Résumé
Abstract
This study investigates the spatial distribution and temporal variability of chlorophyll-a (Chl-a)
biomass in the Algerian Basin (AB) along its meridional and cross-shore dimensions, focusing on
coastal enrichments. After correcting most atmospheric disturbances in the daily MODIS Level-2 data
series between 2003 and 2018, a fortnightly climatology of 1-km resolution Chl-a has been generated
to account for specific coastal features previously poorly evidenced from 4-km level-3 data. The AB is
characterised by two extreme seasons of high and low biomass, separated by sharp transitions, that
characterise the offshore domain. The coastal area (<10 km) reveals an intense and distinct dynamic
associated with highly productive local hotspots rather than seasonal variability. A biomass index is
proposed as the horizontally integrated Chl-a concentration from the coastline to the most offshore
extension of the 0.5 mg m
-3
Chl-a isopleth. This index separately quantifies the cumulative biomass of
both offshore and coastal domains with large alongshore variability. Low values (<5 g m
-2
) were
observed in the offshore area during summer and high values during the spring blooms (up to 40 g m
-
2
), while maximum values (>50 g m
-2
) were locally observed in the coastal domain. The narrow coastal
area alone represents 44% of the total biomass, with coastal hotspots where the enrichment is up to 5
times higher than offshore. Multivariate modelling of the potential factors favouring coastal
enrichments shows that the phytoplanktonic biomass in coastal waters is mainly associated with
enrichments from wadis and seasonally from city sewage as well as by the presence of a bay.
The suspended particulate matter concentration in the AB show strong seasonal and longitudinal
variability in coastal waters, essentially conditioned by the variations in the liquid flow of the wadis
and the coastal morphology (bay, golf, …). Two types of coastal waters have been defined in the AB:
(i) Case-1 waters are in the western part of the basin, where the coastline is generally linear, (ii) Case2 waters are found in the eastern part of the basin, these waters are a mixture of phytoplankton
pigments and suspended particulate matter. In the Case-2 waters, it is recommended to use specific
bio-optical algorithms. In many areas of the Algerian coast, sandy bottoms can significantly bias the
estimation of Chl-a concentration from space.
6
Abstract
This study investigates the spatial distribution and temporal variability of chlorophyll-a (Chl-a)
biomass in the Algerian Basin (AB) along its meridional and cross-shore dimensions, focusing on
coastal enrichments. After correcting most atmospheric disturbances in the daily MODIS Level-2 data
series between 2003 and 2018, a fortnightly climatology of 1-km resolution Chl-a has been generated
to account for specific coastal features previously poorly evidenced from 4-km level-3 data. The AB is
characterised by two extreme seasons of high and low biomass, separated by sharp transitions, that
characterise the offshore domain. The coastal area (<10 km) reveals an intense and distinct dynamic
associated with highly productive local hotspots rather than seasonal variability. A biomass index is
proposed as the horizontally integrated Chl-a concentration from the coastline to the most offshore
extension of the 0.5 mg m
-3
Chl-a isopleth. This index separately quantifies the cumulative biomass of
both offshore and coastal domains with large alongshore variability. Low values (<5 g m
-2
) were
observed in the offshore area during summer and high values during the spring blooms (up to 40 g m
-
2
), while maximum values (>50 g m
-2
) were locally observed in the coastal domain. The narrow coastal
area alone represents 44% of the total biomass, with coastal hotspots where the enrichment is up to 5
times higher than offshore. Multivariate modelling of the potential factors favouring coastal
enrichments shows that the phytoplanktonic biomass in coastal waters is mainly associated with
enrichments from wadis and seasonally from city sewage as well as by the presence of a bay.
The suspended particulate matter concentration in the AB show strong seasonal and longitudinal
variability in coastal waters, essentially conditioned by the variations in the liquid flow of the wadis
and the coastal morphology (bay, golf, …). Two types of coastal waters have been defined in the AB:
(i) Case-1 waters are in the western part of the basin, where the coastline is generally linear, (ii) Case2 waters are found in the eastern part of the basin, these waters are a mixture of phytoplankton
pigments and suspended particulate matter. In the Case-2 waters, it is recommended to use specific
bio-optical algorithms. In many areas of the Algerian coast, sandy bottoms can significantly bias the
estimation of Chl-a concentration from space.
6