Freshwater future: The influence of exposure to extreme summer rainfall events on the resistance and recovery patterns of an estuarine seagrass

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 \u3c 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 \u3e 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

Increased extent of waterfowl grazing lengthens the recovery time of a colonizing seagrass (Halophila ovalis) with implications for seagrass resilience

Herbivore distributions and abundance are shifting because of climate change, leading to intensified grazing pressure on foundation species such as seagrasses. This, combined with rapidly increasing magnitudes of change in estuarine ecosystems, may affect seagrass resilience. While the overall resilience of seagrasses is generally well-studied, the timeframes of recovery has received comparatively little attention, particularly in temperate estuaries. We investigated how the recovery time (RT) of seagrass is affected by simulated grazing in a southwestern Australian estuary. Whilst excluding swans, we simulated different grazing intensities (25, 50, 75, and 100 % removal from 1 m2 plots) at four locations in the Swan-Canning Estuary, Western Australia during summer and tracked the recovery of seagrass over 3 months, using seagrass cover as the main measure of recovery. We found that seagrass recovered within 4–6 weeks from the lower grazing intensities (25 and 50 %) and 7–19 weeks from the higher grazing intensities (75 and 100 %) across the estuary. Increased grazing intensity led to not only longer recovery times (RTs), but also greater variability in the RT among experimental locations. The RT from the higher grazing intensities at one location in particular was more than double other locations. Seagrass recovery was through vegetative mechanisms and not through sexual reproduction. There was a significant grazing treatment effect on seagrass meadow characteristics, particularly belowground biomass which had not recovered 3 months following grazing. As the pressure of climate change on estuarine environments increases, these quantified RTs for seagrass provide a baseline for understanding grazing pressure as a singular disturbance. Future work can now examine how grazing and other potentially interacting pressures in our changing climate could impact seagrass recovery even further

The influence of abiotic and biotic conditions on lifecycle stages is critical for estuarine seagrass resilience

Abiotic and biotic factors influence seagrass resilience, but the strength and relative importance of the effects are rarely assessed over the complete lifecycle. This study examined the effects of abiotic (salinity, temperature, water depth) and biotic (grazing by black swans) factors on Ruppia spp. over the complete lifecycle. Structures were set up in two estuaries ( – 33.637020, 115.412608) that prevented and allowed natural swan grazing of the seagrasses in May 2019, before the start of the growing season. The density of life stage(s) was measured from June 2019 when germination commenced through to January 2020 when most of the seagrass senesced. Our results showed that swans impacted some but not all life stages. Seedling densities were significantly higher in the plots that allowed natural grazing compared to the exclusion plots (e.g. 697 versus 311 seedlings per m-2), revealing an apparent benefit of swans. Swans removed ≤ 10% of seagrass vegetation but a dormant seedbank was present and new propagules were also observed. We conclude that grazing by swans provides some benefit to seagrass resilience by enhancing seedling recruitment. We further investigated the drivers of the different lifecycle stages using general additive mixed models. Higher and more variable salinity led to increased seed germination whilst temperature explained variation in seedling density and adult plant abundance. Bet-hedging strategies of R. polycarpa were revealed by our lifecycle assessment including the presence of a dormant seedbank, germinated seeds and seedlings over the 8-month study period over variable conditions (salinity 2–42 ppt; temperatures 11–28 °C). These strategies may be key determinants of resilience to emerging salinity and temperature regimes from a changing climate

Population-specific resilience of Halophila ovalis seagrass habitat to unseasonal rainfall, an extreme climate event in estuaries

