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 nitroge n production and respiration rate measurements made in the Arabian Sea duri ng the 1995 Northeast (NEM) and Southwest (SWM) Monsoons. Increased biologi cal 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 b etween nearshore and offshore waters, especially during the SWM. Daily C-14 and total N-15 production underestimated gross C production, and at the ma jority of stations C-14 and total N-15 production were either the same as n et C production or between gross and net C production. Moreover, new produc tion ((NO3)-N-15), scaled to carbon, was substantially less than net C prod uction. Approximately 50% of the (POC)-C-14 was metabolized during the phot operiod, with smaller losses (7-11%) overnight. The simplest explanation fo r the discrepancy between gross and total N-15 production and between net C and new production was the loss of N-15-labeled particulate matter as diss olved organic matter. Partitioning of metabolized gross C production into r espiratory and dissolved pools showed distinct onshore-offshore distributio ns that appeared to be related to the composition of the phytoplankton asse mblage and probably reflected the trophodynamics of the ecosystem. The perc entage 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 produ ction further offshore where picoplankton prevailed. In general, stations t hat retained more gross C production as net production (i.e., high net C/gr oss C ratios) had higher rates of DOC production relative to community resp iration. Locations where community respiration exceeded DOC production were characterized by low rates of net C production and had low net C/gross C r atios. In those ecosystems, less net C production was retained because high er metabolic losses reduced gross C production to a greater extent than at the more productive sites. (C) 2001 Elsevier Science Ltd. All rights reserv ed.