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Biostabilization and erodibility of cohesive sediment depositsin wildfire-affected streams

M. Stone a,*, M.B. Emelko b, I.G. Droppo c, U. Silins d

aDepartment of Geography and Environmental Management, University of Waterloo, Waterloo, Ontario, Canada N2L3G1bDepartment of Civil and Environmental Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L3G1cNational Water Research Institute, Environment Canada, Burlington, Ontario, Canada L7R4A6dDepartment of Renewable Resources, University of Alberta, Alberta, Canada T6G2H1

a r t i c l e i n f o

Article history:

Received 10 May 2010

Received in revised form

8 September 2010

Accepted 12 September 2010

Available online 21 October 2010

Keywords:

Wildfire

Sediment transport

Biostabilization

Cohesive sediment

Colmation

Critical shear stress

Treatability

Water treatment

* Corresponding author.E-mail address: mstone@fes.uwaterloo.ca

0043-1354/$ e see front matter ª 2010 Elsevdoi:10.1016/j.watres.2010.09.016

a b s t r a c t

The erosion characteristics and bed stability of wildfire-affected stream sediment were

measured in an annular flume. Biofilms were grown in the flume on cohesive streambed

sediments collected from a wildfire affected stream and a reference undisturbed stream in

southern Alberta, Canada. Examined factors that influence sediment erosion, settling and

bed stability included applied shear stress, geochemical and physical properties of the

sediment, floc structural characteristics and consolidation period (2, 7, 14 days). Erosion

characteristics and sediment properties were strongly influenced by wildfire, consolidation

period and bed biostabilization. The fire-modified sediment was more resistant to erosion

than the referenceunburned sediment. Settling velocitieswere lower in theburnedsediment

due to higher organic content and porosity. The critical shear stresses for erosion were 1.6

and 1.8 times higher for the burn-associated sediment after 7 and 14 days of consolidation.

The differences are related to the greater degree and spatial extent (depth) of biofilm

attachment in the burned sediment. Erosion depths were 4e8 times higher in burned sedi-

ment as a result of wildfire-associated biostabilization.

ª 2010 Elsevier Ltd. All rights reserved.

1. Introduction stability of cohesive sediment deposits is governed by factors

In aquatic systems, many contaminants of concern are bound

to and transported by cohesive sediment (inorganic and

organic particles<63 mm). Depending upon the hydrodynamic

and biogeochemical characteristics of the system, fine-

grained materials flocculate in the water column (Droppo,

2001) and upon settling are the building blocks of sediment

deposits referred to as surficial fine-grained laminae (SFGL)

(Stone and Droppo, 1994). These deposits contribute to both

external and internal colmation that occurs at the interface

between ground water and surface water (Brunke, 1999). The

(M. Stone).ier Ltd. All rights reserved

such as electrochemical reactions (Mehta, 1989), consolida-

tion (Droppo and Amos, 2001), dewatering (Tolhurst et al.,

2000) and biostabilization (Dade and Norwell, 1990; Paterson,

1997). When the critical shear stress for erosion (sc) is excee-

ded (Stone et al., 2008), these deposits are remobilized within

the water column where they can adversely impact down-

stream lotic environments (Wood and Armitage, 1997).

In aquatic systems, sediment resuspension is a function

of shear stress and sediment characteristics such as grain

size, density and mineralogy (Partheniades, 1990). Benthic

flora and fauna are also being increasingly recognized as

.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 2 1e5 3 4522

important contributors to sediment stability (Paterson, 1997).

Benthic bacteria, microalgae and macrofauna secrete poly-

mers (extracellular polymeric substances; EPS) that bind

mineral grains (biostabilization) together in a mucilaginous

matrix (Dade and Norwell, 1990; Paterson, 1997). For bio-

stabilization to occur, threshold concentrations of organic

compounds that drive microbial metabolism must be avail-

able for energy maintenance and microbial enzyme induc-

tion (Schmidt et al., 1985). Biofilms develop on a variety of

interfaces by attachment processes; aquatic sediments, in

particular, provide excellent substrata for biofilm growth

(Neu, 1994).

The development of biofilms on aquatic sediments can

change the characteristics of deposited sediment (i.e. particle

structure, morphology, size, porosity, shape, degree of consol-

idation) and influence erosion rates (Droppo and Amos, 2001;

Droppo et al., 1997; Lau, 1995). Several studies have demon-

strated that biostabilization can significantly increase the

energy required to erode sediments by horizontal shear (Amos

et al., 2004; Gerbersdorf et al., 2008). While the influence of

biogenic sediment stabilization on sediment erosion and

contaminant transport has been extensively examined in

marine environments (Underwood and Paterson, 2003; Friend

et al., 2003), much less is known about the role of biofilms in

stabilizing sediment deposits in freshwater systems (Droppo

et al., 2007; Droppo, 2009; Gerbersdorf et al., 2008); particularly

those formed in streams draining forested landscapes

impacted by severe, landscape-altering disturbances such as

wildfire.

