Effects of two polychaete worms, Nereis diversicolor and Arenicola marina, on aerobic and anaerobic decomposition in a sandy marine sediment

The effects of 2 polychaete species, Nereis diversicolor and Arenicola marina, on benthic metabolism and decomposition processes in organic-poor (0.06 % POC), sandy sediment were experimentally investigated. Sediment O-2 uptake and COP release were stimulated by 80 to 90 % and 260 to 270 % for N. diversicolor and A. marina, respectively, from basal rates of approximately 30 mmol m(-2) d(-1). These enhancements in benthic fluxes were due to both increased solute exchange due to macrofauna irrigation and increases in total benthic metabolism. Although the latter was enhanced by 10 to 35 % and 115 to 123 % by N, diversicolor and A, marina, respectively, both species inhibited anaerobic decomposition as indicated by 66 and 42 % lower sulfate reduction rates. Benthic fluxes and basal rates of benthic metabolism (without macrofauna) were 2 to 3 times higher in sediments enriched with organic matter compared to unamended sediments. Similarly, sulfate reduction rates were 3 times higher in enriched sediments. Total benthic metabolism in enriched sediments was stimulated by N. diversicolor to a similar extent (+ 27 to 34 %) as in unamended sediments, whereas A. marina stimulated total benthic metabolism to a slightly lesser extent (+43 to 55%) in enriched sediments than in unamended sediments. N. diversicolor had little effect on sulfate reduction (-4 %) in enriched sediments, while A, marina reduced sulfate reduction by 85 % in enriched sediments. Porewater profiles reflected the balance between stimulating effects on sediment decomposition processes and removal of porewater solutes by benthic macrofauna. Porewater and sediment profiles of reduced S compounds also showed the effects of these polychaetes on sedimentary S cycling, i.e, lower anaerobic decomposition and increased reoxidation of reduced compounds. Both species affected sediment element cycles, stimulating C cycling and favoring more oxidized species and processes in S cycling. Furthermore, both species affected the fate and distribution of the metabolites produced from decomposition by enhancing the exchange of solutes across the sediment-water interface and porewater flushing. In general, the biogeochemical impact of A. marina was greater than that of N. diversicolor.