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NIRE Annual Report
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 1999

Existence of NO3- Pool Which Is Not Dissolved in Pore Water in a Coastal Marine Sediment (Tokyo Bay, Japan), and Its Availability for Denitrification

Marine Environment Division
Environmental Assessment Department
Objectives
Nitrogen(N) is a limiting nutrient in many coastal marine environments, and N removal via denitrification in the sediments is important in controlling the degree of eutrophication of coastal ecosystems. The availability of nitrate(NO3-) is an important factor controlling denitrification activity in many aquatic sediments. In studies of sediment denitrification, attention has so far only been given to NO3- dissolved in pore water, and NO3- pool which is not dissolved in pore water has been considered to be unlikely to exist in sediments.# In this study, the existence of undissolved NO3- pool in a coastal marine sediment was reported, and its availability for sediment denitrification was discussed.
Results
A seasonal study of NO3- flux, NO3- concentration profile at the sediment-water interface and in situ sediment denitrification activity was carried out at a 10 m depth station in Tokyo Bay. The direction of the flux measured with undisturbed sediment cores were from the overlying water into the sediment at almost all seasons, and there was a statistically significant correlation between the concentration of NO3- in the overlying water and the activity of sediment denitrification. From these results, it was concluded that the major source of NO3- for sediment denitrification was coming from the overlying water at the study site. For determining NO3- concentration, sediment samples were frozen at -30°C, and the pore water was separated by centrifuging the thawed sediments. In conflict with the measured flux, the direction of the NO3- flux predicted from the concentration gradients were from the sediment to the overlying water at all seasons. This contradiction suggests that a large part of the NO3- pool of the frozen and thawed sediments was not dissolved in the pore water under in situ conditions. This hypothesis was supported by the results that the NO3- pool of the unfrozen sediments was much lower than that of the frozen sediments, and the concentration gradient shifted to the inverse direction (Fig. 1). Summarizing the overall results, NO3- metabolism at the sediment-water interface including the undissolved NO3- pool is illustrated in Fig. 2. The undissolved NO3- pool may not be directly available for sediment denitrification.
Fig.1. NO3- concentration profiles of frozen and unfrozen sediment samples collected in October
Fig.1. NO3- concentration profiles of frozen and unfrozen sediment samples collected in October
Fig.2. NO3- metabolism at sediment-water inerface
Fig.2. NO3- metabolism at sediment-water inerface
Selected Publications
1) Existence of insoluble NO3-pool in a coastal marine sediment (Tokyo Bay,Japan), and its availability for denitrification, 6th Int. Workshop on the Measurement of Microbial Activities in the Cycling of Matter in Aquatic Environments (1995).
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