The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 335

(a)

0

300

100

175

O 2
mmol m -3

50

200

(b) 0

50

100

25

POC

mmol m -3

0

200

125º00 W 124º40 W 124º20 W

Longitude

(c)

0 0

ml l -1

120 110

7 6

%

5

50

3

50 50

2

100 100

JFM A M JJ ASONDJ

Month

J FM A M J J A S O N DJ

Month

Figure 10.21 See colour plates version of (a) and (b). (a) Transect of dissolved oxygen
concentration across the shelf and shelf edge off Oregon. The bold white contour line marks
the extent of hypoxic bottom water; (b) Transect of particulate organic carbon (POC) across
the Oregon shelf and shelf edge; (c) Annual cycle of the dissolved oxygen concentration (left)
and % saturation (right) on the Washington shelf. (a) and (b) are high resolution observations
collected in May 2001 during a period of upwelling-favourable winds. After Hales et al., 2006 ,
courtesy of the American Geophysical Union. (c) is adapted from Landry et al. ( 1989 ), with
permission from Elsevier.

surface water achieves super-saturated D.O. conditions in summer, due to enhanced
primary production in response to the upwelling of nutrients. Below the productive
surface layer the oxygen concentration becomes hypoxic (D.O. saturation between
1% and 30%). While upwelling of deep water from off the shelf edge can introduce
water with low oxygen concentrations, the hypoxia in the bottom waters of
the California upwelling system in summer is attributed to the decay of organic
carbon previously fixed in response to nutrients supplied by upwelling (Connolly
et al., 2010 ).