Quantifying the Hydroregime of a Temporary Pool Habitat: A Modelling Approach for Ephemeral Rock Pools in SE Botswana

Ecological and evolutionary processes in temporary rock pools operate within constraints imposed by their hydrologic regimes. These shallow pools flood when seasonal rains accumulate on impermeable substrates. Despite the ecological importance of hydrologic conditions for these ecosystems, we typically lack tools and empirical data required to understand the implications of hydrologic variability and climate change for biotic populations and communities in these habitats. In this study, we developed a hydrologic model to simulate rock pool hydrologic regimes based on rainfall, evapotranspiration, and basin geometry. The model was used to investigate long-term patterns of seasonal and inter-annual variation in hydroregime. In addition, hydrologic conditions associated with potential climate change scenarios were simulated and evaluated with respect to the biological requirements of the anostracan Branchipodopsis wolfi. The model’s output for daily inundation matched with field observations with an overall accuracy of 85% and correctly estimated complete hydroperiods with an overall accuracy of 70%. Simulations indicate large variation in individual hydroperiods (76–115%) as well as in the number of hydroperiods per year (19–23%). Furthermore, this study suggests that climate change may significantly alter the rock pool hydroregime. These findings confirm the hydrologic sensitivity of these ephemeral habitats to precipitation patterns, and their potential sensitivity to future climate change. Modelling indicates that the suitability of average inundation conditions for B. wolfi deteriorates significantly under future climate predictions. High levels of spatial and temporal variation in hydrologic conditions are dominant features of these habitats and an essential consideration for understanding population and community-level ecological processes.     

Community structure in temporary freshwater pools: disentangling the effects of habitat size and hydroregime

1. Hydroregime (duration, frequency and predictability of the aquatic phase) is a key feature of temporary aquatic habitats that not only moulds community structure and diversity (species sorting) but also life history characteristics of the inhabitants (natural selection). However, since hydroregime is a complex multidimensional entity that cannot be estimated from short term observations, morphometric variables are commonly used as proxies for hydroregime, making it impossible to separate effects of habitat size and hydroregime on biota.
2. We have used a simple hydrological model, validated with recent (average r ² = 96%) and historic water level observations (average r ² = 81%), to accurately reconstruct hydroregime based on historical rainfall and evaporation data in a cluster of 36 temporary rock pools in central South Africa.
3. Using the model output, we demonstrated that both hydroregime and habitat size had unique and shared effects on temporary pool biota and that these effects depended on the dispersal modes of the taxa. Model-generated hydrological data explained more variation in community patterns than short-term observations of hydroperiod. Hydroregime was more important for passive dispersers than for active dispersers that can migrate when pools dry up. Basin morphometry was a relatively poor predictor of pool hydroregime. We concluded that simple modelling may greatly improve the resolution of studies linking hydroregime to biological variables.
4. An accurate account of hydrological variation provides a firm foundation to understand community and population structure and dynamics in temporary aquatic habitats. Since many of these habitats have been destroyed or degraded in recent decades, our findings and tools may contribute to the development of reliable conservation guidelines.     

Long-term droughts change the hatching patterns of zooplankton resting eggs from permanent and temporary lakes

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