Characterising biogeochemical fluctuations in a world of extremes

Coastal and intertidal habitats are at the forefront of anthropogenic influence and environmental change. The species occupying these habitats are adapted to a world of extremes, which may render them robust to the changing climate or more vulnerable if they are at their physiological limits. We characterised the diurnal, seasonal and interannual patterns of flux in biogeochemistry across an intertidal gradient on a temperate sandstone platform in eastern Australia over six years (2009–2015) and present a synthesis of our current understanding of this habitat in context with global change. We used rock pools as natural mesocosms to determine biogeochemistry dynamics and patterns of eco-stress experienced by resident biota. In situ measurements and discrete water samples were collected night and day during neap low tide events to capture diurnal biogeochemistry cycles. Calculation of pH_T using total alkalinity (TA) and dissolved inorganic carbon (DIC) revealed that the mid-intertidal habitat exhibited the greatest flux over the years (pH_T 7.52–8.87), and over a single tidal cycle (1.11 pH_T units), while the low-intertidal (pH_T 7.82–8.30) and subtidal (pH_T 7.87–8.30) were less variable. Temperature flux was also greatest in the mid-intertidal (8.0–34.5˚C) and over a single tidal event (14˚C range), as typical of temperate rocky shores. Mean TA and DIC increased at night and decreased during the day, with the most extreme conditions measured in the mid-intertidal owing to prolonged emersion periods. Temporal sampling revealed that net ecosystem calcification (NEC) and production (NEP) were highest during the day and lowest at night, particularly in the mid-intertidal. Characterisation of biogeochemical fluctuations in a world of extremes demonstrates the variable conditions that intertidal biota routinely experience and highlights potential microhabitat-specific vulnerabilities and climate change refugia.