6.8 Anthropogenic effects on ocean chemistry
The activities of humans have had some impacts on both the major and minor
element chemistry of the modern oceans. For example, seawater major ion
budgets mostly assume the estimated riverwater input to seawater is that of the
pristine (pre-human) system. However, anthropogenic processes have altered
some of these fluxes. For example, the riverine Cl-flux may have increased by
more than 40% as a result of human activity and the SO 42 - flux may have doubled,
due mainly to fossil fuel combustion and oxidation of pollution-derived H 2 S.
As one might predict, the most obvious human impacts are manifest in smaller
regional seas that are much less well mixed with respect to the open ocean, or in
continental shelf settings close to land. The impacts in these areas is mainly a
result of human activities on land that affect the chemical composition of the
runoff and/or the mass of suspended load carried by rivers. Deforestation and
increased agricultural activity worldwide make land surfaces more susceptible to
erosion. The effects on the major ion chemistry of seawater are mainly related
to increased input of detrital solids to continental shelves that increase the amount
of ion exchange and other solid–seawater interactions (Section 6.4). However,
this situation is still changing; the increasing use of dams on rivers is now reduc-
ing sediment inputs to the oceans. Less important in terms of global budgets, but
important from an ecological viewpoint, increased suspended loads in tropical
rivers issuing into coastal waters choke coral reefs with detritus and decrease
biological productivity by reducing water clarity.
6.8.1 Human effects on regional seas 1: the Baltic
The Baltic Sea (Fig. 6.30) is a large regional sea, receiving drainage from much
of northern and central Europe. The hydrography of the Baltic is complex, con-
sisting of a number of deep basins separated by shallow sills. As a result, the waters
of the deep basins can be isolated from exchange with one another—and from
the atmosphere—on timescales of years.
There is a long record (almost 100 years) of dissolved phosphorus (P) and
oxygen (O 2 ) concentrations for the waters of the Baltic. The records are ‘noisy’
due to complex water exchange and deep mixing, but the increasing concentra-
tion of dissolved phosphorus over the last 30 years is clear from Fig. 6.30. This
increase in nutrient concentration has fuelled primary production and has
increased the flux of organic matter to the deep waters. Measurements of dis-
solved oxygen in deep waters of the Baltic show a steady decline over the last 100
years (Fig. 6.30), consistent with an increase in rates of oxygen consumption due
to increasing organic matter inputs—overall, a clear example of eutrophication.
The isolated deep waters of the Baltic have probably always had low oxygen
concentrations. However, the declining trend over recent years means that, in
some areas, oxygen concentrations have fallen to zero (anoxic). Under anoxic
conditions, respiration of organic matter by microbial sulphate (SO 42 - ) reduction
has produced hydrogen sulphides (HS-) (plotted as negative oxygen in Fig. 6.30).
The Oceans 233
