Relative influences of bioturbation and physical mixing on degradation of bloom-derived particulate organic matter: Clue from microcosm experiments
Introduction
Mixing processes at the sea floor play an important role in organic matter cycling of marine systems, but it has been unclear how different mixing processes (biological vs. physical) affect the degradation of organic matter (Kristensen and Blackburn, 1987, Arzayus et al., 2002, Sun et al., 2002a). In shallow estuarine and coastal regions, sediments experience intensive mixing due to both physical and biological processes (Hopkinson, 1985, Dellapenna et al., 1998). Different mixing processes change the local environmental conditions and create distinct biogeochemical regimes for organic matter degradation. For example, bioturbation due to benthic macrofaunal activities such as burrowing, irrigating, reworking, and grazing creates an oscillating oxic/anoxic regime in surface sediments (Aller, 1994). As a result, particulate organic matter is continuously mixed through sediments with a range of redox properties. By contrast, strong and episodically physical mixing processes such as hurricanes and severe storms move surface sediments and associated labile particulate organic matter to deep permanent anoxic layers (Hopkinson, 1985). The consequence is a shift in degradation regime for fresh organic matter from oxic conditions in surface sediments to anoxic conditions in deep sediments. Many studies have demonstrated that oxygen plays a complicated and critical role in degradation and preservation of organic matter (Emerson and Hedges, 1988, Harvey et al., 1995, Hartnett et al., 1998). However, our understanding for this role has been equivocal (Henrichs and Reeburgh, 1987, Pedersen and Calvert, 1990, Lee, 1992, Canfield, 1994, Sun et al., 2002b).
Chl-a, the most abundant pigment component in all phytoplankton species, has been commonly used as a biomarker to assess phytoplankton biomass in water column and organic carbon flux into sediments (Furlong and Carpenter, 1988, Sun et al., 1991, Stephens et al., 1997). Phaeophytin-a (Ppt-a), one of major phaeopigments derived from Chl-a, is produced during bacterial, viral, enzymatic, and cell lyses, and also by animal grazing activities (Welschmeyer and Lorentzen, 1985, Bianchi et al., 1988, Buffan-Dubau et al., 1996, Brotas and Plante-Cuny, 1998, Cartaxana et al., 2003). Thus, it has been often used as diagnostic indicator of physiological status, detrital content and grazing processes in natural populations of phytoplankton (Daley and Brown, 1973, Mantoura and Llewellyn, 1983, Strom, 1993). Since Ppt-a has a different lability from Chl-a during overall degradation processes, the ratio of abundance of Chl-a to Ppt-a has been sometimes used to examine degradation pathway and to determine degradation extent (Sun et al., 1993a, Sun et al., 1994, Stephens et al., 1997, Ingalls et al., 2000).
Degradation of Chl-a in sediments is strongly influenced by redox conditions and presence of benthic macrofauna (Bianchi et al., 1988, Ingalls et al., 2000, Chen et al., 2001). For example, Chl-a generally degrades faster and more completely in oxic sediments than in anoxic sediments, but, based on simulated oxic/anoxic incubation experiments, oxygen alone is not enough for faster and complete degradation (Sun et al., 1993b). Macrofauna (Yoldia limatula) can significantly enhance Chl-a degradation in sediments (Ingalls et al., 2000). However, few studies have been conducted, which directly compare Chl-a degradation under different sediment mixing regimes (biological vs. physical). Also the manner in which different macrofaunal species affect Chl-a degradation is poorly known.
In this study, we observed the variability of chloropigment profiles in the Doboy Sound (Georgia, USA) sediments in two seasons. To understand the relative influences of biological vs. physical mixing processes, we conducted a series of microcosm experiments with a focus on the degradation of particulate organic matter under bioturbated, mechanically stirred, and unmixed conditions. Special attention was given to the effects of benthic macrofaunal assemblages, individual species, and the frequency of physical stirring. We followed variations in 13C-POC, 15N-PON (labeled algal particulate organic matter) and Chl-a concentration with time and depth in 1-month incubations to estimate their degradation rate constants. The Chl-a/Ppt-a ratios were determined in various incubations for attempting to understand Chl-a degradation pathways in different sedimentary processes.
Section snippets
Study site, sampling and materials
Sediment, seawater, and benthic macrofauna used in this study were collected from a subtidal site in the Doboy Sound (31°23.52′ N, 81°17.64′ W) near Sapelo Island during March and July 1999. The freshwater discharge from adjacent Altamaha River and several creeks to the Sound varies seasonally (Leiper, 1973), resulting in a variation in salinity from ∼27‰ in March to ∼34‰ in July. The study area is commonly characterized by intensive bioturbation (Howard and Reineck, 1972, Dörjes, 1972).
Profiles of chloropigments in the Doboy Sound sediments
Chloropigments were determined in one sediment core collected from the Doboy Sound in March 1999 and two cores collected in July 1999 (Fig. 1). The Chl-a profile in March was characterized by an apparent subsurface (2–3 cm) maximum while the profiles in July showed a clear exponential decrease with depth although there was a noticeable variability in Chl-a concentration between two cores. Surficial concentrations of Chl-a in spring were lower than that in summer, but the background
Processes affecting seasonal profiles of chloropigments in the Doboy Sound sediments
Seasonal variations of sedimentary chloropigment profiles have been observed in coastal systems and continental margins (Graf, 1989, Sun et al., 1994, Gerino et al., 1998). Similarly, the profiles of chloropigments in the Doboy Sound sediments varied with season (Fig. 1), which were likely caused by seasonal variations of algal organic matter input, mixing, and degradation in sediments. Higher surface Chl-a concentration in summer than in spring might reflect the higher input of algal organic
Conclusions
The experimental results shown here demonstrated the effects of episodic physical mixing and biological mixing (bioturbation) on degradation of fresh algal organic matter in coastal marine sediments. The key difference between the two mixing processes is the alteration of biogeochemical regimes for the degradation. Bioturbation creates an oscillating oxic/anoxic environment, where particulate organic matter and associated Chl-a degrade under variable redox conditions. In contrast, physical
Acknowledgements
We would like to thank the UGA Marine Institute at Sapelo Island for providing logistical support for sampling. We are grateful to W.-J. Cai, Y. Wang, S. Carini, G. LeCleir, and P. Zhao for help with sampling and setting up of incubation experiments. We also thank D. Bishop for help with identification of macrofaunal species. This manuscript was greatly improved by the constructive comments of two anonymous reviewers and careful editorial help from Dr. M. Scranton. This research was supported
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