The frequency and severity of large-scale natural distur-

bances such as wildfire in many forested regions of the globe

has significantly increased in recent decades (Westerling et al.,

2006). Becauseof the severity andmagnitudeofwildfire related

landscape disturbances, sediment fluxes (Silins et al., 2008,

2009) are modified at rates and magnitudes that cause

profound and often irreversible changes in river system func-

tion (DeBanoet al., 1998; Bladon et al., 2008) anddrinkingwater

treatability (Emelko et al., 2011). For example, Blake et al. (2009)

demonstrated that post-fire sediment and nutrient transport

dynamics in streams are strongly related to coarsening of the

effective particle size distributions in burned material via the

aggregation of fines into composite particles. They reported

that burned composite particles (predominantly <63 mm) had

higher concentrations of bio-available phosphorus than

unburned sediment. The subsequent increased flux of bio-

available phosphorus and terrestrial organic matter from hill-

slopes to streamsproducedbywildfiredisturbance increase in-

streamstorageandavailability of particle-associatednutrients

(Petticrew et al., 2006), which increases biofilm formation and

growth rates, particularly in areaswhere the forest canopyhas

been lost (Minshall, 2003). Little information currently exists

regarding biofilm formation and its potential role in stabilizing

cohesive sediment deposits and associated contaminants in

streams draining fire impacted catchments. Given the

increasing severityandspatial extentof landscapedisturbance

by wildfire at the global scale and its impacts on sediment

availability, transport and storage in streams, knowledge of

processes that govern sediment transport is critical to quantify

and model sediment and associated contaminant fluxes from

fire impacted watersheds to downstream environments. Such

information is alsohighly relevant to the impacts ofwildfire on

reservoir management and drinking water treatability.

Here, wildfire and bed age were studied using an annular

flume to elucidate their impacts on the stability of cohesive

stream sediment deposits. The objectives of this work were (1)

to quantify the physical (particle size, morphology, density,

porosity, settling velocity) and geochemical (major element

composition, mineralogy, total carbon) properties of burned

and unburned river sediment (2) to characterize the microbial

communities comprising the sediment-associated biofilms and

(3) determine the effect of biofilms on the transport (sc), depo-

sition (settling behaviour) and erodibility (erosion rate) of the

two sediment types.

2. Methods

2.1. Study area and sample collection

The 2003 Lost Creek wildfire burned >21,000 ha in the

Crowsnest Pass, Rocky Mountain region of southwestern

Alberta. It was particularly severe and consumed most forest

cover and floor organic matter across much of the headwater

regions of theOldmanRiver Basin. One of thewatersheds, Lynx

Creek, was severely impacted (67.3% burned), which dramati-

cally altered the discharge and sediment and nutrient charac-

teristics of the river (Silins et al., 2008, 2009; Bladon et al., 2008).

Increased post-fire light levels and particle-associated nutrient

inputs to the stream have significantly increased the presence

and abundance of biofilms in Lynx Creek compared to neigh-

bouring unburned watersheds (Silins et al., in review). Here,

cohesive sediment and riverwaterwere collected in LynxCreek

(burned watershed) and the Castle River (reference unburned

watershed) in 2007 (4 years post-fire). Surface deposits of fine

sediment were collected with a plastic scoop, immediately

refrigerated at 4 �C then transported within one week to the

Canada Centre for Inland Waters in Hamilton, Ontario where

they were studied using an annular flume.

2.2. Experimental procedure

A stainless steel annular flumewas used tomeasure sc, erosion

rate and erosion depth of burned and unburned river sediment

(Lau, 1995). The outside diameter of the flume is 2 m and the

trough is 20cmwideand12cmhigh.A clear glass top,whichfits

inside the trough, is lowered until it touches the water surface

then rotated to generate flow. Calibration of the flume was

described by Lau and Droppo (2000). Sediment and river water

were placed in the flumeand then the coverwas rotated at high

speed. After the sediment bed was completely entrained and

well mixed, the cover rotation was reduced gradually and then

stopped. Suspended solids in the water column settled under

low shear to form a cohesive sediment bed approximately 1 cm

thick. Thebedwas thenallowed to settle for three consolidation

periods (2, 7 and 14 days) during which eight wide-spectrum

fluorescent grow lights (40Weach totalling ameasured1250 Lx)

were activated above the flume for a 12 h light and 12 h dark

scenario to promote biofilm growth.

The flume experiments were conducted in sequence to

simulate sediment transport conditions that could occur in

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 2 1e5 3 4 523

stream reaches where cohesive sediments are deposited and

biofilms form and subsequently resuspend at conditions of

higher flow. Accordingly, for each sediment type (burned/

unburned), the first consolidation period studied was 2-days.

Thereafter, the flume was rotated and shear stress increased

incrementally (every 10 min) until all of the bed material had

eroded; thereby enabling the calculation of Tcrit. After the

estimated critical bed shear level had been applied for 10 min

the flume was stopped to monitor still water settling. Sus-

pended solids were collected every 2 min for 10 min. Two

subsequent samples were then collected at 5 min intervals

and four samples collected at 10min intervals thereafter. This

process was repeated for subsequent, longer consolidation

periods (i.e. the 7-day consolidation period experiment was

conducted after the 2-day experiment; the 14-day experiment

was conducted after the 7-day experiment). The same sedi-

ment remained in the flume for all three erosion experiments

conducted for a given source sediment. The river water was

not changed except when suspended solids were collected

from the sampling port and then an equal amount of river

water was added to the flume to maintain a constant water

level. This approach is non-ideal for simulating the biogeo-

chemical environment and biofilm growth in natural streams;

nonetheless, it provides a rigorous quantitative assessment of

sediment erosion under controlled hydrodynamic conditions

and a relative comparison of the effects of biofilms on the

erosion of the burned and unburned sediments.

