190
J. E. a. RAYMONT
benthic microflora and macroflora has probably been underestimated.
Oceanic areas a t low latitudes may show, over short periods, high rates
of primary production owing to temporary nutrient enrichment, but in
general the low level of the major plant nutrients (nitrate and phosphate) in tropical and subtropical areas, with the permanent thermocline restricting vertical mixing, markedly limits annual productivity,
despite the much greater depth of the illuminated zone. In temperate
and sub-polar regions, although production rates vary markedly over the
year owing to the great seasonal differences in depth of light penetration,
the greater nutrient concentration leads to increased annual production.
At high latitudes, very high rates of production may hold for short
periods, but the time of abundance tends to be short. Productivity in
the Antarctic is high; in the highest Arctic latitudes, however, the annual
production is very low. Upwelling areas in lower latitudes show greatly
increased production.
The effect of temperature on productivity is essentially indirect since
its main influence is on the stabilization of the upper water layers. In a
few areas salinity differences may also assist in stabilizing the upper
water layers and thus may affect production. The establishment of a
seasonal thermocline sharply limits production but the drop in phytoplankton crop in temperate and higher latitudes during early spring is
more often due to grazing than to nutrient lack. The effect of a discontinuity layer in reducing replenishment of nutrients from deeper
layers has its main effect on phytoplankton production during the
summer period. The direct regeneration of nutrients within the euphotic
zone is therefore of primary importance in production over the summer
period at higher latitudes; it is always of great significance at lower
latitudes. Although trace elements and organic substances are required
by many marine algae, there is little evidence for believing that decreased concentrations of these substances affect total production,
except perhaps temporarily in some tropical oceanic areas. The varying
amounts of such materials between inshore and offshore waters may,
however, influence the composition of the phytoplankton population.
In general, areas of high primary production have large standing
crops of algae; the temporal variations in crop, especially at higher latitudes also usually parallel the changes in production, except that heavy
grazing may reduce the crdp over summer. High densities of zooplankton
are found only in regions of richer phytoplankton; a sparse distribution
of algal cells involves the herbivorous zooplankton in excessive searching for food so that less energy is available for growth and reproduction.
Some degree of “wastefulness” in the grazing of herbivores on the
phytoplankton is likely at higher latitudes and is more generally true at
all latitudes in neritic regions. The “wastefulness” arises mainly from
J. E. a. RAYMONT
benthic microflora and macroflora has probably been underestimated.
Oceanic areas a t low latitudes may show, over short periods, high rates
of primary production owing to temporary nutrient enrichment, but in
general the low level of the major plant nutrients (nitrate and phosphate) in tropical and subtropical areas, with the permanent thermocline restricting vertical mixing, markedly limits annual productivity,
despite the much greater depth of the illuminated zone. In temperate
and sub-polar regions, although production rates vary markedly over the
year owing to the great seasonal differences in depth of light penetration,
the greater nutrient concentration leads to increased annual production.
At high latitudes, very high rates of production may hold for short
periods, but the time of abundance tends to be short. Productivity in
the Antarctic is high; in the highest Arctic latitudes, however, the annual
production is very low. Upwelling areas in lower latitudes show greatly
increased production.
The effect of temperature on productivity is essentially indirect since
its main influence is on the stabilization of the upper water layers. In a
few areas salinity differences may also assist in stabilizing the upper
water layers and thus may affect production. The establishment of a
seasonal thermocline sharply limits production but the drop in phytoplankton crop in temperate and higher latitudes during early spring is
more often due to grazing than to nutrient lack. The effect of a discontinuity layer in reducing replenishment of nutrients from deeper
layers has its main effect on phytoplankton production during the
summer period. The direct regeneration of nutrients within the euphotic
zone is therefore of primary importance in production over the summer
period at higher latitudes; it is always of great significance at lower
latitudes. Although trace elements and organic substances are required
by many marine algae, there is little evidence for believing that decreased concentrations of these substances affect total production,
except perhaps temporarily in some tropical oceanic areas. The varying
amounts of such materials between inshore and offshore waters may,
however, influence the composition of the phytoplankton population.
In general, areas of high primary production have large standing
crops of algae; the temporal variations in crop, especially at higher latitudes also usually parallel the changes in production, except that heavy
grazing may reduce the crdp over summer. High densities of zooplankton
are found only in regions of richer phytoplankton; a sparse distribution
of algal cells involves the herbivorous zooplankton in excessive searching for food so that less energy is available for growth and reproduction.
Some degree of “wastefulness” in the grazing of herbivores on the
phytoplankton is likely at higher latitudes and is more generally true at
all latitudes in neritic regions. The “wastefulness” arises mainly from