Environmental Engineering Reference
In-Depth Information
measurement and modeling of oceanic biogeochemical cycles is essential for
understanding and projecting responses to natural and anthropogenic-induced
climate change.
Introduction
The ocean is the dominant surface feature that has controlled much of the evolution,
distribution, and success of life on earth, and the changes in ocean chemistry reflect
the interaction with biota throughout geological time. The oceans were originally
anoxic, but the evolution of organisms with oxygen-generating processes (photo-
synthesis) resulted in the conversion of the oceans to an oxygenated environment,
which greatly altered the availability of some elements for those organisms. The
cycling of elements within the earth's oceans and the complex relationships among
the biological, chemical, and geological processes are the core of the study of
marine biogeochemistry. Understanding these relationships is difficult and is fur-
ther complicated by the space and time variability of the dominant processes that
control the cycling of the different elements. Understanding the interactions and
linkages among and between the cycles of biogeochemical elements is critically
important for assessing and projecting the nature, degree, and direction of changes
in ocean processes that may result from changes induced by natural and/or anthro-
pogenic activities.
Elements in the ocean have characteristic vertical and horizontal distributions
that result from the processes that regulate their long-term source/sink relationships.
For example, oceanic carbon dioxide (CO
2
) distributions are characterized by
a horizontal concentration gradient that increases from the equator to the poles,
which results from the greater dissolution of CO
2
in colder water. Carbon dioxide
concentrations generally increase with depth due to remineralization in the deeper,
older waters relative to its removal at the surface. Other elements may be controlled
by different factors (e.g., sources from the sediments or hydrothermal vents; atmo-
spheric sources) and have different vertical and horizontal patterns, but all interact to
create the observed vertical distributions in the ocean. Understanding marine bio-
geochemistry requires knowledge not only of specific processes regulating
a particular element, but also an understanding of the interdisciplinary aspects that
control these cycles.
Nutrients are the biogeochemical elements that are required for biological
activity. Some elements are greatly reduced in their concentrations by chemical
or biological processes and can reach such low concentrations that they subse-
quently limit the growth of organisms in the sea. Such elements are thought
of as limiting nutrients in the sense of the German agricultural chemist, Justus
von Liebig, who suggested that the growth of plants is limited not by the total
amount of resources, but by the resource in lowest abundance relative to the others.
Plant growth in the ocean is known to be limited by a small number of nutrients
that include nitrogen, phosphorus, iron, silicic acid, and inorganic carbon. The
cycling and processes that control the concentrations of these limiting nutrients
