Abstract
Abstract. Nitrification is a series of processes that oxidizes ammonia to nitrate, which contributes to hypoxia development in coastal oceans, especially in eutrophicated regions. The nitrification rate of bulk water (NRb) and particle free water (NRpf, particle > 3 μm eliminated) were determined along the Chang Jiang River plume in August 2011 by nitrogen isotope tracer technique. Measurements of dissolved oxygen (DO), community respiration rate (CR), nutrients, dissolved organic nitrogen (DON), total suspended matter (TSM), particulate organic carbon/nitrogen (POC / PON), acid-leachable iron and manganese on suspended particles and both archaeal and β-proteobacterial ammonia monooxygenase subunit A gene (amoA) abundance on size-fractioned particles (> 3 μm and 0.22–3 μm) were conducted. The NRb ranged from undetectable up to 4.6 μmol L−1 day−1, peaking at a salinity of ~ 29. NRb values were positively correlated with ammonium concentration, suggesting the importance of substrate in nitrification. In the river mouth and the inner plume, NRb was much higher than NRpf, indicating that the nitrifying microorganism is mainly particle associated, which was supported by its significant correlation with amoA gene abundance and TSM concentration. The estimated oxygen demands of nitrification accounted for 0.32 to 318% of CR, in which 50% samples demanded more oxygen than that predicted by by the Redfield model (23%), indicating that oxygen might not be the sole oxidant though DO was sufficient (> 58 μmol kg−1) throughout the observation period. The excess nitrification-associated oxygen demand (NOD) showed a tendency to occur at lower DO samples accompanied by higher acid-leachable Fe / Mn, which implied reactive Fe3+ / Mn4+ may play a role as oxidant in the nitrification process. Stoichiometric calculation suggested that reactive Fe on particles was 10 times the oxidant demand required to complete ammonia oxidation in the entire plume. The potential involvement of reactive iron and manganese in the nitrification process in oxygenated water further complicated nitrogen cycling in the turbid river plume.
Highlights
Estuaries and coastal seas receiving natural and anthropogenic materials from rivers serve as an important reactor for chemical/physical transformation
Higher nitrification rate of bulk water (NRb) in those estuaries is likely caused by higher ammonium concentration (> 3 μmol L−1), which again reveals the importance of ammonia supply
The rate of nitrification and its oxygen consumption were investigated along the turbid Chang Jiang River plume
Summary
Estuaries and coastal seas receiving natural and anthropogenic materials from rivers serve as an important reactor for chemical/physical transformation. A growing population and human activities in the past few decades have enhanced riverine nutrient loads more than 5-fold, which has profoundly impacted the aquatic environment of the land–ocean boundary (Dai et al, 2011; Howarth and Marino, 2006). Nitrogen is one of the most influential elements among the enhanced nutrients (Galloway et al, 2004; Gunnarsson et al, 2000; Howarth and Marino, 2006) that may lead to eutrophication. One of the processes most affected is nitrification, in which ammonia is oxidized to nitrate. Nitrification is composed of two steps, i.e., ammonia oxidation and nitrite
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