Regions of freshwater influence (ROFIs) - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 294

Figure 9.16 Frequency of the
timing of peak chlorophyll
concentration from an analysis
of data from northern
temperate coastal regions. After
Cloern and Jassby, 2008 ,
courtesy Wiley-Blackwell.

15

10

5

0

Jan Mar May Jul Sep Nov

Month of peak chlorophyll

southward by the density-driven surface flow to complete the circuit. We might
expect that the length of time spent in the dark during the bottom-layer journey
from the mouth to the head of the Bay would be challenging for the phytoplank-
ton. However, experiments carried out at the time of the surveys showed that
the Prorocentrum cells could remain viable for 4 weeks in the dark (Tyler and
Seliger, 1978 ) .

9.8.2 Responses to cycles in stratification

Given the importance of stratification in coastal waters as a driver of increases in
surface primary production, we might expect that the regular cycles of stratifica-
tion in a ROFI will have consequences for phytoplankton growth. Certainly it is
recognised that the seasonal cycles of primary production in freshwater-influenced
coastal waters look very different from the cycles in the open shelf seas (Cloern and
Jassby, 2008 ) . An analysis of the timing of peak chlorophyll concentrations during
the year for a variety of northern temperate coastal regions, many of which are
influenced by freshwater inputs, is illustrated in Fig. 9.16 . While there is some
tendency towards favouring spring, peak chlorophyll can occur at any time of
the year with a notable broad response through the whole spring-summer period.
This pattern is very different from the dominance of spring growth seen away
from the coast in temperate shelf seas. In conducting this analysis, Cloern and
Jassby noted that the often unpredictable nature of the timing of coastal blooms
will have impacts on grazing communities; unlike the open shelf seas, the coastal
consumers do not have the relative convenience of a consistent annual event in the
arrival of food.
While coastal primary production is often viewed as unpredictable, we have seen
that many ROFIs do have stratification-mixing cycles with regular periodicity. Is
there evidence of the primary producers responding to these cycles?
Consider first the spring-neap cycles in ROFI stratification. We know that phyto-
plankton in the open shelf seas can respond on this time scale, so do they within
ROFIs? In Fig. 9.17 , data from Puget Sound, a fjord on the coast of the north-
west United States, provides an example of a degree of correlation between
the spring-neap tidal cycle,

the strength of stratification, primary production