180
Chapter 9: The Atlantic Ocean
downward. At this redox interface, there is a “null zone”, tens of meters thick, in
which both gases are present at very low concentrations. At this interface, photosynthetic
oxidation of H 2 S by sulfur-oxidizing bacteria (Thiobacillus) occurs, resulting in high
concentrations of elemental sulfur. Other phototrophic, green (bacteriochlorophyll-)
sulfur bacteria, mostly Chlorobium phaeobacterioides, are also active in the redox interface.
The interface between oxygenated water and water that contains H 2 S is, of course, the
lower limit of zooplankton in the water column and of benthos in the sediments.
The pristine Black Sea plankton was overall much less diverse than that of the Mediterranean, including, for example, only 15 species of copepods compared with 304. It also
contained a significant element of freshwater plankton over the northern continental
shelves and in the Sea of Azov, where salinity is very low. The mesozooplankton assemblage in the 1940s comprised only 7 hydromedusae, 2 scyphomedusae, 1 ctenophore,
12 cladocerans, 15 copepods, 1 isopod, 2 chaetognaths, and 6 appendicularians. Thus,
coccoliths, radiolarians, siphonophores, salps, and pteropods were absent. The distribution of mesozooplankton in the Black Sea was also unique (Vinogradov et al., 1985).
By day, the dominant species formed a layer of high abundance (25–380 g m
−3 wet
weight) over a depth interval of only 5–20 m, coinciding exactly with the isopleths for
04–05 ml O 2 liter
−1 that occurs in the null zone (or redox interface) discussed previously. Following this isopleth, the actual depth of the zooplankton layer varied from
50 to 150 m and the distribution of organisms within this narrow layer was predictable
with precision. The upper part was occupied by the tentaculate ctenophore Pleurobrachia
pileus, the middle zone by late copepodite and adult C. helgolandicus, and the lower
zone was occupied by the chaetognath Sagitta setosa. The small Pseudocalanus elongatus might also be present. At night, diel migration altered the pattern. The copepods
and chaetognaths rose into the near-surface layers (0–20 m), whereas the ctenophores
adjusted their depth only slightly, rising by about 20 m. Thus, during each diel cycle, the
copepods ran the minefield of ctenophores passively awaiting their passage while at the
same time avoiding the hunting chaetognaths, which tracked their migration. Seasonal
vertical migration also occurred, but rather anomalously. The cold-water forms, such as
C. helgolandicus, P. elongatus, and Oithona similis, occurred throughout the oxygenated
layer in winter but descended to depths around 50 m during summer. In general, their
depth distribution was constrained between the oxycline and the isotherm of their maximal temperature, usually 10–14
C. Following the precautionary principle, and lacking
current information, I have written this paragraph in the past tense.
In the oxygenated water above the redox layer, a thermocline develops in summer,
with which is associated a DCM due to normal autotrophic phytoplankton. However,
within the oxygenated surface layer, nitrate has a very unusual profile. In the redox
zone the nitrogen cycle is extremely active, and this layer is a sink for nitrate, nitrite,
and ammonium. Consequently, rather than a nitracline, we find a subsurface nitrate
maximum in the Black Sea. Below this maximum, nitrate is utilized by the activity at the
redox layer and, above, by autotrophic cells of the euphotic zone (Sorokin, 1983; Murray
and Izdar, 1989; Murray, 1991). This nitrate maximum consistently had values of about
70 M in 1988 compared to only 3 M in 1970. A similar increase in the integrated rate
of primary production (100 to 300 mg C m
−2 d
−1 ) and chlorophyll concentration (0.2
to 04 mg chl m
−3 ) has also been observed between 1970 and 1990 (see later discussion).
In past decades, a winter bloom has occurred rather generally between November and
March in the Black Sea, replacing the summer community of coccolithophores and
dinoflagellates with a surge of diatoms (Vedernikov and Demidov, 1991; Krupatkina
et al., 1991).
