The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 342

compared to 25% and 35% in the open ocean and on the shelf. In Fig. 10.25d -f a
detailed analysis of the phytoplankton community based on data from a flow
cytometer shows that the open ocean thermocline was dominated by small
cyanobacteria. The smallest were Prochlorococcus at about 0.5-0.7

m size, and were
mainly found in the open ocean. Synechococcus, larger at 0.8-1.5

m

m, had their
maximum concentrations in the surface layer of the shelf sea. Within the shelf sea
thermocline, and particularly at the shelf edge in a region with very few cyanobac-
teria, small eukaryotes (2-5

m

m size) were found in large numbers.
The horizontal and vertical gradients in the phytoplankton community shown in
Fig. 10.25 appear to be consistent with the changes in the vertical supply of nitrate.
Remember from Chapter 5 that small cells are particularly well adapted to life in
nutrient-deplete environments, such as the thermocline of the open ocean where
nitrate fluxes are around 0.1 mmol m 2 d 1 , and the surface layer of the shelf sea
where nitrate is limiting and the phytoplankton are almost entirely dependent on
ammonium as the nitrogen source. At the base of the shelf sea thermocline, where the
nitrate flux is greater at 1-2 mmol m 2 d 1 , the community begins to shift to larger
cells. In the region with the largest supply of nitrate, at the point on the shelf edge
where the internal tide breaks, we find the largest phytoplankton. However, the
contrasts in nitrate supply can only be a part of the explanation. In Chapter 5 we
noted that small cells will always have an advantage in the competition for nutrients;
the growth of larger cells probably requires a relative reduction in the grazing pressure
( Section 5.2.4 ). Preliminary evidence at the Celtic Sea shelf edge has shown that the
numbers of small copepods do not appear to exhibit a response at the shelf edge
(Sharples et al., 2009 ). However, the complete picture of how the phytoplankton
community contrasts develop in this region has yet to be determined, though the
marked gradients in ecology and physics in the region perhaps provide an ideal
environment to assess the dynamics of the phytoplankton and grazer communities.

m

10.8.3 A possible link to the fish at the Celtic Sea shelf edge

There are many examples worldwide of high zooplankton biomass on ocean margins
that are also important fishing grounds (see, for instance, the Bering Sea 'Green Belt'
(Springer et al., 1996 ), off Vancouver Island (Mackas et al., 1997 ), the Mid Atlantic
Bight (Cosper and Stepien, 1984 ), the shelf edge of southeastern Australia (Young
et al., 2001 ) and the Patagonian shelf (Sabatini and Colombo, 2001 ). All of the
research we have been involved in along the shelf edge of the northwest European
continental margin has been carried out against a background of high fishing activity,
targeting spawning stocks of mackerel, horse mackerel, blue whiting, hake, monkfish
and megrim. Regional data on the distribution of fish eggs and larvae show that the
shelf edge is a site for spawning fish along most of the enhanced chlorophyll band of
Fig. 10.24 (Ibaibarriaga et al., 2007 ). The persistent, northward slope current along the
northwest European continental margin is implicated in the transport of the larvae
back to the adult grounds (Reid, 2001 ), analogous to the role of advection around
Georges Bank ( Fig. 8.21 ). The phytoplankton community arising from the internal