TY - JOUR
T1 - Nitrogen assimilation and the f-ratio in the northwestern Indian Ocean during an intermonsoon period
AU - Watts, L. J.
AU - Owens, N. J.P.
N1 - Copyright © 1999 Elsevier Science Ltd. All rights reserved.
Author was not affiliated to SAMS at the time of publication
Funding Information:
This work forms part of a study funded by the UK Natural Environment Research Council (Grant No. GR3/09344).
PY - 1999/3/1
Y1 - 1999/3/1
N2 - 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 approximately 8°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 and chlorophyll concentrations. Euphotic zone integrated nitrogen assimilation rates varied between 1.1 and 23.6 mmol 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 mmol N m-2 day-1 compared to 1.6 and 1.8 mmol 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 (RPI) values which were consistently > 1 for ammonium and < 1 for nitrate. A further examination of the data has lead to an apparent partitioning of the northwestern Indian Ocean into 2 regions; a region north of 17°30'N and a region south of this, to about 8°N. This division is based on: (i) the relationship between the f-ratio and ambient nitrate levels; (ii) nitrogen assimilation and primary production and (iii) the biomass distribution. It is suggested that this partitioning should be investigated further with the development of biogeochemical provinces in mind and the estimation of f-ratios on much larger, horizontal scales.
AB - 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 approximately 8°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 and chlorophyll concentrations. Euphotic zone integrated nitrogen assimilation rates varied between 1.1 and 23.6 mmol 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 mmol N m-2 day-1 compared to 1.6 and 1.8 mmol 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 (RPI) values which were consistently > 1 for ammonium and < 1 for nitrate. A further examination of the data has lead to an apparent partitioning of the northwestern Indian Ocean into 2 regions; a region north of 17°30'N and a region south of this, to about 8°N. This division is based on: (i) the relationship between the f-ratio and ambient nitrate levels; (ii) nitrogen assimilation and primary production and (iii) the biomass distribution. It is suggested that this partitioning should be investigated further with the development of biogeochemical provinces in mind and the estimation of f-ratios on much larger, horizontal scales.
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U2 - 10.1016/S0967-0645(98)00125-8
DO - 10.1016/S0967-0645(98)00125-8
M3 - Article
AN - SCOPUS:0033104131
SN - 0967-0645
VL - 46
SP - 725
EP - 743
JO - Deep-Sea Research Part II: Topical Studies in Oceanography
JF - Deep-Sea Research Part II: Topical Studies in Oceanography
IS - 3-4
ER -