Abstract
Changes in airborne high spectral resolution lidar (HSRL) measurements of scattering, depolarization, and attenuation coincided with a shift in phytoplankton community composition across an anticyclonic eddy in the North Atlantic. We normalized the total depolarization ratio (delta) by the particulate backscattering coefficient (b(b)(p)) to account for the covariance in delta and b(b)(p) that has been attributed to multiple scattering. A 15% increase in delta/b(b)(p) inside the eddy coincided with decreased phytoplankton biomass and a shift to smaller and more elongated phytoplankton cells. Taxonomic changes (reduced dinoflagellate relative abundance inside the eddy) were also observed. The delta signal is thus potentially most sensitive to changes in phytoplankton shape because neither the observed change in the particle size distribution (PSD) nor refractive index (assuming average refractive indices) are consistent with previous theoretical modeling results. We additionally calculated chlorophyll-a (Chl) concentrations from measurements of the diffuse light attenuation coefficient (K-d) and divided by b(b)(p) to evaluate another optical metric of phytoplankton community composition (Chl:b(bp)), which decreased by more than a factor of two inside the eddy. This case study demonstrates that the HSRL is able to detect changes in phytoplankton community composition. High spectral resolution lidar measurements reveal complex structures in both the vertical and horizontal distribution of phytoplankton in the mixed layer providing a valuable new tool to support other remote sensing techniques for studying mixed layer dynamics. Our results identify fronts at the periphery of mesoscale eddies as locations of abrupt changes in near-surface optical properties.
Highlights
An understanding of phytoplankton community composition and its changes through time is critical for determining the ecological and biogeochemical role of water masses (Richardson and Jackson, 2007; Uitz et al, 2010)
Backtracking of water parcels advected by geostrophic currents indicates that the anticyclonic eddy sampled by the ship and aircraft had been recirculating during the previous several weeks, with only a small contribution of waters coming from roughly 45◦S (Figures 3c,d)
Even though the altimetry-derived geostrophic currents do not necessarily represent the currents observed within the mixed layer to which the phytoplankton community is constrained, it is reasonable to assume that advective influences were small compared to those outside the eddy and had only a minor impact on community composition, suggesting that the ship-based measurements should be comparable to airborne high spectral resolution lidar (HSRL) data collected 11 days later
Summary
An understanding of phytoplankton community composition and its changes through time is critical for determining the ecological and biogeochemical role of water masses (Richardson and Jackson, 2007; Uitz et al, 2010). Multiple algorithms for retrieving phytoplankton functional types (PFTs; i.e., phytoplankton groups that share a similar biogeochemical role) are available and applied in both coastal and open ocean environments (Kostadinov et al, 2010; Werdell et al, 2014; Uitz et al, 2015; Mouw et al, 2017) These algorithms use the magnitude and/or spectral shape of satellite remote-sensing reflectance to derive properties (e.g., chlorophyll, inherent optical properties) associated with different PFTs. In contrast, the ocean-optimized high spectral resolution lidar (HSRL) directly measures the particulate backscattering coefficient (bbp) and depolarization ratio (δ), measurements which have previously been used to characterize particles in the ocean but with limited validation with in situ measurements (Fry and Voss, 1985; Loisel et al, 2008; Fournier and Neukermans, 2017). The HSRL returns profiles of ocean properties to approximately 2.5–3 optical depths (Schulien et al, 2017) which can yield new insights on the vertical structure of phytoplankton communities
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