Production and respiration rates in the Arabian Sea during the 1995 Northeast and Southwest Monsoons

In this paper we examine the relationships among oxygen, carbon and nitrogen production and respiration rate measurements made in the Arabian Sea during the 1995 Northeast (NEM) and Southwest (SWM) Monsoons. Increased biological production characterized the SWM, with rates 12–53% higher than the NEM. In most cases, we found remarkable similarity in production rates during the two monsoons and an absence of strong spatial gradients in production between nearshore and offshore waters, especially during the SWM. Daily 14C and total 15N production underestimated gross C production, and at the majority of stations 14C and total 15N production were either the same as net C production or between gross and net C production. Moreover, new production (15NO3), scaled to carbon, was substantially less than net C production. Approximately 50% of the PO14C was metabolized during the photoperiod, with smaller losses (7–11%) overnight. The simplest explanation for the discrepancy between gross and total 15N production and between net C and new production was the loss of 15N-labeled particulate matter as dissolved organic matter. Partitioning of metabolized gross C production into respiratory and dissolved pools showed distinct onshore–offshore distributions that appeared to be related to the composition of the phytoplankton assemblage and probably reflected the trophodynamics of the ecosystem. The percentage of gross C production released as dissolved organic carbon (DOC) was highest in the nearshore waters where diatoms dominated the phytoplankton assemblage, while community respiration was a more important fate for production further offshore where picoplankton prevailed. In general, stations that retained more gross C production as net production (i.e., high net C/gross C ratios) had higher rates of DOC production relative to community respiration. Locations where community respiration exceeded DOC production were characterized by low rates of net C production and had low net C/gross C ratios. In those ecosystems, less net C production was retained because higher metabolic losses reduced gross C production to a greater extent than at the more productive sites.