Conference Agenda

Inland waters provide many important ecosystem services, yet they are under pressure due to ongoing environmental change. Monitoring these aquatic ecosystems is an essential step in understanding their response to environmental pressures and in developing management and conservation strategies. The complex optical properties of inland waters, as well as their high spatial and temporal variation, present challenges for monitoring attempts. In addition to existing satellite missions, which only partly meet the observational requirements for inland waters, the upcoming Fluorescence Explorer (FLEX) imaging spectroscopy mission complements observational capacity to eventually advance aquatic research.

This contribution outlines some recent studies based on in situ measurements in Swiss lakes and demonstrates how FLEX like data can be exploited for aquatic research. We first describe a novel approach to estimate non-photochemical quenching (NPQ) from in situ measured vertical profiles of sun-induced chlorophyll fluorescence (SIF) and chlorophyll-a (CHL-a). We then demonstrate how NPQ impacts estimates of CHL-a from SIF retrievals. We also demonstrate the capacity of hyperspectral data to discriminate phytoplankton species and track seasonal phytiolankton blooms. We discuss insights gained from the above studies related to possible application fields of FLEX mission data, e.g. for estimates of phytoplankton productivity, harmful algae bloom detection, or phytoplankton diversity assessments.

To understand marine ecosystems, assessing phytoplankton physiology and species is critical, as they affect light conversion and primary production. Satellite optical missions traditionally monitor aquatic systems and phytoplankton biomass using multispectral imaging. FLEX's FLORIS instrument will measure Top-of-Atmosphere light with the high spectral resolution (~0.3 nm) needed to isolate fluorescence. Integrating FLEX with Sentinel-3 will advance global aquatic monitoring, but requires novel radiative models and algorithms, particularly utilizing O2 absorption bands, to accurately separate fluorescence.

The FLEX satellite, with moderate spatial (300m) and very high hyperspectral resolution (~0.3 nm, 500–780 nm), will orbit in tandem with Sentinel-3 for crucial atmospheric correction and complementary spectral coverage. Algorithm development and water parameter retrieval require radiative transfer simulations mimicking this synergy. Water-leaving radiance depends on optically active constituents and their optical properties, including inelastic scattering (Raman, fluorescence). To reduce cost, we propose a parameterized look-up table model that accelerates very-high spectral resolution (0.1 nm) computations, incorporating elastic and inelastic scattering to determine radiance/reflectance at the bottom and top of the atmosphere, which is then convolved with FLEX (FLORIS) and Sentinel-3 (OLCI/SLSTR) spectral responses.

A preliminary study identified global priority areas for FLEX based on radiometric/spectral needs and maximizing scientific return, including Marine Protected Areas, aerosol deposition impact, and historical data analyses (Chlorophyll-a, Rrs at 665 nm, clear-sky availability). Seasonal priority rankings were computed, and radiative transfer simulations were tailored to these areas. Simulations are validated using hyperspectral data (PRISMA, EnMAP, PACE) at the top and bottom of the atmosphere, supplemented by field hyperspectral measurements, including HYPERNETS and our own acquisitions from radiometers like FLOX (matching FLORIS's spectral characteristics). This study investigates phytoplankton hyperspectral signatures based on synthetic or actual measurements mimicking the future Sentinel-3/FLEX synergy data. Based on these simulations and in situ data, we present the potential of the FLEX mission for hyperspectral remote sensing of aquatic environments and phytoplanktonic ecosystems.

The AquaValiX consortium brings together scientists from four German research institutes with long-standing expertise in hyperspectral remote sensing of aquatic environments and validation of satellite missions. Its aim is to validate and extend products from the ESA Fluorescence Explorer (FLEX) for inland, coastal and oceanic waters. Validation plans include (1) direct comparison of satellite‑derived reflectance with simultaneous in‑situ measurements; (2) indirect validation through inversion of radiative‑transfer models; and (3) inter‑comparison of FLEX outputs with products from Sentinel‑3 OLCI, Sentinel‑2 MSI, PACE and EnMAP. Over the next years, coordinated field campaigns are planned in Germany, other parts of Europe, the Canadian Arctic and New Zealand. Measurements will cover high-resolution spectral absorption, scattering and water-leaving reflectance, as well as phytoplankton pigment composition and inelastic scattering processes (fluorescence by chlorophyll, phycobilin, and yellow substance). These observations, complemented by drone surveys, laboratory analyses and biogeo-optical modelling, will be integrated with solar radiative-transfer simulations (SCIATRAN, WASI, Hydrolight) to provide optical closure between water constituents, their inherent optical properties and the satellite signal. By characterizing phytoplankton fluorescence and pigment signatures, the project will assess the added value of FLEX +OLCI‑3 synergy and the development of new products such as harmful‑algal‑bloom detection and phenology monitoring. The work plan is organised into four inter‑linked work packages: WP 1 ensures joint campaign coordination, data quality control and FAIR data release. WP 2 generates a public, high‑quality in‑situ reference dataset from lakes (e.g., Lake Constance) and coastal/polar waters using hyperspectral radiometers, UAV imaging, autonomous surface vehicles and comprehensive laboratory analyses. WP 3 focuses on the adaption of radiative‑transfer models to retrieve sun‑induced chlorophyll fluorescence (SIF) at 687 nm and 760 nm and the production of prototype Level‑2B products that can be upscaled with Sentinel‑2 MSI and Sentinel‑3 OLCI imagery. Finally, in WP 4, we will work towards an implementation roadmap for a permanent hyperspectral calibration/validation station at a reference lake, defining technical specifications, maintenance procedures and open-data governance.

In this contribution, we also describe the spectral features that only become visible through high-resolution spectral sensors (in situ and from space). In addition to fluorescence effects, this refers primarily to phytoplankton communities and the effects of pigments, colour, shape and size distribution of algae.