The impact of the polychaete Nereis diversicolor and enrichment with macroalgal ( Chaetomorpha linum) detritus on benthic metabolism and nutrient dynamics in organic-poor and organic-rich sediment

Abstract

The combined impact of the burrow-dwelling polychaete, Nereis diversicolor, and organic enrichment with macroalgal detritus, Chaetomorpha linum, on sediment metabolism (CO 2 production) and nutrient (NH 4 + and NO 3 −) dynamics was assessed in laboratory microcosms over 35 days with homogenized sandy (0.5% LOI) and muddy (8.6% LOI) sediment from a shallow estuary (Kertinge Nor, Denmark). In addition, the gross C and net N mineralization (+DOC evolution) was followed in 4 time series using closed anaerobic jars containing the two sediment types with and without organic addition. The unenriched treatments showed that the sandy sediment exhibited higher C and N mineralization than the muddy, due to a more reactive indigenous organic pool (fresh diatoms vs. degraded macroalgal detritus). A higher excess CO 2 production due to enrichment in the muddy than the sandy sediment indicates a higher instant capacity for carbon mineralization in the former sediment. In contrast to the sandy, the muddy sediment had previously been exposed to extensive input of filamentous algae and may therefore be better adapted to this particular detritus type. The excess NH 4 + production due to enrichment was highest in sandy sediment, suggesting that N was recycled rapidly by the microbial community of the relatively nitrogen-poor muddy sediment, resulting in a low net N mineralization. It also appears that C and N mineralization was uncoupled in sandy sediment, as indicated by the simultaneous initial production of DOC and NH 4 + with no concomitant CO 2 production. The presence of animals enhanced C mineralization in all treatments; highest in the enriched treatments. The enhancement in absolute rates was similar when both sediment types were enriched, indicating that the faunal enhancement with added detritus is independent of sediment type. The faunal enhancement of N mineralization, on the other hand, was highest in sandy sediment irrespective of organic enrichment. Denitrification estimates, based on mass balance, also appeared to be stimulated significantly by the presence of animals, whereas organic enrichment had a minor influence on this particular process.