The Ubiquitous “Horizontal Front’’ at the Shallow Pycnocline
43
regional analysis of f-ratios and by the observations of Horne et al. (1989) that nitrate
uptake in the front was 036 nM m
−2 sec
−1 compared with 0.09 on the mixed side
and 0.18 on the stratified side of the front; in nearby oceanic water, the demand was
only 002 nM m
−2 sec
−1 . Phytoplankton demand at the front was partitioned between
nitrate and ammonium, showing that 61% of the daily production within the front was
new production, based on nitrate introduced into the front by the processes discussed
previously, compared with only 41% and 27% in stratified and mixed water, respectively.
We can therefore have reasonable confidence that the remarkably precise coincidence
that has been observed between shelf sea thermal fronts and chlorophyll, as observed on
the Plymouth-Roscoff section in the western English Channel using towed, undulating
instrumentation (Aiken, 1981), does represent in situ growth rather than accumulation.
This conclusion supports the concept of fronts as ecotones, though at the scale of shelfsea fronts the exchange between adjacent waters will be sufficiently dynamic that we
will not expect that edge-effect species could maintain self-sustaining populations within
the front, except perhaps for species at high trophic levels. Thus, there is also some
explanation of the common observations that shelf sea fronts are attractive feeding zones
for fish and seabirds.
Several general mechanisms have been advanced to account for the enhanced availability of nutrients in shelf sea fronts, always on the assumption that here phytoplankton
growth is nutrient limited. The simplest explanation is that as the lunar month advances,
tidal friction increases and the front advances toward its stratified side, progressively
incorporating nutrients from below the stratification. Two other, more general models
were proposed independently, both in 1981. Holligan (1981) suggested that on the stratified side of fronts, phytoplankton growth is released from constraints of light limitation
that would occur below the stratified layer and from nutrient limitation that would occur
on the mixed side of the front. Tett (1981), on the other hand, suggested that vertical
eddy diffusion from bottom friction should be stronger in the front than on either side,
thus constantly supplying nutrients from the near-bottom layer. The model of Franks
and Chen (1996) of processes at the Georges Bank fronts emphasizes, as noted previously,
the decoupling of phytoplankton and herbivores in the mixed zone alongside the front
(compared with the spatial balance achieved on the stratified side of the front), and the
consequent release from grazing pressure on the autotrophs. This is likely to prove to be
a general phenomenon.
Other potential mechanisms exist to explain linear zones of enhanced plankton biomass
in shelf seas. For example, in the North Sea, where the velocity of tidal streams falls
below a critical value, organic material sinks from suspension and forms linear zones
of soft mud at midshelf depths, of which that below the Friesian Front (Baars et al.,
1991) is a good example. In summer, above such benthic fronts the flux of remineralized
nitrogen from the organic sediments is thought to be responsible for a zone of enhanced
phytoplankton production in the water column. However, in practice it may be difficult
to separate this effect from that of tidal fronts that may be aligned with the linear benthic
remineralization zone.
The Ubiquitous “Horizontal Front” at the
Shallow Pycnocline
As biogeographers have long been aware, the most significant environmental gradient
and discontinuity in the ocean is horizontal, between shallow and deeper layers, rather
than in the vertical plane at the frontal systems discussed earlier. This gradient lies at the
seasonal or tropical pycnocline and is globally associated with the change from epipelagic
43
regional analysis of f-ratios and by the observations of Horne et al. (1989) that nitrate
uptake in the front was 036 nM m
−2 sec
−1 compared with 0.09 on the mixed side
and 0.18 on the stratified side of the front; in nearby oceanic water, the demand was
only 002 nM m
−2 sec
−1 . Phytoplankton demand at the front was partitioned between
nitrate and ammonium, showing that 61% of the daily production within the front was
new production, based on nitrate introduced into the front by the processes discussed
previously, compared with only 41% and 27% in stratified and mixed water, respectively.
We can therefore have reasonable confidence that the remarkably precise coincidence
that has been observed between shelf sea thermal fronts and chlorophyll, as observed on
the Plymouth-Roscoff section in the western English Channel using towed, undulating
instrumentation (Aiken, 1981), does represent in situ growth rather than accumulation.
This conclusion supports the concept of fronts as ecotones, though at the scale of shelfsea fronts the exchange between adjacent waters will be sufficiently dynamic that we
will not expect that edge-effect species could maintain self-sustaining populations within
the front, except perhaps for species at high trophic levels. Thus, there is also some
explanation of the common observations that shelf sea fronts are attractive feeding zones
for fish and seabirds.
Several general mechanisms have been advanced to account for the enhanced availability of nutrients in shelf sea fronts, always on the assumption that here phytoplankton
growth is nutrient limited. The simplest explanation is that as the lunar month advances,
tidal friction increases and the front advances toward its stratified side, progressively
incorporating nutrients from below the stratification. Two other, more general models
were proposed independently, both in 1981. Holligan (1981) suggested that on the stratified side of fronts, phytoplankton growth is released from constraints of light limitation
that would occur below the stratified layer and from nutrient limitation that would occur
on the mixed side of the front. Tett (1981), on the other hand, suggested that vertical
eddy diffusion from bottom friction should be stronger in the front than on either side,
thus constantly supplying nutrients from the near-bottom layer. The model of Franks
and Chen (1996) of processes at the Georges Bank fronts emphasizes, as noted previously,
the decoupling of phytoplankton and herbivores in the mixed zone alongside the front
(compared with the spatial balance achieved on the stratified side of the front), and the
consequent release from grazing pressure on the autotrophs. This is likely to prove to be
a general phenomenon.
Other potential mechanisms exist to explain linear zones of enhanced plankton biomass
in shelf seas. For example, in the North Sea, where the velocity of tidal streams falls
below a critical value, organic material sinks from suspension and forms linear zones
of soft mud at midshelf depths, of which that below the Friesian Front (Baars et al.,
1991) is a good example. In summer, above such benthic fronts the flux of remineralized
nitrogen from the organic sediments is thought to be responsible for a zone of enhanced
phytoplankton production in the water column. However, in practice it may be difficult
to separate this effect from that of tidal fronts that may be aligned with the linear benthic
remineralization zone.
The Ubiquitous “Horizontal Front” at the
Shallow Pycnocline
As biogeographers have long been aware, the most significant environmental gradient
and discontinuity in the ocean is horizontal, between shallow and deeper layers, rather
than in the vertical plane at the frontal systems discussed earlier. This gradient lies at the
seasonal or tropical pycnocline and is globally associated with the change from epipelagic