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 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

Seagrass soils sequester up to half the metal emissions of one of the world\u27s largest smelters

One of the world\u27s largest smelters has been operating in South Australia since 1889, affecting environment and human health. Here we quantified the magnitude of Pb, Zn and Cd emissions from the smelter sequestered in the soil of an adjacent 110 km2 Posidonia australis seagrass meadows. Seagrass core records show that the smelter contaminated the entire area with decreasing sequestration with increasing distance from contamination points. The soil accumulated ~1300 t of Pb, ~3450 t of Zn, and ~ 90 t of Cd since 1889, and sequestered the equivalent of ~20 % of Pb, and ~50 % of Zn and Cd cumulative smelter emissions since 1999, showing that seagrass can be significant, long-term sinks of metal pollution in highly contaminated environments. Conservation efforts should prioritize these seagrass meadows to avoid the potential release of pollutants from their soils following habitat loss, which could turn seagrasses from a sink to a source of pollution

Contribution of seagrass blue carbon toward carbon neutral policies in a touristic and environmentally-friendly island

Estimates of organic carbon (Corg) storage by seagrass meadows which consider inter-habitat variability are essential to understand their potential to sequester carbon dioxide (CO2) and derive robust global and regional estimates of blue carbon storage. In this study, we provide baseline estimates of seagrass extent, and soil Corg stocks and accumulation rates from different seagrass habitats at Rottnest Island (in Amphibolis spp., Posidonia spp., Halophila ovalis, and mixed Posidonia/Amphibolis spp. meadows). The Corg stocks in 0.5 m thick seagrass soil deposits, derived from 24 cores, were 5.1 ± 0.7 kg Corg m–2 (mean ± SE, ranging from 0.05 to 12.9 kg Corg m–2), accumulating at 23.2 ± 3.2 g Corg m–2 year–1 (ranging from 0.22 to 58.9 g Corg m–2 year–1) over the last decades. There were significant differences in Corg content (%) and stocks (mg Corg cm–3), stable carbon isotope composition of the soil organic matter (δ13C), and soil grain size among the seagrass meadows studied, highlighting that biotic and abiotic factors influence seagrass soil Corg storage. Mixed meadows of Posidonia/Amphibolis spp. and monospecific meadows of Posidonia spp. and Amphibolis spp. had the highest Corg stocks (ranging from 6.2 to 6.4 kg Corg m–2), while Halophila spp. meadows had the lowest Corg stocks (1.2 ± 0.6 kg Corg m–2). We estimated a total soil Corg stock of 48.1 ± 8.5 Gg Corg beneath the 755 ha of Rottnest Island’s seagrasses, and a Corg sequestration capacity of 0.81 ± 0.06 Gg Corg year–1, which is equivalent to the sequestration of ∼22% of the island’s current annual CO2 emissions. Our results contribute to the existing global dataset on seagrass soil Corg storage and show a significant potential of seagrass to sequester CO2, which are particularly relevant in the context of achieving carbon neutrality through conservation actions in environmentally-marketed, tourist destinations such as Rottnest Island

Release of dissolved organic carbon from seagrass wrack and its implications for trophic connectivity

ABSTRACT: The export of old leaves and stems (wrack) from seagrass meadows provides a mechanism for trophic connectivity among coastal ecosystems. As little of this wrack is consumed by mesograzers, leached dissolved organic carbon (DOC) may determine the importance of wrack as a trophic subsidy. However, few studies have examined the effect of seagrass type or age on the release of DOC or its bioavailability. We examined the amount and composition of DOC released from different wrack: Posidonia sinuosa, Amphibolis antarctica and the alga Laurencia sp. We then examined the effect of age on DOC leaching from P. sinuosa wrack. The bioavailability of the DOC was also assessed using a bacterial bioassay. The rate of DOC leaching from P. sinuosa leaves decreased exponentially with time. According to that exponential model, ~50% of the total DOC release occurred in the first 14 d and it would require a further 2.94 yr to release the same amount again. Fresh algae Laurencia sp. leached the greatest amount of DOC in the first 16 h (6.7 g kg-1 fresh weight (FW) wrack), followed by fresh P. sinuosa leaves (1.7 g kg-1 FW), A. antarctica leaves (1.1 g kg-1) and stems (0.6 g kg-1), 4 wk old P. sinuosa (67 g kg-1) and fine detritus (74 g kg-1). In all cases, the composition of the DOC was similar and dominated by the hydrophilic component (in P. sinuosa, predominantly sugars and amino acids). Leachates from all fresh wrack supported bacterial growth over 24 h. Leachate from older wrack either failed to support bacterial growth or only supported it for a limited time. Given the exponential decay in DOC release rate, the interacting timescales of transport and leaching will affect the value of wrack as a vector for trophic subsidies

