Variational Autoencoder Framework for Hyperspectral Retrievals (Hyper-VAE) of Phytoplankton Absorption and Chlorophyll a in Coastal Waters for NASA's EMIT and PACE Missions

Phytoplankton is an extremely diverse set of microorganisms, varying in cell morphologies, biogeochemical functions, and physiological responses to environmental disturbances [21]. As the primary producer of ocean's food web, phytoplankton produces approximately 50 percent of Earth's oxygen, regulate the global carbon cycle and climate, and support various ecosystem services, such as fisheries, water quality, and biodiversity [7]. Therefore, knowledge of phytoplankton biomass and their community composition is critical to understanding the food web structure, higher trophic level production (e.g., fisheries), and biological shifts among other complex Earth Science questions, especially in the context of degraded water quality (e.g., eutrophication) and climate change (e.g., warming temperatures), demanding attention at local, regional, and global scales [9]. As such, there is an increasing interdisciplinary interest in studying phytoplankton community dynamics in estuarine-coastal waters, where massive riverine inputs of nutrient-rich freshwaters often lead to eutrophication, harmful algal blooms (HABs), and the annual recurrence of bottom-water hypoxia events, which cause widespread and severe impacts on the aquatic ecosystem [3], [43], [57], [23], [54]. In the field of ocean color remote sensing, the concentration of chlorophyll a (Chl-a) and phytoplankton absorption properties (aphy) are two of the most commonly used metrics for assessing phytoplankton abundance and diversity in aquatic environments [41], [56]. Those phytoplankton-related satellite algorithms are rooted in the physical principle that remote sensing reflectance (Rrs, sr 1)), the ratio of water-leaving radiance to the total downwelling irradiance just above water, is determined by the inherent optical properties (IOPs), most importantly the total backscattering coefficient (btotal, m 1) and the total absorption coefficient (atotal, m 1) [38].