Megaherbivore exclusion led to more complex seagrass canopies and increased biomass and sediment Corg pools in a tropical meadow

In some regions of the Caribbean Sea, seagrasses have been negatively affected by sea turtle overgrazing. Seagrass canopy complexity has declined at a long-term monitoring site in Costa Rica. We deployed megaherbivore exclosures for 13 months and found an increase over time in seagrass cover and maximum canopy height to similar to 75% and 20 cm respectively in the exclosures; while they remained steady in controls at < 25% and similar to 5 cm. Following exclusion, above ground biomass was higher in exclosures (320 +/- 58 g DW m(-2)) compared to controls (171 +/- 60 g DW m(-2)). Leaves were longer and wider in the exclosures (8 +/- 5 cm and 0.8 +/- 0.2 cm) compared to controls (2 +/- 2 cm and 0.5 +/- 0.1 cm). Above ground biomass C(org )pools in exclosures (1.2 +/- 0.2 Mg ha(-1)) were two-times higher than in controls (0.6 +/- 0.2 Mg ha(-1)). Meanwhile, there was no variation between treatments in seagrass shoot density (1,692 +/- 803 shoots m(-2)), below ground biomass (246 +/- 103 g DW m(-2)) and its C-org pool (0.8 +/- 0.4 Mg ha(-1)). Relative sediment level increased up to 4.4 cm within exclosures revealing a net increase in sediment C-org, while surficial sediment C-org percentage was similar between exclosures and controls. Releasing these meadows from megaherbivore grazing therefore led to a clear increase within exclosures of seagrass cover, canopy complexity, above ground biomass, and C-org pools in above ground biomass and sediment. Our study reveals that the decline in canopy complexity over time at this meadow is linked to megaherbivore grazing and has most likely led to a decrease in blue carbon pools. Excessive megaherbivore grazing at this site could lead to a continued decline or potential loss of the meadow, and seagrass conservation and restoration initiatives should include consideration of trophic dynamics.

Automated summary

The negative impact of sea turtle overgrazing on seagrasses in some regions of the Caribbean Sea has led to a decline in seagrass canopy complexity. However, excluding megaherbivores has shown to increase seagrass cover, canopy height, above ground biomass, and carbon pools, while no significant changes were observed in seagrass shoot density, below ground biomass, and below ground carbon pools. This study highlights the importance of considering trophic dynamics in seagrass conservation and restoration efforts.