Regions of freshwater influence (ROFIs) - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 295

Figure 9.17 Variability of
stratification and primary
production during spring and
summer 1966 in Puget Sound.
(a) Tidal amplitude, indicating
the spring-neap variability. (b)
Stratification, expressed as the
density difference between a
depth of 25 metres and the sea
surface. (c) Chlorophyll,
integrated from the surface to
the 1% light level. (d) Rate of
primary production (carbon
uptake) integrated from the
surface to the 1% light level.
Based on (Winter et al., 1975 )
with permission from Springer.

(a)

4

2

(b)

2

0

(c)

100

0

(d)

2

1

0

1 April 1 May 1 June

Date in 1966

rates and the concentrations of surface layer chlorophyll. Note the responses
to the sequence of neap tides between May 14 and June 25. Similar behaviour
in surface chlorophyll responding to a spring-neap cycle has been reported in
the York River, a tributary of Chesapeake Bay (Sin et al., 1999 ), within San
Francisco Bay (Cloern , 1991 ) , and in the coastal plume of the Fraser River (Harrison
et al., 1991 ).
We should not be surprised that phytoplankton growth can respond to stratifica-
tion-mixing cycles driven by the spring-neap tidal cycle in ROFIs. With cell doubling
times of typically 1 to 2 per day, the timing of the spring-neap cycle is well suited to
driving such a growth response. But what about tidal straining? Are there examples
of primary production responding to a modulation of stratification on a semi-diurnal
time scale? This is a difficult question to address, not least because, as we saw in
Chapter 5 , the usual method of measuring primary production rates requires sample
incubations over several hours. One tantalising example comes from Liverpool Bay,
with measurements taken over one tidal cycle in a region we now know to experience
tidal straining. Observations of both autotrophic and heterotrophic urea assimilation
rates (shown in Fig. 9.18a ), alongside knowledge of the cycling of stratification
( Fig. 9.18b ), show increases in assimilation rates correlating with the breakdown
and subsequent re-establishment of stratification either side of a mixed period.