182
E. D. S. CORNER AND ANTHONY G . DAVIES
concentrations were available just below the euphotic zone (50-100 m
depth). In general, conccntrations were found to be approximately
twice those observed in the northern Sargasso Sea. The proximity of
this supply of nutrients to the euphotic zone meant that any physical or
meteorological process which caused turbulence would bring the
nutrients into the surface waters and thus give rise to outbursts of high
plant production. Such phenomena would probably account for the
patchiness of phytoplankton production observed in this area.
Also of importance to the nitrogen budgets of tropical seas are the
abundant colonies of the blue-green alga Trichodesmium found in these
regions. The ability of these organisms to fix nitrogen directly from
the atmosphere was demonstrated by Dugdalc et al. (1964) though it is
not yet certain whether the algae themselves, or associated organisms
such as bacteria, are responsible for this. Trichodesmium is also able to
assimilate ammonium- and nitrate-nitrogen (Goering et al., 1966) but,
as the plant is normally present in waters which are low in nitrate,
ammonium-nitrogen probably represents the chief supply of the element
which is supplemented by direct nitrogen-fixation.
C . Polar regions
The seasonal variation of nutrient levels in the Antarctic has yetto
be described though plant populations (as measured by chlorophyll a)
appear to undergo normal seasonal cycles (El-Sayed, 1970), maximal
levels occurring during the austral summer (Fig. 13). Although this
region is usually considered to be of generally high productivity,
El-Sayed (1970) found that there was, in fact, a significant geographical
variation, the waters nearer to the land ma.sses being considerably more
fertile (by a factor of 5 ) than the oceanic waters.
The surface waters in the Antarctic move northwards away from the
continent and are replaced by the upwelling of deeper waters so that
nutrient supplies in the area are continually being replenished. Nutrient
levels, therefore, are rarely limiting, nitrate-nitrogen averaging 12.4
pg-atoms11 and phosphate-phosphorus 1-15 pg-atomsll. (El-Sayed,
1970). Concentrations are generally lower north of the Antarctic
convergence where the northward moving waters sink below the
surrounding sub-Antarctic waters ; but El-Sayed has pointed out that
even there the levels of nutrients are generally higher than those
corresponding to the winter maxima of temperate regions. Nevertheless, the convergence does noticeably affect the productivity of the area,
the phytoplankton standing crop to the north of it being much smaller
than that to the south.
E. D. S. CORNER AND ANTHONY G . DAVIES
concentrations were available just below the euphotic zone (50-100 m
depth). In general, conccntrations were found to be approximately
twice those observed in the northern Sargasso Sea. The proximity of
this supply of nutrients to the euphotic zone meant that any physical or
meteorological process which caused turbulence would bring the
nutrients into the surface waters and thus give rise to outbursts of high
plant production. Such phenomena would probably account for the
patchiness of phytoplankton production observed in this area.
Also of importance to the nitrogen budgets of tropical seas are the
abundant colonies of the blue-green alga Trichodesmium found in these
regions. The ability of these organisms to fix nitrogen directly from
the atmosphere was demonstrated by Dugdalc et al. (1964) though it is
not yet certain whether the algae themselves, or associated organisms
such as bacteria, are responsible for this. Trichodesmium is also able to
assimilate ammonium- and nitrate-nitrogen (Goering et al., 1966) but,
as the plant is normally present in waters which are low in nitrate,
ammonium-nitrogen probably represents the chief supply of the element
which is supplemented by direct nitrogen-fixation.
C . Polar regions
The seasonal variation of nutrient levels in the Antarctic has yetto
be described though plant populations (as measured by chlorophyll a)
appear to undergo normal seasonal cycles (El-Sayed, 1970), maximal
levels occurring during the austral summer (Fig. 13). Although this
region is usually considered to be of generally high productivity,
El-Sayed (1970) found that there was, in fact, a significant geographical
variation, the waters nearer to the land ma.sses being considerably more
fertile (by a factor of 5 ) than the oceanic waters.
The surface waters in the Antarctic move northwards away from the
continent and are replaced by the upwelling of deeper waters so that
nutrient supplies in the area are continually being replenished. Nutrient
levels, therefore, are rarely limiting, nitrate-nitrogen averaging 12.4
pg-atoms11 and phosphate-phosphorus 1-15 pg-atomsll. (El-Sayed,
1970). Concentrations are generally lower north of the Antarctic
convergence where the northward moving waters sink below the
surrounding sub-Antarctic waters ; but El-Sayed has pointed out that
even there the levels of nutrients are generally higher than those
corresponding to the winter maxima of temperate regions. Nevertheless, the convergence does noticeably affect the productivity of the area,
the phytoplankton standing crop to the north of it being much smaller
than that to the south.