Silicate versus nitrate limitation in the equatorial Pacific estimated from satellite-derived sea-surface temperatures

Abstract

Productivity in the equatorial Pacific may be limited by a variety of factors including the availability of either nitrate (NO 3) or silicate (Si(OH) 4). The large size of this oceanic region makes it difficult to obtain a synoptic view of the nutrient field in traditional ways to assess NO 3 or Si(OH) 4 limitation. However, since concentrations of both nutrients are correlated with temperature, we have been able to employ remotely-sensed sea-surface temperatures (SST) from AVHRR to estimate the variability of surface NO 3 and Si(OH) 4 concentrations, and the ratio of one nutrient to the other which can be used to evaluate nutrient limitation in a Liebig sense. Using zonal transects along the equator, nutrient concentrations obtained from AVHRR-SST data from 1987 and 1988 show Si(OH) 4 values to be typically less than NO 3 and NO 3:Si(OH) 4 > 1, indicating Si(OH) 4 to be the limiting nutrient and the equatorial Pacific to be a HNLSLC (High Nitrate, Low Silicate, Low Chlorophyll) area. However, in warmer climatic conditions, i.e. during 1987—an El Niño year and in the western part of the equatorial swath, NO 3 falls to undetectable levels and is most likely the more limiting macronutrient. Data from 1988 for 15°N to 15°S, 180°W to 90°W shows maximal concentrations of both nutrients to occur to the south of the equator with the ratio values indicating Si(OH) 4 limitation to the south and NO 3 limitation to the north of the equator.