Chapter 4
Physical Control of
Ecological Processes
A
suggestion of Yentsch and Garside, made to a congress of pelagic biogeographers in 1986, might serve as a synopsis of the argument presented in this
book:
Are the large-scale biogeographic distributions of oceanic biology a response to seasonal
and spatial patterns of primary production? Geostrophic currents dictate the shape of the
density structure and what we term the degree of baroclinicity is a global mirror of primary
production. The fact is that for at least fifty years we have recognized that light, wind
and temperature regulate phytoplankton growth. We recognize that seasonality in the
primary processes is controlled by stirring of the upper layers of the ocean, and the nutrient
and density fields. Why, then, are we so hesitant to tie the climatological primary process
together with concepts concerning biogeography? One answer might be that we do not have
enough knowledge of primary productivity over large areas of the world’s ocean.
Now, of course, we do have enough knowledge from satellite imagery to begin the
integration we were encouraged to undertake. A little later, Yentsch gave one of the
earliest demonstrations that the CZCS chlorophyll field could be used to demonstrate
the control of algal growth by irradiance and nutrients. He showed that the front of the
migrating North Atlantic spring bloom, as revealed by CZCS, closely matched predictions
obtained from an independent analysis of the seasonal poleward march of the shoaling
of the mixed layer.
How such thinking had much earlier led to the formulation of Sverdrup’s critical
depth theory is well known. Had Sverdrup not left Scripps, as he did in 1948 to return
to Norway, he might not have become involved in thinking about the Ocean Weather
Station “M” observations, and his formulation would now bear another’s name. It is so
fundamental that with or without his initiative it would have become central to biological
oceanographic theory.
Nevertheless, we tend to forget that Sverdrup’s important contribution was to encapsulate, in a formally stated theorem, several concepts that had already been discussed
more generally. Nathansohn (1909), Gran (1931), Riley (1942), and others made major
advances as the old “nitrification” theory of plankton production came to be replaced
by modern understanding of the nitrogen cycle. Prior to Sverdrup, the concept of a
compensation illuminance (I c ) at which photosynthesis of individual cells would exactly
balance cellular respiration had already been evoked from studies done in the Gulf of
Maine (Gran and Braarud, 1935).
Sverdrup (1953), using time-series observations of daily mixing depths and cell counts
made at OWS “M” (66
N2
E) analyzed the interaction between vertical mixing and I c and
introduced the concept of a critical depth (Z cr ) above which phytoplankton respiratory
51
Physical Control of
Ecological Processes
A
suggestion of Yentsch and Garside, made to a congress of pelagic biogeographers in 1986, might serve as a synopsis of the argument presented in this
book:
Are the large-scale biogeographic distributions of oceanic biology a response to seasonal
and spatial patterns of primary production? Geostrophic currents dictate the shape of the
density structure and what we term the degree of baroclinicity is a global mirror of primary
production. The fact is that for at least fifty years we have recognized that light, wind
and temperature regulate phytoplankton growth. We recognize that seasonality in the
primary processes is controlled by stirring of the upper layers of the ocean, and the nutrient
and density fields. Why, then, are we so hesitant to tie the climatological primary process
together with concepts concerning biogeography? One answer might be that we do not have
enough knowledge of primary productivity over large areas of the world’s ocean.
Now, of course, we do have enough knowledge from satellite imagery to begin the
integration we were encouraged to undertake. A little later, Yentsch gave one of the
earliest demonstrations that the CZCS chlorophyll field could be used to demonstrate
the control of algal growth by irradiance and nutrients. He showed that the front of the
migrating North Atlantic spring bloom, as revealed by CZCS, closely matched predictions
obtained from an independent analysis of the seasonal poleward march of the shoaling
of the mixed layer.
How such thinking had much earlier led to the formulation of Sverdrup’s critical
depth theory is well known. Had Sverdrup not left Scripps, as he did in 1948 to return
to Norway, he might not have become involved in thinking about the Ocean Weather
Station “M” observations, and his formulation would now bear another’s name. It is so
fundamental that with or without his initiative it would have become central to biological
oceanographic theory.
Nevertheless, we tend to forget that Sverdrup’s important contribution was to encapsulate, in a formally stated theorem, several concepts that had already been discussed
more generally. Nathansohn (1909), Gran (1931), Riley (1942), and others made major
advances as the old “nitrification” theory of plankton production came to be replaced
by modern understanding of the nitrogen cycle. Prior to Sverdrup, the concept of a
compensation illuminance (I c ) at which photosynthesis of individual cells would exactly
balance cellular respiration had already been evoked from studies done in the Gulf of
Maine (Gran and Braarud, 1935).
Sverdrup (1953), using time-series observations of daily mixing depths and cell counts
made at OWS “M” (66
N2
E) analyzed the interaction between vertical mixing and I c and
introduced the concept of a critical depth (Z cr ) above which phytoplankton respiratory
51