Abstract
Profiling floats equipped with bio-optical sensors well complement ship-based and satellite ocean color measurements by providing highly-resolved time-series data on the vertical structure of biogeochemical processes in oceanic waters. This is the first study to employ an autonomous profiling (APEX) float in the Gulf of Mexico for measuring spatiotemporal variability in bio-optics and hydrography. During the 17-month deployment (July 2011 to December 2012), the float mission collected profiles of temperature, salinity, chlorophyll fluorescence, particulate backscattering (bbp), and colored dissolved organic matter (CDOM) fluorescence from the ocean surface to a depth of 1,500 m. Biogeochemical variability was characterized by distinct depth trends and local “hot spots”, including impacts from mesoscale processes associated with each of the water masses sampled, from ambient deep waters over the Florida Plain, into the Loop Current, up the Florida Canyon, and eventually into the Florida Straits. A deep chlorophyll maximum (DCM) occurred between 30 and 120 m, with the DCM depth significantly related to the unique density layer ρ = 1023.6 (R2 = 0.62). Particulate backscattering, bbp, demonstrated multiple peaks throughout the water column, including from phytoplankton, deep scattering layers, and resuspension. The bio-optical relationship developed between bbp and chlorophyll (R2 = 0.49) was compared to a global relationship and could significantly improve regional ocean-color algorithms. Photooxidation and autochthonous production contributed to CDOM distributions in the upper water column, whereas in deep water, CDOM behaved as a semi-conservative tracer of water masses, demonstrating a tight relationship with density (R2 = 0.87). In the wake of the Deepwater Horizon oil spill, this research lends support to the use of autonomous drifting profilers as a powerful tool for consideration in the design of an expanded and integrated observing network for the Gulf of Mexico.
Highlights
Long-term monitoring of carbon cycling in the oceans is required to understand oceanic ecosystem response to natural and anthropogenic perturbations, including distinguishing trends due to storm events, climate cycles, oil spills, and global warming
Profiles were often associated with the Loop Current (LC) in the upper layer, especially over the Florida Plain, demonstrating unique T-S characteristics, which were distinct from other profiles
We observed highly dynamic biogeochemistry in both the particulate and dissolved matter pools in deep waters of the southeastern Gulf of Mexico, using an APEX profiling float equipped with bio-optical sensors
Summary
Long-term monitoring of carbon cycling in the oceans is required to understand oceanic ecosystem response to natural and anthropogenic perturbations, including distinguishing trends due to storm events, climate cycles, oil spills, and global warming. As largely contributed to by phytoplanktonic carbon fixation, accounts for approximately half of the global estimated net primary production, an amount roughly equivalent to that on land [1]. This primary production represents the base of the marine food web, supporting most oceanic life and significantly affecting global biogeochemical cycles, including atmospheric CO2 uptake. Given the importance of both the marine particulate and dissolved organic matter pools, improved methods are required for jointly assessing and monitoring long-term changes due to perturbations, especially given linkages between the two components
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