Evaluation of a satellite-derived Astaxanthin Equivalent Index against in situ zooplankton and right whale observations in the Gulf of Maine

Ocean color remote sensing has primarily been used to monitor surface phytoplankton distributions, but recent developments have extended its application to detecting higher trophic-level organisms. The copepod Calanus finmarchicus contains the carotenoid pigment astaxanthin, which has specific optical signals that can be detected by satellite sensors. In the Gulf of Maine (GoM), C. finmarchicus plays a critical ecological role as the dominant copepod species and a key prey item for the endangered North Atlantic right whale (Eubalaena glacialis). This study, focusing on the GoM, evaluates a satellite-derived Astaxanthin Equivalent Index (AEI), as a proxy for surface abundance of astaxanthin-rich zooplankton. MODIS-Aqua satellite AEI estimates from 2003 to 2022 were compared with stage- and species-resolved in situ zooplankton data from Continuous Plankton Recorder near-surface transects, as well as with depth-stratified net tows and conical-net surface and oblique tows from other programs. In situ abundance values were converted into AEI units (μg/m3) using stage- and species-specific pigment scaling. While the AEI was calibrated for C. finmarchicus, comparisons indicated that the satellite signal also included contributions from other astaxanthin-containing taxa, particularly Centropages typicus. Stronger correspondence was observed when astaxanthin content from both C. finmarchicus and C. typicus was considered, but the degree of correlation varied across datasets. This variability likely reflects satellite overestimation due to other astaxanthin-rich signals and may also be influenced by differences in in situ sampling methods. High satellite AEI values co-occurred with behavioral observations of right whale surface skim-feeding in Cape Cod Bay, indicating that AEI products may help characterize potential feeding habitats for the species. These comparisons highlight the potential of astaxanthin-based satellite indices to complement field observations in zooplankton monitoring and right whale habitat assessment.