Award: OCE-1917029

Ecosystems can exhibit ‘tipping points’ whereby an environmental disturbance pushes an ecosystem into a degraded state from which it does not recover, even when the environment normalizes. This may have happened to valuable oyster reefs in Northwest Florida in 2012, when drought and low river flow allowed predators of oysters to flourish and consume nearly all the oysters. Despite subsequent years of normal rainfall and river flow, oysters have not recovered, suggesting the ecosystem may have crossed a tipping point. In this project investigators hypothesized that the strength and timing of Hurricane Michael may have provided a sufficient physical and salinity disturbance to shift the system back into its original, healthy state. Prior observations of hurricane effects supported this prediction, but that earlier research was unable to quantify the underlying ecological mechanisms. In this study, investigators collected new water quality data and combined it with older data to show that Hurricane Michael did lower the salinity of Apalachicola Bay in a manner that could have initiated system recovery by alleviating predation on juvenile oysters (see Figure 1). Immediately after this disturbance, we repeated predator-exclosure experiments throughout the bay. In contrast to our pre-hurricane experimental results, post-hurricane experiments showed a significant reduction in predation on juvenile oysters that had not been observed since the fishery collapsed in 2012 (see Figure 2). In the absence of predation, the number of juvenile oysters on oyster reefs increased significantly, suggesting a potential return shift to a healthy state (see Figure 3c). However, it remains unclear if this recovery will be sustained because Hurricane Michael did not re-expose previously buried reef substrate, which larval oysters require for settlement (Figure 3a). Ongoing analysis of empirical data and mathematical modeling will evaluate whether the hurricane impact to salinity and predation are sufficient for system recovery, given the lack of the predicted impact of increasing reef substrate availability. Intellectual Merit: Identifying and predicting tipping points for ecosystems between healthy and degraded states remains a central goal in ecology. A new approach was recently proposed: replicated field observations and experiments are compared to model simulations in which the system has been pushed past the tipping point. While this approach has provided improved predictions in tests with historical datasets and highly controlled experiments, it has not been applied in the field to identify the mechanisms underlying the bi-directional shift of a natural system beyond a tipping point. This is mainly because observations, experiments, and models are typically not in place to do so at the time of a shift. The effects of Hurricane Michael on Apalachicola Bay provided a unique opportunity to address this knowledge gap. The observational and experimental data generated are being combined with a mathematical model to quantify the boundary between the basins of attraction for the healthy and degraded states under different scenarios of impacts to water salinity, predation on juvenile oysters, and settlement substrate for oyster larvae. Broader Impacts: This research can inform the restoration and management of the Apalachicola Bay oyster fishery, which crashed in 2012. Specifically, this research highlights how post-hurricane alterations to salinity may represent an optimal time period for enhancing the amount of substrate on oyster reefs. This project also identified the optimal amount of substrate to deploy per unit area (Kimbro et al. 2020 Restoration Ecology 28: 1633-1642). Given a limited supply of restoration substrate material and restoration funds, it is important to identify the optimal time and amount of substrate to deploy. To date, this project has produced a nearly eight-year time series on oyster density and size, substrate availability, predation, and larval recruitment on oyster reefs in Apalachicola Bay, with associated meta-data deposits at the Biological and Chemical Oceanography and Data Management Office of the National Science Foundation (links). In coordination with the Apalachicola National Estuarine Research Reserve, meta-data were also deposited at the Statewide Ecosystem Assessment of Coastal and Aquatic Resources Data Discovery of the Florida Department of Environmental Protection (https://dev.seacar.waterinstitute.usf.edu/programs/details/5075). Last Modified: 12/04/2020 Submitted by: Christopher Stallings