Ocean acidification alters meiobenthic assemblage composition and organic matter degradation rates in seagrass sediments

Abstract

Seagrass meadows are an important organic matter (OM) reservoir but, are currently being lost due to global and regional stressors. Yet, there is limited research investigating the cumulative impacts of anthropogenic stressors on the structure and functioning of seagrass benthic assemblages, key drivers of OM mineralization and burial. Here, using a 16‐month field experiment, we assessed how meiobenthic assemblages and extracellular enzymatic activities (as a proxy of OM degradation) in Posidonia oceanica sediments responded to ocean acidification (OA) and nutrient loadings, at CO2 vents. P. oceanica meadows were exposed to three nutrient levels (control, moderate, and high) at both ambient and low pH sites. OA altered meiobenthic assemblage structure, resulting in increased abundance of annelids and crustaceans, along with a decline in foraminifera. In addition, low pH enhanced OM degradation rates in seagrass sediments by enhancing extracellular enzymatic activities, potentially decreasing the sediment carbon storage capacity of seagrasses. Nutrient enrichment had no effect on the response variables analyzed, suggesting that, under nutrient concentration unlikely to cause N or P imitation, a moderate increase of dissolved nutrients in the water column had limited influence on meiobenthic assemblages. These findings show that OA can significantly alter meiobenthic assemblage structure and enhance OM degradation rates in seagrass sediments. As meiofauna are ubiquitous key actors in the functioning of benthic ecosystems, we postulated that OA, altering the structure of meiobenthic assemblages and OM degradation, could affect organic carbon sequestration over large spatial scales.

Resilient consumers accelerate the plant decomposition in a naturally acidified seagrass ecosystem

“…Finally, the loss of other competitors and predators under acidification may have released the species from antagonistic interactions, further boosting their population growth (Kroeker et al, 2011). Interestingly, similar ecological patterns have been reported by other studies conducted in different temperate CO 2 vent systems, where amphipods attain higher population densities and/or richness under acidification (Auriemma et al, 2019; Baggini, 2015; Garrard et al, 2014; Kroeker et al, 2011; Ravaglioli et al, 2019; Scipione et al, 2017; Vizzini et al, 2017). Because gammarids function as important consumers of plant detritus in diverse coastal marine habitats (e.g., Gilson et al, 2021; Haram et al, 2020; Middleton & McKee, 2001; Remy et al, 2018), a broader examination of how this group responds to OA (e.g., metabolic and feeding rates, population dynamics, and community structure; Borges et al, 2018; Ng & Micheli, 2020; Schram et al, 2016; Timmers et al, 2021) may shed further light on detrital dynamics in the future ocean.…”

“…For example, Witt et al (2011) demonstrated that elevated pCO 2 could alter the composition and relative abundance of coral reef‐associated microbes and their CN contents. In one of the vent systems used in this study (Castello), Ravaglioli et al (2019) found that acidification increased microbial enzymatic activities within seagrass sediments directly related to organic matter decomposition. It is possible that the enhanced microbial production and conditioning of detrital leaves under OA resulted in higher detritivory (Fairbanks Jr. et al, 2018).…”

“…However, generally, 26% to 42% of P. oceanica production is decomposed in situ (Pergent et al, 1994(Pergent et al, , 1997, and 10% to 55% may be processed at exported locations (Boudouresque et al, 2016). In addition, belowground organs of P. oceanica (e.g., roots, rhizomes, and sheaths) undergo a slow decay process within sediment that may be affected by OA (Ravaglioli et al, 2019). Hence, further research is needed on how acidification affects the decay processes in different detrital compartments of P. oceanica (e.g., rhizome matte and exported necromass).…”

Comparing Subsurface Seasonal Deoxygenation and Acidification in the Yellow Sea and Northern East China Sea Along the North-to-South Latitude Gradient

“…2− ] are the concentrations of calcium and carbonate ions, respectively, and K sp * is the apparent solubility product for either calcite or aragonite. The declines in pH and could lead to CaCO 3 -undersaturated corrosive seawater conditions, affecting marine calcifying organisms and even the whole marine ecosystem (Fabry, 2008;Jin et al, 2015;Ravaglioli et al, 2020).…”

Effects of widespread human disturbances in the marine environment suggest a new agenda for meiofauna research is needed

“…Studies investigating the effects of low pH on meiofauna organisms in the field, however, showed that shifts in their community structure were driven by the indirect effects of acidification. These included changes to habitat type and structure, and shifts in species interactions resulting from, for example, release from predation pressure and altered quantity and type of food available, rather than physiological intolerance to low pH (Garrard et al, 2014;Ravaglioli et al, 2019).…”