This contribution presents information on nitrogen nutrients dissolved in the sediment pore waters of the oligotrophic northwestern Mediterranean Sea. The areas studied are situated in various geographical environments, adjacent to or offshore from a river mouth and on wide or narrow parts of the continental shelf. Near the pro-delta of the Rhoˆne River, which carries about 5 × 10 6 t y −1 of solid matter, measurements indicate that the rate of nitrogen nutrient production (ammonification) reaches frequently (or even exceeds) 2 × 10 6 μmol cm −3 s −1. The production rate near the mouth of the Siagne River, a mountain stream, is generally lower, at 10 −7 μmol −3 s −1; exceptionally, it reaches 10 6 to 1.5 × 10 −6 μmol cm −3 s −1. Adjacent to any river mouth, the concentrations of inorganic nitrogen species dissolved in the surficial sediments vary with time. A similar pattern occurs at locations far from a river mouth. High levels of organic matter in the sediments in the latter areas is related to planktonic blooms, which occur mainly in spring. There is more temporal variation in concentrations in sediments just below the sediment-seawater boundary, than in the deeper deposits. Ammonia concentrations increase regularly, with depth, within the sediment. In contrast, the nitrate profiles are frequently irregular and show a concentration maximum at 1 or 2 cm below the interface; then, below a minimum value at 5–8 cm, then a second maximum at 10–15 cm. According to incubation experiments, the production of nitrate occurs at 15–20 cm, where oxygen is still present at concentrations higher than 1–1.5 mg l −1. The low nitrate concentrations at about 6–8 cm appear to result from relatively high denitrifying activity at this level within the sediments. Concentrations of the nitrogen nutrients are generally higher in the surficial sediment pore waters, than in the overlying seawater. There is a flux of these species from the sediment to the seawater. In continental shelf areas, the amount of nitrogen nutrients removed from the sediment, into the sea, corresponds to nearly half of that required by planktonic primary production. Nitrate production rates were not measured, but can be evaluated from a model based upon Fick's Laws for diffusion. The fact that nitrate production does not increase with depth (in the sediment) has been taken into account in the modelling; and, that a layer exists where nitrifying and denitrifying activities occur together (approximately from 2 or 3, to 20 cm). Since nitrifying and denitrifying activities vary temporarily and seasonally (in response to water temperature, pycnocline presence, winter storms, etc.), the model has to be of a non-steady state type; this is realistic in considering the decline in dispersivity (or diffusivity) as a function of depth in the sediment. The model is based upon the assumption that the nitrification rate intensity varies as a function of time, according to a Gaussian curve. The nitrification maximum occurs, in spring, in the surficial sediments. The classical partial differential equation can be approximated using finite difference analysis (with an implicity scheme, for example). For areas located at a distance from river mouths, good agreement was obtained between the derived nitrate concentrations and those measured. Maximum nitrate production rate occurs in the uppermost sediment layers (with Rp =0.5–1.5 × 10 −6 μmol cm −3 s −1). These values correspond nearly to those which have been found for surficial sediments in other countries; however, for the continental shelf of the northwestern Mediterranean Sea, the high production rate lasts for only short periods throught the year (mainly, during spring). In the Monaco area, at some distance from the mouth of a river, the bulk production rate of the dissolved inorganic nitrogen species (=the ammonification rate) in the sediment (0–30 cm), expressed as μmol cm −2 s −1, is 2 or 2.5 times higher than the downward flux of the faecal pellet particulate nitrogen. Thus, a greater part of the sedimentary particulate organic matter is supplied from the littoral and adjacent land areas.
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Mar 1, 2003
Guillermo Guerao + 2
Open Access