The long-term efficacy of stormwater treatment systems requires continuous pollutant removal without substantial re-release. Hence, the division of incoming pollutants between temporary and permanent removal pathways is fundamental. This is pertinent to nitrogen, a critical water body pollutant, which on a broad level may be assimilated by plants or microbes and temporarily stored, or transformed by bacteria to gaseous forms and permanently lost via denitrification. Biofiltration systems have demonstrated effective removal of nitrogen from urban stormwater runoff, but to date studies have been limited to a ‘black-box’ approach. The lack of understanding on internal nitrogen processes constrains future design and threatens the reliability of long-term system performance. While nitrogen processes have been thoroughly studied in other environments, including wastewater treatment wetlands, biofiltration systems differ fundamentally in design and the composition and hydrology of stormwater inflows, with intermittent inundation and prolonged dry periods. Two mesocosm experiments were conducted to investigate biofilter nitrogen processes using the stable isotope tracer 15NO3 − (nitrate) over the course of one inflow event. The immediate partitioning of 15NO3 − between biotic assimilation and denitrification were investigated for a range of different inflow concentrations and plant species. Assimilation was the primary fate for NO3 − under typical stormwater concentrations (∼1–2 mg N/L), contributing an average 89–99% of 15NO3 − processing in biofilter columns containing the most effective plant species, while only 0–3% was denitrified and 0–8% remained in the pore water. Denitrification played a greater role for columns containing less effective species, processing up to 8% of 15NO3 −, and increased further with nitrate loading. This study uniquely applied isotope tracing to biofiltration systems and revealed the dominance of assimilation in stormwater biofilters. The findings raise important questions about nitrogen release upon plant senescence, seasonally and in the long term, which have implications on the management and design of biofiltration systems.

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The pathways and fate of C and N in the microbial compartment of a coastal permeable sandy sediment in the photic zone were studied in a pulse-chase experiment.13 C-bicarbonate and 15 N-nitrate were added to the water column on top of 5 sediment cores incubated in 4 transparent and 1 opaque laboratory chambers. After 9 h of labelling in the light and dark, stable isotope incorporation by microphytobenthos (MPB) and bacteria was quantified over a period of 4 d through the analysis of phospholipid-derived fatty acids (PLFA) and hydrolysable amino acids (HAA). 13 C was fixed by MPB, and more than 50% was directed to the production of extracellular polymeric substance (EPS). MPB 15 N incorporation was similar in the dark and in the light. Bacterial activity appeared to have 2 effects that depended on sediment depth: at the sediment surface, there was a steady increase of label in the bacterial PLFA, suggesting metabolism of label-containing EPS; in the subsurface layers, uniform enrichment started immediately after the labelling procedure, indicating continuous incorporation of inorganic C by chemoautotrophic bacteria (Cbac). This experiment demonstrates the efficient transfer of inorganic C and N to the benthic community through the activities of photo-and chemoautotrophic microorganisms, and the role of EPS as a carrier of energy to the benthic microbial food web.KEY WORDS: Microphytobenthos · Cyanobacteria · EPS · PLFA · Biomarkers · Amino acids · Chemoautotrophy · Permeable sediment · 15 N · 13 C · Stable isotopes · Nitrate Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 53: [257][258][259][260][261][262][263][264][265][266][267][268][269] 2008 ( Fig. 1). In the surface layer, MPB often represents the most important component in terms of biomass and turnover and modulates C and N transfer across the sediment -water interface (MacIntyre et al. 1996, Sundback et al. 1996. Although C fixation and N assimilation are often closely related to the buildup of biomass, a large part of the C fixed may be diverted to synthesis of extracellular polymeric substances (EPS) that can account for a large fraction of organic C present in the sediment (Smith & Underwood 1998, Goto et al. 1999, Wieland et al. 2008. The second major pathway is heterotrophy: the utilisation of detritus, MPB, EPS and other organic substrates by heterotrophic bacteria (Hbac), meiofauna and macrofauna. Heterotrophic benthic organisms produce new biomass (secondary production) and govern mineralization of organic matter. The third metabolic pathway is chemoautotrophy: the fixation of CO 2 using the energy released upon oxidation of reduced inorganic compounds. This topic has received limited attention in the literature, yet it may constitute a significant pathway close to redox boundaries (Kristensen & Hansen 1995, Thomsen & Kristensen 1997.Although the biological importance of subtidal permeable sediments was recognized decades ago (Riedl et al. 1972), these systems remain poorly documented. ...

The relative importance of allochthonous phytodetritus deposition and autochthonous microphytobenthos (MPB) production for benthic consumers in an organic carbon (C org )-poor sandy sediment was assessed using a 13 C-stable isotope natural abundance study combined with a dual 13 C-tracer addition approach. In a first experiment (Expt 1), a set of sediment cores received a pulse of 13 C-labelled phytodetritus and the fate of that organic matter was followed in the benthic food web (bacteria, meiofauna and macrofauna) over a period of 72 h. In a second experiment (Expt 2), the MPB present in a set of sediment cores was labelled with 13 C-bicarbonate and the fate of labelled MPB was followed the same way over a period of 96 h. Natural 13 C abundances of sources and consumers revealed that the benthic food web likely relied primarily on MPB. In particular, diatoms contributed at least 40% to the diet of 12 out of the 16 taxonomic groups identified. The dual approach revealed the complexity of the trophic interactions and gave evidence for resource partitioning between 2 species of harpacticoid copepods. Both 13 C-tracer addition experiments showed a fast transfer of label to most heterotrophs. Bacteria, which comprised the largest fraction of the heterotroph biomass, incorporated more 13 C than other consumers. Meiofauna had similar relative incorporations in both experiments and likely relied equally on benthic and pelagic inputs. Macrofauna relied significantly more on MPB. In both experiments, most of the 13 C-label that was incorporated by heterotrophs was respired. While phytodetritus-derived C org consumed by heterotrophs was 41 mg C m −2 , MPB-derived was at least one order of magnitude higher. The benthic community growth efficiency in Expt 2 (40%) was higher than that of Expt 1 (25%), confirming the pivotal role of MPB.KEY WORDS: Stable isotopes · 13 C · Food web · Benthic microalgae · Phytoplankton · Meiofauna · Macrofauna · Bacteria Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 455: [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] 2012 ments (i.e. heterotrophic bacteria, microfauna, meiofauna and macrofauna) undergo frequent reworking due to physical forces. However, recent studies have provided strong evidence of high primary production (Billerbeck et al. 2007, Cook et al. 2007, Evrard et al. 2008) and enhanced remineralization rates supported by advective porewater transport (Huettel & Gust 1992a, Rusch et al. 2001, Ehrenhauss et al. 2004.The relative importance of the heterotrophic compartments has been assessed and some generic distribution patterns related to grain size have been illustrated. Firstly, the biomass of macrofauna (which generally comprises the metazoans larger than 1 mm) shows a decreasing trend towards coarser sediments ). This has been attributed to the greater influence of hydrodynamics on coarser grains as well as the patchiness of resources. Secondly, there is a coupled decrease in densities...