Bioturbator‐stimulated loss of seagrass sediment carbon stocks

Abstract

Seagrass ecosystems are highly productive, and are sites of significant carbon sequestration. Sediment‐held carbon stocks can be many thousands of years old, and persist largely due to sediment anoxia and because microbial activity is decreasing with depth. However, the carbon sequestered in seagrass ecosystems may be susceptible to remineralization via the activity of bioturbating fauna. Microbial priming is a process whereby remineralization of sediment carbon (recalcitrant organic matter) is stimulated by disturbance, i.e., burial of a labile source of organic matter (seagrass). We investigated the hypothesis that bioturbation could mediate remineralization of sediment carbon stocks through burial of seagrass leaf detritus. We carried out a 2‐month laboratory study to compare the remineralization (measured as CO2 release) of buried seagrass leaves (Zostera muelleri) to the total rate of sediment organic matter remineralization in sediment with and without the common Australian bioturbating shrimp Trypaea australiensis (Decapoda: Axiidea). In control sediment containing seagrass but no bioturbators, we observed a negative microbial priming effect, whereby seagrass remineralization was favored over sediment remineralization (and thus preserving sediment stocks). Bioturbation treatments led to a two‐ to five‐fold increase in total CO2 release compared to controls. The estimated bioturbator‐stimulated microbial priming effect was equivalent to 15% of the total daily sediment‐derived CO2 releases. We propose that these results indicate that bioturbation is a potential mechanism that converts these sediments from carbon sinks to sources through stimulation of priming‐enhanced sediment carbon remineralization. We further hypothesized that significant changes to seagrass faunal communities may influence seagrass sediment carbon stocks.

Seagrass Removal Leads to Rapid Changes in Fauna and Loss of Carbon

“…The presence and density of fauna, especially large bioturbators, can be used as an indicator and predictor of the functional status of benthic environments (Eyre, 2011). Faunal impacts on C storage have been documented in many blue carbon habitats, including seagrass (Papaspyrou et al, 2004;Thomson, 2018). Herbivory can have direct impacts on cover and production through consumption, for example green turtles (Heithaus et al, 2014) and urchins (Rose et al, 1999) can remove most aboveground biomass.…”

“…In these anoxic conditions, bioturbation by callianassid shrimp can provide a substantial input of oxygen into the sediment, particularly through processes of bio-irrigation, which is predominantly used in biogeochemical processes and microbial respiration (Webb and Eyre, 2004). The remineralization processes promoted by callianassid burrowing activity can result in a 2 to 5 times increase in CO 2 emission (Thomson, 2018). Whilst callianassid shrimp are the most obvious bioturbators, seagrass removal resulted in a general shift in community structure toward larger bodied, more active taxa; perhaps as the physical barriers of dense seagrass canopy and rhizomes were removed, facilitating predation, or as the sediment chemistry changed, permitting higher sedimentary oxygen levels.…”

High variability of Blue Carbon storage in seagrass meadows at the estuary scale

“…Variability of sedimentary C org stocks has been associated with multiple interrelated biological and environmental factors: type of seagrass species 7 , depth and light availability 10,11 , landscape configuration 12 , physical disturbances 13 , wave height and turbidity 14 , and even faunal presence such as bioturbators and top predators 15,16 Most of these studies highlighted the type of carbon sources and the sediment grain size as the main factors influencing carbon storage in seagrass sediments, suggesting that the processes affecting these factors explain the high variability found in seagrass sedimentary C org stocks 17 .…”

Land use impacts on benthic bioturbation potential and carbon burial in Brazilian mangrove ecosystems

“…Benthic invertebrates are abundant in mangrove forests and their bioturbation activities are essential to biogeochemical cycles, ultimately influencing the diagenesis of OC within soils (Aller 1982;Kristensen and Alongi 2006;Kristensen et al 2008). Bioturbation has only recently been considered important for OC dynamics in coastal wetland soils, with growing evidence that large benthic bioturbators may increase or decrease soil carbon stocks (Andreetta et al 2014;Atwood et al 2015;Mehring et al 2017;Thomson et al 2019). Increased remineralization of soil carbon may occur when bioturbation increases soil oxygen supply, whereas burial of plant detritus into deep anoxic soil layers by benthic fauna may increase OC sequestration (Lee 1998;Kristensen et al 2008;Andreetta et al 2014;).…”