J. MARTIN HERNANDEZ-AYON and ALBERTO ZIRINO, Instituto de Investigaciones Oceanologicas, Universidad Autónoma de Baja California, Ensenada, B.C., Mexico
OSMUND HOLM-HANSEN, Polar Research Program, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California 92093-0202
Previous studies in the AMLR survey grid have shown that much inorganic nutrient recycling occurs in the upper water column (Koike, Holm-Hansen, and Biggs 1986). The major processes responsible for these reactions involve grazing of autotrophic phytoplankton by heterotrophic protozoans and microzooplankton, in addition to catabolism of dissolved organic compounds by bacterioplankton. Because the sum of these heterotrophic reactions will affect the food resources available to krill ( Euphausia superba ), it is important to understand more fully the dynamics of the many routes involved in the microbial food web in antarctic waters. Space and time onboard ship during the Antarctic Marine Living Resources (AMLR) cruises, however, do not allow for comprehensive studies of the biomass and metabolic activity of heterotrophic organisms. Other methods must, therefore, be used to improve our understanding of the relative rates of primary production by phytoplankton as compared to the rates at which the photosynthate is used by heterotrophic organisms.
During photosynthesis, carbon dioxide is consumed and oxygen is liberated; the reverse is true during respiration in heterotrophic organisms. One way to approach the problem of the relationship between the relative rates of autotrophic versus heterotrophic metabolism is, thus, to examine the relative concentrations of dissolved oxygen and inorganic carbon in water samples. Dissolved oxygen and inorganic carbon can be measured on discrete water samples, but for upper water column studies, it is desirable to have in situ sensors so that detailed profiles can be measured in profiles extending throughout the euphotic zone. Oxygen electrodes are suitable for this task, but no methods are available for direct in situ measurement of total inorganic carbon. The concentration of inorganic carbon, however, will be directly related to the hydrogen ion activity (pH) of the water, and in situ pH electrodes are suitable for such profiling studies.
During Leg II of the AMLR studies in 1997, measurements were made of pH in addition to temperature and salinity on a continuous-flow system using water pumped from 5 meters depth while the ship was steaming and also in profiles at selected stations from the surface to 240 meters depth. The profile data were obtained with a Sea-bird Seacat conductivity-temperature-depth (CTD) unit equipped with a pH electrode (model SEB-16).
Data from the pumped seawater line are shown in figure 1, which illustrate that pH values of antarctic surface waters varied considerably (all data after day 15.5, figure 1), as did the values for temperature, salinity, and the derived values for density. Values of pH tend to change in the same direction as temperature and inversely to changes in salinity and density, indicating vigorous vertical mixing. However, as pH is affected to a relatively large extent by biological activity whereas temperature and salinity are not, considerable scatter is evident in the correlations between pH and the physical variables. Drake Passage waters and stations in Water Zone I (Station D62) tend to have relatively high pH values as compared to the lower pH values found in Bransfield Strait waters (e.g., Station D64).
Representative data from the CTD-pH profiles are shown in figure 2. Amos, Wickham, and Rowe ( Antarctic Journal , in this issue) have classified the stations in the AMLR study area into five water zones that can be differentiated on the basis of temperature and salinity characteristics. Stations D62, D63, and D64 are found in Water Zone I (Drake Passage waters), Water Zone II (a mixing zone), and Water Zone IV (Bransfield Strait origin), respectively (Amos personal communication). Station D62 shows relatively high pH values in surface waters (>8.07), a steep gradient in pH between 75 to 125 meters, and low values at 200 meters (<7.9). The depth region of the steep gradient in pH corresponds to the location of the remnants of Winter Antarctic Surface Water; the water below that has its origin in Circumpolar Deep Water. Stations D63 and D64 have lower pH values in surface waters (<8.07), no sharp gradient around 100 meters depth, and values greater than 7.9 at 200 meters depth. It appears that determination of pH in situ thus might be useful not only in estimating relative rates of photosynthesis and respiration but also as an indicator of different water zones found in the AMLR study area around Elephant Island.
This research was supported by National Oceanic and Atmospheric Administration (NOAA) contract number 50ABNF600013. Grateful acknowledgment is extended to the officers and crew of the R/V Yuzhmorgeologiya for their excellent support during all field operations. We thank the Physical Oceanography group for kindly providing their CTD data. Shipboard personnel included M. Hernandez.
Amos, A.F. 1997. Personal communication.
Amos, A.F., A.R. Wickham, and C.R. Rowe. 1997. AMLR program: Midsummer 1997 in the Elephant Island area—A month of warm surface waters and calm winds. Antarctic Journal of the U.S., 32(5).
Koike, I., O. Holm-Hansen, and D.C. Biggs. 1986. Inorganic nitrogen metabolism by antarctic phytoplankton with special reference to ammonium cycling. Marine Ecology Progress Series , 30, 105-116.