Abstract
We investigated the effects of Manila clam aquaculture on the rates and pathways of anaerobic organic carbon (OC) oxidation in highly bioturbated (HB) and poorly bioturbated (PB) sediment in Keunso Bay, Yellow Sea. Due to the labile organic matter supply via sediment reworking by Manila clams, the anaerobic OC oxidation rate in HB sediment (38.8 mmol m-2 d-1) was ~1.5 times higher than that in PB sediment (26.8 mmol m-2 d-1). Microbial Fe(III) reduction (FeR) dominated OC oxidation pathways in HB sediment, comprising 55 to 76% of anaerobic OC oxidation, whereas sulfate reduction (SR) was the dominant oxidation pathway in PB sediment, accounting for up to 92% of anaerobic OC oxidation. Despite higher anaerobic respiration rates at the HB site, concentrations of NH4+, PO43-, oxalate-extractable iron (Fe(II)(oxal)), and total reduced inorganic sulfur were 2 to 3 times lower in HB than in PB sediment. Conversely, the concentration of reactive Fe(III)(oxal) at the HB site (2243 mmol m-2) exceeded that at the PB site (1127 mmol m-2) by a factor of 2. These results indicate that bioturbation by Manila clams enhances the re-oxidation processes of reduced metabolites in the sediment, thereby prohibiting SR and promoting FeR. Overall, the results suggest that aquaculture activities of Manila clams shift the dominant OC oxidation pathways in sediment from SR to FeR, which generates relatively oxidized and less sulfidic environments.
Highlights
Aquaculture has expanded rapidly over the last 20 yr due to progressively impoverished natural fish stocks and increased human demand for seafood (Klinger & Naylor 2012, FAO 2018, Ahmed & Thompson 2019)
We investigate the biogeochemical impact of burrows created by Manila clam aquaculture on (1) the distribution of chemical constituents in pore water and sediment and (2) the rates and partitioning of organic carbon (OC) oxidation by sulfate reduction (SR) and Fe(III) reduction (FeR), the 2 most dominant anaerobic OC oxidation pathways in intertidal sediment
Total organic carbon (TOC) concentration with sediment depth was higher at the highly bioturbated (HB) site than at the poorly bioturbated (PB) site (2-way ANOVA, IBM SPSS Statistics, p = 0.037), and the averaged TOC to a depth of 0 to 10 cm was higher at the HB site than at 185 ± 21 and 173 ± 62 mg m−2 at the HB and PB sites, the PB site (Table 2)
Summary
Aquaculture has expanded rapidly over the last 20 yr due to progressively impoverished natural fish stocks and increased human demand for seafood (Klinger & Naylor 2012, FAO 2018, Ahmed & Thompson 2019). A major environmental concern associated with intensive aquaculture activities is the accumulation of organic matter in sediments (Holmer & Kristensen 1992, 1994, Holmer et al 2005, Hyun et al 2013, Choi et al 2018). Bivalves eject suspended organic matter trapped from the water column during filter feeding either as pseudo-feces or fecal pellets (Zhou et al 2006, Zhang et al 2013, Galimany et al 2017). Ejected organic matter is larger and more prone to deposition on the sediment, since pseudo-feces and fecal pellets are excreted as mucus-bound aggregates (Giles & Pilditch 2004). Accumulation of organic matter and intense sediment reworking (i.e. physical mixing) can have a substantial influence on sediment biogeochemistry (Welsh 2003, McKindsey et al 2011, Kristensen et al 2012, Wendelboe et al 2013, Koo & Seo 2017, Powilleit & Forster 2018)
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