The light conditions (intensity and duration) in forested

regions of the southern Rockies are highly variable and

dependent upon site specific conditions such as the time of

year, slope, aspect, canopy cover, forest age class and degree

of burn. In this study, fluorescent lights were not used to

simulate actual light conditions present at the stream surface

at a specific location in the complex forested environment

from which the sediment samples were obtained, but rather

to generate biofilm growth in the flume under controlled

conditions (12 h on and 12 h off) to compare and contrast

potential biofilm generation in the two systems.

Changes in sediment bed stability were observed through

windows on the side of the flume and with a video imaging

Lennox� boreoscope. The suspended sediment concentration

wasmonitoredcontinuouslyusinganoptical backscatter (OBS)

turbidimeter. Suspended solids concentrations were also

measured gravimetrically from samples collected directly

from theflume througha sampleport at 1, 5 and9minafter the

start of every 10-min shear interval.

Type I erosion is defined as an erosion rate that exponen-

tially decays to zero and is comprised of both Type 1A (erosion

of the loosely bound floc layer referred to as SFGL) and Type 1B

(erosion of the stronger bed [or armour layer] below the SFGL)

erosion (Villaret and Paulic, 1986). Here, the critical shear

stress for erosion (sc) for Type 1B erosion was determined

using visual and boreoscope observations as well as the

temporal suspended sediment concentration data for each

sediment type during each of the annular flume experiments.

2.3. Settling analysis

Total suspended solids concentrations over time were plotted

and the characteristic kinetic plots for still water settling were

examined. Still water settling was considered as the period

after the flume was stopped and the flow of water could no

longer be observed (360 s and thereafter). Mass settling during

each experiment was evaluated by calculating the appropriate

(second order in all cases as discussed below in the results and

discussion) kinetic decay constant (k), which is obtained by

regressing the inverse of the suspended solids concentration

(L mg�1) at each sampling point (360, 480, 600, 900, 1200 and

1800 s; in some cases, also 2400, 3000, and/or 3600 s) on time (s)

and determining the slope by least squares linear regression

to yield k (L mg�1 s�1). The TSSmass settling rate at each point

in time was then calculated by multiplying k by the square of

the measured suspended solids concentration. The rates of

change in mass settling rate during still water settling in the

burned and unburned systems with the various consolidation

periods were compared using an F-test for equality of slopes

(SAS/STAT� Version 9.1).

2.4. Physical and geochemical sediment properties

Sediment bulk density after the growth/consolidation periods

was used to evaluate the influence of consolidation (and

potentially biofilm formation) on bed stability. Sediment

samples were settled in 10 cm of river water in each of three

glass beakers to form beds with similar thickness to those

formed in the flume. Bulk density profiles were measured in

1 mm increments (Berkhout, 1994).

Eroded floc settling velocity was determined using

a settling column interfaced with a Nikon SMZ-2T (Nikon

Canada Inc., Mississauga, Canada) stereo-scopic microscope

and Open Lab image analysis system (Improvision, Coventry,

UK) to size and track particle settling trajectories for the

measurement of settling velocity (Droppo et al., 1997). Stokes’

law was used to obtain density and porosity estimates (Li and

Ganczarczyk, 1987).

Surface deposits of cohesive sediment were collected

from a wildfire-affected stream and a reference undisturbed

stream and immediately submitted to a commercial labora-

tory for grain size and geochemical analyses to characterize

and compare the sediment geochemistry between the two

rivers. The grain size distribution of the two sediment types

was determined using a Malvern Mastersizer Model 2000.

The concentration of major elements (Al2O3, CaO, Cr2O3,

Co3O4, CuO, Fe2O3, K2O, MgO, MnO, Na2O, NiO, P2O5, SiO2,

TiO2) was determined by X-ray fluorescence spectrometry.

The sediment loss on ignition (LOI) was determined at 475 �Cfor 12 h. Themineralogical composition of the sedimentswas

evaluated by X-ray diffraction (Philips X’pert PW3040-PRD

diffractometer with a Cu X-ray source operated at 40 kV and

50 mA). Certified reference materials USGS GXR-1, GXR-2,

GXR-4 and GXR-6 were analyzed at the beginning and end of

each batch of samples. Internal control standards were

analyzed after every 10 samples and a duplicate was

analyzed after every 10 samples. Volatile organic carbon was

determined by heating a 0.1 g sample in a pure oxygen

environment at 380 �C, releasing the volatile organic carbon

species, binding them with the oxygen to form CO and CO2,

the majority being CO2. Carbon was measured as carbon

dioxide in the IR cell.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 2 1e5 3 4524

2.5. Microscopy

Eroded floc morphology (size distribution and structure) was

examined by light microscopy (Zeiss Axiovert 100 fitted with

a flow cell interfaced directly to the flume) (Droppo et al.,

2007). Environmental scanning electron microscopy (ESEM;

ElectroScan model 2020 operated at 20 kV, ElectroScan

Corporation, Wilmington, MA, USA) was used to observe fine-

scale structural characteristics of the eroded flocs using

a Peltier stage cooled to 1 �C (Leppard, 1986). Transmission

electron microscopy (TEM) was used to determine the micro-

structure and internal composition of the eroded flocs.

