Benthic primary production in emerged intertidal habitats provides resilience to high water column turbidity

Abstract

Increasing inputs of terrigenous sediments from anthropic land use change elevates water column turbidity, reducing light availability to benthic primary producers that sustain estuarine food webs. For intertidal habitats, photosynthesis during low tide periods of emergence may provide resilience against this temporally displaced stressor, yet the significance of low tide production (PP) has largely been overlooked. Emerged and submerged benthic PP was measured in adjacent seagrass (Zostera muelleri) and microphytobenthos-dominated (sandflat) soft-sediment habitats at three locations encompassing a turbidity gradient. Light and dark benthic incubation chambers were used to measure the flux of CO2 across emerged sediments, and dissolved oxygen across submerged sediments to derive net (NPP) and gross (GPP) PP. Seagrass NPP and GPP exceeded sandflat habitats (by 3–8 times) at low turbidity sites during submergence and at all sites during emergence (p < 0.01), whereas habitat differences were absent (p > 0.1) in turbid locations. Emerged NPP and GPP were higher (2–16 times) than submerged in all habitats and locations (p < 0.01). When standardised by mean incident photosynthetically active radiation (PAR), the difference between emerged and submerged seagrass PP increased with site turbidity, from 2 to 26 times greater. In a global context of increasing inputs of terrigenous sediment to estuaries, emerged PP may be crucial for providing resilience against benthic productivity losses in highly turbid environments.