(^) Because phytoplankton blooms in spring occur in temperate coastal areas and in the
subarctic North Atlantic, where scientific ocean observations began, explanations of
the seasonal cycles of productivity and standing stocks of phytoplankton and grazers
became a central dogma of “classical” biological oceanography. In Chapter 11 we will
consider spring blooms in more detail. Briefly, the sequence of events and their
supposed causes are as follows:
(^1) Phytoplankton stock is low through the winter because strong vertical
mixing keeps net losses from the photic zone greater than the possible net
growth, despite sufficient nutrients. Low sun angles and short days
contribute to the balance at low stock by keeping algal growth rates slow.
2 In spring, increased illumination and reduced winds generate some
vertical stratification. This reduces the loss rate from the now better-lighted
surface layer, and a population of phytoplankton builds up. This is called
the spring bloom.
3 As spring wears into summer, the growth of phytoplankton depletes the
nutrients that are no longer rapidly supplied from depth because of density
stratification. Productivity becomes nutrient limited and falls off.
Simultaneously and subsequently, the increase in algae allows an increase in
animals, which eat up the algae. Grazing, reduced algal growth rates, and
sinking of algal cells due to nutrient starvation and agglutination produce a
mid-summer low in algal stocks.
4 By fall, the grazing animals have declined, or have entered non-feeding
resting stages for the forthcoming winter. The first, intermittent storms of
the coming winter usually stir up some nutrients without completely mixing
away the density stratification. The daylength is still moderately long, the
sun is still high, and the gray cloud banks of winter hesitate on the horizon.
The result is a brief but substantial fall bloom.
5 The fall bloom is mixed downward out of existence by the storms of early
winter, which also resupply the surface with nutrients. The sea is plowed for
the next spring’s bloom.
(^) The details of this scenario, and the variant processes in large ecosystems that work
differently, are the principal subjects of many studies and, thus, of models in pelagic
ecology.
Rate Equation Modeling
(^) Rate equations are the inner machinery of most ecosystem models, so rudimentary
understanding of these equations and their solution is essential. A.J. Lotka (1925), an
American mathematical ecologist, and Vito Volterra (1926a & b), an Italian