Ultrathin sectionswere imaged in transmissionmode (TEM) at

an accelerated voltage of 80 kV (JEOL 1200 Ex II TEMSCANTEM)

(Leppard, 1986).

2.6. Biofilm mass

Biofilm is a heterogeneous andamorphousmaterial comprised

of living organisms and other solid materials (e.g., organic

substancessuchasEPS, inorganicsolids suchasclays). Liuetal.

(1994) suggested determination of total attached poly-

saccharide (carbohydrate) as an indirect measure of biofilm

mass because biofilms contain relatively large amounts of

polysaccharide. Here, water samples from the flume were

collected in a polyethylene bottle and filtered onto Whatman

GF/C glass-fiber filters and frozen (220 �C) for subsequent

analysis (performed on the filters within one week). The filter

residue was extracted with acetone and assessed at a wave-

length of 663 nm (Pacepavicuis et al., 1997). This method is

particularly suitable for the determination of total poly-

saccharide content in suspended biofilm because it is sensitive

(i.e. a few mg is sufficient) and specific (i.e. there is little inter-

ference from environmental factors such as salt and water

hardness). The amount of eroded biofilmmass is expressed as

mass of total polysaccharide as glucose per liter of water

(mg L�1) and is determined directly from a glucose calibration

curve. The contribution of biofilm mass (as glucose) to sus-

pended solids concentration in the burned and unburned

sediment was evaluated using least squares linear regression

and an F-test for equality of slopes (SAS/STAT� Version 9.1).

2.7. Microbial community analysis

Phospholipid fatty acids (PLFA) are the primary constituents of

cell membranes of all living cells. Different groups of micro-

organisms synthesize varieties of PLFA through different

biochemical pathways. Some PLFAs can be used as “signa-

tures” to analyze changes in viable microbial biomass and

community structure (Tunlid and White, 1992); accordingly,

changes in PLFA profiles are indicative of changes in the total

viable microbial community, thereby making PLFA an effec-

tive tool for monitoring microbial responses to environmental

conditions. Here, lipid extraction and PLFA analyses were

performed using the modified Bligh and Dyer-method (1959)

as described by White et al. (1979).

Themicrobial communities comprising the biofilm formed

on the two sediment types (burned and unburned) after 2, 7

and 14 days of consolidation were determined using a phos-

pholipid fatty acid (PLFA) technique (White et al., 1979).

Biofilmwas formed using a plastic bin containing a 1 cm layer

of bed sediment covered by a water layer of 2 cm. The bins

were placed under the grow lights and frosted glass slides

were placed on the surface of the sediment to allow biofilm

growth for the designated consolidation periods of 2-, 7-, and

14-days.While biofilm growth on slides is likely different from

that formed in streams where flow conditions, light and

nutrient levels are variable over short temporal scales (days to

weeks), this method provides a comparison of relative differ-

ences in biofilm growth that may form on various sediment

types (Lau, 1995; Lau and Liu, 1993). Twenty-four slides were

added to the plastic bins (8 for each consolidation period). Of

these eight slides: the biofilm from four was combined and

analyzed by PLFA (Microbial Insights; Rockford, Tennessee) to

yield a composite determination of viable microbial biomass

and a PLFA profile reflecting the proportions of organisms

present in the sample, according to six PLFA structural groups

(monoeonic, terminally branched saturated, branched mon-

oeonic, mid-chain branched saturated, normal saturated, and

polyeonic). Biofilm from the remaining four slides was

analyzed by TEM and ESEM.

3. Results and discussion

3.1. Physical and geochemical sediment properties

The geochemical composition and claymineral assemblage of

sediment in aquatic systems are largely dependent upon the

general characteristics of the source area which include

geology, weathering, vegetation, soils, mass wasting

processes and land use (Griggs and Hein, 1980). Resulting

differences in the sediment properties (density, grain size,

geochemistry) will influence sediment transport characteris-

tics. Themajor element composition of the study sediments is

presented in Table 1 and indicates that Lynx Creek sediment

(burned) has higher concentrations of CaO (9.16%) and carbon

(LOI ¼ 16.7%) but lower concentrations of SiO2 (53.74%) and

Al2O3 (8.79%) than Castle River (unburned) sediment. Lynx

Creek sediment contains less quartz than Castle River sedi-

ment, but elevated levels of dolomite andmuscovite (Table 2).

The measured volatile organic carbon content of Lynx Creek

was 4.73% compared to <0.05% in Castle River.

The median density, porosity and settling velocity of the

burned and unburned sediments after consolidation are pre-

sented in Table 3. While the geochemical composition of both

sediments varied slightly, their densities were similar. The

median settling velocity and porosity of the unburned sedi-

ment were higher than of burned sediment. In contrast to the

results presented herein, Blake et al. (2007) examined the

impact of wildfires on the effective size distribution of burned

and unburned soils and reported that burned soils had signif-

icantly higher settling velocities than unburned particles of

equivalent diameter; they attributed this to increased density

and decreased organic matter and pore space in burned soils.

