Search

Iron has been measured for 16 months with ∼21 day resolution at three stations in the upwelling ecosystem of central California, providing the first detailed assessment of the annual iron cycle in the coastal zone. A large pulse of iron occurs during the first spring upwelling event of the year. Iron concentrations then decay up to 100-fold over several months, although upwelling continues. Excess surface nitrate and low iron are the result during the summer and fall at the two stations furthest offshore (20 and 45 km), while nitrate is depleted and iron high nearshore (5 km). Phytoplankton biomass, primary production and community structure appear to be controlled by iron concentrations in offshore waters during this period.

[1] Oceanic CO2 levels are expected to rise during the next 2 centuries to levels not seen for 10–150 million years by the uptake of atmospheric CO2 in surface waters or potentially through the disposal of waste CO2 in the deep sea. Changes in ocean chemistry caused by CO2 influx may have broad impacts on ocean ecosystems. Physiological processes animals use to cope with CO2-related stress are known, but the range of sensitivities and effects of changes in ocean chemistry on most ocean life remain unclear. We evaluate the effectiveness of various designs for in situ CO2 release experiments in producing stable perturbations in seawater chemistry over experimental seafloor plots, as is desirable for evaluating the CO2 sensitivities of deep sea animals. We also discuss results from a subset of these experiments on the impacts of hypercapnia on deep sea meiofauna, in the context of experimental designs. Five experiments off central California show that pH perturbations were greatest for experiments using “point source” CO2 pools surrounded by experimental plots. CO2 enclosure experiments with experimental plots positioned within a circular arrangement of CO2 pools had more moderate pH variation. The concentration of dissolution plumes from CO2 pools were related to the speed and turbulence of near-bottom currents, which influence CO2 dissolution and advection. Survival of meiofauna (nematodes, amoebae, euglenoid flagellates) was low after episodic severe hypercapnia but lower and variable where pH changes ranged from 0 to 0.2 pH units below normal.

Oceanic dimethylsulfide (DMS) emissions to the atmosphere are potentially important to the Earth's radiative balance. Since these emissions are driven by the surface seawater concentration of DMS, it is important to understand the processes controlling the cycling of sulfur in surface seawater. During the third Pacific Sulfur/Stratus Investigation (PSI-3, April 1991) we measured the major sulfur reservoirs (total organic sulfur, total low molecular weight organic sulfur, ester sulfate, protein sulfur, dimethylsulfoniopropionate (DMSP), DMS, dimethylsulfoxide) and quantified many of the processes that cycle sulfur through the upper water column (sulfate assimilation, DMSP consumption, DMS production and consumption, air-sea exchange of DMS, loss of organic sulfur by particulate sinking). Under conditions of low plankton biomass ( 8 μM nitrate), 250 km off the Washington State coast, DMSP and DMS were 22% and 0.9%, respectively, of the total particulate organic sulfur pool. DMS production from the enzymatic cleavage of DMSP accounted for 29% of the total sulfate assimilation. However, only 0.3% of sulfate-S assimilated was released to the atmosphere. From these data it is evident that air-sea exchange is currently only a minor sink in the seawater sulfur cycle and thus there is the potential for much higher DMS emissions under different climatic conditions.