Continental Shelf Research 232 (2022) 104629
Available online 5 December 2021
0278-4343/© 2021 Elsevier Ltd. All rights reserved.
Spatio-temporal variability of a chlorophyll-a based biomass index and
influence of coastal sources of enrichment in the Algerian Basin
Romaissa Harid
a, b, *
, Herv´ e Demarcq
b
, Mehdia-Asma Keraghel
a
, Malik Ait-Kaci
a
,
Mohamed Zerrouki
a
, Nour-El-Islam Bachari
c
, Fouzia Houma
a
a
ECOSYSMarL: Laboratoire des ´
Ecosyst` emes Marins et Littoraux, ´
Ecole Nationale Sup´ erieure des Sciences de la Mer et de l’Am´ enagement du Littoral (ENSSMAL),
Campus Universitaire de Dely Ibrahim Bois des Cars, B.P. 19, 16320, Alger, Algeria
b
MARBEC, IRD, Ifremer, CNRS, Univ Montpellier, S` ete, Avenue Jean Monnet, CS 30171, 34203, S` ete cedex, France
c
USTHB: Universit´ e des Sciences et Technologie Houari Boumedien, D´ epartement ´
Ecologie et Environnement, BP 32 Bab Ezzouar, 16111, Alger, Algeria
ARTICLE INFO
Keywords:
Ocean colour
Alongshore variability
Wadi
Anthropic enrichment
MODIS.
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. A separate source of enrichment is undoubtedly associated with the
presence of aquaculture cages.
1. Introduction
Chlorophyll-a (Chl-a) biomass is associated with the net primary
production in marine ecosystems, and marine physical and biochemical
processes strongly influence its variability. A precise description of the
spatio-temporal variability of Chl-a biomass is necessary to understand
coastal marine systems functioning. The Chl-a concentration in the
southwestern Mediterranean Sea (Med) is closely related to winter
mixing and summer stratification. The Med ecosystem is increasingly
threatened by human activities in coastal areas, as well as by a
continuing warming trend (Vargas-Y´ a˜ nez et al., 2010). A recent study
conducted by Keraghel et al. (2020) highlights that the southwestern
Med is a net sink of carbon dioxide, even compared to the Med as a
whole, which is a significant contributor at the global level (Khatiwala
et al., 2013). Scientists are still trying to understand the evolution of the
Med ecosystem to better assess current and future changes and consider
solutions to mitigate some impacts of global warming.
The Algerian Basin (AB) is classified as mesotrophic (D’Ortenzio and
Ribera d’Alcal` a, 2009; Harid et al., 2018; O’Reilly and Werdell, 2019).
Water exchange across Gibraltar has a significant influence on its general circulation (B´ eranger et al., 2005; Millot, 1989; Peliz et al., 2009)
and controls its nutrient content (Bethoux et al., 2002; Crispi and
* Corresponding author. ECOSYSMarL: Laboratoire des ´
Ecosyst` emes Marins et Littoraux, ´
Ecole Nationale Sup´ erieure des Sciences de la Mer et de l’Am´ enagement
du Littoral (ENSSMAL), Campus Universitaire de Dely Ibrahim Bois des Cars, B.P. 19, 16320, Alger, Algeria.,
E-mail addresses: r.harid@enssmal.dz, romaissa.harid@hotmail.fr (R. Harid), herve.demarcq@ird.fr (H. Demarcq), ma.keraghel@enssmal.dz (M.-A. Keraghel), m.
ait-kaci@enssmal.dz (M. Ait-Kaci), m.zerrouki@enssmal.dz (M. Zerrouki), n.bechari@usthb.dz (N.-E.-I. Bachari), f.houmabachari@enssmal.dz (F. Houma).
Contents lists available at ScienceDirect
Continental Shelf Research
journal homepage: www.elsevier.com/locate/csr
https://doi.org/10.1016/j.csr.2021.104629
Received 8 June 2021; Received in revised form 10 September 2021; Accepted 27 November 2021
Available online 5 December 2021
0278-4343/© 2021 Elsevier Ltd. All rights reserved.
