INTER-COMPARISON OF OCEAN COLOUR DATA PRODUCTS DURING ALGAL BLOOMS IN THE NORTH SEA

ABSTRACT Several satellite-borne ocean colour Earth observation (EO) sensors are presently collecting data on an operational basis, including the Envisat MERIS sensor. This study aims at detecting differences in ocean colour EO sensor performances for ocean monitoring and in particular the study of algal bloom situations. The motivation for the present study is to explore how data from different sensors can be utilized in one system for HAB detection and monitoring. Ocean products from the MERIS, MODIS/Aqua, and SeaWiFS sensors have been processed and inter-compared for data acquired during the development of an early spring algal bloom in 2004 in the North Sea region. The study assesses the comparability of these OC sensors in the cases of bloom and non-bloom situations. 1. INTRODUCTION The Nansen Environmental and Remote Sensing Centre (NERSC) has since 1998 developed and operated a near real time system for algal bloom monitoring based on ocean colour satellite data from the Sea-viewing Wide Field of View (SeaWiFS) sensor. The system provides daily information about the abundance of phytoplankton in the North Sea and Skagerrak regions, provided via the web. In cooperation with Plymouth Marine Laboratory images of the surface distribution of chlorophyll are used in conjunction with other observations and data. In the event of harmful algal blooms (HABs) such information is essential for the national fishery authorities in order to implement mitigation actions as well as for the fish farming industry. The main goals of the system are to early detect and subsequently monitor the spatial distribution and intensity of phytoplankton bloom development and decay. In case of identified HAB events the satellite data are used to initiate and optimize dedicated field observations in order to improve the understanding the HAB triggering and growth mechanisms. With the launch of several new ocean colour satellite sensors, an increased amount of Earth Observation (EO) data is available. However, the sensors have different design and specifications, all with the aim to among other map the phytoplankton distribution. The Medium Resolution Imaging Spectrometer Instrument (MERIS) onboard Envisat was launched in 2002, and the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Aqua and Terra satellites were launched in 2002 and 1999, respectively. SeaWiFS has been operational since 1997. By merging information from these sensors, improved temporal resolution and spatial coverage can be obtained, while an extensive time series of ocean colour EO data is gathered. The motivation for the present study is to explore how ocean colour data from different sensors can contribute in improving a pre-operational system for HAB detection and monitoring. Ocean products from the MERIS, MODIS/Aqua, and SeaWiFS sensors have been processed and compared. In particular the consistency between the standard chlorophyll products of the three sensors have been evaluated. For the end user it is essential that regular and equivalent information is made available independent of the EO sensor used. The validity of the used ocean colour products is limited to clear (Case 1) waters while the region under consideration contains sub-regions that often are optically complex (Case 2) waters. However, there are several reasons why we still restrict the present analysis to standard chlorophyll