The Impacts of Seasonality on Coastal Water Systems in Southeast Côte d'Ivoire

Abstract

To have a compendium of responses from coastal water systems along the east Ivorian coast under different seasons, samples of rainwater (n= 30), groundwater (wells and boreholes, n= 81 and surface water (Ébrié lagoon and Atlantic coastline, n= 69) were collected for geochemical, isotopic and biological analyses during the dry (January/February) and rainy (September/October) seasons of 2014. From geomorphological viewpoint, the slope of the study area is between 0 and 70 % relative to the sea. Present land use cover includes farmlands (50 %), water bodies (32 %), settlements (13 %) and forests (5 %). Within 25 years (1989/90 - 2014/15), urbanization has claimed 31.83 % of forestlands and 37.42 % of farmlands. As regards hydrology, moderate hydrological drought spells were revealed for 1987, 1998/99, and 2013/14 based on Standardised Precipitation Evapotranspiration Indices (SPEI) of multi-decadal (1971 – 2014) meteorological data. The aquifer materials comprise up to 98 % silicate. The implications of this is groundwater acidification (pH: 3.9 – 7.4) on dissolution. From hydrogeological perspective, changing hydrological regimes exerts strong influence on coastal water chemistry and contaminant export. Based on geochemical and stable isotope analyses, marine influence dominates during the dry season, while continental (fluvial and precipitation) processes dominate during the rainy season. Piper diagrams showed that majority (dry season: 87 %; rainy season: 80 %) of groundwater samples belong to the Na-Cl facies, depicting saltwater intrusion. Maximum chloride concentration was 1,088 mg/L and 785.2 mg/L for the groundwater during dry and rainy seasons, respectively. Conversely, nitrate concentrations in groundwater ranged from 0.3 – 139.5 mg/L and 0.0 – 165.9 mg/L for the dry and rainy seasons, respectively and for the surface waters, between 0.01 – 3.6 mg/L and 0.01 - 332.3 mg/L for the dry and rainy seasons, respectively. From the viewpoint of biology, seasonal influx of nutrients into the coastal surface waters also influences phytoplankton distribution. Maximum phytoplankton biomass was 11.3*106 and 15.4*106 cells/mL for the dry and rainy seasons respectively with floristic growth response ranging between 16.4 and 80.8 %. Highest temporal variations in biomass were recorded in areas of the Ébrié lagoon with high marine influence. Diatoms were the dominant taxa, while cyanobacteria are the most abundant. The overall low phytoplankton diversity (Shannon index, H’ less than 1) and localized monospecies dominance are vital signs of disturbances. Finally, survey results show that the riverine communities are sensitive to coastal water degradation. These waters constitute the primary source of domestic water for 53 % and a secondary source for 97 % of the population. Additionally, about 30 % of the population depend directly on aquatic resources from these coastal waters for livelihood.