553 Journal of Oceanography, Vol. 60, pp. 553 to 562, 2004 Keywords: ⋅ Nutrient depletion, ⋅ variability, ⋅ winter mixing, ⋅ phytoplankton, ⋅ North Atlantic Ocean, ⋅ BATS. * Corresponding author. E-mail: s010103d@mbox.nagoya-u. ac.jp † Present address: School of Environmental Science, The University of Shiga Prefecture, 2500 Hassaka-cho, Hikone, Shiga 522-8533, Japan. Copyright © The Oceanographic Society of Japan. Temporal Variations in the Dissolved Nutrient Stocks in the Surface Water of the Western North Atlantic Ocean INSAF S. BABIKER*, MOHAMED A. A. MOHAMED, KAORI KOMAKI, KEIICHI OHTA † and KIKUO KATO Hydrospheric Atmospheric Research Center, Nagoya University, Chikusa-ku, Furo-cho, Nagoya 464-8601, Japan (Received 26 February 2003; in revised form 10 July 2003; accepted 15 July 2003) Changes in patterns of undetectability and molar ratios of dissolved nutrients in the euphotic zone of the oligotrophic western North Atlantic Ocean were investigated utilizing the Bermuda Atlantic Time-series Study (BATS) data set of the US Joint Global Ocean Flux Study (JGOFS). Our aim was to examine the temporal dynamics of nutrient stocks over a decade (1989~1998) and to gain insight into the interactions between the different biotic and abiotic factors underlying BATS. Patterns of nutri- ent undetectability clearly revealed the depleted nature of the nutrients in surface water at the BATS location, particularly phosphorous. The N:P ratio was consistently far above the nominal Redfield ratio (mean, 38.5) but was significantly lower during the 1993~1994 period (22.1). Over the same period the proportion of samples de- pleted in N only increased while the proportion of samples depleted in P only de- creased. This indicates an overall reduction of N relative to P in the surface water at BATS during the 1993~1994 period, the reasons for this anomaly, though, are not clear. The correlation analysis between the biotic and abiotic variables at BATS has indicated some interesting relationships that can help understand some of the pa- rameters affecting nutrient stocks in the euphotic zone and their consequent impacts on marine biota. Although nutrient stocks in the oligotrophic environment are lim- ited, they might be subject to interannual variation that may become anomalous in some cases. These variations might underlay significant feedback mechanisms by af- fecting marine productivity, the prime factor controlling the sequestration of atmos- pheric CO 2 by the oceans. be depleted in the major nutrients necessary to support oceanic production (nitrate, phosphate and silicate). These are termed oligotrophic ocean systems. Oligotrophic oceans, which are always supposed to support a biologi- cal community in a steady state, have already witnessed some major ecosystem shifts over various time scales. The reasons for this variation are not well understood, however, and are sometimes attributed to short-term or long-term climate variability (Karl et al ., 1995; Karl et al., 2001). Nevertheless, the Redfield ratios of nutrients in the ocean, which are assumed to be constant, were found to change in response to a range of dynamic proc- esses (Siegel et al., 2001). For example, the North Atlan- tic Ocean exhibited an exceptionally high N:P ratio, partly due to the deposition of fixed nitrogen from urbanized North America (Fanning, 1992), while in the North Pa- cific Ocean high C:N and C:P ratios have been attributed to the intensified export production (Pahlow and 1. Introduction There has been recently an increasing demand for an understanding of the variability of ocean systems and this feedback response in relation to the global carbon cycle. Over the last few decades the ocean is estimated to have taken up about 30% of the anthropogenic CO 2 re- leased to the atmosphere. Ocean ecosystems are subject to variability over a wide range of time and space scales (Dickey, 1991). A combination of chemical, physical and biological processes is responsible for this variability. Of the world’s ocean surface waters, 80% are considered to