Bioturbators: Friend or foe to seagrass carbon stocks?

Seagrass is one of the most proficient carbon sequestration ecosystems in the world. Their ability to capture and store carbon for long periods of time is largely due to a constant supply of organic matter, and maintained... [ view full abstract ]

Seagrass is one of the most proficient carbon sequestration ecosystems in the world. Their ability to capture and store carbon for long periods of time is largely due to a constant supply of organic matter, and maintained benthic anoxia. The process of estimating carbon stocks takes these factors into consideration, but generally these systems are treated as static, and the carbon stock is only reassessed after a disturbance (i.e. a storm or dredging event). However, there are natural and persistent disturbances ubiquitously found in seagrass ecosystems; bioturbators.
We performed a mesocosm experiment comparing the susceptibility of surface seagrass sediment (young, highly labile) with deep (ancient, highly bound to minerals) sediment, to determine the stability of its carbon stock against bioturbation by Callianassid shrimp. Over 2 months, bioturbation reduced surface carbon stocks by 35%, while deep stocks appeared to be stable. We assessed the microbial density as well as sediment oxygenation, and determined that the reduction in surface carbon stock was likely due to be a function of both factors – increased microbial density (and activity), as well as increased sediment oxygen penetration and concentration. It appears that through bioturbation, chemical and microbial processes within the surface sediment are altered, fueling an increase in carbon remineralisation. The stability of deep sediment carbon stocks against bioturbation was a likely result of this carbon being highly bound to minerals, and therefore difficult to remineralise.
This result has implications for continued seagrass carbon stock potential, with a reduction in surface carbon putting at risk the persistence of carbon stock through time. This has further implications on a wider ecosystem scale; if bioturbators populations increase in density, we may predict their further negative impact on future coastal seagrass carbon stocks.