Pros & Cons of satellite chlorophyll-a for assessments.

What are the strengths of chlorophyll-a values from ocean color satellites?

Chlorophyll-a values from ocean color satellites can offer a very useful and complementary source of data, for marine assessment as they provide a cost-effective technology to monitor large and distant areas of the world ocean at time scales than enable to capture the regional and local variability derived from the dynamics of marine processes.

In the last decades, various missions with optical sensors on polar orbiting satellites (CZCS, SEAWIFS, AQUA/MODIS, TERRA/MODIS, ENVISAT/MERIS, VIIRS, etc) have provided a very valuable collection of data suitable to detect the concentration of chlorophyll-a in the surface layers. Other parameters can also be derived from these data such as water transparency, total suspended matter, pigmented fraction of dissolved organic matter, harmful algal blooms, etc.

Which precautions have to be taken to use these data in assessments?

Satellite data also have some limitations and uncertainties that have to be taken into account to use this data in an accurate way. The first one concerns the fact that only surface layers are sensed, hence subsurface peaks of chlorophyll may be underestimated or completely missed. Another limitation concerns the accuracy of the estimation of chlorophyll-a values, among all in waters with complex optical properties such as coastal waters.

In this context, what will be proposed by the CHLO4MSFD Service?

The CHLO4MSFD service will aim to demostrate the potential, usefulness, and limits of satellite data to be incorporated in the estimation of indicators and then into the assessments, highligthing their strengths into providing a monitoring coverage over large and distant areas at time scales than enable to capture the temporal variability derived from the dynamics of marine processes that affect the achievement of the Good Environmental Status targeted by MSFD.

The use of this data can also potentiate coherent and integrative assessment across EEZs and Marine regions avoiding inconsistent assessment results issued from dispare monitoring techniques and assessment methodologies.