The ability of Ruppia polycarpa to regenerate from seed depends on seasonal porewater salinity dynamics and declining winter rainfall could delay recruitment

For many plants, regeneration from seed is vital for population recovery. Climate change is modifying the global hydrological cycle and a primary factor controlling germination of marine plants: salinity. How altered salinity regimes, and especially altered porewater salinity regimes, will regulate early life history stages of estuarine seagrasses is poorly understood. Here, we quantified the porewater salinity dynamics in two ephemeral estuaries that are dominated by the cosmopolitan genus Ruppia. Seedbank, germlings (germinated seeds) and seedlings were found in salinities ranging from 5 to 110 over an annual cycle. To understand the germination ecology of the dominant species, Ruppia polycarpa, seeds were exposed to treatments simulating current salinity regimes and those predicted under climate change. Seeds underwent a Dormancy treatment (15, 60, 150) followed by a Germination treatment (10, 20, 80). Generally, early life history stages were positively affected by hypersaline dormancy conditions if the subsequent Germination salinity was ≤ 20. Germination success was significantly higher for seeds transferred to 10 (65%) compared to 20 (49%) whilst no seeds germinated in 80 highlighting the risk of lower germination as estuaries become drier and more hypersaline with declining winter rainfall. However, germlings were found in situ in salinities ≥ 80 suggesting aspects of the salinity dynamics, not captured by our experimental conditions, may broaden tolerances. Dormant seeds were continuously present in situ and seedlings were observed throughout the whole of the growing season. These results are indicative of bet-hedging strategies. Future research should explore the capacity of these strategies to afford resilience to R. polycarpa to salinity variability under climate change

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

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