Wildfires can influence the structure and stability of soil

aggregates but the degree of aggregationwill vary as a function

burn temperature, soil depthaswell as themineral andorganic

properties of the soil (Andreu et al., 2001; Fox et al., 2007).

However, once soil aggregates enter receiving streams they are

Table 1 e Major element composition of fine sedimentdeposits in Burned and Unburned Watersheds (%).

Major element Burned Unburned

Al2O3 8.79 10.49

CaO 9.16 2.02

Cr2O3 0.01 0.01

Co3O4 0.005 0.005

CuO 0.005 0.005

Fe2O3 2.92 3.59

K2O 3.05 3.24

MgO 4.87 2.88

MnO 0.055 0.052

Na2O 0.86 0.99

NiO 0.003 0.003

P2O5 0.12 0.11

SiO2 53.74 71.26

TiO2 0.33 0.51

LOI 16.66 4.44

Table 3 e Physical characteristics of eroded sediment.

Consolidationtime(days)

Median settlingvelocity(mm s�1)

Medianporosity

(%)

Mediandensity(g cm�3)

Burned 2 2.22 84.7 1.09

7 2.81 85.6 1.09

14 2.96 93.3 1.05

Unburned 2 3.18 89.3 1.08

7 3.26 91.3 1.07

14 3.82 93.4 1.04

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 2 1e5 3 4 525

influenced by a range of physical, chemical and biological

processes that alter the grain size distribution and biogeo-

chemical nature of the particles (Blake et al., 2009). Here, flocs

were formed within the flume (i.e. not eroded soil aggregates)

with a significant microbial component (biofilm interactions).

The higher organic content (Table 1) for Lynx Creek is likely

responsible for the larger floc size and increased floc porosity

resulting in a slower settling velocity (although the density

analysis showed no difference between rivers). Moreover, the

river sediments assessed in this study are four years post-fire

whereas Blake et al. (2009) investigated recently burned soils.

3.2. Morphology and physical characteristics oferoded flocs

Eroded flocs were collected during the flume experiments to

assess changes in floc structure between rivers and over

different consolidation times. Within these experiments it is

difficult to compare eroded floc morphology, size, settling

velocity, porosity and density for a given shear, because for

a given shear value, flocs will originate from different depths

within the bed where different physicochemical and biological

properties may exist. Further, a comparison of floc character-

istics at sc also provides little insight as these comparisons are

different for each consolidation period and bed type (burned

and unburned) resulting in a wide range of properties given the

different forces affecting the floc. Accordingly, differences in

floc properties can only be viewed on an average trend basis.

Table 2 eMineralogy of fine sediment deposits in Burnedand Unburned Watersheds (%).

Burned Unburned

Quartz 23 42

Calcite 1 1

Dolomite 8 2

Albite 7 12

Microcline 15 14

Muscovite 41 22

Chlorite 5 7

In general, fire modified eroded flocs were larger than

unburned flocs and both floc types decreased in size with

increasing shear levels.Using threedifferentmicroscopeswith

a range of resolution, it was observed (e.g., Fig. 1) that Castle

Creek exhibited more inorganic networks relative to Lynx

Creek, which possessed substantially more organic networks

encompassing both cellular material and extracellular poly-

meric substances in the form of fibrils (see TEM image Fig. 1c

and f). COM images (Fig. 1a and d) show the eroded Lynx Creek

flocs to be more diffuse and irregular in shape while the ESEM

images (Fig. 1b and e) showmore organic coatings on the Lynx

Creek flocs relative to the inorganic dominated Castle Creek

flocs. The above differences relative to floc structure were

consistent over time, however, the level of organic dominance

increased with time of consolidation/growth.

3.3. Biofilm analysis

Visual inspection of streambeds draining unburned (Fig. 2a)

and burned (Fig. 2b) catchments even 5 years post-fire clearly

indicated the possibility of significant differences in

streambed-associated biomass amount, composition and

activity. Biomass can attach to sediment in the form of biofilm

and several studies have demonstrated that biostabilization

can significantly increase the energy required to erode sedi-

ments by horizontal shear (Amos et al., 2004; Gerbersdorf et al.,

2008; Droppo, 2009). A simple estimate of biofilmaccumulation

on the sediment bed within the flume expressed as mass of

total polysaccharide as glucose per liter of water indicated that

burned sediments had significantly higher biomass (glucose

mass) per suspended solids mass than unburned sediments

( p < 0.0001; Fig. 3). PLFA analyses of the same biofilms did not

yield a clear difference in accumulated biomass associated

with the burned and unburned sediments (Fig. 4). However,

only a very limited number of samples could be processed

thereby precluding a more thorough assessment of intra- and

inter-sample variability.

Themicrobial community structure comprising the biofilms

formed on the unburned and burned sediments with various

consolidation periods is organized according to the PLFA

structural groups and presented in Fig. 5. As observed with the

total accumulated biomass evaluated using PLFA (Fig. 4), few

differences in microbial community structure between the

unburned and burned catchments and the various consolida-

tion periods were observed. The only notable difference

appeared tobean increase in themid-chainbranchedsaturated

PLFA structural group indicative of Actinobacteria (e.g.,

Fig. 1 e Eroded Castle River (a) and Lynx Creek (b) flocs using conventional optical microscopy. Representative ESEM images

of 14-day Castle River (c) and 14-day Lynx Creek sediment (d). TEM images of 14-day Castle River (e) and 14-day Lynx Creek

sediment (f).

