Spatial and temporal variations of microinvertebrates across temporary floodplains of the lower Okavango Delta, Botswana

Abstract

The connectivity of the Okavango Delta to its temporary floodplains may reduce due to threats of increasing human water demands and climate change. Microinvertebrates in aquatic systems have a regulatory effect on juvenile fish production. Microinvertebrates in this study refer to both non- and planktonic microscopic invertebrates such as rotifers, cladocerans, copepods and ostracods. To understand the response of microinvertebrates to flooding of different types of temporary floodplains of the Delta, selected floodplains from primary, secondary and rarely inundated floodplains were sampled for microinvertebrates between May 2009 and February 2010. Field sampling coincided with different hydrological phases of flooding; flood rising, peak flooding and flood recession phase. Vegetation cover, woody debris, and a suite of physicochemical water parameters were assessed at each study floodplain. Woody debris, oxygen saturation (%), turbidity and conductivity were all significantly highest in rarely flooded floodplains (ANOVA, p<0.05) whereas aquatic vegetation cover was highest in the primary floodplains. Fifty-five species and four taxa of microinvertebrates were identified in the study floodplains. Copepods were the most abundant by density across the study floodplains. During the early stages of flooding microinvertebrate species richness and diversity were significantly highest in frequently flooded floodplains (Kruskal–Wallis, p<0.05) and they were lower in rarely flooded floodplains. Soon after inundation of the temporary floodplains until peak flooding, highest microinvertebrate densities (ANOVA, p<0.05) were observed in the rarely flooded floodplains. Findings of this study indicate that, high flooding frequency of the primary floodplains may be important for the maintenance of high microinvertebrate species richness whereas flooding of rarely flooded floodplains might contribute the highest densities of microinvertebrates to the temporary floodplain food webs. Findings of this study imply that substantial reduction of flooding regimes in the Delta may negatively affect both composition and density of microinvertebrates. Therefore, management and conservation efforts in the Okavango Delta are encouraged to consider balancing human water needs and water needed for the ecological functioning of the system.