Including the invisible: Deep depth-integrated chlorophyll estimates from remote sensing may assist in identifying biologically important areas in oligotrophic coastal margins

Abstract. Deep chlorophyll maxima (DCM) are common in stratified water columns and may support higher trophic levels. Yet, it is challenging to include DCM contributions in studies aiming to identify marine animal foraging habitats and hotspots, because these studies often rely on satellite remote sensing data restricted to the surface. Previously established quantitative relationships between surface and depth-integrated chlorophyll within the euphotic zone of the open ocean and a eutrophic coastal margin encouraged us to assess whether such relationships are also present within the Western Australian intermittent-oligotrophic coastal margin. We also assessed whether the relationships could be extended to greater depths to capture DCMs below the euphotic zone. Based on ~9600 ocean glider profiles, our analyses demonstrate that such a relationship similarly exists off Western Australia and can be extended to twice the euphotic zone depth. Regression parameters were fine-tuned for three different conditions: 1) stratified waters in summer-transition months (September–April), characterised by relatively deep biomass maxima; 2) stratified waters in mid-winter (May–August) in which DCMs were less common and more likely a photo-acclimation maximum; and 3) mixed waters. While mean absolute errors increased in relationships over twice the euphotic zone depth (i.e., for estimates of deep depth-integrated chlorophyll), they remained low (i.e., max 16.5 %). These results and an observed chlorophyll increase in summer, unique to deep depth-integrated values, highlight the necessity to include deep depth-integrated chlorophyll estimates from satellite remote sensing in studies that aim to identify biologically important areas and productivity anomalies in (intermittent) oligotrophic environments.