Simple Summary
Globally, there is a decline in aquatic bird diversity in deltaic floodplains. We investigated the spatial and numerical dynamics of colonial waterbird species breeding in five colonies, using them as biotic indicators of recent changes in temperature and flood regimes in the Danube Delta Biosphere Reserve, Romania. Within the 2016–2023 study period, systemic shifts in hydrological regimes, progressive habitat degradation, and heightened meteorological variability have driven significant transformations in colony abundance, notably contributing to a systemic decline in total breeding pairs from peak values recorded in 2019. The integration of in situ measurements and remote sensing confirms that spring thermal anomalies and water-level fluctuations are the primary environmental forcing factors for colonies’ dynamics. Temperature plays a dual role: rising temperatures stimulate the development of food resources in April, yet they also enhance evaporation and increase the frequency of droughts later in the breeding season. Water levels and the extent of aqueous areas determine the availability of feeding habitats for colonial birds. These highlight the need for appropriate conservation measures to mitigate the effects of climate change through adaptative hydrological management. Future conservation must prioritize sustainable GIS-guided solutions to maintain the minimum foraging levels, buffering extremes while preserving natural pulses through monitoring the aqueous surface extent.
Abstract
Climate change and altered hydrological regimes are restructuring wetland habitats globally, triggering cascading effects on colonial waterbirds. This study investigates how environmental drivers, including thermal anomalies, water-level fluctuations, and aqueous surface extent, influence the distribution and size of waterbird colonies (Ardeidae, Threskiornithidae, and Phalacrocoracidae) in the Danube Delta Biosphere Reserve. We integrated colony census data (2016–2023) with remote-sensing-derived habitat metrics, in situ meteorological and hydrological measurements to model colony abundance dynamics. Our results indicate that elevated early spring temperatures and water level variability are the primary determinants of numerical population dynamics. Spatial analysis revealed a heterogeneous response to hydrological stress: while the westernmost colony exhibited high site fidelity due to its proximity to persistent aquatic surfaces, the central colonies suffered severe declines or local extirpation during extreme drought periods (2020–2022). A discernible eastward shift in bird assemblages was observed toward zones with superior hydrological connectivity and proximity to anthropogenic hubs, suggesting an adaptive spatial response that was consistent with behavioral flexibility. We propose an adaptive management framework prioritizing sustainable solutions for maintaining minimum lacustrine water levels to preserve critical foraging zones. This integrative framework highlights the pivotal role of remote sensing in transitioning from reactive monitoring to predictive conservation of deltaic ecosystems.