Carbon and nitrogen cycling on intertidal mudflats of a temperate Australian estuary. II. Nitrogen cycling

Abstract

Benthic fluxes of dissolved nitrogen, rates of denitrification, N 2 fixation and NH 4 + upward flux within the sediment (calculated from porewater profiles) were measured on the upper and lower mudflats at 2 study sites, 1 in the upper, river-dominated part of the estuary, and 1 in the lower, more marine part of the Huon Estuary, Tasmania, Australia. The calculated upward flux of NH 4 + from within the sediment based on porewater profiles was generally in excess of measured benthic fluxes, suggesting that NH 4 + was reassimilated at the sediment surface by microphytobenthos (MPB). The ratio of total CO 2 (TCO 2 ):NH 4 + produced within the sediment was generally in excess of 15, and in some cases in excess of 60. Significant influxes and effluxes of dissolved organic nitrogen (DON) were measured where the activity of MPB was highest. At times, DON influxes and effluxes were well in excess of dissolved inorganic nitrogen (DIN) fluxes, highlighting the importance of measuring DON fluxes where the activity of MPB is high. Rates of denitrification were very low, and represented only a small loss of N from the sediment, most probably as a consequence of the activity of MPB. Estimates of nitrogen assimilation by MPB showed that N 2 fixation was likely to be the major source of nitrogen during the summer at the study site in the upper estuary. There was also a high estimated C:N ratio (~20) of TCO 2 and nitrogen assimilated at this site, suggesting that a significant proportion of primary production was exuded as dissolved organic carbon rather than cellular production.KEY WORDS: Microphytobenthos · Denitrification · Intertidal · Porewater · Dissolved organic nitrogen · Sediment · Nitrogen fixation · Sediment -water exchange Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 280: [39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] 2004 Microphytobenthos (MPB) are ubiquitous on mudflats and may exert an important influence on nitrogen cycling processes in these environments (Underwood & Kromkamp 1999). Assimilation of nitrogen by MPB means that effluxes of NH 4 + and NO 3 -may be drastically reduced or even reversed in the light (Sundbäck et al. 1991, Cabrita & Brotas 2000, Magalhaes et al. 2002. In addition, MPB have been found to compete effectively with nitrifying/denitrifying bacteria for NH 4 + and NO 3 -. Assimilation:denitrification ratios of between 2 and 10 have been reported for eutrophic mudflats, whilst ratios of between 7 and >100 have been reported for more oligotrophic sediments (Cabrita & Brotas 2000, Dong et al. 2000. Furthermore, MBP have also recently been shown to have a significant negative impact on denitrification, most probably via competition for nitrogen (Risgaard-Petersen 2003). These findings are significant, as they suggest that a far greater proportion of nitrogen remineralised within the sediments will be assimilated rather than denitrified, resulting in a retention of bioavailable nitrogen within the system. Recen...

Carbon and nitrogen cycling on intertidal mudflats of a temperate Australian estuary. III. Sources of organic matter

“…This accords with the high rates of N 2 fixation observed here at times (Cook et al 2004b). The δ 15 N of 7.1 ‰ for the MPB at Site PC was much more enriched than at Site CF, reflecting the abundance of diatoms at this site and lower rates of N 2 fixation (Cook et al 2004b). The δ 15 N of the diatoms, measured at Port Cygnet, is in the upper range of δ 15 N literature values reported for benthic diatoms (Riera 1998 and references therein).…”

“…Low δ 15 N values in benthic and pelagic algae are most likely to arise from nitrogen fixation by cyanobacteria (Currin et al 1995, MacGregor et al 2001). This accords with the high rates of N 2 fixation observed here at times (Cook et al 2004b). The δ 15 N of 7.1 ‰ for the MPB at Site PC was much more enriched than at Site CF, reflecting the abundance of diatoms at this site and lower rates of N 2 fixation (Cook et al 2004b).…”

Diatoms control nutrient cycles in a temperate, wave‐dominated estuary (southeast Australia)

“…whereas C : N ratios of up to 12 were found in extracellular polymeric substances formed by planktonic diatoms under N-depleted conditions (Engel et al 2002) and C : N ratios of up to 40 were inferred for extracellular polymeric substances formed by microphytobenthos (Cook et al 2004).…”

In situ soft sediment nutrient enrichment: A unified approach to eutrophication field experiments

“…We thank one anonymous reviewer for their constructive comments that have improved the paper. (2000) Tagus Estuary Portugal 18-40 Percuoco et al (2015) Great Bay Estuary USA 50-1400 De Vittor et al (2012) Marano-Grado Lagoon Italy 52-900 Zhang et al (2013) Pearl River Estuary China 64-321 Vidal and Morgui (1995) Alfacs Bay Spain 100-600 Magni et al (2014) Shinkawa-Kasugawa Estuary Japan 200-500 Lohrer et al (2010) Mahurangi Estuary New Zealand 257-1542 Pérez- Villalona et al (2015) San Juan Bay Estuary Puerto Rico 461-572 Cook et al (2004) Huon Estuary Australia 500 Clavero et al (2000) Palmones River Estuary Spain 500-3500 Bally et al (2004) Seine Estuary France 1940 Gonçalves et al (2012) Santos-Cubatao Estuarine System Brazil 2495-4989 This study Application rate 150 g N m −2 Application rate 600 g N m −2 64-10,275 11-18,842…”