No wonder, then, that chlorophyll images show that the range (both temporal
and spatial) of chlorophyll values is now relatively limited. Over the deep basin of
the Black Sea, indicated surface chlorophyll concentration varies only from 0.5 to
Chapter 9: The Atlantic Ocean
downward. At this redox interface, there is a “null zone”, tens of meters thick, in
which both gases are present at very low concentrations. At this interface, photosynthetic
oxidation of H 2 S by sulfur-oxidizing bacteria (Thiobacillus) occurs, resulting in high
concentrations of elemental sulfur. Other phototrophic, green (bacteriochlorophyll-)
sulfur bacteria, mostly Chlorobium phaeobacterioides, are also active in the redox interface.
The interface between oxygenated water and water that contains H 2 S is, of course, the
lower limit of zooplankton in the water column and of benthos in the sediments.
The pristine Black Sea plankton was overall much less diverse than that of the Mediterranean, including, for example, only 15 species of copepods compared with 304. It also
contained a significant element of freshwater plankton over the northern continental
shelves and in the Sea of Azov, where salinity is very low. The mesozooplankton assemblage in the 1940s comprised only 7 hydromedusae, 2 scyphomedusae, 1 ctenophore,
12 cladocerans, 15 copepods, 1 isopod, 2 chaetognaths, and 6 appendicularians. Thus,
coccoliths, radiolarians, siphonophores, salps, and pteropods were absent. The distribution of mesozooplankton in the Black Sea was also unique (Vinogradov et al., 1985).
By day, the dominant species formed a layer of high abundance (25–380 g m
−3 wet
weight) over a depth interval of only 5–20 m, coinciding exactly with the isopleths for
04–05 ml O 2 liter
−1 that occurs in the null zone (or redox interface) discussed previously. Following this isopleth, the actual depth of the zooplankton layer varied from
50 to 150 m and the distribution of organisms within this narrow layer was predictable
with precision. The upper part was occupied by the tentaculate ctenophore Pleurobrachia
pileus, the middle zone by late copepodite and adult C. helgolandicus, and the lower
zone was occupied by the chaetognath Sagitta setosa. The small Pseudocalanus elongatus might also be present. At night, diel migration altered the pattern. The copepods
and chaetognaths rose into the near-surface layers (0–20 m), whereas the ctenophores
adjusted their depth only slightly, rising by about 20 m. Thus, during each diel cycle, the
copepods ran the minefield of ctenophores passively awaiting their passage while at the
same time avoiding the hunting chaetognaths, which tracked their migration. Seasonal
vertical migration also occurred, but rather anomalously. The cold-water forms, such as
C. helgolandicus, P. elongatus, and Oithona similis, occurred throughout the oxygenated
layer in winter but descended to depths around 50 m during summer. In general, their
depth distribution was constrained between the oxycline and the isotherm of their maximal temperature, usually 10–14
C. Following the precautionary principle, and lacking
current information, I have written this paragraph in the past tense.
In the oxygenated water above the redox layer, a thermocline develops in summer,
with which is associated a DCM due to normal autotrophic phytoplankton. However,
within the oxygenated surface layer, nitrate has a very unusual profile. In the redox
zone the nitrogen cycle is extremely active, and this layer is a sink for nitrate, nitrite,
and ammonium. Consequently, rather than a nitracline, we find a subsurface nitrate
maximum in the Black Sea. Below this maximum, nitrate is utilized by the activity at the
redox layer and, above, by autotrophic cells of the euphotic zone (Sorokin, 1983; Murray
and Izdar, 1989; Murray, 1991). This nitrate maximum consistently had values of about
70 M in 1988 compared to only 3 M in 1970. A similar increase in the integrated rate
of primary production (100 to 300 mg C m
−2 d
−1 ) and chlorophyll concentration (0.2
to 04 mg chl m
−3 ) has also been observed between 1970 and 1990 (see later discussion).
In past decades, a winter bloom has occurred rather generally between November and
March in the Black Sea, replacing the summer community of coccolithophores and
dinoflagellates with a surge of diatoms (Vedernikov and Demidov, 1991; Krupatkina
et al., 1991).
No wonder, then, that chlorophyll images show that the range (both temporal
and spatial) of chlorophyll values is now relatively limited. Over the deep basin of
the Black Sea, indicated surface chlorophyll concentration varies only from 0.5 to