Relative influences of bioturbation and physical mixing on degradation of bloom-derived particulate organic matter: Clue from microcosm experiments

Abstract

Chloropigment profiles observed in coastal marine sediments imply that the fate of bloom-derived organic matter is simultaneously affected by physical and biological mixing processes, but these influences are poorly differentiated by field measurements. To test relative influences of biological versus physical mixing processes on organic matter degradation, we conducted a series of microcosm experiments in three simulated mixing regimes: bioturbated, episodically physically mixed, and unmixed. Algal materials (uniformly 13C and 15N-labeled) were added on the surface of homogenized (pre-sieved) sediment cores as a simulation of natural deposition of bloom-derived organic matter (with an increase of 30–40 nmol chlorophyll- a in 1 g of dry sediment). Biological mixing was initiated by adding a group of macrofauna or individual species into the sediment cores while physical mixing was manipulated by mechanically stirring the surface sediment at variable frequency. We followed the time-dependent and depth-dependent variations of algal organic matter ( 13C-POC and 15N-PON) and chloropigments in simulated mixing regimes over 1-month incubations. The analytical results showed that algal organic matter and chlorophyll- a (Chl- a) degraded coincidentally but the degradation rates varied in different mixing regimes, which was likely related to variable redox conditions created by different mixing processes. In general, physical mixing immediately transported the fresh particulate organic matter from the oxic surface to the deep anoxic sediments, where 13C-POC, 15N-PON and Chl- a degraded at a 3–5× slower rate than those in the unmixed and bioturbated sediments. No matter how frequently the sediments were stirred, similar amounts of 13C-POC, 15N-PON, and Chl- a remained in all physically mixed cores after 1 month. On the other hand, biological mixing created an oscillating oxic/anoxic environment for degradation of algal organic matter through irrigation to deepen dissolved oxygen penetration and via reworking to mix particles between oxic and anoxic sediments. However, the influences of biological mixing on organic matter degradation varied with macrofaunal species and their behaviors. In the unmixed regime, 13C-POC, 15N-PON, and Chl- a from the added algal materials degraded under continuous oxic conditions on the sediment–water interface at similar rates to those in the bioturbated sediments.