Metacommunity Structure and Assemblage Processes of Fish, Macroinvertebrate and Phytoplankton in the Lower Niger River Basin, Nigeria

Abstract

Understanding ecological processes that drive metacommunity dynamics is essential for elucidating the mechanisms of community assembly and for guiding biodiversity conservation. To this end, the relative influence of local environmental and spatial factors in structuring the community composition of fish, macroinvertebrate and phytoplankton at 15 sampling sites along the longitudinal gradient of the Lower Niger River Basin (LNRB) was investigated in dry and rainy seasons. In addition, the beta diversity and its components (species replacement and richness difference) of the species and their underlying drivers were examined. The result of Redundancy analysis indicated that the community compositions (fish, phytoplankton and macroinvertebrate) in the upper region were influenced by high nutrient concentrations, attributable to arable and nomadic farming within the region. In contrast, downstream sites were characterized by high concentrations of suspended solids indicative of flooding and mining effects. Fish and phytoplankton community compositions strongly differed between the upper and lower regions probably because of the presence of dams along the channel of the river that may have interrupted migration and dispersal. However, macroinvertebrate community composition was homogenized despite the presence of barrier (dam) in the river channel. Probable explanation to this observation may be the arial dispersal mode of most macroinvertebrate species that allow them to track suitable environment for their survival. Variation partitioning revealed higher contributions of spatial than environmental predictors on fish and phytoplankton community compositions (revealing the effect of dispersal limitations), but the total predicted variance was higher during dry season. Differences in flow dynamics between upper and lower regions as attributable to black and white floods in the LNRB modify the connectivity between sites. Dispersal among sites may be more limited downstream than in the upper region, particularly in dry season, certainly because damming in the upper region also interrupts the natural flood regime such that there are low water levels in the lower region, which spatially isolate fish and phytoplankton communities at certain sampling sites. Environmental predictors were the determinants of macroinvertebrate community composition reflecting the strength of species sorting in structuring macroinvertebrate community in the LNRB. This study revealed weak beta diversity for all the species studied indicating low species turnover. The replacement component contributed most to the overall beta diversity and its components for all the studied species in the LNRB, an indication of the importance of species sorting. Whereas, spatial predictors explained fish total beta diversity and its components, environmental filtering was the primary driver of macroinvertebrate and phytoplankton total beta diversity and its replacement component. However, their richness difference components were explained by spatial factors. The varying local contribution to biodiversity (LCBD) by different sites shows that some sites along the longitudinal gradient of the LNRB requires restoration measures (e.g. Awuru) while others require conservation actions (e.g Jebba). Overall, the results of this study indicated that damming induced changes in physiochemical variables (e.g. nutrients), accompanied by alterations in flow regime and longitudinal connectivity, increased replacement and loss of taxa. These changes have consequently led to alteration of species community dissimilarity and affected the relative effects of environmental and spatial factors on local community compositions, beta diversity and its components. These findings suggested that conservation of fish, phytoplankton and macroinvertebrate diversity in afro-tropical rivers requires maintenance of both habitat heterogeneity (spatial variation in habitat conditions) and connectivity at the scale of entire river basins.