Longer hydroperiods intensify predation effects on planktonic size spectra in Mediterranean temporary ponds

Abstract Mediterranean temporary ponds are ecologically vital ecosystems that support numerous endemic and rare species. These shallow ponds undergo unpredictable inter-annual fluctuations, with flooding and dry phase cycles driven largely by meteorological conditions. This study investigates how changes in the physical and chemical environment, resource availability (bottom-up), and predation (top-down) throughout the hydroperiod influence the structure and dynamics of planktonic communities. We analyzed the size spectrum (i.e., the relationship between individual body size and abundance) including phytoplankton and zooplankton communities in three freshwater temporary ponds in NE Spain, sampled monthly over four consecutive hydrologic cycles (2016–2020). Hydroperiod ranged from 2 to 8 months, allowing us to assess whether hydroperiod duration modulates the relative influence of environmental, bottom-up, and top-down controls. Results indicate that macroinvertebrate predation was the main driver of a declining size spectrum intercept, suggesting a general reduction in abundance across all planktonic size classes. Longer hydroperiods intensified top-down effects, also steepening the slope of the spectrum. In contrast, environmental fluctuations across the hydroperiod did not significantly influence the size spectrum shape, regardless of hydroperiod length. These findings highlight size spectra as useful indicators of the temporal changes in the trophic structure and biomass transfer in ephemeral aquatic systems.