Shipboard Lidar as a Tool for Remotely Measuring the Distribution and Bulk Characteristics of Marine Particles

• Main Author: Brian Leigh, Brian Leigh
• Format: Text
• Language: unknown
• Published: Old Dominion University Libraries 2021
• Subjects: North Atlantic
• Online Access: https://dx.doi.org/10.25777/bt06-xm51; https://digitalcommons.odu.edu/oeas_etds/183/

Light detection and ranging (lidar) can provide remote estimates of the vertical distribution of optical properties in the ocean, potentially revolutionizing our ability to characterize the spatial structure of upper ocean ecosystems. However, challenges associated with quantifying the relationship between lidar measurements and biogeochemical properties of interest have prevented its adoption for routinely mapping the vertical structure of marine ecosystems. To address this, we developed a shipboard oceanographic lidar that measures attenuation (α) and linear depolarization (δ) at scales identical to those of in-water optical and biogeochemical measurements. The instrument’s ability to resolve the distribution of optical and biogeochemical properties was characterized during a series of field campaigns in the Mid-Atlantic Bight (MAB) and Gulf of Maine (GoM). α resolved vertical and horizontal gradients in absorption and chlorophyll concentration associated with the Chesapeake Bay outflow and distinct water masses in the GoM. δ was related to the particulate backscattering ratio, an optical proxy for particle size and composition, suggesting that δ could provide information on the material properties of marine particles. After initial characterizations, we conducted a 13-day deployment in the GoM and western North Atlantic to sample a mesoscale coccolithophore bloom. Bloom features were mapped at sub-kilometer scales and δ was used to distinguish coccoliths/coccolithophores from non-calcified particles. Finally, a model parameterized with in-water optical measurements from the bloom and laboratory linear depolarization measurements was used to explore the influence of multiple scattering and particle characteristics on measurements of δ. Single scattering measurements of δ exhibited a complex dependency on particle shape, size, and composition that was consistent with scattering calculations for non-spherical particles. Model results suggested that variability in δ was driven predominantly by shifts in particle concentration rather than their bulk characteristics. However, the behavior of δ when backscattering became decoupled from calcite could only be reproduced by including a separate coccolith particle class. Taken as a whole, this work provides new insights into the scattering nature of marine particles and the complex response of the lidar return signal to water column optical properties, and is an important demonstration of the sampling capabilities afforded by shipboard lidar.