Freshwater future: The influence of exposure to extreme summer rainfall events on the resistance and recovery patterns of an estuarine seagrass
Date of Award
Bachelor of Science Honours
School of Science / Centre for Marine Ecosystem Research (CMER)
Dr Kathryn McMahon
Restoring and maintaining the ecological resilience of seagrass ecosystems will be a major challenge of the 21st century. The decline of seagrasses worldwide is attributed to the erosion or elimination of their ecological resilience driven by human impacts, extreme climate events and climate change. Ecological resilience refers to the ability of ecosystems to resist or recover from disturbances whilst maintaining their integral structure and function. Seagrass resilience is influenced by life history, meadow form (transitory or enduring) and habitat type. The purpose of this honours project was to investigate the influence of extreme climate events and meadow form on a seagrass species located in temperate Western Australia.
Extreme climate events typically precede extensive seagrass decline with recovery times of ≥ 2 years and in extreme cases, no recovery at all. The first part of this study investigated the response of the dominant seagrass Halophila ovalis to an extreme rainfall event which produced flooding in the Swan-Canning Estuary, Western Australia in early summer of 2017. This event resulted in rapid changes in salinity, light and nutrient delivery causing seagrass decline and adverse impacts to estuarine condition. Utilising an approach developed for marine heat waves, we defined this extreme event as the period during which salinity differed from background conditions by ≥ 5% and quantified metrics relating to the duration and intensity of the flood. Plant traits that reflected resistance (e.g. cover and biomass) and recovery (e.g. growth potential and seed bank density) were measured at six sites along the estuarine gradient, before, during and after the ECE. The purpose of this was to test the following hypotheses: 1) that the intensity and duration of the flood event would influence the patterns of resistance and recovery and 2) upstream meadows that are more frequently exposed to freshwater would be more resistant and recover faster from heavy rainfall events.
Site and temporal variation was significant (p < 0.05). Widespread defoliation was evident within one month at upstream meadows where the event persisted for 88 days with salinity declines of 20 PSU. At downstream meadows, the event duration was shorter (79 days) and changes in salinity were less severe (±15 PSU). Despite experiencing lower physical impact, biomass at downstream sites declined by 72% (±15SE) within two months compared to 24- 57% (±14SE) at upstream sites. The period of resistance was similar across sites with significant declines in plant traits evident within one to two months. In contrast, the recovery time for plant traits to return to pre-flood levels varied between sites, with longer times at downstream sites. Biomass and leaf density returned to pre-flood levels (p > 0.05) over five to nine months at the upstream meadows but not at the downstream sites during the period of this honours project. Consistently higher seed bank densities at the downstream meadows did not appear to accelerate the recovery process. Acclimation to freshwater exposure may explain the greater resilience of upstream meadows compared to downstream meadows.
The concept of meadow form proposes that management strategies be tailored according to whether seagrass meadows are enduring, and more effective at resistance, or transitory and more dependent on recovery. To test this, meadows were classified as enduring or transitory and the same plant traits linked to the resistance and recovery processes used in part one of the study were compared to test an additional hypothesis: that meadow form would influence resilience and specifically that resistance traits would be greater in enduring meadows whilst recovery traits would be higher in transitory meadows. The results did not support this 4 hypothesis, however, this was based on a limited sample size due to the occurrence of the flood which prevented further sampling for this aspect of the study. Amid ongoing seagrass decline, opportunities remain to improve our current management strategies and this concept should be explored in future work.
Webster, C. (2017). Freshwater future: The influence of exposure to extreme summer rainfall events on the resistance and recovery patterns of an estuarine seagrass. https://ro.ecu.edu.au/theses_hons/1525