Biogeochemical feedbacks may amplify ongoing and future ocean deoxygenation: a case study from the Peruvian oxygen minimum zone

Abstract

A new box model is employed to simulate the oxygen-dependent cycling of nutrients in the Peruvian oxygen minimum zone (OMZ). Model results and data for the present state of the OMZ indicate that dissolved iron is the limiting nutrient for primary production and is provided by the release of dissolved ferrous iron from shelf and slope sediments. Most of the removal of reactive nitrogen occurs by anaerobic oxidation of ammonium where ammonium is delivered by aerobic organic nitrogen degradation. Model experiments simulating the effects of ocean deoxygenation and warming show that the productivity of the Peruvian OMZ will increase due to the enhanced release of dissolved iron from shelf and slope sediments. A positive feedback loop rooted in the oxygen-dependent benthic iron release amplifies, both, the productivity rise and oxygen decline in ambient bottom waters. Hence, a 1% decline in oxygen supply reduces oxygen concentrations in sub-surface waters of the continental margin by 22%. The trend towards enhanced productivity and amplified deoxygenation will continue until further phytoplankton growth is limited by the loss of reactive nitrogen. Under nitrogen-limitation, the redox state of the OMZ is stabilized by negative feedbacks. A further increase in productivity and transition to sulfidic conditions is only possible if the rate of nitrogen fixation increases drastically under anoxic conditions. Such a transition would lead to a wide-spread accumulation of toxic sulfide with detrimental consequences for fishery yields in the Peruvian OMZ that currently provides a significant fraction of the global fish catch.

Sediment Release in the Benguela Upwelling System Dominates Trace Metal Input to the Shelf and Eastern South Atlantic Ocean

“…The positive feedback loop would continue until NO x losses due anammox and denitrification in subsurface waters depleted available N stocks. The offshore region would thus potentially become fixed N limited (Wallmann et al., 2022). The biological significance of increasing Fe and Co fluxes off‐shelf may therefore be strongly dependent on the response of the N‐cycle to intensification of deoxygenation.…”

“…The input of shelf sediment‐derived micronutrients such as Fe to overlying waters is therefore critical for determining the identity of the (micro)nutrient proximally constraining primary production in OMZ regions (Browning et al., 2017, 2018). Yet quantifying how vertical and lateral fluxes of TMs scale with benthic release into the water column remains a key challenge in assessing to what extent OMZ expansion or contraction may be mediated by biogeochemical feedbacks (Landolfi et al., 2013; Wallmann et al., 2022). Observations regarding transport of TMs across shelf to the open ocean in OMZ regions are sparse limiting our understanding of the underlying processes defining spatial/temporal trends in TM (co)‐limitation of primary production.…”

The Shelf‐To‐Basin Transport of Iron From the Northern U.S. West Coast to the Pacific Ocean

“…The physical and biogeochemical processes that govern the shelf‐to‐basin Fe transport, such as benthic release and biological uptake, are likely to undergo significant changes in a future climate, following widespread oceanic warming, changes in circulation, and O 2 loss (Kwiatkowski et al., 2020; T. Liu et al., 2022; Wallmann et al., 2022). In addition, the shelf‐to‐basin pathways described here are expected to affect the cross‐shore transport of non‐dissolved Fe pools (in particular suspended Fe minerals), as well as other trace elements (Richon et al., 2020; Weber et al., 2018), which can also modulate primary production in far‐field regions.…”

Nitrogen cycling in sediments on the NW African margin inferred from N and O isotopes in benthic chambers

“…Long-term monitoring of sediment fluxes is needed to address this issue more precisely, preferably using autonomous methodologies such as bottom crawlers that can operate independently of costly ship-based surveys (Wenzhöfer et al, 2016). Until the technology to do so becomes more accessible, future projections of the nutrient budgets and the economic sustainability of fisheries in tropical upwelling regions will carry larger uncertainties (Wallmann et al, 2022).…”