Wetlands support some of the highest biodiversity in the world. Macrophytes (aquatic plants which grow in/near water) are of particular importance in aquatic ecosystems, as they link the sediment with the overlying water. They provide a habitat for fish and substrate for aquatic invertebrates, offering protection against both currents and predators. They also play a role in nutrient and carbon cycles (CO2 and CH4).

Phytoplankton are another critical component of water-related ecosystems. As prolific primary producers they play a key role in sustaining life and conditioning the underwater light climate around the globe.

Phytoplankton are an indicator of trophic status (water ecosystem productivity); a temporal shifting of bloom phenology as well as the spreading of cyanobacterial blooms might be an indicator of a lake’s response to climate change. The occurrence of harmful algal blooms (HABs) – also often associated with cyanobacteria – might hinder the use of water resources. The measurement of Phytoplankton production/biomass is therefore a key part of water quality monitoring programs worldwide.

Earth Observation Data Use

• Landsat
• Sentinel-2/3
• Historical digital archives of satellite imagery (Landsat, Envisat, Sentinel-2/3, hyperspectral sensors).
• High-resolution airborne imaging spectrometry
• In-situ data (from Europe, China; LIMNADES, LTER)
• Ocean colour radiometry

Methodology

Satellite imagery is used to map the extent and distribution of macrophytes; and to assess biomass, biodiversity, and the presence of invasive species. Rule-based classifications of floating/emerging macrophyte types are based on atmospherically corrected, multi-temporal satellite data. Vegetation indices and in-situ data are used to map fractional cover, leaf area index (LAI), and above-water biomass. Hyperspectral inversion of bio-optical modelling is used for mapping the fractional cover of submerged macrophytes.

Phytoplankton are observed by proxy, via biomass pigment concentrations, mainly the concentration of Chlorophyll-a (through sun-induced fluorescence), secondary pigments, and harmful algal blooms (HABs). Scattering-based approaches in red wavelengths are also applicable in high-biomass waters.

Key Issues and Results

Using Earth observation to monitor macrophytes and phytoplankton supports the implementation of several EU policies (EC Habitat/WFD directives, and EC WFD and Bathing directives, respectively), and the generation of maps and forecasts is central to a number of Horizon 2020 projects (e.g., EOMORES, ECOPOTENTIAL; and CYANOLAKE, respectively).

Analysis, Status, and Outlook

Earth observation techniques are a step forward for macrophyte and phytoplankton mapping, going beyond the local-scale and supporting regional to continental monitoring of the spatial and temporal dynamics of primary producers in freshwater ecosystems. Considering the vast expanse of inland waters, their spatial heterogeneities and the high-degree of temporal changes, EO is a powerful tool for assessing water-related ecosystems.

Provided that suitable instruments remain in service, methods will continue to improve as they mature into generally accepted best-practice science, offering considerable advantages for a wide range of practical and scientific applications. The continuity of image archives and polar orbiting, global satellite missions to capture trends and ensure repeatable monitoring for ecosystem studies is critical.