Seagrass meadows provide 3D habitat for reef fish

For large fishes, seagrass canopies typically provide a relatively flat habitat on seabeds, but seagrasses in the genus Posidonia can provide additional habitat complexity by forming organic-rich deposits known as mats. Erosional processes can scour channels through the mats, resulting in the formation of escarpments with caves. Here we report that reef fishes, such as groupers, inhabit the caves found within mat escarpments. The characteristics of the cavities are highly variable, ranging from small-elongated holes to deep caves with large entrances. The origin of these caves (biological and/or geological) is unknown, but it is possible that fish behavior enhance their formation. Posidonia seagrass escarpments provide a complex 3D habitat for reef fish that is not provided by typical canopy structure of seagrass. Further studies are required to gain insights into the natural history of seagrass escarpments and their ecological importance

Ranking the risk of CO2 emissions from seagrass soil carbon stocks under global change threats

Seagrass meadows are natural carbon storage hotspots at risk from global change threats, and their loss can result in the remineralization of soil carbon stocks and CO2 emissions fueling climate change. Here we used expert elicitation and empirical evidence to assess the risk of CO2 emissions from seagrass soils caused by multiple human-induced, biological and climate change threats. Judgments from 41 experts were synthesized into a seagrass CO2 emission risk score based on vulnerability factors (i.e., spatial scale, frequency, magnitude, resistance and recovery) to seagrass soil organic carbon stocks. Experts perceived that climate change threats (e.g., gradual ocean warming and increased storminess) have the highest risk for CO2 emissions at global spatial scales, while direct threats (i.e., dredging and building of a marina or jetty) have the largest CO2 emission risks at local spatial scales. A review of existing peer-reviewed literature showed a scarcity of studies assessing CO2 emissions following seagrass disturbance, but the limited empirical evidence partly confirmed the opinion of experts. The literature review indicated that direct and long-term disturbances have the greatest negative impact on soil carbon stocks per unit area, highlighting that immediate management actions after disturbances to recover the seagrass canopy can significantly reduce soil CO2 emissions. We conclude that further empirical evidence assessing global change threats on the seagrass carbon sink capacity is required to aid broader uptake of seagrass into blue carbon policy frameworks. The preliminary findings from this study can be used to estimate the potential risk of CO2 emissions from seagrass habitats under threat and guide nature-based solutions for climate change mitigation

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

Patch dynamics driven by wave exposure in subtidal temperate seaweeds are exacerbated by warming oceans

Over the past decades, ocean temperatures have been steadily increasing and are projected to continue to do so, stressing many temperate marine organisms. Changing temperatures do not affect ecosystems in isolation, but interact with many other factors in shaping ecological communities. We investigated the changes over 2 decades in subtidal temperate seaweed communities over a wave exposure gradient in Western Australia, a global warming hotspot. We found higher diversity in the seaweed community and a higher proportion of biomass of species with a warm affinity (expressed as the tropicalization index: TI) over time. There was no decline in biomass of the dominant habitat-forming kelp Ecklonia radiata on low wave exposure reefs, while it was patchier and comprised a lower proportion of the total seaweed biomass on the medium and high wave exposure reefs. Furthermore, the presence of E. radiata was disproportionally associated with low abundances of seaweeds with warm affinity. The increasing patchiness of E. radiata likely provided a competitive release for other seaweeds, and the increase in abundance of Scytothalia dorycarpa likely provided a compensatory effect which resulted in a lower than expected TI. We found no indication of an ameliorating effect by wave exposure, and conclude that the patch dynamics driven by wave exposure are more likely exacerbated by increasing ocean temperatures on subtidal temperate reefs. If this continues, the reduction in E. radiata and increase in warm affiliated seaweeds will result in a more diverse seaweed community, but one with a lower standing biomass