Ocean acidification modifies the impact of warming on sediment nitrogen recycling and assimilation by enhancing the benthic microbial loop

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 (NOx) fluxes and 15N-nitrate assimilation in shallow unvegetated estuarine sediments. Dissolved inorganic nitrogen (DIN = NH4+ + NOx) 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.

Coastal acidification alters estuarine sediment nitrous oxide and methane fluxes

“…Perhaps the labile OM from macroalgae MP drives N 2 O production under acidification while the more recalcitrant, salt marsh OM at SLP lessened the effect on N 2 O under low pH (Table 1). The availability of inorganic N can fuel N 2 O production (Murray et al 2015) and a recent study reported that acidification increased sediment DIN fluxes (Simone et al 2022). However, in our study water column concentrations were similarly low at each site (Table 1) and in most cases we observed no relationship between N 2 O flux and inorganic N concentrations (Supporting Information Tables S4, S5).…”

The need for proactive environmental management of offshore aquaculture

“…Such models will need to be informed by (pelagic and benthic) monitoring programs that can quantify the sediment‐solute exchange rates of the receiving environment. For example, the relative sediment–seawater exchanges of dissolved oxygen, and inorganic carbon and nitrogen species, can be used to infer the metabolic pathways likely occurring in the sediments, 11,26,27 and whether sediments are retaining, remineralising, or removing carbon and nitrogen from the ecosystem 11 . Environmental managers may then use such measures of metabolic activity to define the limits of the seafloor ecosystem for accommodating organic waste 18 …”

“…Techniques that measure the sediment–seawater exchange of metabolic solutes provide a starting point for a proactive management framework. For instance, ex situ sediment core incubations are a common tool that has been used for decades to measure the sediment–seawater exchange of dissolved oxygen, carbon and nitrogen, revealing sediment metabolic functions 27,35–37 . Such incubations have also been used to study the seafloor near fish farms 38–40 .…”

“…For instance, ex situ sediment core incubations are a common tool that has been used for decades to measure the sedimentseawater exchange of dissolved oxygen, carbon and nitrogen, revealing sediment metabolic functions. 27,[35][36][37] Such incubations have also been used to study the seafloor near fish farms. [38][39][40] Similarly, in situ techniques such as non-invasive gradient flux, 41 eddy covariance measurements 42,43 and benthic chamber incubations, [44][45][46] are wellestablished methods that may prove useful in an environmental monitoring framework.…”