MEPS 681:53-69 (2022)  -  DOI: https://doi.org/10.3354/meps13913

ABSTRACT: Nitrogen that has been recycled in the benthos supports high rates of primary and secondary production in estuaries. However, little is known about the effect of future climate on benthic nitrogen recycling and assimilation. An ex situ core incubation was used to assess the impact of combinations of warming (8°C range) and ocean acidification (OA) (i.e. increased pCO₂ and decreased pH) on ammonium (NH₄⁺) and nitrate/nitrite (NO_x) fluxes and ¹⁵N-nitrate assimilation in shallow unvegetated estuarine sediments. Dissolved inorganic nitrogen (DIN = NH₄⁺ + NO_x) fluxes were significantly affected by the interaction of warming and OA, highlighting the importance of considering combined stressor treatments when investigating ecosystem responses to future climates. Warming alone increased DIN efflux from the sediments. At current mean ambient temperatures (23°C) and below (Δ-3°C), OA significantly increased DIN effluxes, but there was little to no effect of OA on DIN fluxes at warmer temperatures (Δ+3°C and Δ+5°C). OA reduced the ¹⁵N assimilation/retention of the sediments across all temperatures, suggesting that nitrogen retention in bacterial biomass was reduced, despite OA also increasing primary productivity. As such, under the projected future climate of ~3°C warming and doubling of pCO₂ (~1000 µatm), unvegetated estuarine sediments are likely to have a more rapid turnover of DIN driven by greater microphytobenthos production and recycling.

KEY WORDS: Ocean acidification · Marine sediments · Climate change · Microphytobenthos · Estuaries · Nitrogen cycling