7 Water Constituents Assessment at the Sassandra River Mouth (Côte d’Ivoire)
143
and suggests that CDOM and CHL have similar higher contributions, in comparison
to DET. However, the contribution of CDOM would appear to be much higher, if
not for the SCS/GCS upwelling seasons, which increase substantially that of CHL.
As shown in Table 7.1, the percent contributions of the relative proportions of
the main water constituents does vary quite a bit in each of the “marine” seasons
described above. The relative proportion of CDOM, CHL and DET in each of the
seasons and transition periods considered is shown in the ternary plots of Fig. 7.6,
panels a to f. In general, we observed three different sets of conditions: periods of
CHL prevalence (as in the SCS and GCS); periods of CDOM prevalence (as in the
GWS and SWS); and periods in which we find the presence of all three components
(as in two transition periods, between the other seasons).
The first set of conditions concerns the “cold” seasons, which take place from
January to February (SCS) and from July to September (GCS). These periods display
a high percent contribution of CHL, in both cases greater than 50 %. However, the
SCS percentage (Fig. 7.6a) varies between 60 % and 90 %, while the GCS percentage
(Fig. 7.6d) varies between 50 and 70 % only (see also Table 7.1). Both the cold seasons
are characterized by upwelling episodes, which increase nutrients availability and
phytoplankton productivity (Demarcq and Aman 2002; Djagoua 2003). The higher
percent contribution of CHL in the SCS might suggest that this period can be more
productive than the GCS. As this is not the case (Bakun 1978; Binet 1997), it would
seem that the estimated CHL contribution doesn’t always represent real conditions
along the Cote d’Ivoire coastal zone. Possibly, the high CDOM contribution in the
GSC, with respect to the SCS—one third (28,09 %) vs one tenth (10.52 %) of the total,
in Table 7.1, while the DET contribution remains rather constant, at ∼ 14 %—might
indicate that the CHL contribution was underestimated in this particular season.
The second set of conditions is linked to a high percent contribution of CDOM
and concerns the “warm” seasons, which take place from March to May (GWS)
and from November to December (SWS). During these periods (Fig. 7.6d, f), the
CDOM contribution is nearly the same (∼ 60 %; see also Table 7.1). But that of
CHL (∼ 37 %) is not negligible either. Indeed, during both the SWS and especially
the GWS, the Cote d’Ivoire coastal waters are characterized by nutrients too low to
sustain a high productivity. However, in spite of the generally poor phytoplankton
field, one or more episodes of higher productivity may occur during the GWS, when
the thermocline rises to the surface (Sevrin-Reyssac 1993).
The third set of conditions is that of the two transition periods (June and October),
immediately before and after the GCS, which present similar percent contributions
of both CDOM and CHL (Fig. 7.6c, e). The relatively high CHL values may be
related to (early or late) upwelling events of the GCS, and/or high river discharges
occurring in these periods (especially in October). Data about the monthly mean
discharge from the Sassandra River, for the period from 1986 to 1999, are shown in
Fig. 7.7, lower panel. The seasonal variations of river runoff can be compared with
the relative proportions of RS-derived water constituents plotted in Fig. 7.6, upper
panel, at the same monthly scale.
Le Loeuff and Marchal (1993) ascribe the prevalence of one or the other water
constituent, during the Cote d’Ivoire “marine” seasons, to either the shift between
143
and suggests that CDOM and CHL have similar higher contributions, in comparison
to DET. However, the contribution of CDOM would appear to be much higher, if
not for the SCS/GCS upwelling seasons, which increase substantially that of CHL.
As shown in Table 7.1, the percent contributions of the relative proportions of
the main water constituents does vary quite a bit in each of the “marine” seasons
described above. The relative proportion of CDOM, CHL and DET in each of the
seasons and transition periods considered is shown in the ternary plots of Fig. 7.6,
panels a to f. In general, we observed three different sets of conditions: periods of
CHL prevalence (as in the SCS and GCS); periods of CDOM prevalence (as in the
GWS and SWS); and periods in which we find the presence of all three components
(as in two transition periods, between the other seasons).
The first set of conditions concerns the “cold” seasons, which take place from
January to February (SCS) and from July to September (GCS). These periods display
a high percent contribution of CHL, in both cases greater than 50 %. However, the
SCS percentage (Fig. 7.6a) varies between 60 % and 90 %, while the GCS percentage
(Fig. 7.6d) varies between 50 and 70 % only (see also Table 7.1). Both the cold seasons
are characterized by upwelling episodes, which increase nutrients availability and
phytoplankton productivity (Demarcq and Aman 2002; Djagoua 2003). The higher
percent contribution of CHL in the SCS might suggest that this period can be more
productive than the GCS. As this is not the case (Bakun 1978; Binet 1997), it would
seem that the estimated CHL contribution doesn’t always represent real conditions
along the Cote d’Ivoire coastal zone. Possibly, the high CDOM contribution in the
GSC, with respect to the SCS—one third (28,09 %) vs one tenth (10.52 %) of the total,
in Table 7.1, while the DET contribution remains rather constant, at ∼ 14 %—might
indicate that the CHL contribution was underestimated in this particular season.
The second set of conditions is linked to a high percent contribution of CDOM
and concerns the “warm” seasons, which take place from March to May (GWS)
and from November to December (SWS). During these periods (Fig. 7.6d, f), the
CDOM contribution is nearly the same (∼ 60 %; see also Table 7.1). But that of
CHL (∼ 37 %) is not negligible either. Indeed, during both the SWS and especially
the GWS, the Cote d’Ivoire coastal waters are characterized by nutrients too low to
sustain a high productivity. However, in spite of the generally poor phytoplankton
field, one or more episodes of higher productivity may occur during the GWS, when
the thermocline rises to the surface (Sevrin-Reyssac 1993).
The third set of conditions is that of the two transition periods (June and October),
immediately before and after the GCS, which present similar percent contributions
of both CDOM and CHL (Fig. 7.6c, e). The relatively high CHL values may be
related to (early or late) upwelling events of the GCS, and/or high river discharges
occurring in these periods (especially in October). Data about the monthly mean
discharge from the Sassandra River, for the period from 1986 to 1999, are shown in
Fig. 7.7, lower panel. The seasonal variations of river runoff can be compared with
the relative proportions of RS-derived water constituents plotted in Fig. 7.6, upper
panel, at the same monthly scale.
Le Loeuff and Marchal (1993) ascribe the prevalence of one or the other water
constituent, during the Cote d’Ivoire “marine” seasons, to either the shift between