Going under: The implications of sea-level rise and reduced light availability on intertidal primary production

Estuarine intertidal flats are known for high rates of primary production, however, globally these ecosystems are under threat from rising sea-level and accelerated inputs of terrestrial sediments. These stressors simultaneously alter the duration of submersion/emersion periods and water-column turbidity impacting seafloor photoperiods. To assess how changes in submergence time and water-clarity can affect benthic primary production, we derived in situ seasonal photosynthesis-irradiance (P-I) curves over 2 yr. These were conducted during submerged and emerged periods in a seagrass meadow (Zostera muelleri) and an unvegetated sandflat (dominated by microphytobenthos). P-I curves showed strong light responses during submergence in both habitats and seagrass during emergence, but in the exposed sandflat there was little relationship between light availability and photosynthesis. Light-saturated measures of gross primary production (GPP(sat)) were 2.1 (submerged) and 4.1 times (emerged) higher in the seagrass than the sandflat habitat. Submerged GPP(sat) was always higher than emerged GPP(sat), but this difference was much more pronounced in the sandflat habitat (2.4 vs. 1.3 times for seagrass). Integrating seasonally averaged P-I curves for both tidal states into a simple model suggests that increasing submergence time could increase primary production in both habitats. However, reductions in water-clarity will decrease submerged primary production; an effect that will be exacerbated with sea-level rise. The sandflat trophic state was more sensitive to these environmental stressors because emerged production could not compensate for reductions during submergence. Collectively, regional management of water-clarity may be key to providing resilience to intertidal habitats, against the inevitable effects of sea-level rise.