15.3 Oceanic Fronts and Biological Productivity
459
as the biological factors are distinguished. In Fig. 15.3, the comparison of a
calculated and measured profile, taken before midday, is shown. Neglecting the
biological factor, S, in the modelling causes the largest discrepancy for water
depths between 6 and 16 m. The effect of the differential sinking of individual
particles was found to be of negligible importance for observed profile changes.
A more comprehensive model of the chlorophyll a distribution, including production and loss of phytoplankton mass will be discussed in Sect. 15.5.
15.3 Oceanic Fronts and Biological Productivity
When the temperature, salinity or density do not vary gradually, regions of
high gradients of these properties are observed. These regions are known as
fronts, following the name of similar structures in the atmosphere. There is a
large variety of sizes of oceanic fronts. For example, the front associated with
. the Gulf Stream is long, exhibiting a temperature change of woe in 50 km
near the surface. On the other hand, there are small scale fronts of the order
of kilometres, such as those associated with island wakes or tidal jets between
reefs (Wolanski and Hamner, 1988; Mann and Lazier, 1996).
Fronts are important for the marine environment because of their higher
biological productivity. Dense aggregations of phytoplankton and zooplankton
are usually associated with fronts (Le Fevre, 1986). The physics of large-scale
fronts and their biological productivity have been documented in many papers
and a comprehensive review is given in Mann and Lazier's book (1996). The
influence of tidally mixed fronts on phytoplankton production is also discussed
in Sect. 15.6.
Small-scale frontal systems have not been studied in such detail as the largescale fronts. However, fronts associated with island wakes are common in shallow waters. Eddies resulting downstream of the island generate a secondary
circulation which brings the fine sediments and benthic organisms to the surface near the eddy centre (Wolanski and Hamner, 1988). On coral reefs, the
fronts associated with reef wakes play an important role in the redistribution
of coral eggs and larvae. Initially after spawning, eggs are distributed over the
whole area of the reef. After a few hours, the eggs and larvae usually aggregate
in slicks trapped along the outer edges of reef wakes.
Tide-induced currents in the passages between reefs are usually strong and
generate localized upwelling on the upper continental slope elevating nitrate
and phosphate to the surface (Wolanski et al., 1988). Simultaneously, a tidal jet
through the passage advects the nutrients to the shallow water area. Wolanski
et al. (1988) reported results of measurements and modelling of the tidal jets
transporting nutrients towards the calcareous alga Halimeda banks situated
several kilometres inshore of the reefs. Halimeda can gradually accumulate
nutrients and over a certain time the quantity of upwelled nitrogen finally
becomes more than sufficient to supply the total nitrogen requirements of the
Halimeda vegetation.
459
as the biological factors are distinguished. In Fig. 15.3, the comparison of a
calculated and measured profile, taken before midday, is shown. Neglecting the
biological factor, S, in the modelling causes the largest discrepancy for water
depths between 6 and 16 m. The effect of the differential sinking of individual
particles was found to be of negligible importance for observed profile changes.
A more comprehensive model of the chlorophyll a distribution, including production and loss of phytoplankton mass will be discussed in Sect. 15.5.
15.3 Oceanic Fronts and Biological Productivity
When the temperature, salinity or density do not vary gradually, regions of
high gradients of these properties are observed. These regions are known as
fronts, following the name of similar structures in the atmosphere. There is a
large variety of sizes of oceanic fronts. For example, the front associated with
. the Gulf Stream is long, exhibiting a temperature change of woe in 50 km
near the surface. On the other hand, there are small scale fronts of the order
of kilometres, such as those associated with island wakes or tidal jets between
reefs (Wolanski and Hamner, 1988; Mann and Lazier, 1996).
Fronts are important for the marine environment because of their higher
biological productivity. Dense aggregations of phytoplankton and zooplankton
are usually associated with fronts (Le Fevre, 1986). The physics of large-scale
fronts and their biological productivity have been documented in many papers
and a comprehensive review is given in Mann and Lazier's book (1996). The
influence of tidally mixed fronts on phytoplankton production is also discussed
in Sect. 15.6.
Small-scale frontal systems have not been studied in such detail as the largescale fronts. However, fronts associated with island wakes are common in shallow waters. Eddies resulting downstream of the island generate a secondary
circulation which brings the fine sediments and benthic organisms to the surface near the eddy centre (Wolanski and Hamner, 1988). On coral reefs, the
fronts associated with reef wakes play an important role in the redistribution
of coral eggs and larvae. Initially after spawning, eggs are distributed over the
whole area of the reef. After a few hours, the eggs and larvae usually aggregate
in slicks trapped along the outer edges of reef wakes.
Tide-induced currents in the passages between reefs are usually strong and
generate localized upwelling on the upper continental slope elevating nitrate
and phosphate to the surface (Wolanski et al., 1988). Simultaneously, a tidal jet
through the passage advects the nutrients to the shallow water area. Wolanski
et al. (1988) reported results of measurements and modelling of the tidal jets
transporting nutrients towards the calcareous alga Halimeda banks situated
several kilometres inshore of the reefs. Halimeda can gradually accumulate
nutrients and over a certain time the quantity of upwelled nitrogen finally
becomes more than sufficient to supply the total nitrogen requirements of the
Halimeda vegetation.