Seagrass Coverage
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Halt loss of Posidonia oceanica seagrass meadows
No change observed
Gökova Bay to Cape Gelidonya
Türkiye
Problem: Anchoring of boats is the largest threat to seagrass meadows in this seascape.
Action: Akdeniz Koruma Derneği (AKD) are engaging in outreach with boat owners and operators to educate and share information on the impacts of anchor damage. AKD also monitor the health and distribution of Posidonia oceanica meadows and share this information with relevant public institutions.
Indicator: An increase in mean coverage and shoot density of seagrass indicates that degradation of seagrass meadows due to boat anchoring has been reversed.
Methods
Posidonia oceanica was primarily monitored through annual in-water dive surveys, monitoring shoot density, coverage and level of anchor damage, both inside and outside No-Fishing Zones (NFZs).
Eight sites were selected across the three project areas of Gökova (five sites within NFZs and one outside NFZs), Kaş (one site within an NFZ) and Göcek (one site within an NFZ and one site outside NFZs). A cement marker was placed to locate each site for annual visits. Eleven reference markers (one every 5 m, for 50 m) were placed along the edge of the seagrass meadow at each site.
At each marker, a 60 x 60 cm quadrat was placed, divided into nine 20 x 20 cm squares. Empty squares were counted to estimate average P. oceanica cover. In addition, three 20 x 20 cm squares within each quadrat were chosen randomly and the number of shoot counts within them were counted; these counts were then combined to calculate average shoot densities.
Water temperature was also measured during surveys. Expert opinion on the level of anchor damage, as well as evidence of other stressors, including grazing, turbidity, and signs of temperature stress, were also collected to understand possible threats to P. oceanica in the seascape.
Data was collected annually from 2020 to 2023, although some sites were monitored for fewer years.
Diver completes Posidonia oceanica seagrass monitoring, tracking seagrass meadow expansion and regression. Photo: AKD.
Diver sets up permenant monitoring marker for Posidonia oceanica seagrass monitoring. Photo: AKD.
Results
Across sites, there were no statistically significant differences in Posidonia shoot density (Fig. 1) or cover (Fig. 2) inside and outside NFZs.
Comparing between years, there was a significant difference in shoot density between 2022 and 2023, with lower density in 2023. This is believed to have been caused by unusually high water temperatures leading to thermal stress. Among other years there were no significant changes.
Expert observation of conditions at monitoring sites estimated that there was no observable change in the level of anchor damage between the baseline year (2020) and the final year of monitoring (2023).
Figure 1: Mean Posidonia shoot density (shoots / m2) inside and outside No-Fishing Zones. Error bars denote standard deviation.
Figure 2: Mean % cover of Posidonia inside and outside No-Fishing Zones. Error bars denote standard deviation.
Interpretation
The lack of a significant difference between sites inside and outside NFZs reflects the fact that anchoring (a main cause of damage to Posidonia) is permitted within NFZs, so they do not directly reduce threats to seagrass.
As Posidonia is notably slow growing, recovery will be a long-term process requiring ongoing data collection over a longer timescale to observe change. The data collected so far will provide a robust baseline which future data can be compared against to track seagrass recovery.
Although this meant it was unlikely that change would be detected in the short-term, monitoring Posidonia provided a useful threat detection system, so that any rapid or widespread losses of seagrass could be quickly identified.
Diver completes Posidonia oceanica seagrass quadrat monitoring. Photo: AKD.
Lessons learned
Alongside the in-situ monitoring, work was also undertaken to map seagrass using remote sensing data, with ground truthing data collected at in-situ monitoring sites. The initial attempt returned only 32% accuracy, while a further attempt using a different framework returned higher accuracy (>70%). However, this was still considered too low to use the results with confidence.
This highlighted the challenges of gaining an accurate and high-resolution understanding of seagrass habitat using remote sensing, in an area where seagrass can establish at significant depths (> 30 m). Constraints remain in using these mapping outputs. Significant areas were masked due to factors such as cloud cover and turbidity from river inlets. There was also potentially indiscriminate detection of different seagrass species, with implications for blue carbon estimates, blue carbon credit planning and low perceived accuracy by local experts.
Next steps
A longer timeseries is needed to understand more about the condition of this slow-growing priority habitat, and the effects of restoration activities on recovery. In future, more comprehensive data will be collected on anthropogenic impacts including boat anchoring and a broader range of environmental stressors to establish causal relationships, and to incorporate adaptive management strategies to address localised challenges more effectively.
Citations
RAC/SPA – UNEP/MAP (2014). Monitoring protocol for Posidonia oceanica beds. By Guala I, Nikolic V, Ivesa L, Di Carlo G,Rajkovic Z, Rodic P, Jelic K. Ed. RAC/SPA – MedMPAnet Project, Tunis.
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