# Long-term impacts of salinity and temperature changes on Brachionus calyciflorus populations: understanding the role of intraspecific variability

## TL;DR

This study explores how long-term exposure to increased salinity and temperature affects the population resilience of the rotifer Brachionus calyciflorus.

## Contribution

The study reveals that intraspecific variability influences long-term population responses to combined salinity and temperature stressors.

## Key findings

- Some salinity-tolerant clonal lineages of Brachionus calyciflorus were extirpated earlier under long-term salinity exposure.
- Negative effects of salinity were enhanced at lower and higher temperatures, leading to earlier extirpation of tolerant lineages.
- Short- and long-term salinity sensitivity of clonal lineages showed no consistent association, indicating complex adaptation dynamics.

## Abstract

Worldwide, many coastal freshwater ecosystems suffer from seawater intrusion. In addition to this stressor, it is likely that the biota inhabiting these ecosystems will also need to deal with climate change-related temperature fluctuations. The resilience of populations to long-term exposure to these stressors will depend on their genetic diversity, a key for their adaptation to changing environments. Accordingly, this study aimed to understand the long-term effects of salinity and temperature on the population density dynamics of the rotifer Brachionus calyciflorus by considering intra-specific variability. Six clonal lineages of B. calyciflorus, exhibiting differential lethal sensitivity (LC50,24 h) to salinity, were exposed for at least 34 days, to a control and to artificial seawater (at a conductivity corresponding to the LC70,24 h for the most tolerant clonal lineage = 9.89 mS/cm), under three temperatures: 17, 20 (standard) and 23 °C. Long-term exposure to artificial seawater affected population densities, leading to the extirpation of some salinity-tolerant clonal lineages earlier than that of salinity-sensitive lineages. This inversion in short- and long-term sensitivity may suggest a higher susceptibility of populations when exposed to long periods of increased salinity. The negative effects caused by artificial seawater were enhanced at 17 °C and 23 °C, with an even earlier occurrence of extirpation of some clonal lineages, namely, two clonal lineages considered tolerant to artificial seawater. The results suggest the potential synergistic effects of the two abiotic stressors when combined. Overall, a lack of association between the clonal lineages’ short- and long-term sensitivity to salinity or their sensitivity to salinity under different temperature scenarios was observed. These results suggest an increased risk to the resilience of B. calyciflorus populations exposed to climate change-related scenarios of increased salinity and temperature fluctuations owing to an enhanced reduction in their genetic variability.

The online version contains supplementary material available at 10.1007/s11356-025-35995-3.