Abstract
Abstract Extreme climate events are predicted to alter estuarine salinity gradients exposing habitat‐forming species to more frequent salinity variations. The intensity and duration of these variations, rather than the mean salinity values ecosystems are exposed to, may be more important in influencing resilience but requires further investigation. Precipitation, including the frequency, intensity and timing of occurrence, is shifting due to climate change. A global analysis on the timing of rainfall in estuarine catchments was conducted. In 80% of the case studies, the maximum daily rainfall occurred in the dry season at least once over the 40‐year period and could be classified as an extreme event. We selected an estuary in southwestern Australia and investigated the effects of an extreme rainfall event in 2017 resulting in an excess discharge of freshwater on seagrass Halophila ovalis. Adapting an approach applied for marine heatwaves using salinity data, we quantified metrics and characterised the event along the estuarine gradient. We assessed seagrass resilience by calculating resistance times based on the comparisons of biomass and leaf density data prior to, and during the event, and recovery times through assessment against historical condition. Where salinity is historically more variable, reductions in biomass were lower (higher resistance via plasticity in salinity tolerance) and meadows recovered within 9–11 months. Where salinity is historically more stable, loss of biomass was greatest (low resistance) post‐event and recovery may exceed 22 months, and potentially due to the rapid decline in salinity (−3 PSU/day). As estuaries become more hydrologically variable, these metrics provide a baseline for retrospective and future comparisons. Our results suggest seagrass resilience to hyposalinity is population specific. This understanding enables more accurate predictions about ecological responses to climate change and identifies which populations may ‘future proof’ ecosystem resilience. Synthesis. Following an extreme rainfall event, we found seagrass populations that are exposed to variable salinities recovered while those from a stable salinity environment were unable to recover within the study time frame. These findings expand upon existing evidence, derived primarily from other ecosystems, that show new sources of resilience may be uncovered by accounting for between‐population variation.
Highlights
The occurrence of extreme climate events (ECEs) has increased since 1950, a trend projected to continue to the end of the century (Christensen & Christensen, 2004; IPCC, 2014; Trenberth & Fasullo, 2012). Smith (2011) defined an ECE ‘as an episode or occurrence in which a statistically rare or unusual climatic period alters ecosystem structure and/or function well outside the bounds of what is considered typical or normal variability’
This has been well-documented for heatwaves (Smale et al, 2019) but our global analysis has identified that unseasonal rainfall events are another extreme climate event with the potential for significant ecological impacts, in estuarine ecosystems, and they occur across all climate regions
The metrics we developed to define this extreme event varied along the estuary
Summary
The occurrence of extreme climate events (ECEs) has increased since 1950, a trend projected to continue to the end of the century (Christensen & Christensen, 2004; IPCC, 2014; Trenberth & Fasullo, 2012). Smith (2011) defined an ECE ‘as an episode or occurrence in which a statistically rare or unusual climatic period alters ecosystem structure and/or function well outside the bounds of what is considered typical or normal variability’. | Journal of Ecolo gy 3 and predict ecosystem response and resilience, for example, in coral reefs (Hughes & Stachowicz, 2004; Kayanne, 2017), seagrass (Strydom et al, 2020) and forest ecosystems (Tatarinov et al, 2016) Such a framework for extreme rainfall events, and the associated changes in environmental conditions and ecosystem responses in coastal and marine ecosystems, has not been developed. Development of metrics to characterise the environmental change in estuaries from unusual rainfall events following the approach of Hobday et al (2016) for marine heatwaves, but using salinity rather than temperature We applied this metric approach to an unseasonal and large summer rainfall event that occurred in a temperate estuary in 2017 and assessed the ecosystem response using a resilience framework. These events are likely to create a rapid change in salinity and lead to significant ecosystem impacts constituting an extreme event (Levinton et al, 2011)
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