Phytoplankton initiation bloom magnitude in Algerian continental shelf waters using 11 years of ocean color observations

摘要

Phytoplankton are photosynthetic organisms that live in the upper part of the water's surface. A rapid growth in a short time cause the appearance of blooms which can long-term impact marine ecosystem and fisheries. Chlorophyll-a (chl-a) concentrations, the proxy pigment of phytoplankton biomass, is measured with conventional in-situ analysis methods and retrieved from satellite observations. Currently, remotely sensed ocean color data constitute a rich and important spatio-temporal database that has been exploited in many scientific studies and has been shown to be relevant in phytoplankton dynamic metrics. Here in this study, we analyze 11 years of ocean color observations, MERIS (2003-2012) and MODIS (2012-2013) archives, over Algerian coasts in order to compute the chl-a concentrations corresponding to the start and end of bloom event. The determination is based on one of the most used definition of bloom in pelagic ocean, when chl-a concentrations rise above and fall below the full time series median for each pixel plus 5%. Satellite data were preprocessed considering flags to eliminate contaminated pixels, land, cloud and open water pixels. Spatial distribution mapping of results was done showing that the highest magnitudes are concentrated near the coasts. For statistical comparison purpose, the shelf waters were divided into six regions, limited from north by the 1000 m bathymetric curve and from south by Algerian coastline. The east and west limits for each region are chosen considering the marine meteorological regions. This study highlights for the first time the spatial distribution, at high resolution (250 m). of bloom initiation magnitudes along Algerian shelf waters, taking into account the influence of important wadis (small Mediterranean river) flowing there, the marine meteorology and north Algeria pluviometry. Statistics, particularly means, based on the proposed regions, seems to be relevant because they are consistent with the dominant regional pluviometry. In fact, the precipitation causes a runoff of the lands and this flow enters the sea loaded with nutrients. The obtained values can be used to determine the bloom start timing and others phenological parameters.