THE INFLUENCE OF SALTWORKS’ WASTE-BRINE ON DISTRIBUTION OF MANGROVE CRABS (DECAPODA, PORTUNIDAE) WITHIN THE GONGONI-KURAWA INTERTIDAL AREA, KENYA

Abstract

Evaporation is one of the oldest methods employed in sea salt production, a process that involves pumping seas water into a series of ponds where solar evaporation concentrates it into brine, and precipitates the salt; which is then harvested manually. Lack of baseline information on the effect of the discharged brine at the Gongoni-Kurawa region caused the need to determine its impact on the marine ecosystem. The objectives of this study were to compare the productivity of the coastal water, identify the effect of brine on mangrove crab species diversity and distribution and to determine the impact of the physic-chemical parameters and productivity on distribution and diversity of the mangrove crabs. The impact of the discharged hyper-saline waste-brine waters on mangrove crabs along the intertidal habitats bordering two of the biggest salt works; Krystalline and Kurawa salt industries was investigated in this study. Sampling was done during both spring and neap season using 1m2 quadrats, to crabs collected from the quadrats used to estimate the densities. A total of 34 mangrove crab species were recorded, with abundances significantly higher (p>0.05) within the inlets’ habitats as compared to the outlets (discharge-point) habitats. Six species Macrophthalmus grandidieri, Uca chlorophthalmus, Terebralia palustris, Macrophthalmus latreillei, Uca tetragonon and Amaea acuminata were most dominant and occurred in all of the transect samples. Higher species diversity and evenness were recorded in inlet habitats at Kurawa compared to the outlets. The inlet habitats reported higher Maximum Shannon-Wiener diversity, whereas outlets recorded lower diversity, with Marereni recording considerably lower Hmax, at 0.95. Species distribution showed a significant reduction of the genera Uca and M. grandidieri (p<0.05) at Marereni outlet habitats, but an increase in U. vocans at the inlet habitats. Large numbers of small sized species were recorded at the outlet compared to the inlet at both Kurawa and Marereni. Similarly, there was a higher abundance of genera Uca, M. ovalina, M. grandidieri, M. latreillei, Amaea acuminata and Cerithidea decollata (p<0.05) during the spring tide period while the abundances of U. vocans dropped during the same period. The higher densities of M. ovalina and Uca inversa were mainly associated with spring tides. PCA showed dominance of M. grandidieri (PC-1 score = 0.9129) at the inlet and outlet of the Kurawa site during both spring and neap tide, thereby accounting for most of the inertia. CCA showed that there was a strong positive loading of Chlorophyll-a on the abundance of M. ovalina at the Kurawa inlet habitats during the spring tide, but negative loading of salinity, pH and total phosphorus on Pseudograpus elongatus and T. palustris at Kurawa outlet habitats during neap tide. This asymmetric distribution between inlets and outlets was explained by significant variations in salinity as well as site specific salinity gradients at the two study sites; Marereni and Kurawa in north coast, Kenya. Suggestions for improving salt production and quality while minimizing adverse environmental effects through frequent monitoring of the water quality parameters and the mangrove crabs before and after brine discharged were recommended.