The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 334

Southern-Oscillation (ENSO). These ENSO events, which occur every few years,
involve a large-scale change in the atmospheric pressure gradient between the south-
east Pacific and Indonesia. The change in pressure gradient is accompanied by a
reduction in the strength of the trade winds which are normally responsible for
maintaining a shallow thermocline along the equatorial and subtropical coast of South
America and a deeper thermocline in the west Pacific. Weaker trade winds allow the
thermocline to relax, becoming deeper on the South American coast so that any coastal
upwelling brings relatively warm, nutrient-poor water to the surface. There is a strong
link between the years of positive temperature anomalies at the Peruvian coast and
reductions in the catch of anchovies (Escribano et al., 2004 ).
The ENSO is a large-scale phenomenon that we can see has impacts on fisheries
because it alters the dynamics of upwelling over a long period; i.e. in a positive
ENSO phase all upwelling events will be poor at supplying nutrients to the sea
surface because of the consistent change in the depth of the thermocline. There are
also ways in which the frequency of short-term wind events within an otherwise
upwelling-favourable system could affect the survival and growth of fish. Within the
California upwelling system observations have shown that first-feeding anchovy
larvae have a better chance of survival if they have access to high densities of large
dinoflagellates within the sub-surface chlorophyll maximum (Lasker, 1975 ).
While the winds are important for supplying nutrients to the shelf, episodes of
calm weather are required for this layer of dinoflagellates to form. Strong winds
(e.g. during an upwelling event) disperse the layer and substantially reduce the food
available to the larvae. The necessary 4-day period of winds 5ms

1 has been

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termed a 'Lasker Event'. 3

Upwelling and bottom water hypoxia
So far we have concentrated on the impacts of nutrient upwelling on the growth of
phytoplankton and subsequent responses of higher trophic levels, including commer-
cially important fish stocks. There are of course other routes that the organic carbon
fixed by the phytoplankton could take. Remember the case of the Louisiana 'Dead
Zone' (Fig. 9.19), where supplies of nutrients from rivers led to increased surface
phytoplankton growth, but also to reduced oxygen concentrations in the bottom
waters as the sinking organic material decayed. Upwelling systems exhibit analogous
behaviour. Figure 10.21a , b illustrate a clear link between dissolved oxygen (DO) and
organic carbon distributions over the Oregon shelf and shelf edge. Organic carbon is
fixed in the surface water in response to upwelled nutrients, but flocculates and sinks
towards the seabed, with hypoxic conditions developing in the bottom water as a
result of the decay of this sinking organic material. This development of low oxygen
bottom waters is a seasonal feature of the western American shelf, correlated with the
seasonal upwelling-favourable winds which prevail in the summer months. The
typical annual cycle in the vertical distribution of oxygen concentration mid-shelf
off Washington, northwestern United States, is shown in Fig. 10.21c . Note how the

3 After Reuben Lasker who suggested this condition for larval survival.