Seasonal stratification and the spring bloom - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 196

on fish larvae for their food, can also be affected by the timing of the spring bloom;
for instance seabird breeding success is thought to be partially driven by bloom
timing (Frederiksen et al., 2006 ) and the survival of young shrimp in the northwest
Atlantic has been linked to the timing and duration of the spring bloom (Ouellet
et al., 2011). This apparent dependence of some organisms on bloom timing forms
the basis of the well-known match-mismatch hypothesis proposed by the fisheries
scientist David Cushing (Cushing, 1990 ). We do not expect fish larvae or seabirds to
be capable of predicting the onset of stratification; instead it is more likely that they
will have adapted to some long-term mean bloom timing.
We have already noted that Equation (6.26) provides a way of determining when
the condition for the onset of thermal stratification is just met. Inter-annual variabil-
ity in this energy balance reflects variations in Q i (a more cloudy spring would delay
the onset of stratification) and W (a windier spring would delay the onset of
stratification). More subtly, Q i can vary in response to changes in air temperature,
with warmer air helping to increase heat flux into the ocean. Similarly Q i is influenced
by sea surface temperature; a cooler sea (controlled by heat exchanges in the months
prior to spring) will lead to an increase in heat supplied to the sea surface. Year to
year variability in meteorological conditions is the principal source of inter-annual
fluctuations in bloom timing in the open ocean (Waniek, 2003 ), and plays a signifi-
cant role in bloom timing in shelf seas. In the Gulf of Maine, variations in cloudiness
and in the intrusions of nearby slope water (the latter probably a response to non-
local meteorology and the North Atlantic Oscillation) are known to drive changes in
bloom timing (Townsend et al., 1994 ; Thomas et al., 2003 ). In the northern North
Sea the onset of spring stratification has been found to vary over a total range of one
month, with much of the variability attributable to inter-annual changes in wind
stress and air temperature during the spring months (Sharples et al., 2006 ). Such
strong control by atmospheric conditions has important, far-reaching implications in
the context of our changing climate.
There is an additional, important, source of variability in the timing of shelf sea
stratification and the spring bloom. Most shelf seas experience significant changes in
tidal current amplitude in the form of the 14.8 day spring-neap cycle, due to the
interaction of the M 2 and S 2 tidal constituents (see Section 2.5.1 ). Inter-annual shifts
in the phase of the spring-neap cycle can influence the timing of spring stratification,
for instance by delaying the onset of stratification if the stronger mixing of a spring
tide coincides with the long-term mean date of stratification. A double, or prolonged,
spring bloom is possible if the spring tidal currents are able to halt the early progress
of the bloom as shown in the example of Fig. 6.12 . The initial spring bloom, triggered
at the neap tide of April 24, is interrupted and redistributed throughout the whole
water column by strong spring tide mixing around May 3. The establishment of
spring stratification is more likely to occur during the transition from spring to neap
tides (i.e. as the tidal mixing is reducing) than as tides increase from neaps to springs
(Sharples et al., 2006 ).
Note that you can investigate the influences of different tidal and meteorological
conditions on the timing of the spring bloom using the model associated with this topic.