A long-term study on zooplankton in two contrasting cascade reservoirs (Iguaçu River, Brazil): effects of inter-annual, seasonal, and environmental factors

Abstract

Background In reservoirs, zooplankton strongly interact with the physical and chemical characteristics of water, and this interaction is mainly influenced by climate variation and the different methods used to manage the dam water level. Therefore, the aim of this study was to evaluate how the distinct operating modes of two cascade reservoirs affected the richness, abundance, and composition of zooplankton, both spatially (intra and inter-reservoirs) and temporally (annual and seasonal). In this study, the upstream reservoir (Salto Santiago) operates using the storage method, with a water retention time (WRT) of 51 days, whereas the downstream reservoir (Salto Osório) operates using the run-of-river method, with a WRT of 16 days. Methods Zooplankton samples were collected for 16 consecutive years from the two reservoirs located on the Iguaçu River, Brazil. A total of 720 samples were collected. Four-way ANOVAs were used to determine the differences in richness and abundance of the zooplankton among years, periods, reservoirs, and environments. Multidimensional non-metric scaling (NMDS) and an analysis of similarities (ANOSIM) were used to describe similarity patterns in species composition. Finally, a canonical correspondence analysis (CCA) was used to select the environmental predictors that best explained the variation in zooplankton abundance data. Results We identified a total of 115 taxa in this study, and rotifers were the richest group. In contrast, the copepods were the most abundant. The four-way ANOVA results showed significant differences in the species richness and abundance of the zooplankton among years, periods, reservoirs, and environments. The NMDS ordination and ANOSIM test indicated that the largest differences in zooplankton species composition were annual and seasonal differences. Finally, the CCA showed that these differences were mainly associated with changes in water transparency, temperature, and the chlorophyll a, phosphorus, and total dissolved solids concentrations. Discussion Inter-annual changes in zooplankton species composition showed that over time, large filters-feeders (e.g., large daphinids and calanoid copepods) were replaced by small cladocerans (e.g., bosminids) and generalist rotifers. The highest species richness was associated with the fluvial environment, whereas the highest abundance was associated with the transitional and lacustrine reservoir environments. Variations in water temperature, nutrients, and food availability explained the annual and seasonal changes in community structure, whereas variations in the water flow characteristics of the environments explained the longitudinal changes in the richness and abundance of zooplankton in reservoirs. The differences in zooplankton structure between the two reservoirs can be explained by the functional differences between the two systems, such as their WRTs and morphometrics.

Warming exacerbates the impacts of ultraviolet radiation in temperate diatoms but alleviates the effect on polar species

“…UVR is known to cause major damage to the photosynthetic apparatus, carbon fixation, DNA, and other metabolic pathways. 36,37 In the present work, exposure under the relatively low light level used for growth would not induce significant nonphotochemical quenching, while UVR caused a significant decrease in the photochemical quantum yields of all species, probably due to damage to PSII, which can be inactivated even at relatively low levels of UVR. 2 Interestingly, the UV inhibition was correlated with the latitude where the species was isolated, T. weissflogii (isolated from the lowest latitude) was less sensitive to UVR compared with the other two species, the polar species showed the greatest sensitivity, indicating that the light history might play a role in the differences in their UV resistance.…”

“…UVR is known to cause major damage to the photosynthetic apparatus, carbon fixation, DNA, and other metabolic pathways 36,37 . In the present work, exposure under the relatively low light level used for growth would not induce significant nonphotochemical quenching, while UVR caused a significant decrease in the photochemical quantum yields of all species, probably due to damage to PSII, which can be inactivated even at relatively low levels of UVR 2 .…”

A variação da diversidade de microcrustáceos (Cladocera e Copepoda) a jusante de pequenos reservatórios é influenciada por táxons litorâneos

“…O fato de os ambientes estudados estarem localizados numa escala espacial pequena, dos reservatórios serem parecidos em termos de área, profundidade, formato e ainda conectados no sentido montante-jusante, podem ter favorecido o resultado de pouca influência sobre os organismos. Se comparados, por exemplo, a reservatórios de grande área, volume e profundidade, onde ocorre ampla compartimentalização espacial, as variáveis limnológicas tendem a exercer maior efeito sobre as assembleias de microcrustáceos, como temperatura, transparência, nutrientes e clorofila (Nogueira et al, 2008;Illyová & Pastuchová, 2012;Perbiche-Neves & Nogueira, 2013;Picapedra et al, 2020).…”

Taxonomic and functional homogenisation of zooplankton after river damming in Central Brazil

“…Changes in the structure of this community may alter ecosystem function (Deosti et al, 2021; Rizo et al, 2020). Studies have pointed out the effects of damming on these communities (Shen et al, 2022; Souza et al, 2019), such as reduced species richness and increased density of resistant species (Picapedra et al, 2020) and altered species distribution (Schmidt et al, 2020). However, there is a knowledge gap when comparing studies of biotic responses to dams between temperate and tropical regions.…”