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

Global dataset on seagrass meadow structure, biomass and production

Seagrass meadows provide valuable socio-ecological ecosystem services, including a key role in climate change mitigation and adaption. Understanding the natural history of seagrass meadows across environmental gradients is crucial to deciphering the role of seagrasses in the global ocean. In this data collation, spatial and temporal patterns in seagrass meadow structure, biomass and production data are presented as a function of biotic and abiotic habitat characteristics. The biological traits compiled include measures of meadow structure (e.g. percent cover and shoot density), biomass (e.g. above-ground biomass) and production (e.g. shoot production). Categorical factors include bioregion, geotype (coastal or estuarine), genera and year of sampling. This dataset contains data extracted from peer-reviewed publications published between 1975 and 2020 based on a Web of Science search and includes 11 data variables across 12 seagrass genera. The dataset excludes data from mesocosm and field experiments, contains 14271 data points extracted from 390 publications and is publicly available on the PANGAEA® data repository (10.1594/PANGAEA.929968; Strydom et al., 2021). The top five most studied genera are Zostera, Thalassia, Cymodocea, Halodule and Halophila (84 % of data), and the least studied genera are Phyllospadix, Amphibolis and Thalassodendron (2.3 % of data). The data hotspot bioregion is the Tropical Indo-Pacific (25 % of data) followed by the Tropical Atlantic (21 %), whereas data for the other four bioregions are evenly spread (ranging between 13 and 15 % of total data within each bioregion). From the data compiled, 57 % related to seagrass biomass and 33 % to seagrass structure, while the least number of data were related to seagrass production (11 % of data). This data collation can inform several research fields beyond seagrass ecology, such as the development of nature-based solutions for climate change mitigation, which include readership interested in blue carbon, engineering, fisheries, global change, conservation and policy

Environmental variability generates sources of resilience in seagrasses

When we ‘look harder’ to quantify and understand the differences among individuals and populations that produce in variation in resilience within species we achieve better outcomes; successes from medical and agricultural industries are testament to this. Such successes have not yet been achieved for many ecosystems as the variation in resilience within species remains largely unexplored. This knowledge gap is acute for seagrasses which are key determinants of coastal ecosystem structure and function. The overarching goal of this research was to assess the role of environmental variability for influencing the resilience within species of seagrasses and to test these in the context of improving predictions under climate change. Using a combination of field and controlled experiments, this work examined how resilience differs among populations and across life history stages of multiple colonising seagrass species in temperate estuaries of Western Australia. The biological response of Halophila ovalis populations was compared along an estuarine-marine gradient to an extreme rainfall event in a ‘natural’ experiment and to salinity changes in a subsequent controlled experiment. I examined the impacts of abiotic and biotic factors on the response of Ruppia polycarpa over the complete lifecycle in a field manipulative experiment and experimentally tested the effects of salinity on seed dormancy release, germination and seedling establishment. Resilience to hyposalinity varied among H. ovalis populations along the estuarine-marine gradient. Estuarine populations were more resilient to low salinity stress than marine populations consistent with generally being exposed to a more variable salinity regime. In the field, upper-estuary populations recovered to historical baseline conditions whereas lower-estuary populations did not. These differences in recovery were verified by the experimental results which showed that upperestuary populations had greater survival and growth compared to lower-estuary populations. The upper-estuary populations represent an important source of adaptive capacity for the species. These results confirmed that environmental variability associated with the salinity gradient exerts strong selective pressure on seagrass populations and influences their resilience. The response of individual life history stages of R. polycarpa to abiotic and biotic factors varied. Salinity shifts from high to low followed by gradual increases promoted seed germination. Increasing temperature positively impacted seedlings but after a point, caused declines in adults. Swan grazing had minimal impact across life history stages but benefited seedling recruitment. Bet-hedging strategies, including the presence of dormant seeds, were also identified. These results indicated that species persistence results from a combination of environment dependent selection that ‘tailors’ individual life history stages to the conditions they are most likely to be exposed to and strategies that reduce the risk of complete mortality associated with a highly variable habitat. Management of habitats should reflect an understanding of the requirements of each and every life history stage and move away from an emphasis on adults. Overall, the findings of this research indicate that environmental variability and life history stages can generate variation in resilience within seagrass species. This variation matters when predicting the vulnerability of species under climate change. Populations that are naturally exposed to variable conditions are likely to be more resilient to emerging disturbances and reduce the overall vulnerability for the species. In habitats where conditions are highly seasonal, individual life history stages may have greater resilience to disturbances than others and be key modifiers of species’ vulnerability to climate change. These populations and/ or life history stages represent important sources of resilience that may have been underestimated in previous predictions. To move forward, conservation ecologists and managers need to consider the variation in resilience within seagrass and begin testing approaches that leverage this knowledge to reinforce these important species for future climate change

Journal of ecology 2021 global rainfall-seagrass resilience-Swan River

The excel file contains: summary of daily maximum precipitation from global rainfall analysis, monthly change in seagrass indicators (biomass, leaf density etc); water quality data including salinity, temperature and light data. There are also .nc files containing precipitation data from 1979-2019