(Based on
Lalli C M & Parsons T R 1993, Biological Oceanography: An Introduction,
Butterworth Heineman )
1 The marine
phytoplankton is composed of several diverse groups of algae that carry out
autotrophic production and begin the pelagic marine food chain. Photosynthesis
results in the production of high-energy organic materials from carbon dioxide
and water plus inorganic nutrients.
2
Photosynthesis involves a series of interrelated chemical reactions. The light
reactions depend upon chlorophyll and accessory pigments capturing photons of
light, so that radiant energy is converted to chemical energy. The dark reactions
do not require light; they reduce the carbon dioxide and produce high-energy
carbohydrates as end products. Respiration in plants and animals is the reverse
process of photosynthesis, and oxygen is used to release the energy contained
in carbohydrates and carbon dioxide is liberated.
3 All
phytoplankton species require certain inorganic substances to carry out
photosynthesis, including sources of nitrogen and phosphorus (and silica for
diatoms) which may be in concentrations that are low enough to be limiting to
plant production. Some species also require certain organic substances (e.g.
vitamins) for auxotrophic growth, and these also may be present in limiting
concentrations.
4
Estimates of the total phytoplankton crop (standing stock or biomass) in a
particular locality can be determined by measurements of cell numbers, total
volume, or most commonly, by quantity of chlorophyll-a. The rate of
primary production is most often measured by following the uptake of
radioactive 14C in samples of seawater containing phytoplankton.
5 The
amount of photosynthesis increases with light intensity up to a maximum value
known as Pmax which is specific for each species. When light
intensity increases beyond this value, the rate of photosynthesis declines due
to photoinhibition. The light intensity at which plant photosynthesis
(production) exactly equals plant respiration (losses) is the compensation
intensity. Gross photosynthesis describes total photosynthesis; net
photosynthesis is gross photosynthesis less respiratory losses.
6
Photosynthetic responses of phytoplankton species to light can show different
types of adaptation2 of the most common are a) increase in the number of
photosynthetic units (PSU's) and b) increase in the size of the antenna
portions of the existing PSU's
7
Phytoplankton are exposed to differing light intensities as light changes over
the course of a day and as the algae are mixed vertically in the surface layers
of the sea. At the critical depth, photosynthetic gains throughout the water
column are just balanced by respiratory losses in the phytoplankton. If the
depth of water mixing is greater than the critical depth, no net primary
production can take place. Net production occurs only when the critical depth
exceeds the depth of mixing.
8 Growth
rates of phytoplankton are also controlled by the concentrations of essential
nutrients in seawater. Oligotrophic regions have low concentrations of
essential nutrients and therefore low productivity per unit area or volume of
water. Eutrophic waters contain high nutrients and support high numbers of
phytoplankton. Cells in the oligotropihic areas may exhibit high productivity
per cell and be growing near to mmax for
that species( as suggested by the
Redfield ratio of C:N:P of 106:16:1 shown by such organisms) but be rapidly
grazed. The nutrients released from the grazers can be utilized to sustain this
so called "regenerated production" in what is likely to be a closely
coupled system.
9 Each
species of phytoplankton has a particular response to different concentrations
of limiting nutrients, and each has a different maximum growth rate. These
differences and the species-specific responses to different light intensities,
temperatures, salinities and other parameters, mean that heterogeneous and
fluctuating environmental conditions favour different species at different
times and allow many species to coexist in the same body of water. Thus
phytoplankton species diversity can be high in what appears superficially to be
a homogeneous aqueous environment.
10 Solar
radiation and essential nutrient availability are the dominant physical factors
controlling phytoplankton production in the sea. The amount of light varies
with latitude, and the amount of nutrients contained in the euphotic zone(
bottom of which is defined approximately as the depth to which 1% of surface
irradiance penetrates) is largely determined by physical factors controlling
vertical mixing of water.
11
Despite year-round high light intensity, tropical regions are generally low in
productivity because solar heating stabilizes the water column and nutrients
remain at low concentrations within the euphotic zone. Conversely, polar
regions are generally high in nutrients but low in solar radiation except for a
brief period in the summer. Maximum annual productivities are generally found
in temperate latitudes where light and nutrients are both reasonably abundant.
12 The
general latitudinal patterns of primary productivity are altered by a number of
different physical processes that lead to nutrients being redistributed in the
water column in discrete areas. These processes occur on scales varying from
very large (e.g. gyres and continental upwelling), to smaller (e.g. tidal
fronts and rings), to the very small scales of Langmuir circulation in which
only the top few metres of the water column are mixed.
13
Oceanic primary productivity ranges from < 40 to > 100 g C m-2
yr-1. Coastal upwelling regions have the highest productivity with
values of up to 350 g C m-1 yr-1. The standing stock of
phytoplankton in the surface layers of the sea ranges from less than 1 mg chlorophyll-a m-3 to about 20 mg
m -3 during a phytoplankton bloom.
14 The
vertical profile of phytoplankton production changes with season and with
latitude. High surface productivities generally occur in temperate latitudes in
spring and autumn, whereas chlorophyll and productivity maxima occur
considerably deeper in tropical waters.