Thomas O. Manley

Lakebed Pockmarks in Burlington Bay, Lake Champlain: I. Hydrodynamics and Implications of Origin

Using side-scan sonar and sub-bottom profiles, a 40 m diameter, ~4 m deep lakebed pockmark was investigated in Burlington Bay, Lake Champlain, Vermont. Five piston cores (2 inside, 1 on the rim and 2 outside of the pockmark) were collected and analyzed for magnetic susceptibility, physical properties and 210Pb and 132Cs concentrations. A yearlong subsurface mooring, equipped with an Acoustic Doppler Current Profiler (ADCP) and vertical temperature chain, was placed in the deepest section of the pockmark (27.7 m). A second temperature chain was placed along the pockmark floor and over its rim. At the base of the mooring, 35 mm stereo cameras and a Savonius rotor current meter ~ 0.3 m above the lake floor) were installed. Bottom photographs show episodic events of suspended sediment. Temperature data show the presence of anomalously cold temperature water near the rim of the pockmark during stratified periods and anomalously warmer water temperatures during isothermal winter type conditions. Coarser grain size and bulk density occur inside the pockmark with an associated decrease in water content in comparison to sediment outside the pockmark. Radionuclide results show no detectable net accumulation of sediment within the rim of the pockmark, slight accumulation in the deepest part of the pockmark, and only a few tenths of a mm/yr accumulation outside. This pockmark was most likely formed by groundwater seepage and is subsequently modified by continuing groundwater seepage and bottom-flowing currents. Taylor column and frictional boundary layer dynamics are believed to play a role in the circulation within this feature.

Numerical studies of the 4-day oscillation in Lake Champlain

The summer thermocline of Lake Champlain, which is found at depths of 20-30 m, oscillates with typical vertical amplitudes of 20-40 m and periods of ∼4 days. Fluctuations at the ends of the lake are opposite in phase and accompanied in the central lake by strong shears across the thermocline. Thcsc arc basin-wide baroclinic disturbances which are forced by wind. A numerical, one-dimensional, two-laycr, shallow-water model incorporating nonlinear and frictional effects in a rectangular basin forced by wind was first tested with idealized wind impulses. The results do not resemble the observed thermocline motion. However, when this simple model is forced with wind data from a nearby shore site, there is rcasonable agreement between the model results and observed long-period thermocline motions in Lake Champlain. Dispersion effects appear to be negligible hcre. This contrasts with othcr long, narrow lakes whcre dispersion effects are important and internal surges are followed by wave trains resembling the soliton solutions of the Korteweg-deVries equation. A possiblc explanation for the different regime in Lake Champlain may be found in its unique bathymetry with sloping bottom at the ends and numerous embayments on the sides that provide traps to collect wind-driven warm water and then release it slowly during recovery of cquilibrium, prevcnting the formation of steep fronts and soliton wave trains.

Lakebed Pockmarks in Burlington Bay, Lake Champlain II. Habitat Characteristics and Biological Patterns

Pockmarks are common features on the bottom of Lake Champlain, especially in the central part of the lake, between Burlington, Vermont and Plattsburgh, New York. Pockmarks have the potential to provide a unique bottom environment for biota because of the different water quality and habitat conditions that may exist inside and outside the pockmark. In Burlington Bay, the “General” is a large pockmark, roughly 40 meters in diameter and 4 meters in depth, located in about 28 meters of water. We collected benthic invertebrate samples inside and outside the “General” pockmark, and collected sediment, pore and overlying water for analysis. We also caged zebra mussels inside and outside the pockmark to determine whether varying water quality or food availability in the pockmark would influence these mussels.