Ocean deoxygenation caused non-linear responses in the structure and functioning of benthic ecosystems

The O2 content of the global ocean has been declining progressively over the past decades, mainly because of human activities and global warming. Nevertheless, how long-term deoxygenation affects macrobenthic communities, sediment biogeochemistry and their mutual feedback remains poorly understood. Here, we evaluate the response of the benthic assemblages and biogeochemical functioning to decreasing O2 concentrations along the persistent bottom-water dissolved O2 gradient of the Estuary and Gulf of St. Lawrence (QC, Canada). We report several of non-linear biodiversity and functional responses to decreasing O2 concentrations, and identify an O2 threshold that occurs at approximately at 63 μM. Below this threshold, macrobenthic community assemblages change, and bioturbation rates drastically decrease to near zero. Consequently, the sequence of electron acceptors used to metabolize the sedimentary organic matter is squeezed towards the sediment surface while reduced compounds accumulate closer (as much as 0.5-2.5 cm depending on the compound) to the sediment-water interface. Our results illustrate the capacity of bioturbating species to compensate for the biogeochemical consequences of hypoxia and can help to predict future changes in benthic ecosystems.

Benthos response to nutrient enrichment and functional consequences in coastal ecosystems

As land use intensifies, many coastal waters are becoming enriched with otherwise limiting nutrients, leading to eutrophication. While the extreme effects of eutrophication on benthic communities are well documented, there is still a lack of knowledge about how nutrient enrichment alters biogeochemical interactions occurring at the sediment-water interface. Using ex-situ experiments, this study explores the consequences of nutrient enrichment on sediment characteristics, macrofauna community and benthic fluxes. The quantity of sedimentary organic matter and porewater concentration of NH₄⁺, NO_x and PO₄^3- increased in enriched treatments. These changes did not affect the macrobenthic community structure. However, macroinfauna buried less deep and increased their ventilation activity. As consequences, nutrient efflux increased, thereby favouring eutrophication processes. These effects were reduced in presence of seagrass, thus illustrating the buffering capacity of seagrass in the context of environmental changes, and particularly, of eutrophication. Overall, this study highlights that the functional consequences of nutrient enrichment involve interconnected processes that are variable in space and time.

Arsenic enrichment in sediment on the eastern continental shelf of Brazil

This study focuses on the vertical distribution of total and reactive As in two contrasted coastal sedimentary environments: the Abrolhos Continental Shelf (ACS), a carbonate and siliciclastic shelf sediment, and the Doce River Continental Shelf (DRCS), a submerged delta system. The Doce River was the location of a massive ore tailings dam collapsed in November 2015. Millions of liters of tailings were dumped into the river and reached the continental shelf, causing the country's biggest environmental disaster. We evaluated the As content in sediment of the DRCS before the dam collapse. At both sites, the total As background measured in bottom sediment revealed relative natural enrichment (above 8mg/kg). Content of As decrease with depth; reactive As showed surficial peaks which were associated with Fe and Mn oxides. The ACS sediment did not show significant enrichment or contamination of As, with an enrichment factor (EF) of approximately 2 and a geoaccumulation index (Igeo) near 0. In contrast, the DRCS exhibited severe As enrichment (EF=15) and contamination (Igeo between 3-4). This enrichment is attributed to long-term iron and gold exploitation in the Doce River watershed. The high levels of reactive As, up to 108 mg/kg, alert us to an environmental risk due to potential As bioaccessibility. These data provide an important perspective on the As contamination in continental shelves and encourage the monitoring of the ore mine environmental impacts.

Dose-dependent response of a benthic system to biodeposition from suspended blue mussel (Mytilus edulis) culture

This study reports the results of a field experiment using benthic mesocosms that examined dose-dependent effects of mussel biodeposition on the benthic environment. Mesocosms were placed in the natural sea bottom and subjected to one of eight levels of biodeposition (from 0 to 1400 mussels m⁻²). Most analyses indicated non-linear (i.e., threshold) effects. Sediment characteristics changed significantly between 200 and 400 mussels m⁻² as did multivariate community structure. Community structure effects were characterised by changes in abundances of species that are very sensitive or tolerant to organic loading. The multivariate AZTI Marine Biotic Index (M-AMBI) indicated that the benthic status changed from High to Good in all mesocosms receiving biodeposits. Sediments acted as a sink for oxygen (O₂), but results suggest O₂ sediment demand was not sensitive enough to evaluate organic loading impacts. Results from this and improved experiments can be used to determine the environmental carrying capacity of sites for bivalve culture.