Spatial and Temporal Variations of Total Organic Carbon and Inorganic Nutrient in the Northeast Equatorial Pacific

Abstract

As a part of the KODES program, total organic carbon (TOC) and inorganic nutrients were determined in photic zone of 10 stations located from 131 to 135 W along 10.5 N in the northeast equatorial Pacific during June, 2002. Photic zone in the study area was extended down to 100 m depth and was divided into surface mixed layer (SML) and seasonal thermocline that was formed at between 40 and 60 m depth. The nutrient concentrations in top 40 m intervals were low, and increased with depth at all stations. SML was high in water temperature and showed the characteristics of oligotrophic water. Longitudinal variation was minor in the vertical profiles of nutrient concentrations. In the photic zone, integrated nitrogen, phosphorus, and silicon concentrations were 23.43 gN/m2, 3.88 gP/m2, and 33.31 gSi/m2 in average, respectively. The values of integrated nitrogen in the top 40 m interval varied in a narrow range from 0.03 to 0.06 gN/m2, while those in the thermocline increased exponentially to 2.69 gN/m2 up to 50 m depth. Phosphorus values ranged from 3.1 to 88.1 mgP/m2 in the top 40 m interval and then increased exponentially to 595.9 mgP/m2 up to 50 m depth. However, integrated silicon were different from case of nitrogen and phosphorus. Silicon values ranged from 0.3 to 16.3 gSi/m2 in the photic zone, and slowly increased with depth. TOC contents ranged from 0.48 to 1.47 mgC/L with an average of 0.93 mgC/L in the photic zone. TOC contents of SML were highest in the photic zone, and varied in a wide range reaching up to 0.87 mgC/L. At 50 to 60 m interval below thermocline, integrated TOC content decreased drastically. Integrated TOC content in the photic zone was 92.39 gC/m2. Consequently, the distributions of inorganic nutrients, especially nitrogen and phosphorus, and TOC in the study area are likely the result of close coupling between microbial activity and physical environment.