Nitrogen assimilation and the f-ratio in the northwestern Indian Ocean during an intermonsoon period
Watts, L.; Owens, N. (1996). Nitrogen assimilation and the f-ratio in the northwestern Indian Ocean during an intermonsoon period, in: Baeyens, J. et al. (Ed.) Integrated Marine System Analysis. European Network for Integrated Marine System Analysis. FWO Vlaanderen: minutes of the first network meeting (Brugge, 29.02.96-02.03.96). pp. 7-28 In: Baeyens, J. et al. (Ed.) (1996). Integrated Marine System Analysis. European Network for Integrated Marine System Analysis. FWO Vlaanderen: minutes of the first network meeting (Brugge, 29.02.96-02.03.96). Vrije Universiteit Brussel. Laboratorium voor Analytische Chemie: Brussel. 217 pp., more |
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| Abstract | Rates of nitrogen assimilation by phytoplankton were measured at 13 stations along a transect in the northwestern Indian Ocean, from the Gulf of Oman, southwards to ~ 08 °N, during November and December 1994. Nitrate (NO3), ammonium (NH4) and urea assimilation were measured using simulated in situ 15N incubation techniques. These measurements were supported by simultaneous rate measurements of primary production using 14C incubation techniques and detailed vertical distributions of temperature, chlorophyll and photosynthetically active radiation. Euphotic zone integrated nitrogen assimilation rates varied between 1.1 and 23.6 mmoles N m-2 day-1, with generally higher rates occurring at the northern and southern ends of the transect. At the majority of stations ammonium was the preferred nitrogen substrate assimilated; the average integrated assimilation rate of ammonium being 3.7 mmoles N m-2 day-1 compared to 1.6 and 1.8 mmoles N m-2 day-1 for urea and nitrate respectively. This general preference is reflected in the low f-ratios which were £ 0.52 for all stations and in the relative preference indices (R.P.1.) values which were consistently >1 for ammonium and <1 for nitrate. A further examination of the data has lead to a partitioning of the northwestern Indian Ocean into 2 regions, an "inshore" and an "offshore" region. This division is based on: (i) the relationship between the f-ratio and ambient nitrate levels; (ii) nitrogen assimilation and primary production; (iii) the biomass distribution; and (iv) the depth of light penetration. It is suggested that such partitioning could be combined with remote sensing techniques to allow the estimation of f-ratios on much larger, horizontal scales. |
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