Modeling the effects of climate change on eelgrass stability and resilience: future scenarios and leading indicators of collapse

Abstract

Seagrass meadows influence local hydrodynamics in coastal bays, resulting in a decrease in the shear stress acting on the underlying bed sediment. The reduced sediment suspension and water column turbidity creates a more favorable light environment for further seagrass growth. This positive feedback is strong enough to induce depth-dependent bistable dynamics with 2 possible stable states, an extant meadow and a bare sediment surface. A coupled vegetation-growth hydrodynamic model was used to investigate eelgrass stability and leading indicators of ecosystem shift under the effects of sea-level rise and increases in water temperature associated with climate change. The model was applied to Hog Island Bay, a shallow coastal bay within the Virginia Coast Reserve, USA, where eelgrass restoration efforts are ongoing. The results indicate that while extant eelgrass meadows are likely to tolerate sea-level rise, an increase in the frequency of days when summer water temperature exceeds 30°C will cause more frequent summer die-offs. This increase in the number of higher temperature disturbance events is likely to push a dense meadow initially located within the bistable depth range (1.6 to 1.8 m mean sea level) toward and eventually past a critical bifurcation point, from which recovery is not possible. We identified 2 leading indicators of a meadow nearing this bifurcation point, both associated with the number of leaves per shoot: 'flickering,' which reflects conspicuous fluctuations from one attractor to the other across the threshold, and 'slowing down,' which is the decreased recovery from perturbations as a system gets close to a threshold. Our model indicates that the eelgrass in these coastal bays has limited resilience to increases in water temperatures predicted from current climate change models.

Multiple Stable States and Catastrophic Shifts in Coastal Wetlands: Progress, Challenges, and Opportunities in Validating Theory Using Remote Sensing and Other Methods

“…Two alternative stable states are known to exist for seagrass meadows, the vegetated state and the bare sediment (tidal flat) state ( Figure 5) [146,[211][212][213][214][215]. The positive feedback that enables the vegetated state to persist is related to maintaining low water turbidity, enabling the submerged seagrass plants to intercept enough light to perform photosynthesis.…”

Cover versus recovery: Contrasting responses of two indicators in seagrass beds

“…Several recent studies have argued that seagrass systems follow alternative stable state theory, implying hysteresis in the transition between vegetated and unvegetated states (van der Heide et al, 2007Carr et al, 2010Carr et al, , 2012. This has profound effect on the resilience of the system, i.e.…”

Twenty-first century climate change and submerged aquatic vegetation in a temperate estuary: the case of Chesapeake Bay

“…Similar losses will occur in the Bogue Sound-Back Sound in North Carolina (USA) (Micheli et al 2008). Restored eelgrass meadows are also vulnerable (Tanner et al 2010;Carr et al 2012). The resiliency of natural versus restored eelgrass meadows is an unanswered question.…”

“…They also forecasted that longer-term temperature increases of 1-4°C will significantly reduce or eliminate Zostera marina from the Chesapeake Bay . Further, more frequent summer heat waves where water temperatures reach 30°C are likely to trigger permanent phase change in shallow waters, which would prevent future recovery of eelgrass meadows (Carr et al 2012). Similar losses will occur in the Bogue Sound-Back Sound in North Carolina (USA) (Micheli et al 2008).…”

“…This is the case for the Chesapeake Bay, a North American temperate estuary fed by 150 tributary rivers and streams, which has been the focus of intensive conservation and management efforts for decades (Figure 1). Future climate forces will complicate these efforts because they will have profound impacts on submerged aquatic vegetation (SAV), which are foundational species (Carr et al 2012).…”