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 2 1e5 3 4526

Actinomycetes spp.) and sulfate-reducing bacteria in the biofilms

formed on burned relative to unburned sediments (Fig. 6).

Increasedamounts ofbacteria likeActinomycetes spp. inbiofilms

obtained on burned relative to unburned sediments are not

surprising given that Actinomycetes spp. form conidia (asexual

spores) that may help them better survive harsh soil environ-

ments such as those exposed to dessication and heat

(Alexander, 1998) that result fromwildfire.

Increases in Actinomycetes spp. may be of concern when

impacted waters are used as drinking water sources because

thesebacteria are associatedwithearthy/mustyodors inwater

and may contribute to formation of geosmin and 2-

methylisoborneol (MIB), which are major sources of taste and

odor causing compounds in drinking water (Zaitlin and

Watson, 2006). Biofilm growth in plastic bins occurred under

quiescent (no flow) conditions on a glass substratum which

likely resulted in differences observed between the biomass

amount, composition andactivity in the lab compared to those

found in nature. Glass is a relatively smooth substratum;

however, in many circumstances biofilms accumulate more

readily on rougher substrata (e.g., Percival et al., 1999).

Accordingly, biomass growth in the source watersheds was

likely underpredicted by the simple approaches used herein to

estimate biomass accumulation. Nonetheless, caution should

Fig. 2 e Typical biofilm accumulation in riverbeds draining (a) reference and (b) burned catchments five years post-fire.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 2 1e5 3 4 527

be exercised when considering increases (or lack thereof) in

amounts of total estimated biomass (Figs. 3 and 4) or in

biomass associated with specific structural groups such as

those indicative of Actinomycetes spp. (Fig. 6) because differ-

ences in biomass are not necessarily indicative of biomass

activity. For example, even when substantial differences in

substratum roughness exist and may contribute to significant

differences in biomass accumulation, differences in biomass

activity may still be insignificant (e.g., Emelko et al., 2006).

Regardless of how representative biofilm mass formed on

a glass substratum is of the biofilm formed in the source

watersheds, it is essential to note that secondary metabolite

(geosmin and MIB) production by Actinomycetes spp. in a given

situation is also not necessarily linked to the amount of

biomass present (Zaitlin and Watson, 2006). Accordingly,

further investigation of the potential risks (e.g., increased

propensity for taste and odor events as related to sediment-

associated microorganism transport) resulting from land

disturbances to downstream drinking water supply and

further in situ analysis and characterization of biofilms in fire

impacted streams is warranted.

Fig. 3 e Estimated contribution of biofilm mass (as glucose)

to suspended solids concentration arising from the Castle

River (unburned) and Lynx Creek (burned) sediments with

various bed consolidation periods.

3.4. Critical erosion values

Flume experiments were conducted to determine sc of cohe-

sive sediment deposits in a wildfire impacted and reference

stream. Suspended solids concentrations increased exponen-

tially with applied bed shear stress, but the degree of erosion

was influenced by bed age and sediment type (Fig. 7). The scvalues for Type 1A and 1B erosion varied both within (i.e.

different consolidation/biostabilization periods) and between

sediment types (Table 4). The level of shear stress required for

Type 1A erosion increased with consolidation time for the

burned and unburned sediment, but higher shear stresses

were required to erode the SFGL of the burned sediments

compared to unburned (Fig. 7). During the 2-day consolidation

experiments, the level of shear stress required to produceType

1B erosion was comparable for both sediment types; however,

the measured sc was 1.6 and 1.8 times higher for the burn-

associated sediment after 7 and 14 days of consolidation,

respectively than for unburned sediment (Table 4). Accord-

ingly, sediment resistance to erosion generally increased with

increased consolidation time regardless of sediment type;

however, this increase was more pronounced for the burn-

associated sediment.

The observed increase in shear stress required for Type 1A

and 1B erosion to occur with increasing bed age is strongly

related to the nature of the biofilm and its associationwith the

underlying sediments. Boreoscope observations of sediment

movement as a function of applied shear stress show that

surface biofilm began to erode before the underlying sedi-

ments. The process of biofilm erosion typically began at low

shear with the creation of small fractures on the biofilm

surface. With increasing shear, segments of biofilm would

partially dislodge and the biofilm would roll up upon itself in

long narrow segments up to a few cm in length until it

detached completely from the sediment bed. As it was being

dislodged from the bed, the eroded Lynx Creek biofilm

appeared to contain more sediment than Castle River biofilm.

