Chasing windows of opportunity for habitat formation: Spatial and temporal determinants of oyster settlement rates

Abstract The establishment and recovery of biogenic habitats in marine and terrestrial environments often relies on the co‐occurrence of periods of suitable environmental conditions and propagule availability (e.g. seeds, larvae). However, despite the importance of these ‘windows of opportunity’ for species colonization, the temporal and spatial variability of these periods across environmental gradients remains poorly understood. Eastern Australian estuaries transit between periods of floods and droughts, which strongly affect their salinity regimes. Here, we investigated how variations in salinity and temperature influenced the occurrence of windows of opportunity for settlement and survival of the reef‐building oyster Saccostrea glomerata across an estuarine salinity gradient in eastern Australia. Oyster settlement probabilities were calculated using empirical data from a 2‐year settlement study performed at 3 estuarine sites along a salinity gradient. Time series of salinity and temperature from 2010 to 2024 were analysed for windows of opportunity for oyster settlement, defined as periods in which the probability of oyster settlement was equal to or higher than 0.25. Oyster habitat cover was measured at seven intertidal sites along the estuarine salinity gradient to assess how differential settlement patterns and ‘windows of opportunity’ shape oyster habitat formation. We found that in eastern Australia, the windows of opportunity for oyster settlement occurred during the warmer months and were more frequent and of longer duration at downstream (seaward) portions of estuaries, where salinity is higher due to greater oceanic influence. We also found that, although oysters can settle and survive at upstream locations, oyster habitat cover decreased from downstream to upstream locations along the estuarine salinity gradient, reflecting fewer and shorter windows of opportunity for settlement. Synthesis and applications . Temporal fluctuations in salinity and temperature are critical in determining when and where oyster settlement will occur within estuarine salinity gradients. Consequently, to maximize the chances of ecological restoration success, oyster reef restoration projects should focus on not only prioritizing areas for restoration that have higher likelihoods of species establishment, but also timing restoration activities to occur during periods of suitable environmental conditions.