(^) Profiles from oxygen electrodes (Fig. 14.20) and optodes match the deep water
column [O 2 ] well above the water–sediment interface. Descending to within a few
millimeters of that surface, approaching the level of no water motion right where
water meets mud, the probe enters a diffusive boundary layer (DBL) in which the
slope of the [O 2 ] matches the initial subsurface profile. This slope, d[O 2 ]/dz, can be
used to estimate the diffusive oxygen uptake rate, DOU = DO d[O 2 ]/dz. DO is the
molecular diffusivity of oxygen in water, a function of temperature – slower when
cold (Armstrong 1979):
(^) Temperature (°C)DO (cm (^2) s−1 × 10−5)
0 0.99
5 1.27
10 1.54
15 1.82
20 2.10
25 2.38
30 2.67
(^) More sophisticated diffusion-reaction models take account of both the fractional
interstitial porosity and an effect of particle shape and packing called tortuosity, which
can be measured from profiles of sediment resistance to electrical current (e.g. Berg et
al. 1998). In general, these models are “zero order”, that is they assume no
dependence on oxygen concentration, and, in fact, biological oxygen utilization does
not depend upon oxygen availability except at very low concentrations (Fig. 14.20),
only on other factors, primarily the availability of organic matter (Cai & Reimers
1995). Typically, TOU is somewhat greater than DOU, because the metabolism of
dispersed larger organisms is included and the larger area is more likely to incorporate
the metabolic “hot spots” in the microbial microstructure. Macrofauna can generate
local sediment ventilation, and electrodes occasionally show effects of this ventilation
as subsurface peaks in oxygen (Glud et al. 1994).
(^) Neither canisters nor electrodes can be readily pressed into sand or gravel
sediments. In part, that problem stimulated development by Peter Berg et al. (2003) of
a non-penetrating TOU method: eddy correlation. An acoustic Doppler velocimeter
(ADV) is suspended from a stand next to an oxygen electrode, both at 10 to 15 cm
from the sediment. Records are accumulated of vertical velocity, w, and [O 2 ]. This is
well above the DBL (Fig. 14.20). Extremely small, but measurable, drops in [O 2 ] due
to eddy advection and mixing out of the lower concentration in the DBL correlate
with upward velocity pulses (∼0.5 cm s−1); rises correspond to oxygen being mixed
down by downward velocity pulses. The magnitudes of these changes are minute but
recordable at high frequencies (e.g. 64 Hz with running averages at 8 Hz used to
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