These visual observations coupledwith the flumedata suggest

that the degree and spatial extent (depth) of biofilm attach-

mentwere greater on the burned (Lynx Creek) sediment. Table

4 shows that burned sediment was approximately twice as

Fig. 4 e Estimated amount of bacterial and eukaryotic biomass on Castle River (reference/unburned) and Lynx Creek

(burned) sediment with various bed consolidation periods as determined by phospholipid fatty acid (PLFA) analysis.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 2 1e5 3 4528

resilient to the applied bed stress for the 7-day (1.6 times) and

14-day (1.9 times) deposit compared to the unburned sedi-

ment. The higher sc values required to erode Lynx Creek sedi-

ment suggests that the sediment-pore biofilmcomplex ismore

integrated and the biological community associated with the

SFGL and eroded flocs in these deposits is more active than in

unburned materials. Networks of EPS can permeate void

spaces and promote inter-particle linkages (Gerbersdorf et al.,

2008, 2009) and the resultant increased level of attachment

within and between deposited flocs can lead to increased bed

stability (Droppo, 2009). Recent sediment biostabilization

studies in freshwaterwavedominated (Droppoet al., 2007) and

Fig. 5 e Relative percentages of total PLFA structural groups

in Castle River (reference/unburned) and Lynx Creek

(burned) sediments with various bed consolidation

periods. Structural groups are assigned according to PLFA

chemical structure, which is related to fatty acid

biosynthesis.

riverine (Gerbersdorf et al., 2008, 2009) environments show

that bacterial EPS production correlateswellwith bed stability.

Droppo (2009) examined biofilm structure and bed stability of

five contrasting freshwater sediments and demonstrated that

Fig. 6 e Biomass composition of mid-chain branched

saturated structures of PLFA (indicative of Actinobacteria

[e.g., Actinomycetes spp.] and sulfate-reducing bacteria)

comprising the biofilms formed on Castle River (reference/

unburned) and Lynx Creek (burned) sediments with

various bed consolidation periods.

Fig. 7 e Changes in suspended solids concentration as a function of applied bed shear stress for the 2-, 7-, and 14-day

consolidation (Lynx Creek e A, B, C; Castle River e D, E, F).

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 2 1e5 3 4 529

Table 4 e Comparison of erosion depths at Type 1A and 1B erosion for Castle River (unburned) and Lynx Creek (burned)sediment.

Consolidationtime (days)

1A Shear(Pa)

1A Erosiondepth (mm)

1B Shear(Pa)

1B Erosion depth(mm)

Burned 2 0.08 0.0295 0.12 0.336

7 0.16 0.041 0.23 0.426

14 0.18 0.095 0.31 1.54

Unburned 2 0.039 0.0024 0.105 0.0126

7 0.097 0.0009 0.141 0.008

14 0.094 0.0009 0.165 0.014

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 2 1e5 3 4530

the resultant increased level of attachment within and

between deposited flocs increased bed stability.

Wetweight bulkdensitymeasurementsof the twosediment

typesweremade toevaluate the influenceof consolidation (and

potentially biofilm formation) on bed stability. The wet density

of sediment from the unburned watershed (Castle River) was

approximately 1.28 g cm�3 and changed very little with depth.

In contrast, some compaction of the Lynx Creek sediment

deposit is suggested by the increase in density of deposited

sediment with depth (Fig. 8). For example, at a depth of 5 mm,

the density of the burned sediment was 0.19, 0.16 and

0.14 g cm�3 after 2, 7 and 14 days of consolidation, respectively.

However,at adepthof11mm, thedensity increasedto1.16, 1.05

and 0.89 g cm�3 after the respective consolidation periods.

Droppo andAmos (2001) developed a general three-layermodel

to describe the formation processes and characteristics of SFGL

deposits. Themodel describesa surfaceorganicfloc layer (Layer

1), amiddle collapse zone (Layer2) anda lowerconsolidatedbed

(Layer 3). They describe SFGL as a porous, low density and high

water content deposit with high yield resistance due to bio-

stabilization. Here, the bulk density (Fig. 8), the sc (Table 4) and

floc morphology analyses indicate that cohesive sediment

deposits in wildfire-impacted streams consist primarily of

porous, low density flocs that are associated with an active

biological community. Accordingly, the formational processes

Fig. 8 e Bulk density (g mL3) of Ly

and erosion dynamics in streams draining wildfire-impacted

landscapes are consistentwith the conceptualmodel proposed

by Droppo and Amos (2001).

3.5. Erosion rates

The erosion rates and depths for each flume experiment are

shown in time series plots in Fig. 9. For all runs, the initial

erosion event occurred at a lower applied shear than for

subsequent runs with longer consolidation periods (Table 4).

The peak in erosion rate occurred at the beginning of each

shear increment but then decreased as the underlying more

stable bed sediment was exposed. The maximum erosion

rates for the 2, 7 and 14-day runs occurred at 0.22, 0.34 and

0.39 Pa for unburned sediment (Castle River) and 0.25, 0.33 and

0.30 Pa for burned sediment (Lynx Creek), respectively. The

erosion depths at 0.25 Pa for the 2, 7 and 14-day consolidation

periods were 0.084, 0.051 and 0.033 mm for Castle River and

2.78, 0.426 and 0.181 mm for Lynx Creek sediment, respec-

tively. The time series plots show that increasing shear stress

was required to erode the wildfire-affected Lynx Creek sedi-

ment over time with increasing bed age. However, this effect

was less pronounced in the Castle River sediment.

Erosion depths for Castle River sediments never exceeded

0.5mmwhilemaximumerosiondepths in thewildfire-affected

nx Creek (burned) sediment.

Fig. 9 e Changes in erosion rate and depth as a function of applied bed shear stress for the 2-, 7-, and 14-day consolidation

(Lynx Creek e A, B, C; Castle River e D, E, F).

