389
Primary Production in the Oceans
35 m, where it was confined in a 5- to 10-m layer at the top of the thermocline. After subduction, the patch could still be detected by its distinct salinity and low light transmission.
Hydrographic sampling of these areas confirmed the presence of SF 6 by direct chemical
analysis. SF 6 concentrations did not decrease after subduction. The highest values found
each day were a constant 40 to 50 fM over the entire 9-day duration of the experiment. The
constancy of the SF 6 suggests that unfertilized waters did not penetrate into the core of
the patch even after subduction.
Dissolvable iron concentrations (DFe) as high as 6.2 nM were determined in the core of
the patch 4 h after fertilization. These values decreased rapidly when the water column
mixed, with the highest values on the subsequent day being 3.6 nM, in good agreement
with predicted concentrations (3.8 nM Fe). The concentrations of DFe in the core of the
patch decreased by about 15% each day. Despite this decrease, contour plots of iron and
SF 6 in the surface layer were still in good spatial agreement 3 days after fertilization. Iron
dropped below the detection limit in the subducted layer after the fourth day. Remnants
of the patch also remained on the surface, as demonstrated by SF 6 measurements. The iron
concentrations extrapolated from the amount of SF 6 that was found at the surface would
have been too low to detect at sea.
Three distinct pools of iron were formed on injection. Most of the Fe(II) injected into the
patch was oxidized in minutes to Fe(III), and colloidal iron oxyhydroxides should have
precipitated with a first- order rate constant of 0.1 h. Of the three, colloidal Fe(III) is not
thought to be bioavailable. Laboratory experiments, however, indicated that these colloids
would remain suspended where they could be photoreduced and rendered bioavailable.
The changes in the chlorophyll in the patch became apparent within a day after the addition of Fe. Changes in chlorophyll (Figure 9.17) and primary productivity (Figure 9.18) as a
function of depth in the water column were quite apparent a few days after the addition of
Fe. The chlorophyll increased from values of 0.24 μg L –1 to a maximum of about 0.65 μg L –1 ,
a factor of 3 increase. The maximum primary production values in the mixed layer were
initially 15 to 25 μgC L –1 day –1 and increased to values of 48 μgC L –1 day –1 . This two- to
threefold productivity increase was seen in both small and large phytoplankton.
Microscopic examination of the water collected from within the patch indicated an
increase in the biomass of all classes counted. Total autotrophic biomass calculated from
cell numbers and volumes increased from 16 to 33 μgC L –1 , a 100% change. The largest
contributors to the plankton biomass were Synechococcus, a red fluorescing picoplankton,
and autotrophic dinoflagellates. Diatoms were a small fraction of the initial total plankton
biomass (17%) yet showed increases similar to the other groups. Heterotrophic biomass
increased from 4.7 to 7.3 μg C L –1 and was associated with heterotrophic dinoflagellates
followed by ciliates.
Photosynthetic energy conversion efficiency (Fv/ Fm) was the first biological response
detected (Kolber et al., 1994). Values of Fv/ Fm increased by 60% on the first sampling transect through the patch, indicating a large physiological response within the first 24 h (the
period required to fertilize the patch). This parameter increased from 0.3 to 0.60 by day 2
of the experiment, approaching the maximum value attainable for phytoplankton grown
under nutrient- replete conditions (0.65). The distributions of Fv/ Fm and chlorophyll were
also coincident with the area enriched in iron (Figure 9.19). The rapid increases in photosynthetic energy conversion efficiency in all sizes of phytoplankton in the patch confirm
that the ambient populations in all size classes are physiologically limited by lack of available iron.
The changes in the chemical constituents that occurred in the surface waters are
shown in Figure 9.20. The nutrients showed little or no systematic differences in nitrate,
Primary Production in the Oceans
35 m, where it was confined in a 5- to 10-m layer at the top of the thermocline. After subduction, the patch could still be detected by its distinct salinity and low light transmission.
Hydrographic sampling of these areas confirmed the presence of SF 6 by direct chemical
analysis. SF 6 concentrations did not decrease after subduction. The highest values found
each day were a constant 40 to 50 fM over the entire 9-day duration of the experiment. The
constancy of the SF 6 suggests that unfertilized waters did not penetrate into the core of
the patch even after subduction.
Dissolvable iron concentrations (DFe) as high as 6.2 nM were determined in the core of
the patch 4 h after fertilization. These values decreased rapidly when the water column
mixed, with the highest values on the subsequent day being 3.6 nM, in good agreement
with predicted concentrations (3.8 nM Fe). The concentrations of DFe in the core of the
patch decreased by about 15% each day. Despite this decrease, contour plots of iron and
SF 6 in the surface layer were still in good spatial agreement 3 days after fertilization. Iron
dropped below the detection limit in the subducted layer after the fourth day. Remnants
of the patch also remained on the surface, as demonstrated by SF 6 measurements. The iron
concentrations extrapolated from the amount of SF 6 that was found at the surface would
have been too low to detect at sea.
Three distinct pools of iron were formed on injection. Most of the Fe(II) injected into the
patch was oxidized in minutes to Fe(III), and colloidal iron oxyhydroxides should have
precipitated with a first- order rate constant of 0.1 h. Of the three, colloidal Fe(III) is not
thought to be bioavailable. Laboratory experiments, however, indicated that these colloids
would remain suspended where they could be photoreduced and rendered bioavailable.
The changes in the chlorophyll in the patch became apparent within a day after the addition of Fe. Changes in chlorophyll (Figure 9.17) and primary productivity (Figure 9.18) as a
function of depth in the water column were quite apparent a few days after the addition of
Fe. The chlorophyll increased from values of 0.24 μg L –1 to a maximum of about 0.65 μg L –1 ,
a factor of 3 increase. The maximum primary production values in the mixed layer were
initially 15 to 25 μgC L –1 day –1 and increased to values of 48 μgC L –1 day –1 . This two- to
threefold productivity increase was seen in both small and large phytoplankton.
Microscopic examination of the water collected from within the patch indicated an
increase in the biomass of all classes counted. Total autotrophic biomass calculated from
cell numbers and volumes increased from 16 to 33 μgC L –1 , a 100% change. The largest
contributors to the plankton biomass were Synechococcus, a red fluorescing picoplankton,
and autotrophic dinoflagellates. Diatoms were a small fraction of the initial total plankton
biomass (17%) yet showed increases similar to the other groups. Heterotrophic biomass
increased from 4.7 to 7.3 μg C L –1 and was associated with heterotrophic dinoflagellates
followed by ciliates.
Photosynthetic energy conversion efficiency (Fv/ Fm) was the first biological response
detected (Kolber et al., 1994). Values of Fv/ Fm increased by 60% on the first sampling transect through the patch, indicating a large physiological response within the first 24 h (the
period required to fertilize the patch). This parameter increased from 0.3 to 0.60 by day 2
of the experiment, approaching the maximum value attainable for phytoplankton grown
under nutrient- replete conditions (0.65). The distributions of Fv/ Fm and chlorophyll were
also coincident with the area enriched in iron (Figure 9.19). The rapid increases in photosynthetic energy conversion efficiency in all sizes of phytoplankton in the patch confirm
that the ambient populations in all size classes are physiologically limited by lack of available iron.
The changes in the chemical constituents that occurred in the surface waters are
shown in Figure 9.20. The nutrients showed little or no systematic differences in nitrate,