Indian Ocean Coastal Biome
317
mixed layer in boreal summer. Mixed-layer chlorophyll is 03–06 mg chl m
−3 during the
Southwest Monsoon and 05–10 mg chl m
−3 during the Northeast Monsoon. A nitratenitrogen maximum in the premonsoon period (February–March) may be associated
with nitrogen fixation by heavy blooms at that season of Oscillatoria erythraea. Where
the converging flows of the East African Current and the reversed Somali Current of
boreal winter leave the coast, upwelling occurs in the retroflection area at about 2
S
(Kabanova, 1968); an algal bloom associated with this retroflection is seen in some
satellite images, at the base of an eastward filament aligned along the equator (Longhurst,
1993). The mesozooplankton population achieves maximum biomass toward the end of
the Northeast Monsoon and—not coincidentally—it has been noted that fish and large
invertebrates tend to maximize their reproductive effort at this season.
On the Madagascar shelves, at all seasons except the boreal summer monsoon, chlorophyll accumulation is stronger on the west coast, facing the Madagascar Channel, than
on the east coast that faces the open ocean. The seasonal cycle of accumulation on the
east coast is but a part of the strong bloom that occurs within the SEC in boreal summer
(see ISSG), in which large eddies are seen to propagate toward the west, enveloping the
entire eastern seaboard of Madagascar from May until October. Off the western coast, the
chlorophyll biomass associated with the eddy field is almost permanent, and oligotrophic
conditions obtain only very briefly during the boreal autumn intermonsoon period; by
mid-December, accumulation recommences (Saetre and Jorge Da Silva, 1984). However,
coastal blooms observed in level 3 images may simply reflect the effluent from the rivers
draining the coastal lowlands on the western side of the island. Certainly, on the northeast coast, near Nosy-Bé, it has been observed that estuarine circulation on the shelf
dominates nutrient dynamics, and the duration of the local bloom. The picoplankton
fraction in the offshore blooms in the NE Madagascar Channel has been assessed at
∼48% of biomass and as providing ∼54% of total primary production (Magazzu et al.,
1985). The upwelling cell that occurs intermittently off the southeast cape of Madagascar
(see earlier discussion) is quite frequently observed in satellite images to induce enhanced
chlorophyll that is subsequently entrained toward the east.
In the south, over the Agulhas Bank, there is good information on the seasonal succession of primary production and how it is partitioned between nanophytoplankton
and >15 − m cells of the “net” phytoplankton. This is a typical temperate shelf production cycle even if, as Huggett and Richardson (2000) note, inshore coastal upwelling
and dynamic upwelling along the eastern margin do support relatively high chlorophyll
biomass. In winter and spring, the mixed layer is deeper than the euphotic zone; nitrate
and chlorophyll are thoroughly mixed throughout the euphotic zone while productivity is low (3–4 mgC m
−3 hr
−1 ), and restricted to the upper 5–15 m of the mixed layer
(McMurray et al., 1993). In austral summer (December–January), the euphotic zone is
thermally stratified and a deep chlorophyll maximum (< 60 mg chl m
−3 ) forms at the
depth of maximum nitrate gradient, at about half the euphotic depth of 70–80 m. The
DCM lies at the nitracline and is approximately coincident with the depth of maximum
production rate (5–6 mgC m
−3 hr
−1 ). Between these two seasons, a spring diatom bloom
occurs when irradiance at the sea surface increases, and when turbulence in the mixed
layer decreases before the summer, shallow thermocline is established. Thus, production
is light-limited in winter, nutrient-limited in summer.