Spatio-temporal variability of a chlorophyll-a based biomass index and
influence of coastal sources of enrichment in the Algerian Basin
Romaissa Harid
a, b, *
, Herv´ e Demarcq
b
, Mehdia-Asma Keraghel
a
, Malik Ait-Kaci
a
,
Mohamed Zerrouki
a
, Nour-El-Islam Bachari
c
, Fouzia Houma
a
a
ECOSYSMarL: Laboratoire des ´
Ecosyst` emes Marins et Littoraux, ´
Ecole Nationale Sup´ erieure des Sciences de la Mer et de l’Am´ enagement du Littoral (ENSSMAL),
Campus Universitaire de Dely Ibrahim Bois des Cars, B.P. 19, 16320, Alger, Algeria
b
MARBEC, IRD, Ifremer, CNRS, Univ Montpellier, S` ete, Avenue Jean Monnet, CS 30171, 34203, S` ete cedex, France
c
USTHB: Universit´ e des Sciences et Technologie Houari Boumedien, D´ epartement ´
Ecologie et Environnement, BP 32 Bab Ezzouar, 16111, Alger, Algeria
ARTICLE INFO
Keywords:
Ocean colour
Alongshore variability
Wadi
Anthropic enrichment
MODIS.
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. A separate source of enrichment is undoubtedly associated with the
presence of aquaculture cages.
1. Introduction
Chlorophyll-a (Chl-a) biomass is associated with the net primary
production in marine ecosystems, and marine physical and biochemical
processes strongly influence its variability. A precise description of the
spatio-temporal variability of Chl-a biomass is necessary to understand
coastal marine systems functioning. The Chl-a concentration in the
southwestern Mediterranean Sea (Med) is closely related to winter
mixing and summer stratification. The Med ecosystem is increasingly
threatened by human activities in coastal areas, as well as by a
continuing warming trend (Vargas-Y´ a˜ nez et al., 2010). A recent study
conducted by Keraghel et al. (2020) highlights that the southwestern
Med is a net sink of carbon dioxide, even compared to the Med as a
whole, which is a significant contributor at the global level (Khatiwala
et al., 2013). Scientists are still trying to understand the evolution of the
Med ecosystem to better assess current and future changes and consider
solutions to mitigate some impacts of global warming.
The Algerian Basin (AB) is classified as mesotrophic (D’Ortenzio and
Ribera d’Alcal` a, 2009; Harid et al., 2018; O’Reilly and Werdell, 2019).
Water exchange across Gibraltar has a significant influence on its general circulation (B´ eranger et al., 2005; Millot, 1989; Peliz et al., 2009)
and controls its nutrient content (Bethoux et al., 2002; Crispi and
* Corresponding author. ECOSYSMarL: Laboratoire des ´
Ecosyst` emes Marins et Littoraux, ´
Ecole Nationale Sup´ erieure des Sciences de la Mer et de l’Am´ enagement
du Littoral (ENSSMAL), Campus Universitaire de Dely Ibrahim Bois des Cars, B.P. 19, 16320, Alger, Algeria.,
E-mail addresses: r.harid@enssmal.dz, romaissa.harid@hotmail.fr (R. Harid), herve.demarcq@ird.fr (H. Demarcq), ma.keraghel@enssmal.dz (M.-A. Keraghel), m.
ait-kaci@enssmal.dz (M. Ait-Kaci), m.zerrouki@enssmal.dz (M. Zerrouki), n.bechari@usthb.dz (N.-E.-I. Bachari), f.houmabachari@enssmal.dz (F. Houma).
Contents lists available at ScienceDirect
Continental Shelf Research
journal homepage: www.elsevier.com/locate/csr
https://doi.org/10.1016/j.csr.2021.104629
Received 8 June 2021; Received in revised form 10 September 2021; Accepted 27 November 2021