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Observations of pockmark flow structure in Belfast Bay, Maine, Part 1: current-induced mixing

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Abstract

Field observations of current profiles and temperature, salinity, and density structure were used to examine vertical mixing within two pockmarks in Belfast Bay, Maine. The first is located in 21 m water depth (sea level to rim), nearly circular in shape with a 45 m rim diameter and 12 m rim-to-bottom relief. The second is located in 25 m water depth, more elongated in shape with an approximately 80 m (36 m) major (minor) axis length at the rim, and 17 m relief. Hourly averaged current profiles were acquired from bottom-mounted acoustic Doppler current profilers deployed on the rim and center of each pockmark over successive 42 h periods in July 2011. Conductivity–temperature–depth casts at the rim and center of each pockmark show warmer, fresher water in the upper water column, evidence of both active and fossil thermocline structure 5–8 m above the rim, and well-mixed water below the rim to the bottom. Vertical velocities show up- and down-welling events that extend into the depths of each pockmark. An observed temperature change at both the rim and center occurs coincident with an overturning event below the rim, and suggests active mixing of the water column into the depths of each pockmark. Vertical profiles of horizontal velocities show depth variation at both the center and rim consistent with turbulent logarithmic current boundary layers, and suggest that form drag may possibly be influencing the local flow regime. While resource limitations prevented observation of the current structure and water properties at a control site, the acquired data suggest that active mixing and overturning within the sampled pockmarks occur under typical benign conditions, and that current flows are influenced by upstream bathymetric irregularities induced by distant pockmarks.

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Acknowledgements

Data collected as part of this study are available at the Center for Coastal and Ocean Mapping, University of New Hampshire under the experiment name “2011 Belfast Bay Pockmark Experiment”. J. Kelley of the University of Maine provided the multibeam bathymetry map from which candidate pockmarks were identified for further consideration. This study was based on a model conceived and developed by P. Koons of the University of Maine, and the many observations and interpretations made by J. Kelley, D. Bleknap (University of Maine), W. Barnhardt (USGS), and B. Andrews (USGS). Field assistance was provided by Capt. Emily Terry, Capt. Ben Smith, and Jon Hunt. Comments by Diane Foster, Zafere Defne, Ø. Hammer, and an anonymous reviewer greatly improved the manuscript. This study was supported by the National Oceanic and Atmospheric Administration under NOAA grant NA10NOS4000073. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US Government.

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  1. Christina L. Fandel

    Present address: Hydrographic Surveys Division, Office of Coast Survey, National Oceanic and Atmospheric Administration, 1315 East-West Hwy, Silver Spring, MD, 20910, USA

Authors and Affiliations

  1. University of New Hampshire, Center for Coastal and Ocean Mapping, 24 Colovos Rd., Durham, NH, 03824, USA

    Christina L. Fandel, Thomas C. Lippmann & James D. Irish

  2. U.S. Geological Survey, 384 Woods Hole Rd., Woods Hole, MA, 02543, USA

    Laura L. Brothers

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  1. Christina L. Fandel
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  2. Thomas C. Lippmann
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Correspondence to Thomas C. Lippmann.

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Fandel, C.L., Lippmann, T.C., Irish, J.D. et al. Observations of pockmark flow structure in Belfast Bay, Maine, Part 1: current-induced mixing. Geo-Mar Lett 37, 1–14 (2017). https://doi.org/10.1007/s00367-016-0472-4

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  • DOI: https://doi.org/10.1007/s00367-016-0472-4

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