means of volcanic activity and the weathering of stones. Naturally, they are present
in seawater. It was not until particular questions arose about ocean pollution that
attention was directed to the geochemical processes. But the few years of research in
this field have not yet produced sufficient information to determine the relationship
of quantities supplied by erosion and precipitation and by sedimentation. To estimate
whether increased amounts of man-made input will be matched by increased amounts
of sedimentation, so that increased concentrations do not take place in the ocean,
seems an important task.
Knowledge is quite insufficient on the fate of pollutants at the water-sediment interface, and what exchanges may occur between the overlying water, the pore water in
the oxidized surface layer and the pore water in the anoxic deeper layer of the sediment. Quite insufficient, too, is knowledge on the fate of pollutants at the freshwaterseawater border in rivers and estuaries where the situation is complicated by the formation of a turbidity cloud (Fig. 5). Probably a reasonable fraction of the contaminants which originally are dissolved in river water, or are bound to colloidal material,
will go into the flakes which form at the brackish water border, and accumulate in the
brackish water region. It may be that only a fraction of the pollutant load of a river
(Table 8) really reaches the open sea, but that a large fraction accumulates with mud
and clay in the estuarine region. As a matter of fact, fine material which sedimentates
off the mouth of a polluted river contains tremendous concentrations of heavy metals,
more than upriver and more than farther in the sea (Fig. 74; see Chap. 3.2).
Experimentally it is easy to demonstrate changes in the chemical form of heavy
metals when introduced into the marine environment. If one adds 1 p.g/l of inorganic
mercury to seawater in a large, 100 m 3 tank, 12-24 h later one finds about 70% of
the mercury in a nonreactive form (see Fig. 80), probably bound to particles which
exist in seawater as suspension, and which will become incorporated into the sediment after sedimentation (Topping and Davies 1980). Plankton organisms are helping
in the process to eliminate heavy metals from the water column by sedimentation to
the sea bottom (Table 31).
A remarkable correlation exists between cadmium, phosphate, and nitrate concentrations in seawater (Bruland et a1. 1978). In deep seawater high nutrient concentrations are combined with a cadmium concentration of about 100 ng/l. In the biologically active surface zone of the sea, however, cadmium concentrations are as
much depleted as phosphate and nitrate concentrations, and figures as low as 4 ng/l
are measured (Fig. 75). All three elements are, on the other hand, concentrated
in plankton and other living organisms. The correlation between cadmium and
phosphorus in plankton samples is not as close as in seawater, but can nevertheless
be demonstrated. Mean cadmium concentration in plankton collected off Baja
California was 5.8 mgt kg on dry weight basis. One can imagine that with dead plankton phosphorus as well as cadmium is transported to the sea bottom. When under
special conditions phosphate ores are formed on the sea bottom, they automatically
include reasonable concentrations of cadmium. In this way it can be explained why
synthetic fertilizer contains more cadmium which then is transported into agricultural crops, than one would like conSidering the health risk (see Chap. 8.6).
As regards air-sea interaction, interesting phenomena have been discovered in recent
years, but they have not yet been conclusively explained. Air-borne dust samples
139
in seawater. It was not until particular questions arose about ocean pollution that
attention was directed to the geochemical processes. But the few years of research in
this field have not yet produced sufficient information to determine the relationship
of quantities supplied by erosion and precipitation and by sedimentation. To estimate
whether increased amounts of man-made input will be matched by increased amounts
of sedimentation, so that increased concentrations do not take place in the ocean,
seems an important task.
Knowledge is quite insufficient on the fate of pollutants at the water-sediment interface, and what exchanges may occur between the overlying water, the pore water in
the oxidized surface layer and the pore water in the anoxic deeper layer of the sediment. Quite insufficient, too, is knowledge on the fate of pollutants at the freshwaterseawater border in rivers and estuaries where the situation is complicated by the formation of a turbidity cloud (Fig. 5). Probably a reasonable fraction of the contaminants which originally are dissolved in river water, or are bound to colloidal material,
will go into the flakes which form at the brackish water border, and accumulate in the
brackish water region. It may be that only a fraction of the pollutant load of a river
(Table 8) really reaches the open sea, but that a large fraction accumulates with mud
and clay in the estuarine region. As a matter of fact, fine material which sedimentates
off the mouth of a polluted river contains tremendous concentrations of heavy metals,
more than upriver and more than farther in the sea (Fig. 74; see Chap. 3.2).
Experimentally it is easy to demonstrate changes in the chemical form of heavy
metals when introduced into the marine environment. If one adds 1 p.g/l of inorganic
mercury to seawater in a large, 100 m 3 tank, 12-24 h later one finds about 70% of
the mercury in a nonreactive form (see Fig. 80), probably bound to particles which
exist in seawater as suspension, and which will become incorporated into the sediment after sedimentation (Topping and Davies 1980). Plankton organisms are helping
in the process to eliminate heavy metals from the water column by sedimentation to
the sea bottom (Table 31).
A remarkable correlation exists between cadmium, phosphate, and nitrate concentrations in seawater (Bruland et a1. 1978). In deep seawater high nutrient concentrations are combined with a cadmium concentration of about 100 ng/l. In the biologically active surface zone of the sea, however, cadmium concentrations are as
much depleted as phosphate and nitrate concentrations, and figures as low as 4 ng/l
are measured (Fig. 75). All three elements are, on the other hand, concentrated
in plankton and other living organisms. The correlation between cadmium and
phosphorus in plankton samples is not as close as in seawater, but can nevertheless
be demonstrated. Mean cadmium concentration in plankton collected off Baja
California was 5.8 mgt kg on dry weight basis. One can imagine that with dead plankton phosphorus as well as cadmium is transported to the sea bottom. When under
special conditions phosphate ores are formed on the sea bottom, they automatically
include reasonable concentrations of cadmium. In this way it can be explained why
synthetic fertilizer contains more cadmium which then is transported into agricultural crops, than one would like conSidering the health risk (see Chap. 8.6).
As regards air-sea interaction, interesting phenomena have been discovered in recent
years, but they have not yet been conclusively explained. Air-borne dust samples
139