Spatial variability in seawater chemistry across an intertidal mangrove–coral reef boundary

Context Edge effects at habitat boundaries can generate strong physicochemical gradients that structure coastal ecosystems. Although environmental gradients within coexisting mangrove–coral habitats are well documented, spatial variability in seawater chemistry across intertidal mangrove–reef boundaries remains under-characterised. Aim We examined fine-scale spatial variability in seawater chemistry across a mangrove–reef ecotone to assess how nearshore physicochemical conditions vary with proximity to mangroves. Methods Seawater chemistry was measured over 2 weeks in Pioneer Bay, Orpheus Island (Great Barrier Reef, Australia), using sequential deployments of an autonomous sensor package and discrete daytime high- and low-tide sampling along a fine-scale mangrove-proximity gradient and beyond. Key results Temperature, total-scale pH, salinity, PAR and wave power showed strong spatial structuring, whereas oxygen varied primarily with depth and time of day. Discrete sampling showed tide-dependent spatial patterns in temperature, total alkalinity and chlorophyll-a concentration, with the largest physicochemical shifts occurring at shallow nearshore sites closest to mangroves. Conclusions Fine-scale physicochemical gradients occur across intertidal mangrove–reef boundaries without direct habitat coupling, modulated by depth and hydrodynamic exchange. Implications The observed heterogeneity in seawater chemistry highlighted the need for seasonal, multi-site studies to resolve the extent and ecological significance of mangrove-associated processes on coral reef environments.