Geoscience Reference
In-Depth Information
water, a reduction in pH in the latter will put addi-
tional constraints on the animals' ability to compen-
sate. This is discussed further in Section 9.5.
Glud 2004). Such temporal variability in dissolved
constituents, particularly O
2
, gives rise to the devel-
opment of microniches where sharp gradients in
pH and CO
2
may be found. Hulth
et al.
( 2002 )
reported a sharp pH gradient of 1.5 units (NBS
scale) over a distance of 1.1 mm across a ribbon
worm (phylum Nemertea) burrow. In a similar
study, Zhu
et al.
(2006b) showed the development of
pH minima around the walls of actively ventilated
burrows of the polychaete
Nereis succinea
with a pH
in the burrow similar to that of the overlying water.
The authors termed this feature a growing 'low pH
radial halo' while the pH in uninhabited burrows
reverted to the surrounding porewater values
within 2 days (Zhu
et al.
2006b ). Burrow ventilation
by the polychaete presumably enhanced H
+
production in this region of the sediment by intro-
ducing more energetically favourable electron
acceptors for organic matter mineralization. In
addition to ventilation, faunal activity may trans-
port organic matter into subsurface sediments lead-
ing to enhanced H
+
production. For example, a
small (5 mm × 15 mm), low-pH hot spot in a coastal
sediment sample was attributed to the mineraliza-
tion of faecal aggregates in an abandoned
Nemertea
spp. burrow (Hulth
et al.
2002 ). Similarly, Zhu
et al.
(2006b) showed a rapid reduction of pH associated
with the decay of a dead
Nereis
spp. polychaete
(pH
T
minimum ~5.9). This low-pH feature was
indistinguishable from the surrounding sediments
within 5 days (pH
T
~ 6.6).
Two-dimensional measurements using optodes
have now revealed that whilst the burrows may
reduce the pH within the surrounding sediments,
the pH of the water in the burrow itself is often
maintained above that of the surrounding sedi-
ment (Fig. 9.3). However, burrow-water pH also
differs substantially between organisms. For exam-
ple, pH
T
in active burrows of
Nereis succinea
is
maintained at the same level as overlying water
(~8.2), while burrows of the polychaete
Nephthys
incisa
were maintained at lower pH
T
(~7.4; Zhu
et al.
2006b ). Whilst Hulth
et al.
( 2002 ) showed that
active nemertean burrows are maintained at an
even lower pH
NBS
of 6.5, Zhu
et al.
( 2006a ) showed
that the
p
CO
2
in active
N. incisa
burrows was ele-
vated with respect to overlying water, but lower
than in the surrounding sediments. They also
9.3 The impact of macrofaunal activity
on microbially driven geochemical
processes
As stated above, the distribution of pH and
p
CO
2
within the sediment is largely determined by micro-
bial and abiotic processes which in turn depend on
the supply of various substrates (e.g. organic mat-
ter, NO
3
-
, etc.). The supply of these substances is
highly variable both spatially and temporally, and
this variability is often associated with the activity
of sediment fauna. One activity that has a large
effect on pH is the building and ventilation of per-
manent or semi-permanent burrows. These bur-
rows increase the surface area between the reduced
sediment and the overlying water, create additional
habitats for important microbial groups, and pro-
vide a mechanism for the active transport of organic
matter and solutes into and out of the sediment. All
of these effects enhance the degradation of organic
matter and turnover and potentially lead to a reduc-
tion in sediment pH. For example, Aller and Yingst
(1978) showed that redox cycling in the burrow
walls of the polychaete
Amphitrite ornata
was sub-
stantially enhanced compared with surrounding
sediments. The timescale over which macrofauna
affect the chemical environment of marine sedi-
ments is relatively fast. A study following the
in situ
transport and biological incorporation of
13
C-la-
belled organic material in sediments showed that
this process is very rapid; the tracer was incorpo-
rated into macrofauna and bacteria to a depth of 10
cm within 3 days (Witte
et al.
2003 ).
In addition to the effects mentioned above, bur-
row ventilation can also drive pH down by actively
enhancing the supply of O
2
to anoxic sediments. For
example, burrow ventilation by the polychaete
Arenicola marina
was estimated to account for up to
25% of the O
2
supplied to intertidal sediments in the
southern North Sea (de Beer
et al.
2005 ). Burrow
ventilation can also be temporally variable with diel
patterns in O
2
penetration depth and O
2
uptake by
sediments having been attributed to the activity of
the polychaete
Nereis diversicolor
( Wenzhöfer and
