Remote sensing’s contribution to evaluating eutrophication in marine and coastal waters

(2002) Remote sensing’s contribution to evaluating eutrophication in marine and coastal waters. Technical Report. European Environment Agency, Copenhagen, Denmark.

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Summary

The objective of this task was to evaluate estimates of ’chlorophyll-a-like-pigments’ by satellite with focus on the SeaWIFS sensor (Sea-viewing Wide-Field-of-View Sensor). Chlorophyll-a maps were controlled against in situ data from the Skagerrak, Kattegat and the North Sea for the period September 1997 to the end of 1999. Based on in situ data the chlorophyll-a maps were tuned to reflect ’true surface concentrations’ of ’chlorophyll-a-like-pigments’. The information and concentrations in the maps were locally compared with in situ data and a priori general knowledge of the area. The algorithms to produce the ’chlorophyll-a-like-pigments’ from the water using radiance from SeaWIFS observations are presently overestimating the ’true’ concentration. In the open sea of the tested areas the overestimation is in the order of 60-70 %. It was not the objective of this study to investigate the scientific rationale behind these findings. The information in terms of necessary in situ data such as the different optical quantities of suspended and dissolved organic material as well as atmospheric data to do such a study was either not available or limited as this kind of data is not collected in ordinary monitoring programmes. We have therefore tuned the satellite images to in situ concentrations from empirical statistical analysis only. The most severe problems retrieving a correct chlorophyll-a concentration from satellite images was in the near coastal areas, and the problems with overestimation of chlorophyll-a were increasing when approaching less saline water as at the Norwegian coast. The water masses in this area have normally high concentrations of dissolved organic material that influences the retrieval algorithms for chlorophyll-a. But knowing the limitation of the data both before and after rescaling to in situ concentrations the satellite data gives important information on the environment. The satellite data gives good information not only on the relative phytoplankton distribution, but also the concentrations when tuning the SeaWIFS-data to the surface concentrations using in situ data from the area. Using monthly mean maps in the period May to August gives much information on the phytoplankton biomass in the period when the production most often is nutrient limited. The mean or median value in this period would be the best and most appropriate expression for the level of eutrophication. Very limited satellite data was available from the winter period November to February in this area. If the goal is to use the satellite data to map eutrophication this is not critical, since this is a period when the phytoplankton is light-limited, and is therefore not related to the nutrient input. The chlorophyll-a maps from the period March to April/May reflect realistically the magnitude and geographical distribution of the spring bloom, and therefore also the level of eutrophication. Since the spring bloom could reflect the magnitude of the available winter nutrient pool, a chlorophyll-a map showing the maximum concentration in each pixel should be useful for studying the eutrophication level. In general, the satellite data and information that has been investigated in this study gives promising possibilities to improve monitoring by a combination of in situ data and satellite data. New satellite sensors with improved spatial and spectral resolution, and new insight in solving the problems with the algorithms will improve the accuracy of the satellite data products. From a eutrophication monitoring point of view the number of stations may be reduced in the open sea areas when the quality of the satellite data are controlled on other stations with in situ data.

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