Investigating thermal regime on seawalls to maximise nature positive outcomes for encrusting tropical oysters

Ecological niche theory poses that the arrangement of species in the environment is arranged by thresholds and tolerances to settings, though, when these conditions fall outside these (for example, temperature) a species is absent. Using an infra-red thermal sensor, we characterise the surface temperature in summer and autumn on the sun facing (no oysters present) and the shaded side of rock boulders (oysters present) used to build coastal seawall structures in central Queensland, Australia. The sun facing boulder surface was significantly hotter, up to 15 °C at any one time, compared with the boulder shaded side during both the summer and autumn surveys. Diel logging (30 min intervals) of surface temperature of sun facing boulder surfaces without oysters ranged between 24 and 50 °C, while boulders with oysters ranged between 25 and 35 °C. A Principal Component Analysis constructed using boulder dimensions (length, width, height, and surface angle orientation) showed a close positive correlation between length and width, however, height was poorly correlated with the other two dimensions. When this information is used to construct a boulder index no relationship is evident between the boulder size and the observed temperatures on the two sides of boulders. For seawalls to achieve nature positive outcomes in tropical settings, requires simply taking into consideration rock surface temperature settings during the design and construction phase. Implementing this ecological engineering consideration gives oysters a greater chance of colonisation, and thereby providing the ecosystem services that they are well known for. • Seawalls engineered using rock boulders provide shoreline protection. • Oysters were only found on the cooler, shaded faces of boulders. • Sun facing boulder surfaces are up to 15 °C hotter than shaded faces of boulders. • More shaded boulder surfaces increases native positive outcomes.