Assessment of the Vulnerability of the Kribi Coastal Zone (South Cameroon) to Ocean Acidification Based on Zooplankton and Carbonate Chemistry

Abstract

Assessing the vulnerability of a coastal area to an anthropogenic factor is crucial for developing adaptation or mitigation strategies. As other countries, Cameroon is exposed to ocean acidification (OA), but very little is known about the extent of its vulnerability. This study aim was to assess the vulnerability of the coastal ocean ecosystem of Kribi (Southern Cameroon) to ocean acidification. Thus, simultaneous monitoring of carbonate system parameters, zooplankton communities, and others environmental variables was conducted in the area between September 2021 and August 2022. Three monitoring stations and a seasonal sampling strategy were established to understand organism’s exposition to changes in the carbonate chemistry and OA. Seawater physicochemical parameters were measured in situ using a calibrated water quality multimeter probe. Nutrients, chlorophyll-a, and total alkalinity ( T A ) were measured in the laboratory using appropriates methods, with water samples for T A analysis fixed using mercuric chloride ( HgCl 2 ). Based on the pH, T A , salinity, and temperature values, the others parameters of the carbonate system were calculated using the CO2SYS_Xls program. Zooplankton was collected at the surface with a 64-micron plankton net and fixed in 5% formaldehyde, then transported to the laboratory for counting and identification. The Coastal Ocean Acidification Vulnerability Index (COAVI) was then used to determine the area’s vulnerability level. As results, 45 zooplankton species, belonging to 7 phylum and 32 families were identified. Copepod was the dominant group in all sampling stations during all the seasons. Environmental conditions were characterized by high temperature (28.56 ± 1.85°C) and a salinity that varied according to the rainfall. The carbonate system indicates that the critical threshold of OA was not reached in the area, as seawater remained saturated with respect to aragonite and calcite saturation (Ω ˃ 1). Although, lowest value of pH, aragonite and calcite were observed during the small and large rainy seasons. Low pH and high pCO 2 values were accompanied by a drop in diversity, while salinity variation drives selective presence of some taxa in the area. The COAVI index showed an overall value of 0.53, indicating moderate vulnerability to OA in the study area. Station Bp, with an index of 0.81 was the most vulnerable, while the rainy seasons, with an index of 0.76 are at the highest risk. The study highlighted the area’s vulnerability to OA, which calls for raising awareness and developing local environmental management strategies.