Estuarine Sediments Exhibit Dynamic and Variable Biogeochemical Responses to Hypoxia

Abstract

Estuarine ecosystems are considered among the most valuable regions worldwide because of the wealth of ecosystem services they provide. Given their proximity to the land, they are also vulnerable to excessive nutrient inputs, which can lead to eutrophication and hypoxia. Water column hypoxia can dramatically alter sediment biogeochemistry, yet the response of specific processes varies widely. Predicting whether sediments are sources or sinks of ecologically and climatologically relevant nutrients and gases remains elusive, particularly for shallow coastal regions. In this study, we conducted experiments with sediments collected from Waquoit Bay (Massachusetts, United States) and measured dissolved nutrient and gas fluxes across the sediment-water interface to investigate the impact of water column hypoxia (defined here as dissolved oxygen ≤3 mg/L) on three important ecosystem functions: nutrient regeneration, removal of reactive nitrogen, and the regulation of greenhouse gases. Under hypoxia we observed variable responses in sediment biogeochemical cycling, including little to no change in important nitrogen cycling processes such as ammonium efflux and nitrogen removal through denitrification. As expected, low oxygen conditions stimulated sediment phosphate efflux. This caused nutrient ratios for both nitrogen to phosphorus and silica to phosphorus in the overlying water to decrease by 50%. Such changes can alter both the composition of water column primary producers as well as the rate of primary production. Hypoxia led to an almost 60% decrease in sediment nitrous oxide consumption but had little impact on sediment methane emissions. Taken together, these experimental results suggest sediment biogeochemical cycling has variable and dynamic responses to hypoxia.

A synthesis of methane emissions from shallow vegetated coastal ecosystems

“…For subtidal seagrass ecosystems, we can use estuarine systems to help predict the impacts of hypoxia on CH 4 emissions. Estuarine responses to hypoxia are variable with some studies finding increases in CH 4 emissions under hypoxic conditions (Gelesh et al, 2016; Ye et al, 2016) and others finding decreases in CH 4 emissions under hypoxia (Bange et al, 2010; Foster & Fulweiler, 2019). The response of a system to hypoxic events will likely depend on the availability of labile OM in the system with systems containing high concentrations of labile OM experiencing increased CH 4 emission under hypoxic conditions.…”

The truncate soft-shell clam,Mya truncata, as a biomonitor of municipal wastewater exposure and historical anthropogenic impacts in the Canadian Arctic

“…xenobiotics (Fabbri et al, 2008). In this study, the low abundances of HSP90 in the gills of clams nearest the WWTP suggests that expression is more related to oxidative stress than other stimuli as chronic exposure to contamination could inhibit its function (Fabbri et al, 2008;Foster and Fulweiler, 2019). The conditions of oxidative stress at the WWTP is further supported by the expression pattern of MnSOD, a chief ROS scavenging enzyme (Holley et al, 2011).…”

Seasonal patterns of benthic-pelagic coupling in oyster habitats

“…Sediment O 2 consumption and phosphate (PO 4 3− ) release can be enhanced in oyster habitats (Mazouni et al 1996, Kellogg et al 2013 and are also likely to follow patterns of water column productivity. Greater OM loading to sediments can drive aerobic decomposition, reducing O 2 concentrations and leading to PO 4 3− release (Ingall & Jahnke 1994, Foster & Fulweiler 2019. Thus, we expect greater O 2 consumption and PO 4 3− release immediately following blooms.…”

“…Coastal sediment N cycling is dynamic, with multiple, often competing, processes co-occurring (Fulweiler et al 2013, Kuypers et al 2018, Foster & Fulweiler 2019, Ray et al 2020. For example, coastal sediments can rapidly (days to months) switch from N 2 fixation -which introduces new N to the system -to denitrification following the addition of labile OM (Fulweiler et al 2007(Fulweiler et al , 2008(Fulweiler et al , 2013.…”