Search

Equimolar Tris (2-amino-2-hydroxymethyl-propane-1,3-diol) buffer prepared in artificial seawater media is a widely accepted pH standard for oceanographic pH measurements, though its change in pH over pressure is largely unknown. The change in volume (Δ V ) of dissociation reactions can be used to estimate the effects of pressure on the dissociation constant of weak acid and bases. The Δ V of Tris in seawater media of salinity 35 (Δ V Tris ⁎ ) was determined between 10 and 30 °C using potentiometry. The potentiometric cell consisted of a modified high pressure tolerant Ion Sensitive Field Effect Transistor pH sensor and a Chloride-Ion Selective Electrode directly exposed to solution. The effects of pressure on the potentiometric cell were quantified in aqueous HCl solution prior to measurements in Tris buffer. The experimentally determined Δ V Tris ⁎ were fitted to the equation Δ V Tris ⁎ = 4.528 + 0.04912 t where t is temperature in Celsius; the resultant fit agreed to experimental data within uncertainty of the measurements, which was estimated to be 0.9 cm − 3 mol − 1 . Using the results presented here, change in pH of Tris buffer due to pressure can be constrained to better than 0.003 at 200 bar, and can be expressed as: ∆ pH Tris = − 4.528 + 0.04912 t P ln 10 RT . where T is temperature in Kelvin, R is the universal gas constant (83.145 cm 3 bar K − 1 mol − 1 ), and P is gauge pressure in bar. On average, pH of Tris buffer changes by approximately − 0.02 at 200 bar.

A recent analysis of full water column hydrographic data revealed a pH-dependent discrepancy between spectrophotometrically measured pH using purified meta-cresol purple and pH calculated from dissolved inorganic carbon (DIC) and total alkalinity (TA). The discrepancy (pHspec – pHTA,DIC) is approximately −0.018 and 0.014 at pH 7.4 and 8.2, respectively. This discrepancy has a wide range of implications for marine inorganic carbon measurements, such as establishing robust calibration protocols for pH sensors operating on profiling floats. Here, we conducted a series of lab based experiments to assess the magnitude of pH-dependent errors for spectrophotometric pH measurements in seawater by directly comparing its performance to pH measured by an Ion Sensitive Field Effect Transistor (ISFET) pH sensor known to have Nernstian behavior. Natural seawater was titrated with high CO2 seawater while simultaneously measuring pH using spectrophotometry and an ISFET sensor over a large range in pH (7–8.5) and temperature (5–30 °C). The two pH measurements were consistent to better than ±0.003 (range) at all temperatures except at 5 and 10 °C and very low and high pH, where discrepancies were as large as ±0.005. These results demonstrate that pH-dependent errors in spectrophotometric pH measurements can be rejected as the cause of the pH-dependent discrepancy between pHspec and pHTA,DIC. The cause of this discrepancy is thus likely due to our incomplete understanding of the marine inorganic carbon model that could include errors in thermodynamic constants, concentrations of major ions in seawater, systematic biases in measurements of TA or DIC, or contributions of organic compounds that are not accounted for in the definition of total alkalinity. This should be a research priority for the inorganic carbon community.

We report on a direct and autonomous measure of new production based on time series observations with ISUS nitrate sensors deployed for several years on oceanographic moorings offshore of Monterey Bay, California. The amplitude of diel cycles of surface nitrate is correlated with rates of primary production measured by 14C uptake. The drawdown of nitrate averaged over a year is about 70% of the fixed nitrogen needed for biomass production. Phytoplankton biomass predicted from the diel nitrate uptake and a fixed rate constant for nitrate loss (grazing and export) matched observations over the annual cycle. New production rates determined with the moored sensors are highly correlated with nitrate concentration and the intensity of upwelling. The implication is that arrays of moorings with chemical sensors can now be used to estimate new production of biomass and ecosystem processes over multiple temporal and spatial scales.

We report measurements of dissolvable and particulate iron, particulate Al, nutrients and phytoplankton biomass in surface waters during the termination of one upwelling event and the initiation of a second event in August 2000. These events occurred in the area of the Ano Nuevo upwelling center off the coast of central California. The first event was observed after ∼8 days of continuous upwelling favorable winds, while the second event was observed through the onset of upwelling favorable winds to wind reversals ∼3 days later. Coincident with the upwelling signatures of low temperature and high salinity were significantly elevated concentrations of nitrate and silicate with average concentrations greater than 15 and 20 μM, respectively, during both upwelling events. Dissolvable Fe concentrations (TD-Fe) were significantly higher in the second event, 6.5 versus 1.2 nM Fe found in the first event. Nitrate was reduced by ∼5 μM day −1 within this second upwelled plume as compared to a drawdown of ∼2 μM day −1 within the first plume. Silicate was reduced in a ratio of 1.2 mol Si:mol NO 3 in the high Fe waters of the second plume as compared to a ratio of 2.2 in the lower Fe waters of the first plume. The observed differences in nutrient utilization are consistent with some degree of iron limitation. The area of increased dissolvable Fe in the second upwelling event was coincident with elevated particulate Fe concentrations, indicating the particulate pool as a possible source of the observed increase in TD-Fe. The elevated particulate Fe in surface waters was a result of resuspended sediments in the bottom boundary layer (BBL) of the shallow shelf being transported to the surface during upwelling. Particulate (and dissolvable) iron concentrations were significantly reduced as upwelling continued. This was most probably due to a decoupling of the BBL from upwelled source waters as the upwelling front moved offshore and/or reduced turbulence in the BBL as upwelling continued. The observed reduction in both particulate and dissolvable Fe, as upwelling continued to deliver macronutrients to surface waters, may result in varying levels of Fe limitation.