Sea ice algae chlorophyll a concentrations derived from under‐ice spectral radiation profiling platforms

Abstract Multiscale sea ice algae observations are fundamentally important for projecting changes to sea ice ecosystems, as the physical environment continues to change. In this study, we developed upon previously established methodologies for deriving sea ice‐algal chlorophyll a concentrations (chl a ) from spectral radiation measurements, and applied these to larger‐scale spectral surveys. We conducted four different under‐ice spectral measurements: irradiance, radiance, transmittance, and transflectance, and applied three statistical approaches: Empirical Orthogonal Functions (EOF), Normalized Difference Indices (NDI), and multi‐NDI. We developed models based on ice core chl a and coincident spectral irradiance/transmittance ( N  = 49) and radiance/transflectance ( N  = 50) measurements conducted during two cruises to the central Arctic Ocean in 2011 and 2012. These reference models were ranked based on two criteria: mean robustness R 2 and true prediction error estimates. For estimating the biomass of a large‐scale data set, the EOF approach performed better than the NDI, due to its ability to account for the high variability of environmental properties experienced over large areas. Based on robustness and true prediction error, the three most reliable models, EOF‐transmittance, EOF‐transflectance, and NDI‐transmittance, were applied to two remotely operated vehicle (ROV) and two Surface and Under‐Ice Trawl (SUIT) spectral radiation surveys. In these larger‐scale chl a estimates, EOF‐transmittance showed the best fit to ice core chl a . Application of our most reliable model, EOF‐transmittance, to an 85 m horizontal ROV transect revealed large differences compared to published biomass estimates from the same site with important implications for projections of Arctic‐wide ice‐algal biomass and primary production.