Abstract
Abstract: Aim An attempt to justify the variations in the species composition of zooplankton over the day time period was made using the freshwater lakes of Assam, India, as a model study area. Methods The water bodies were sampled using a conical plankton net five times between 6 am and 6 pm in a day during July 2014. Apart from diversity analysis (SHE analysis), discriminant function analysis and a regression tree were built to justify the variations in the zooplankton species composition in the different time periods in a day. Results Repeated sampling during day time indicated the presence of 34 zooplankton species with varied composition over the different time periods. The species richness and abundance of Copepoda, Cladocera and Rotifer differed significantly revealed through the ANOVA (for rotifers – F4, 170 = 28.792; for cladocerans – F4, 170 = 16.343; for copepods - F4, 170 = 9.027). Apparently, the rotifer and copepod abundance was prominent during the early (6am) and late time (6pm) periods of a day while the cladoceran abundance was significant during the middle time periods (12 noon to 3pm) of a day. Variations in the relative abundance of the zooplankton species within a day were sufficient to discriminate the time periods significantly. The regression tree indicated the rotifer abundance (node 1), followed by the cladoceran and the copepod abundances (node 2). The nodes representing cladocerans explained 30.86% of the data while the copepod explained 69.14% data as a split from the rotifer groups. Conclusions A correspondence of high and low abundances of rotifers and cladocerans was characteristic of the species composition of the zooplankton in the study area. The variations in the species composition suggests that the zooplankton exhibits movements either in horizontal or vertical directions induced by physical and/or biological factors. Further studies may be adopted to elucidate the effects of the predators and environmental variables that shape the community of the zooplankton in shallow tropical lakes.
Highlights
The spatial distribution of freshwater zooplankton varies with the time in a day, owing to the movements in the vertical or the horizontal plane (Doulka & Kehayias 2008)
A correspondence of high and low abundances of rotifers and cladocerans was characteristic of the species composition of the zooplankton in the study area
The relative abundance of the representative species varied with the time period, with similar patterns being exhibited by the rotifers, the cladocerans and copepods (Table 2), in terms of the relative abundance in the time periods
Summary
The spatial distribution of freshwater zooplankton varies with the time in a day, owing to the movements in the vertical or the horizontal plane (Doulka & Kehayias 2008). Observations suggest that this process is repeated over time and qualify as a diel periodicity or rhythm. Predator induced shifts in the spatial occupancy of the zooplankton species is considered as an important reason for the diel movement (George & Fernando, 1970; Braghin et al, 2016). Water circulation induced changes in the micronutrient availability and environmental factors like anoxia are considered as probable reason for the movement of the zooplankton in the freshwater bodies (George & Heaney 1978; Lampert 2005). Irrespective of the spatial dimension and the causative factors, the movements of the zooplanktons within the space are observed for almost all lentic systems, reflecting the mechanism to be an adaptive feature of the zooplankton
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