Document Type

Report

Publisher

Western Australia Marine Science Institution

School

Centre for Marine Ecosystems Research / School of Science

RAS ID

71573

Funders

Western Australian Government, Department of Transport

Comments

Webster, C., McMahon, K., Ross, C., Afrifa-Yamoah, E., Said, N., Hovey, R., Martin., B., & Strydom, S. (2024). Two decades of seagrass monitoring data show drivers include ENSO, climate warming and local stressors. Report to WAMSI Westport Marine Science Program.

Abstract

The drivers contributing to the trajectories of seagrass ecosystems is a key knowledge gap which limits our effectiveness in preventing their decline. Here, we present the key findings from the regional assessment of seagrass condition (shoot density) over time, which was undertaken to inform environmental impact assessment and monitoring as well as mitigation for the Westport program. To assess regional trends and drivers of these trends, data was collated from five regional seagrass monitoring programs, including the Cockburn Sound State Environmental Policy Seagrass Monitoring Program and marine park monitoring programs conducted by the Department of Biodiversity, Conservation and Attractions. These monitoring programs are located in the temperate waters of Western Australia (WA) along a latitudinal gradient (30-33°S, covering ~365 kms) and focus on the dominant habitat forming species, Posidonia sinuosa. WA has been designated as a hotspot for climate pressures, influenced by ocean warming and extreme climatic events and there are localised anthropogenic pressures that seagrasses are susceptible to. Hence, this regional approach enables an assessment of the relative importance of global (e.g. warming) and local (e.g. coastal development) pressures for seagrasses.

The five regions assessed included the highly industrialised marine embayment of Cockburn Sound and four non-industrialised marine parks: Jurien Bay, Marmion, Shoalwater Islands and Geographe Bay within Ngari Capes (hereafter, Ngari Capes). Environmental predictors were selected based on an understanding of the major factors that affect seagrass condition, which included: depth, turbidity, sea surface temperature anomaly, mean sea surface summer temperature and habitat type. Hierarchical Generalised Additive Models (HGAMs) were run to examine the spatiotemporal trends in seagrass condition and to identify significant drivers as identified above, with the addition of region, site and geographic position.

The key findings were:

  • Across regions Posidonia sinuosa condition varied over time with a downward trend until 2017, after which there was a reversal in abundance. The variations aligned partly with El Niño Southern Oscillation.
  • Spatial and temporal shifts in Posidonia sinuosa shoot density were reasonably explained by region, year, water depth, turbidity, mean sea surface temperature in summer (mean_summer) and maximum sea surface temperature anomaly (max_over).
  • In some cases, there were differences in the effects of predictor variables on seagrass condition within each region (i.e. localised marginal effects). For example, mean_summer led to seagrass declines in Cockburn Sound but had very little effect in Jurien Bay.
  • Seagrass condition differed among regions, with shoot densities in Ngari Capes significantly higher than Cockburn Sound. Increased mean_summer temperatures were a strong driver of the observed declines in Posidonia sinuosa condition, especially in Cockburn Sound and Marmion.
  • Cooler sea surface temperature regime associated with higher latitude (Ngari Capes) appears to be ‘buffering’ Posidonia sinuosa condition from climate change effects.
  • Oscillations in Posidonia sinuosa abundance were most strongly linked to the El Niño Southern Oscillation cycle in Jurien Bay and to a lesser extent, Cockburn Sound.

Our results suggest that condition of Posidonia sinuosa across WA has declined in response to ocean warming and this was most pronounced in mid-latitude regions (31-32°S). Overall, it appears that in response to future climate change there will be variation in Posidonia sinuosa condition within regions and safeguarding healthy meadows as potential climate refuge sites is important to consider. Dredging and other coastal development activities that increase turbidity levels are likely to worsen Posidonia sinuosa condition given that low light impact seagrasses, and Cockburn Sound is quite susceptible to this. The variation in relationships between predictors and seagrass condition within regions highlights the importance of tailoring management actions to specific stressors and locations. Importantly, this work emphasises the value of long-term seagrass monitoring programs to evaluate ecosystem status in an era of rapid global change.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

Download

Share

 
COinS