Comparison of HPLC Derived Phytoplankton Pigments from Autonomously Collected Samples and CTD Methods to Evaluate the Integration of Autonomous Vehicles as Platforms for Enhancing Ocean Time-series Programs.

Phytoplankton pigment concentrations at the Bermuda Atlantic Time-series Study (BATS) site have been measured consistently every month for the past 30 years providing an excellent framework for understanding seasonal cycling, inter-annual variability and diagnosing long term changes in the phytoplankton community. Standard sampling consists of filtering 4l of seawater at 12 standard depths in the upper ocean from a single CTD profile and though maintaining consistent sampling techniques ensures comparability of results such practices may also be restrictive in exploring higher vertical resolution or spatial gradients. Typically each BATS cruise involves multiple CTD profiles at the nominal BATS location plus a number of profiles at spatial stations over a 5-7 day period, which reveals extensive temporal and spatial variability, in CTD fluorometry. This variability is particularly evident in the depth and structure of the Deep Chlorophyll Maximum (DCM), which often exhibits sharp gradients. Clio is an autonomous underwater vehicle specifically developed for the purpose of collecting high volume particulate seawater samples at high depth resolution and thus has the potential to collect vertical profiles more accurately and efficiently during a research cruise. In order to evaluate the effectiveness of integrating an autonomous system such as Clio into a time series, we investigate the differences in HPLC phytoplankton pigment from samples collected by Clio and BATS methods during a transect cruise from the BATS site to Woods Hole in June 2019. We assess the differences in the major and minor pigments and potential impacts on evaluating the community composition, while highlighting the enhanced vertical definition provided by the Clio samples and the potential role of autonomous vehicles in collecting particulate type samples on time series and transect cruises.