) concentrations in interstitial pore-water samples.
Purpose and Scope
Methods of Data Collection and Analysis
Geophysical Mapping of the Lake Floor
Acoustic Ground Discrimination System Survey
Water Column, Interstitial Pore Water, and Surficial-Sediment Sampling
Quality Assurance and Quality Control
Characterization of the Lake Environment
Submerged Aquatic Vegetation
Water Quality of the Water Column and of Interstitial Pore Water
Chemical Characteristics of the Surficial Sediment on the Lake Floor
Characterization of the Distribution of Variable Leaf Water-Milfoil
Summary and Conclusions
Appendix 1. Site Names, Locations, Water Depths, Textural Compositions, Median Surficial-Sediment Grain Sizes, Standard Deviations, Skewness, Kurtosis, and “Verbal Equivalents” (Shepard, 1954) for Sediment Samples Collected in Moultonborough Bay, Lake Winnipesaukee, New Hampshire, July 2005
2–3. Maps showing—
|2.||Study area for Moultonborough Bay, Lake Winnipesaukee, New Hampshire.|
|3.||Water- and sediment-sampling sites in Moultonborough Bay, Lake Winnipesaukee, New Hampshire.|
|4.||Schematic diagram showing geophysical and sampling tools used to map lake-floor environments.|
|5.||Map showing sidescan-sonar imagery from Moultonborough Bay, Lake Winnipesaukee, New Hampshire, and the locations and names of the sites where samples were taken to provide ground truth for sonar data.|
|6.||Graph showing measures chosen to describe the grain-size distribution of sediment samples include the 16th (d16: fine fraction), 50th (d50: median grain size), and 84th (d84: coarse fraction) percentiles of the cumulative-frequency distribution curves.|
|7.||Map showing bathymetry for Moultonborough Bay, Lake Winnipesaukee, New Hampshire.|
|8.||Graphs showing seismic-reflection profile in Moultonborough Bay, Lake Winnipesaukee, New Hampshire: (A) original image and (B) interpreted profile showing bedrock, glaciolacustrine sediments, and lacustrine sediments.|
9–12. Maps showing—
|9.||Depth to bedrock as measured from the lake surface in Moultonborough Bay, Lake Winnipesaukee, New Hampshire, determined through the use of seismic-reflection data.|
|10.||Total sediment thickness in Moultonborough Bay, Lake Winnipesaukee, New Hampshire, determined through the use of seismic-reflection data.|
|11.||Three lake-floor environments in Moultonborough Bay, Lake Winnipesaukee, New Hampshire, as interpreted from geophysical and sediment-sample data.|
|12.||Distribution and percentage of the lake floor covered by submerged aquatic vegetation in Moultonborough Bay, Lake Winnipesaukee, New Hampshire.|
|13.||Graphs showing a comparison of data from the acoustic ground-discrimination system survey reports for (A) vegetative cover, (B) reflected acoustic intensity of the sediments and vegetation, and (C) fractal dimension among variable leaf water-milfoil sites, other-SAV sites, and no-SAV sites in Moultonborough Bay, Lake Winnipesaukee, New Hampshire.|
|14.||Map showing distribution and percentage of the lake floor covered by variable leaf water-milfoil and areas susceptible to variable leaf water-milfoil growth in Moultonborough Bay, Lake Winnipesaukee, New Hampshire.|
15–19. Graphs showing—
|15.||A comparison of water depth among the variable leaf water-milfoil sites, other-SAV sites, and no-SAV sites in Moultonborough Bay, Lake Winnipesaukee, New Hampshire.|
|16.||A comparison of iron concentrations in water-column samples among the variable leaf water-milfoil sites, other-SAV sites, and no-SAV sites in Moultonborough Bay, Lake Winnipesaukee, New Hampshire.|
|17.||A comparison of surficial-sediment grain size for (A) the fine fraction, 16th percentile (d16), (B) the median grain size, 50th percentile (d50), and (C) the coarse fraction, 84th percentile (d84) among variable leaf water-milfoil sites, other-SAV sites, and no-SAV sites in Moultonborough Bay, Lake Winnipesaukee, New Hampshire.|
|18.||Water depth and the median (d50) surficial-sediment grain size for all study sites in Moultonborough Bay, Lake Winnipesaukee, New Hampshire.|
|19.||Concentrations of total ammonia plus organic nitrogen from interstitial pore-water samples and the median (d50) surficial-sediment grain size for all study sites in Moultonborough Bay, Lake Winnipesaukee, New Hampshire.|
Argue, D.M., Kiah, R.G., Denny, J.F., Deacon, J.R., Danforth, W.W., Johnston, C.M., and Smagula, A.P., 2007, Relation of lake-floor characteristics to the distribution of variable leaf water-milfoil in Moultonborough Bay, Lake Winnepesaukee, New Hampshire, 2005: U.S. Geological Survey Scientific-Investigations Report 2007–5125, 38 p.
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U.S. Geological Survey
New Hampshire-Vermont Water Science Center
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Pembroke, NH 03275
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