Use of ship-board optical measurements to quantify plankton biomass and productivity across multiple trophic levels in the Northern Gulf of Alaska LTER program

We present a new ocean optics monitoring program for the newly-funded Northern Gulf of Alaska Long Term Ecological Research (NGA-LTER) program. Building upon prior results from the southern Gulf of Alaska region, we outline plans for the inaugural expedition aboard the UNOLS ship R/V Sikuliaq in spring 2020. Namely, the building and installation of an underway optical platform for surface water absorption/attenuation (via AC-S) and back-scatter (via BB-3) measurements to estimate phytoplankton physiology (via carbon-to-chlorophyll ratios) and productivity (NPP via CbPM) at high-resolution. Coupling these measurements to high-resolution surface nitrate (via ISUS) and iron (via trace-metal clean surface sampler) measurements, we will explore the complex interplay of light and macro/micro-nutrient limitation on the phytoplankton community both regionally, and across small scale hydrographic features. Direct on-station measurements of chlorophyll, phytoplankton carbon, POC, and net primary productivity, already in-place as part of the NGA-LTER, will ground-truth the optical data and facilitate generation of regionally-tuned bio-optical models. We will also continue to investigate the potential for high-frequency backscatter data to quantify zooplankton size and abundance. As part of the NGA-LTER, optical datasets will span multiple seasons for the next several years, providing an opportunity to establish seasonal and inter-annual variability in a region undergoing rapid change. We also present potential future directions for this program, including expansion of the underway optical system (e.g. acidification modules and imaging capability) and collection of radiometric data for remote-sensing applications, with the goal of inciting collaborative discussions.