Surface seawater distributions of inorganic carbon and nutrients around the Galapagos Islands: results from the PlumEx experiment using automated chemical mapping

During the second leg (PlumEx) of the 1993 IRONEX cruise, the partial pressure of CO2 and the concentrations of nitrate and silicate in the surface waters around the Galapágos Islands were continuously measured using automated underway systems. Based on salinity-versus-constituent mixing diagrams, physical mixing processes dominate the pCO2 and nutrient distributions upstream of the Galápagos Islands. Downstream of the islands, slight removal of nitrate and CO2 can be discerned because of the high resolution of the underway measurements. The high spatial resolution of the underway measurements allowed evaluation of fine features such as sharp fluorescence peaks on the “warm” side of frontal boundaries. In the waters immediately adjacent to Fernandina and Isabela islands (Bolivar channel), dramatic drawdown of pCO2 and nutrients was measured, coincident with the highest measured levels of iron (3 nM) and chlorophyll (>13 μg l-1) (Martin et al., 1994). The nearly constant alkalinity of the waters was combined with the measured pCO2 to calculate total carbon dioxide in the waters. Based on mixing diagrams, the ratio of ΔTCO2 to ΔNO-3 was found to be highly variable, ranging from approximately 6.4 to >10 in the waters near Isabela Island. The ratio of ΔTCO2 to ΔNO-3 is approximately 8.5 in the waters west of the Galápagos where slight removal of nitrate and TCO2 occurs. In these waters, the physical process of mixing and CO2 degassing due to warming of the water becomes significant relative to the biological uptake and the ratio is driven higher.