Atlantic Coastal Biome
219
Norway to the west of Ireland. Early in the year, until the end of February, chlorophyll
biomass was uniformly higher over the shelf than over the open ocean, so that almost
the entire shelf break was delimited by the abrupt difference in biomass. During March
1999, a discrete spring bloom occurred on the deep shelf to the southwest of Ireland,
and in April the northern North Sea sustained a spring bloom. By the middle of July
1999, the entire central North Sea had evolved very low surface chlorophyll while the
Norwegian Coastal Current remained a prominent high-chlorophyll feature, separated
from higher chlorophyll over much of the North Sea by blue water that follows the line of
the Norwegian Trench. Finally, in the last months of 1999, chlorophyll biomass in the
Central North Sea increased from the very low values of July and remained in this state
through the following winter.
In April 1998, a strong bloom occurred over the ocean west and north of the British
Isles that was restricted very precisely by the shelf edge, landward of which chlorophyll
took much lower values. This was the opposite condition from that obtaining a few
weeks earlier, when values over shelf depths exceeded those over the deep ocean. But
the shelf edge is by no means always a line of demarcation between different conditions.
In June 1999, an incursion of clear blue water Atlantic water flooded landward over the
wide shelf regions to the west of Cornwall. And on many occasions, large areas of high
chlorophyll concentration cover both shelf and contiguous deep ocean regions.
Such a diverse range of processes represents the interaction between an ideal seasonal
production cycle, modeled as a response to climatological conditions, and between-year
differences in physical forcing. In the ideal model, we can recognize four ecological
seasons: (i) autotrophic growth constrained by light limitation in winter, (ii) a nutrientlimited spring bloom, (iii) stratified conditions during summer with localized dynamic
zones of high chlorophyll, and (iv) renewed autotrophic growth in autumn if stratification breaks down while surface irradiance is still relatively strong. This is, of course, a
shorthand version of the classical plankton calendar for the continental shelf that was
worked out many decades ago at the old European marine biological stations.
We should not expect to observe this ideal sequence in many regions: for instance,
in areas of permanently mixed water where tidal streams are too strong and water is
too shallow for summer stratification to develop. Here, because tidal friction is constantly supplying nitrogen to the water column from benthic regeneration processes, and
because the limiting tidal fronts are constantly transporting nitrogen into the mixed areas,
we should not expect nitrogen to limit a bloom. Nor, under such circumstances, should
we expect the bloom to begin as early in the spring as it does offshore, because of greater
light limitation due to suspensoid: rather, we may expect a midsummer bloom in which
the rate of primary production is a simple function of irradiance. In the mixed area of
the western English Channel, in March the rate of primary production starts to increase
from low values in winter and this increase is maintained steadily until a maximum
rate is achieved in July. An isotonic decrease is subsequently observed until the winter
minimum is reached again in November. During the whole period, diatoms dominate
the large cells and dinoflagellates are negligible. The same seasonal cycle occurs in the
permanently mixed, highly turbid Severn estuary and the macrotidal Bristol Channel,
with a progressive seasonal bias toward a spring bloom in the outer, less turbid region.
Another situation that diverges from the ideal sequence is offered by the anomalous
blooms that may be observed wherever and whenever stratification is locally imposed on
a previously mixed water column; this occurs most frequently as a result of freshwater
buoyancy, imparted by river effluents, provided that surface irradiance is sufficient at the
time to support plant growth. This may occur very early in the year, as discussed later.
A very useful compendium of location-specific time series (1960–1984) of nitrate,
productivity, and zooplankton biomass is offered by Bot et al. (1996). The ideal phytoplankton cycle seems to occur most frequently in the central North Sea. In the southern
219
Norway to the west of Ireland. Early in the year, until the end of February, chlorophyll
biomass was uniformly higher over the shelf than over the open ocean, so that almost
the entire shelf break was delimited by the abrupt difference in biomass. During March
1999, a discrete spring bloom occurred on the deep shelf to the southwest of Ireland,
and in April the northern North Sea sustained a spring bloom. By the middle of July
1999, the entire central North Sea had evolved very low surface chlorophyll while the
Norwegian Coastal Current remained a prominent high-chlorophyll feature, separated
from higher chlorophyll over much of the North Sea by blue water that follows the line of
the Norwegian Trench. Finally, in the last months of 1999, chlorophyll biomass in the
Central North Sea increased from the very low values of July and remained in this state
through the following winter.
In April 1998, a strong bloom occurred over the ocean west and north of the British
Isles that was restricted very precisely by the shelf edge, landward of which chlorophyll
took much lower values. This was the opposite condition from that obtaining a few
weeks earlier, when values over shelf depths exceeded those over the deep ocean. But
the shelf edge is by no means always a line of demarcation between different conditions.
In June 1999, an incursion of clear blue water Atlantic water flooded landward over the
wide shelf regions to the west of Cornwall. And on many occasions, large areas of high
chlorophyll concentration cover both shelf and contiguous deep ocean regions.
Such a diverse range of processes represents the interaction between an ideal seasonal
production cycle, modeled as a response to climatological conditions, and between-year
differences in physical forcing. In the ideal model, we can recognize four ecological
seasons: (i) autotrophic growth constrained by light limitation in winter, (ii) a nutrientlimited spring bloom, (iii) stratified conditions during summer with localized dynamic
zones of high chlorophyll, and (iv) renewed autotrophic growth in autumn if stratification breaks down while surface irradiance is still relatively strong. This is, of course, a
shorthand version of the classical plankton calendar for the continental shelf that was
worked out many decades ago at the old European marine biological stations.
We should not expect to observe this ideal sequence in many regions: for instance,
in areas of permanently mixed water where tidal streams are too strong and water is
too shallow for summer stratification to develop. Here, because tidal friction is constantly supplying nitrogen to the water column from benthic regeneration processes, and
because the limiting tidal fronts are constantly transporting nitrogen into the mixed areas,
we should not expect nitrogen to limit a bloom. Nor, under such circumstances, should
we expect the bloom to begin as early in the spring as it does offshore, because of greater
light limitation due to suspensoid: rather, we may expect a midsummer bloom in which
the rate of primary production is a simple function of irradiance. In the mixed area of
the western English Channel, in March the rate of primary production starts to increase
from low values in winter and this increase is maintained steadily until a maximum
rate is achieved in July. An isotonic decrease is subsequently observed until the winter
minimum is reached again in November. During the whole period, diatoms dominate
the large cells and dinoflagellates are negligible. The same seasonal cycle occurs in the
permanently mixed, highly turbid Severn estuary and the macrotidal Bristol Channel,
with a progressive seasonal bias toward a spring bloom in the outer, less turbid region.
Another situation that diverges from the ideal sequence is offered by the anomalous
blooms that may be observed wherever and whenever stratification is locally imposed on
a previously mixed water column; this occurs most frequently as a result of freshwater
buoyancy, imparted by river effluents, provided that surface irradiance is sufficient at the
time to support plant growth. This may occur very early in the year, as discussed later.
A very useful compendium of location-specific time series (1960–1984) of nitrate,
productivity, and zooplankton biomass is offered by Bot et al. (1996). The ideal phytoplankton cycle seems to occur most frequently in the central North Sea. In the southern