Abstract
The ocean’s mesopelagic zone is largely uncharacterized despite its vital role in sustaining ocean ecosystems. The composition, cycling, and fate of particle fields in the mesopelagic lacks an integrative multi-scale understanding of organism migration patterns, distribution, and diversity. This problem is addressed by combining complementary technologies with overlapping size spectra, including profiler mounted optical scattering sensors, profiler, and ship mounted acoustic devices, and a custom Unobtrusive Multi-Static Lidar Imager (UMSLI). This unique sensor suite can observe distributions of particles including organisms over a six order of magnitude dynamic size range, from microns to meters. Overlapping size ranges between different methods allows for cross-validation. This work focuses on the lidar imaging measurements and optical backscattering and attenuation, covering a combined particle size range of 0.1 mm to several cm. Particles at the small end of this range are sized using an existing backscattering time series inversion method after Briggs et al. (2013). Larger particles are resolved with UMSLI over an expanding volume using three-dimensional photo-realistic laser serial imaging. UMSLI’s image rectifying ability over time allows for derivation of particle concentration, size, and spatial distribution. Technical details on the development and post-processing methods for the novel UMSLI system are provided. Image resolved particle size distributions (PSDs) revealed a size shift from smaller to larger particles (>0.5 mm) as indicated by flatter slopes from dawn (slope = 2.6) to dusk (slope = 3.0). PSD trends are supported by an optical backscatter and transmissometer time series inversion analysis. Size shifts in the particle field are largely attributed to aggregation effects. Images support evidence of temporal variation between dusk and dawn stations through statistical analysis of particle concentrations for particle sizes 0.50–5.41 mm. Spatial analysis of the particle field revealed a dominantly uniform distributed marine snow background. The importance and potential of integrated approaches to studying particle and organism dynamics in ocean environments are discussed.
Highlights
The Earth’s oceans are a reservoir for an estimated 31% of anthropogenically produced CO2
Throughout this section, emphasis is on the dense and densest lidar scan modes, which have the highest resolution for particle sizes and the most overlap given their relatively close scan angles
The sparse scan mode, while still valuable, is better adapted toward identifying marine fish and larger organisms, which is beyond the size range focus of this analysis
Summary
The Earth’s oceans are a reservoir for an estimated 31% of anthropogenically produced CO2. Of the 100 Gt of organic carbon taken up by ocean surface waters per year, up to 10% is transferred to the mesopelagic (similar magnitude as fossil fuel emissions) (Gardner et al, 2000; Giering et al, 2014, 2018) Without this carbon export mechanism, the concentration of CO2 in the atmosphere would be an estimated 200 ppm higher than present day levels (McDonnell, 2011; Davison et al, 2013; Volk and Hoffert, 2013). Understanding the role of the mesopelagic zone (bottom of euphotic zone to 1,000 m) in mediating remineralization and transport to the deep ocean is currently a subject of intense research (Giering et al, 2019; Briggs et al, 2020)
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