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 2 1e5 3 4 531

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 2 1e5 3 4532

LynxCreek sedimentwere 1.4, 4 and 3.7mm for 2, 7 and 14-day

runs respectively.

3.6. Still water settling

Still water settling rates have been described using zero (Amos

et al., 2003; Droppo, 2009) and first order kinetic models else-

where (Amos et al., 2004 based on Einstein and Krone, 1962). In

contrast to single point-based approaches that have been

utilized in the literature, the approach used herein is based on

all of the collected still water settling data (5e8 points per land

disturbanceeconsolidation period combination). Character-

istic zero, first, and second order kinetic plots were made for

still water mass settling of total suspended solids and indi-

cated that still water settling (the change in TSS concentration

over time) was best described as a second order reaction. For

this type of reaction, the rate of reaction decreases rapidly

(faster than linearly) as the suspended solids concentration

decreases. All combinations of land disturbance (unburned/

burned) and bed consolidation period (2, 7, and 14 days)

resulted in high (i.e. >98%) coefficients of determination for

second order kinetic models.

The still water TSS mass settling rates (mg TSS L�1 s�1)

derived from the Castle River (unburned) and Lynx Creek

(burned) eroded sediments after the various consolidation

periodswerecalculatedandevaluatedover time.TheTSSmass

settling rates associated with the burned eroded sediments

duringstillwater settlingwerehigher than thoseobservedwith

unburned eroded sediments (Fig. 10). The temporal changes in

TSS mass settling rates of the unburned and burned eroded

sediments in still water (Fig. 10) were significantly different

( p ¼ 0.0053, p ¼ 0.0295, p ¼ 0.0054 for the 2, 7, and 14-day

consolidation periods respectively). These analyses suggest

that TSSmass settling rates of the unburned eroded sediments

decreasedmore rapidly than those associated with the burned

sediments, possibly implying that settling velocities of the

unburned sediment were higher than those of the burned

sediments, a result consistent with the settling velocities

reported in Table 3. Differences in mass settling rates for the

two sediment types are likely explained by differences in the

Fig. 10 e Temporal changes in still water TSS mass settling

rates of Castle River (unburned) and Lynx Creek (burned)

sediments after various bed consolidation periods.

floc size distribution and characteristics (i.e. porosity, settling

velocity, floc strength and composition). Here, it is hypoth-

esised that burned sediment and attached biofilm eroded into

the water column formed organic rich (Table 1; Fig. 3), low

porosity (Table 2) flocs that settled more slowly than those

formed in the unburned system. From a water management

perspective, mass settling rate data imply that suspended

solids in wildfire-affected streams will remain in suspension

longer than in streams draining unburned landscapes.

3.7. Implications for water management

Results of the present study suggest that biogenic sediment

stabilization will have a significant effect on the rates and

magnitudes of sediment erosion and associated contaminant

transport in wildfire-affected streams. The initial influx of fire

modified sediment and associated nutrients from hillslopes

(Blake et al., 2009) combined with the loss of forest canopy in

wildfire-affected watersheds can create conditions which

promote biostabilized in-channel deposits of cohesive sedi-

ment. Silins et al. (in review) deployed fixed area ceramic tiles

in the wildfire-affected streams and reported that the mean

algal biomass in these streamswas 10e14 times higher than in

reference streams. The flume experiments show that the

erosion depth of wildfire-influenced sediments is higher,

suggesting that more material will likely be eroded once the

biofilm is removed compared to streams in unburned water-

sheds. Accordingly, once sc in the wildfire-affected streams is

exceeded, the eroded cohesive materials, which have low

settling velocities, will remain in the water column for pro-

longed periods of time and will more likely be transported to

downstream reservoirs, potentially contributing to significant

drinking water treatment challenges (Emelko et al., 2011).

4. Conclusions

1. Burned sediment had lower settling velocities, increased C

and carbohydrate levels compared to unburned sediment.

2. Biofilm formation on burned sediment increased sc for 7

and 14-day deposits by a factor of 1.6 and 1.8, respectively.

3. Erodibility (erosion depth) of burned sediment after 2, 7,

and 14 days of consolidationwas respectively 26, 53 and 110

times greater than unburned sediment.

4. Lower settling velocities of burned sediment are related to

increased organic content and higher porosity compared to

unburned sediment.

5. Changes in suspended solids concentrations as related to

shear stress indicated that sc increased with consolidation

period andas a result ofwildfire-associatedbiostabilization.

6. Erosion depths significantly decreased with bed age;

however, these depths were significantly greater as a result

of wildfire-associated biostabilization.

7. As a result of upstream land disturbance, fine sediments

(and their associated-contaminants) will stay in riverbeds

for longer periods of time due to disturbance-associated

biostabilization.

8. Increases in biofilm communities of Actinomycetes spp. (and

other species) may be associated with wildfire and may be

indicative of an increased propensity for taste and odor

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 2 1e5 3 4 533

events related to sediment-associated microorganism

transport to downstream water suppliers.

9. Temporal changes in TSS mass settling rates of the

unburned and burned eroded sediments in still water were

significantly different for the 2, 7, and 14-day consolidation

periods, respectively.

Acknowledgements

The assistance of S. Deignan, B. Trapp and C. Jaskot in con-

ducting the flume experiments and K. Bladon in collecting

sediment and water samples is greatly appreciated.

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