Imposed on this classical midlatitude cycle on the central Agulhas Bank, episodic uplift
of the thermocline to <30 m occurs during shelf-edge upwelling events and the intrusion
of cold bottom water up over the bank. These events result in episodic blooms of diatoms
during the relatively oligotrophic summer period. During most of the year, small cells
(<15 m) contribute 60–95% of all production, but during the onset of stabilization in
spring and the summer bloom events, 60–85% of total production is by larger diatoms
(Chaetoceros, Bacteriastrum, and Nitzschia). Examination of the serial chlorophyll images
317
mixed layer in boreal summer. Mixed-layer chlorophyll is 03–06 mg chl m
−3 during the
Southwest Monsoon and 05–10 mg chl m
−3 during the Northeast Monsoon. A nitratenitrogen maximum in the premonsoon period (February–March) may be associated
with nitrogen fixation by heavy blooms at that season of Oscillatoria erythraea. Where
the converging flows of the East African Current and the reversed Somali Current of
boreal winter leave the coast, upwelling occurs in the retroflection area at about 2
S
(Kabanova, 1968); an algal bloom associated with this retroflection is seen in some
satellite images, at the base of an eastward filament aligned along the equator (Longhurst,
1993). The mesozooplankton population achieves maximum biomass toward the end of
the Northeast Monsoon and—not coincidentally—it has been noted that fish and large
invertebrates tend to maximize their reproductive effort at this season.
On the Madagascar shelves, at all seasons except the boreal summer monsoon, chlorophyll accumulation is stronger on the west coast, facing the Madagascar Channel, than
on the east coast that faces the open ocean. The seasonal cycle of accumulation on the
east coast is but a part of the strong bloom that occurs within the SEC in boreal summer
(see ISSG), in which large eddies are seen to propagate toward the west, enveloping the
entire eastern seaboard of Madagascar from May until October. Off the western coast, the
chlorophyll biomass associated with the eddy field is almost permanent, and oligotrophic
conditions obtain only very briefly during the boreal autumn intermonsoon period; by
mid-December, accumulation recommences (Saetre and Jorge Da Silva, 1984). However,
coastal blooms observed in level 3 images may simply reflect the effluent from the rivers
draining the coastal lowlands on the western side of the island. Certainly, on the northeast coast, near Nosy-Bé, it has been observed that estuarine circulation on the shelf
dominates nutrient dynamics, and the duration of the local bloom. The picoplankton
fraction in the offshore blooms in the NE Madagascar Channel has been assessed at
∼48% of biomass and as providing ∼54% of total primary production (Magazzu et al.,
1985). The upwelling cell that occurs intermittently off the southeast cape of Madagascar
(see earlier discussion) is quite frequently observed in satellite images to induce enhanced
chlorophyll that is subsequently entrained toward the east.
In the south, over the Agulhas Bank, there is good information on the seasonal succession of primary production and how it is partitioned between nanophytoplankton
and >15 − m cells of the “net” phytoplankton. This is a typical temperate shelf production cycle even if, as Huggett and Richardson (2000) note, inshore coastal upwelling
and dynamic upwelling along the eastern margin do support relatively high chlorophyll
biomass. In winter and spring, the mixed layer is deeper than the euphotic zone; nitrate
and chlorophyll are thoroughly mixed throughout the euphotic zone while productivity is low (3–4 mgC m
−3 hr
−1 ), and restricted to the upper 5–15 m of the mixed layer
(McMurray et al., 1993). In austral summer (December–January), the euphotic zone is
thermally stratified and a deep chlorophyll maximum (< 60 mg chl m
−3 ) forms at the
depth of maximum nitrate gradient, at about half the euphotic depth of 70–80 m. The
DCM lies at the nitracline and is approximately coincident with the depth of maximum
production rate (5–6 mgC m
−3 hr
−1 ). Between these two seasons, a spring diatom bloom
occurs when irradiance at the sea surface increases, and when turbulence in the mixed
layer decreases before the summer, shallow thermocline is established. Thus, production
is light-limited in winter, nutrient-limited in summer.
Imposed on this classical midlatitude cycle on the central Agulhas Bank, episodic uplift
of the thermocline to <30 m occurs during shelf-edge upwelling events and the intrusion
of cold bottom water up over the bank. These events result in episodic blooms of diatoms
during the relatively oligotrophic summer period. During most of the year, small cells
(<15 m) contribute 60–95% of all production, but during the onset of stabilization in
spring and the summer bloom events, 60–85% of total production is by larger diatoms
(Chaetoceros, Bacteriastrum, and Nitzschia). Examination of the serial chlorophyll images