Mediation of nitrogen by post-disturbance shelf communities experiencing organic matter enrichment

Marija Sciberras,Rachel Hale,Guillaume Brochain,Karen Tait,Martin Solan,Jasmin A Godbold,Jan G Hiddink

doi:10.1007/s10533-017-0370-5

Abstract

Microbes and benthic macro-invertebrates interact in sediments to play a major role in the biogeochemical cycling of organic matter, but the extent to which their contributions are modified following natural and anthropogenic changes has received little attention. Here, we investigate how nitrogen transformations, ascertained from changes in archaeal and bacterial N-cycling microbes and water macronutrient concentrations ([NH4–N], [NO2–N], [NO3–N]), in sand and sandy mud sediments differ when macrofaunal communities that have previously experienced contrasting levels of chronic fishing disturbance are exposed to organic matter enrichment. We find that differences in macrofaunal community structure related to differences in fishing activity affect the capacity of the macrofauna to mediate microbial nitrogen cycling in sand, but not in sandy mud environments. Whilst we found no evidence for a change in ammonia oxidiser community structure, we did find an increase in archaeal and bacterial denitrifier (AnirKa, nirS) and anammox (hzo) transcripts in macrofaunal communities characterized by higher ratios of suspension to deposit feeders, and a lower density but higher biomass of sediment-reworking fauna. Our findings suggest that nitrogen transformation in shelf sandy sediments is dependent on the stimulation of specific nitrogen cycling pathways that are associated with differences in the composition and context-dependent expression of the functional traits that belong to the resident bioturbating macrofauna community.

Highlights

Marine soft-sediments cover almost 70% of the earth’s surface and play a fundamental role in the remineralization of organic carbon and nutrient cycling (Olsgard et al 2008)
Deposit feeders such as the polychaete Lagis koreni and the echinoderms Leptosynapta inhaerens, Echinocardium cordatum and Echinocyamus pusillus were more abundant than suspension feeders in communities that had previously experienced a low frequency of bottom fishing, whereas suspension feeders such as Phoronis sp., Owenia fusiformis and Abra alba were more abundant in communities that had previously experienced a high frequency of bottom fishing (Table 3a)
Sediment reworking group biomass was dependent on the independent effects (Model 6, Table 2) of organic matter enrichment (Fig. 2b) and sediment reworking group identity (Fig. 2c), with a greater biomass attributed to biodiffusors (e.g. E. cordatum, Sigalion mathilde, Lumbrineris sp.) and when sediments were enriched with organic matter

Summary

Introduction

Marine soft-sediments cover almost 70% of the earth’s surface and play a fundamental role in the remineralization of organic carbon and nutrient cycling (Olsgard et al 2008). Bottom fishing that uses demersal gear such as trawls and dredges to catch fish, crustaceans and bivalves living in, on or in association with the seabed, exerts a number of pressures on benthic systems that might influence sedimentary nutrient generation and budgets (Pilskaln et al 1998; Olsgard et al 2008) Physical processes such as sediment resuspension and sediment mixing caused by trawling alter grain size distribution, sediment sorting and porosity (Trimmer et al 2005) that may in turn disrupt nitrification and denitrification processes (Rysgaard et al 1994; Kitidis et al 2017) through changes in oxygen penetration depth within the sediment (Warnken et al 2003) and burial of organic matter to anoxic layers before aerobic remineralisation can take place (Mayer et al 1991; Pilskaln et al 1998). The combination of removal of surficial sediments and mixing or burial of organic matter to depth (Duplisea et al 2001; Warnken et al 2003) occurs on different time-scales to those of alterations in community structure; changes in porewater and bottom water nutrient levels due to sediment resuspension return to pre-trawling levels within minutes to hours (e.g. Falcao et al 2003; Trimmer et al 2005; Goldberg et al 2014), whilst microbial assemblage structure and biomass in the surficial sediment layers (upper 1 cm) return to pre-disturbance levels within days (Fiordelmondo et al 2003) to Biogeochemistry (2017) 135:135–153

Methods

Results

Conclusion