Variability in carbon availability and eelgrass (Zostera marina) biometrics along an estuarine gradient in Willapa Bay, WA, USA

Because photosynthesis requires CO2, carbon limitation in aquatic environments could restrict primary production and provide signals in tissue chemistry. We took advantage of spatial variability of aqueous [CO2] in estuaries to examine within-estuary variation in biometrics of intertidal eelgrass (Zostera marina) during peak summer production. As expected from the sensitivity of carbonate equilibria to pH, aqueous [CO2] increased along an ocean-to-river gradient in Willapa Bay, WA, USA. The scale of pH variability also changed, reflecting weather-driven upwelling near the ocean, tidal advection near rivers, and reduced diel fluctuation up-estuary. Z. marina studied at eight sites in the bay integrated across these different temporal fluctuations in water chemistry to exhibit increased tissue carbon and depleted delta 13C up-estuary. However, seagrass production did not change as expected from aqueous [CO2]. Instead, small standing biomass occurred at sites with organic-rich sediments or high wave energy, investment in branching showed trends along the estuarine gradient that changed seasonally, and specific growth rates based on leaf extension did not shift with the estuarine gradient or with standing biomass. These results reinforce that estuarine seagrasses are likely to experience modified mean pH and variability due not only to ocean acidification in the strict sense (anthropogenic CO2 absorbed from the atmosphere) but also from land use, upwelling, and feedbacks from biological processes. However, responses via productivity may be less evident than in tissue chemistry.