Analysis of Optical Spikes Reveals Dynamics of Aggregates in the Twilight Zone

• Main Author: Briggs, Nathan
• Format: Text
• Language: unknown
• Published: DigitalCommons@UMaine 2010
• Subjects: Algal blooms; North Atlantic Ocean; Biogeochemical cycles; Phytoplankton; Ecology and Evolutionary Biology; Oceanography; Oceanography and Atmospheric Sciences and Meteorology; Terrestrial and Aquatic Ecology; Iceland; North Atlantic
• Online Access: https://digitalcommons.library.umaine.edu/etd/161; https://digitalcommons.library.umaine.edu/context/etd/article/1168/viewcontent/BriggsN2010.pdf

The "biological pump," whereby phytoplankton grow in the surface ocean, aggregate, and sink, is a critical process contributing to global atmospheric CO2 drawdown and provides the vast majority of food for deep ocean and benthic ecosystems. The strength of this pump hinges on the amount of material that stick together to form larger aggregates, the sinking rates of these aggregates, and the rate at which they are consumed as they sink. However, marine aggregates, also called "marine snow," are often fragile and notoriously difficult to sample, their sinking rates are highly variable and difficult to quantify, and their concentrations can vary greatly over short periods of time and space during a phytoplankton bloom. Here we present a method for addressing some of these problems and through the analysis of "spikes" that aggregates cause in the signals of low-power optical instruments. As part of the North Atlantic Bloom 2008 project, optical backscatter, attenuation, and fluorescence data were measured on four Seagliders and four cruises south of Iceland for three months beginning April 2008. Ships and gliders followed a Lagrangian mixed-layer float that tracked a single patch of water. We first compare the timing and density of spikes recorded on different optical instruments aboard gliders and ships and find strong agreement in relative spike signals. We then use the optical spike signals to make inferences about aggregate dynamics and produce the following estimates. Aggregates are produced in large numbers during the height of the spring bloom and sink at a rate of -75 m d"1. They produce a peak 2-day average 200 m carbon flux of -540-740 mg C m"2 d"1, which decreased by -50% by 900 m. These results broadly agree broadly with previous results from the literature and independent carbon export estimates from the North Atlantic Bloom 08 project.