Ecomorphodynamics of Oyster Reefs and Their Influence on Oyster Reef Morphology

Abstract

Over 85% of oyster reefs have been lost globally due to disease, overharvesting, global warming, and pollution. Consideration of the ecosystem services provided by healthy oyster reefs (e.g., coastal protection, water purification and carbon burial) has driven recent research and restoration efforts worldwide. However, hydrodynamic studies, specifically looking at the effects of different levels of wave exposure on the ecomorphodynamics of oyster reefs, are scarce. In this study, we consider oyster reefs under different levels of wave exposure to determine how hydrodynamics may shape reef morphology and how reef morphology affects wave dissipation. We quantify oyster reef morphology through spatial analysis, using morphometrics and spatial density and relate these to the ability of oyster reefs to dissipate wave energy. Field campaigns were undertaken at three sites in southeast Australia with different levels of hydrodynamic exposure and morphology: Gamay (Botany Bay), Port Hacking and Crookhaven River. We found that reef morphology and orientation is related to estuarine hydrodynamic conditions and thus we propose an ecomorphodynamic model with a continuum of morphologies from sparse reefs aligned with the tidal currents and incoming waves (patch reefs), through broken up barriers semi-aligned to the tidal flows (string reefs), to the total barrier that exists under the lowest hydrodynamic conditions (fringing reefs). The highest dissipative ability of locally generated wind waves occurred at Crookhaven (patch reef, 165 kW/m2), and lowest at Gamay (string reef, 11.66 kW/m2). Our results suggest that reef morphology, and orientation to currents and waves, influence wave dissipation, and that hydrodynamic conditions in turn influence reef morphology. These findings are important to inform future reef restoration under increasingly severe climate change conditions to optimise ecosystem services on restored oyster reefs.

Keywords: wave dissipation, wave exposure, estuary, Australia