Factors Terminating Peak Fluxes during the Late NE and SW Monsoon

Within the framework of JGOFS, profiles of total inorganic nitrogen concentration (TN — NO3+NO2+NH4) were measured along a transect from the Oman coast via the trap site WAST into the open southern Arabian Sea (Fig. 1). The TN concentrations averaged for the upper 20 m of the water column along the whole transect are enhanced during the upwelling season (3.6-6.6 mmoll-1) and approach zero during the late NE monsoon (0.12limolT1, Fig. 3a). The mean d15N values of the sinking matter decrease during the upwelling season and increase during the late NE monsoon. Since photoautotrophic organisms prefer the uptake of the lighter 14N in comparison to heavier 15N, increasing d15N values could reflect nitrogen-limiting condition during plankton growth (Altabet and Francois, 1994). Consequently, increasing d15N values in addition to low-TN concentrations suggest a progressive consumption of TN leading to nitrogen-limiting conditions at the end of late NE monsoon. During the following oligotrophic spring inter-monsoon inputs of nitrogen via nitrogen fixation could terminate nitrogen-limiting conditions, but so far it has only been observed in the central and eastern Arabian Sea (Devassy et al., 1978; Capone et al., 1997). However, the decreasing d15N values in May/June seem to indicate inputs of recently fixed and thus isotopically light nitrogen that could in part sustain the biological production and the organic carbon export in the WAST (Fig. 3a). During the SW monsoon, decreasing d15N values in addition to high TN concentrations imply non-nitrogen limiting conditions, which in turn suggest other factors to be responsible for the decline of the upwelling-driven bloom in the western Arabian Sea.

During the SW and the late NE monsoon, low carbonate/biogenic opal ratios within the exported matter indicate an enhanced share of diatoms in the exported matter (Fig. 3b). Plankton counts during the SW monsoon 1995 and surface ocean nutrient concentrations along the Oman transect show a decline of the contribution of diatoms to the photoautotrophic organisms when the silicate concentration approaches 2 mmoll-1 (Rixen et al., 2005). A silicate concentration of 2 mmoll-1 was identified in other studies (Egge and Aksnes, 1992) as the threshold below which diatoms lose their ecological advantage over other plankton. The effect of nutrient concentration (N) on plankton growth rate (m) is generally described by a Michaelis-Menton type of kinetics: m — (mmax x NAKW+N), whereas mmax is the maximum growth rate of plankton and 'KN' is the half-saturation constant for nutrient uptake (Lalli and Parsons, 1993). This implies that plankton growth rates decrease (m<mmax) when the nutrient concentration drops below a certain threshold. The plankton bloom may be terminated when the plankton growth rates fall below the zooplankton grazing rates.

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Figure 3: (a) Mean bi-weekly averaged organic carbon fluxes (bold line, see Fig. 2) and d15N values (broken line) at the station WAST. Mean total inorganic nitrogen (TN) concentrations in the surface water (water depth < 20m) averaged along the Oman transect (see Fig. 1) for the six US JGOFS cruises ttn 43, 45, 49, 50, 53 and 54 (black horizontal lines; data are obtained from the US JGOFS database). (b) Bi-weekly averaged organic carbon to calcium carbonate carbon (POC/PIC) ratios (bold line) and carbonate to biogenic opal ratios (broken lines). Horizontal lines representing the mean (solid line) and the range (broken line) of POC/PIC ratios obtained from other sediment trap studies in the carbonate-dominated ocean (Klaas and Archer, 2002). The shaded area reveals the periods during which the mixed layer is deepest (MMLD; Dickey et al., 1998) and upwelling occurs along the Oman coast.

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Figure 3: (a) Mean bi-weekly averaged organic carbon fluxes (bold line, see Fig. 2) and d15N values (broken line) at the station WAST. Mean total inorganic nitrogen (TN) concentrations in the surface water (water depth < 20m) averaged along the Oman transect (see Fig. 1) for the six US JGOFS cruises ttn 43, 45, 49, 50, 53 and 54 (black horizontal lines; data are obtained from the US JGOFS database). (b) Bi-weekly averaged organic carbon to calcium carbonate carbon (POC/PIC) ratios (bold line) and carbonate to biogenic opal ratios (broken lines). Horizontal lines representing the mean (solid line) and the range (broken line) of POC/PIC ratios obtained from other sediment trap studies in the carbonate-dominated ocean (Klaas and Archer, 2002). The shaded area reveals the periods during which the mixed layer is deepest (MMLD; Dickey et al., 1998) and upwelling occurs along the Oman coast.

In order to study such a system, we developed a small numerical model in which a constant mixed-layer volume is assumed. Diatom growth and zooplankton grazing is calculated as m (mmolh-1) — (15 x N)/(1500+N), and grazing — (20 x Nd)/(200+Nd), respectively, where W is the silicate concentration in 'mmoll-1'and 'Nd' the diatom biomass in 'mmol Si l-1'. After digestion of soft tissue by zooplankton, biogenic opal (silica) shells are assumed to be exported into the deep sea as faecal pellets. Owing to vertical mixing, the silica entrainment is set to 1.5 mmoll-1 and upwelling raises the silica concentration to values of 14 mmoll-1 at the coast. The velocity with which the upwelled water advects offshore was set to 0.6 ms-1 (Rixen et al., 2000a). The model results agree quite well with observations (Fig. 4a, b) and imply that a diminishing diatom bloom can be caused by diatom growth rates falling below zooplankton growth rates when the silica concentration drops below approximately 6 mmoll-1 (Fig. 4c). At silica concentration of 1-2 mmoll-1, characteristic for the oligotrophic inter-monsoon season in the WAST (Rixen et al., 2005), the system reaches a steady state in which silica input via vertical mixing is balanced by biogenic opal export and the diatom growth rate equals the zooplankton grazing rate.

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