Supporting Information
Dissolved organic carbon uptake by biofilms: some insights into the potential
impact of loss of large woody debris from lowland rivers
DARREN S. BALDWIN*†, KERRY L. WHITWORTH*‡, CLAIRE L. HOCKLEY‡
* The Murray–Darling Freshwater Research Centre, PO Box 991, Wodonga, Vic., Australia,
3689
†
CSIRO Land and Water, PO Box 991, Wodonga, Vic., Australia, 3689
‡
La Trobe University, PO Box 821, Wodonga, Vic., Australia, 3689
S1. Estimating loss of surface area available for biofilm colonisation:
Systematic desnagging, to facilitate river navigation in the middle reaches of the Murray
River in south-eastern Australia, began in 1855 (Treadwell et al. 1999). Initially the work was
conducted by hand or with the aid of bullock teams and shortly thereafter using specialised
snag boats. Three boats working between about 1865 and 1924 removed 3−400 snags per
month (Phillips 1972). A newer desnagging boat, launched in 1911 and in service until at
least 1971, is said to have removed three million snags over its working life (Phillips 1972;
Treadwell et al 1999), although this number may be somewhat exaggerated. In a more recent
channel clearance program 24,500 snags were removed from a 180 km stretch of the Murray
River from Lake Hume to Yarrawonga between 1976 and 1987 (Treadwell et al. 1999). This
gives a total of approximately 3.25 million snags removed from the river system over a
period of 130 years. Assuming that these had been distributed uniformly along the 2200 km
length of river channel from the Hume Dam down to the river mouth, this equates to removal
of approximately 1500 pieces of large woody debris per river kilometre. Given the prolonged
half-life of river red gum wood in water and the reduced input of fresh snags due to extensive
clearing of the riparian zone, it is reasonable to assume that this value represents the current
reduction in snag density from historical levels. Although this may seem an extreme value,
similar densities were recorded by Marsh et al. (1999) in the Edward River in New South
Wales (1300 km−1) and the Albert River in Queensland (1533 km−1); although it must be
noted that they recorded all woody debris greater than 1 m in length and 0.1 m in diameter. In
stark contrast to the historical observation that snags in the Murray River channel were
‘standing up like a regiment of soldiers’ (Mudie 1961), extant large snags (those visible from
aerial surveys) in the central reaches of the Murray River now occur at a density of less than
100 per kilometre (Koehn et al. 2004, Hughes and Thoms 2002).
Log-book records from desnagging boats (reproduced in Phillips 1972) suggest that these
boats focussed on the removal of large pieces of woody debris. From 26 records, the average
snag length was 14 m and the average basal diameter 2 m. The surface area of these snags
can be calculated if we assume a fractal (self-similar) tree design and employ Da Vinci’s
principle that “all the branches of a tree at every stage of its height are equal in thickness to
the trunk” (Eloy 2011). Using an average height of 14 m and diameter of 1.5 m (accounting
for tapering from the measured basal diameter), with division into two daughter branches,
each half the length and diameter of the mother branch, at each node, up to a total of four
branching levels (smaller branches are not likely to persist for an extended period after the
death of the tree), we calculate a total surface area of 66 m2. However, this does not account
for the fissures that occur in aged wood nor for the root mass or any hollows, which may
substantially increase the surface area available for biofilm colonisation. To account for this,
we double the calculated surface area to give an approximate average surface area per snag
removed of 130 m2. Therefore, we estimate that removal of 1500 snags per kilometre has
resulted in loss of 195,000 m2 km−1 of surface area for potential biofilm colonisation.
References:
Eloy C. (2011) Leonardo’s rule, self-similarity, and wind-induced stresses in trees. Physical
Review Letters, 107, 258101.
Hughes V. and Thoms M.C. (2002) Associations between channel morphology and large
woody debris in a lowland river, in: The structure, function and management
implications of fluvial sedimentary systems. Proceedings of an international
symposium, Alice Springs, Australia, September 2002. IAHS publication number 276.
Koehn J.D., Nicol S.J. and Fairbrother P.S. (2004) Spatial arrangement and physical
characteristics of structural woody habitat in a lowland river in south-eastern Australia.
Aquatic Conservation: Marine and Freshwater Ecosystems, 14, 457–464.
Marsh N., Rutherford I. and Jerie K. (1999) Large woody debris in some Australian streams:
Natural loading, distribution and morphological effects. Paper presented at the Second
Australian Stream Management Conference, Adelaide, South Australia, 8–11 February.
Mudie I.M. (1961) Riverboats. Rigby Limited Adelaide, South Australia.
Phillips P. (1972) River Boat Days on the Murray, Darling and Murrumbidgee. Landsdowne
Press Melbourne, Victoria.
Treadwell S., Koehn J. and Bunn S. (1999) Large woody debris and other aquatic habitat, in
Lovett S. and Price P. (eds): Riparian Land Management Technical Guidelines Volume
One: Principles of Sound Management. Canberra: Land and Water Resources Research
and Development Corporation.
S2. Experimental setup
Supporting Information Figure 1. Photograph of the experimental set-up. Note that during
experiments the columns would have been wrapped with aluminium foil.
S3. Arrhenius Plot
-11.5
-12.0
Ln k
-12.5
-13.0
-13.5
-14.0
-14.5
-15.0
0.0033
0.0034
0.0035
1/T (oK)
Supporting Information Figure 2. Arrhenius plot of the natural logarithm of the mean first
order rate constant (as s-1) for the experiments with 5 bioballs as a function of the inverse of
the absolute temeperature. The Arrhenius plot is linear between 15 and 30 oC but deviates
from linearity between 10 and 15 oC.