Nitrogen and Phosphorus Storage in Contrasting Reaches of a Sub-tropical River System

Nitrogen and Phosphorus Storage in Contrasting Reachesof a Sub-tropical River System

Jason Grainger Kerr & Michele Burford &

Jon Olley & James Udy

Received: 5 March 2010 /Accepted: 11 August 2010 /Published online: 1 September 2010# Springer Science+Business Media B.V. 2010

Abstract This study investigated the storage of nitro-gen (N) and phosphorus (P) in the biomass, bedsediments and water column of representative reachesof a sub-tropical river, the upper Brisbane River (UBR),Queensland, Australia, and contrasted instream storagewith total wet season exports. In reaches whichcontained accumulated fine sediments, more than 87%of total P and between 50% and 92% of total N werestored in the surface sediments. The lower proportion ofN in sediment at some sites was attributed to substantialdifferences in the N/P ratios of sediments and macro-

phytes. At one site, the riverbed was dominated bycobbles and boulders and total nutrient stocks werecomparatively low and dominated by the biomass. Inreaches with a narrow channel and intact riparian cover,biomass N and P were stored predominately in leaflitter, while in wider unshaded reaches, macrophytesdominated. Total instream storage in the mid to lowerreaches of the UBR was ∼50.9 T for N and ∼18.1 T forP. This was considerably higher than total wet season N(∼15.6 T) and P (∼2.7 T) exports from the UBR. Thefirst flow event in the river after a prolonged period ofno flow resulted in the export of free-floating, emergentspecies Azolla. The estimated biomass of Azolla in themid to lower reaches of the river was equivalent toapproximately 24% and 9% of the total N and P flux,indicating that this may be a significant, previouslyunaccounted for, source at peak flow.

Keywords Nitrogen . Phosphorus . Rivers .

Sediment . Macrophytes

1 Introduction

The storage and transfer of nutrients between sediment,biomass and the water column are important componentsof nutrient cycling in rivers (Mainstone and Parr 2002;Mulholland et al. 2000; Newbold 1996; Reddy et al.1999). Despite this, most studies and models ofcatchment nutrient loads focus on inputs to, and outputs

Water Air Soil Pollut (2011) 217:523–534DOI 10.1007/s11270-010-0606-7

J. G. Kerr :M. Burford : J. OlleyAustralian Rivers Institute, Griffith University,Nathan, Queensland, Australia

M. Burforde-mail: [email protected]

J. Olleye-mail: [email protected]

J. UdySeqwater,240 Margaret Street,Brisbane, Queensland, Australiae-mail: [email protected]

Present Address:J. G. Kerr (*)Department of Geography, Trent University,1600 West Bank Drive,Peterborough, Ontario K9J 7B8, Canadae-mail: [email protected]

from, river systems (Young et al. 1996). The omissionof instream nutrient stores is important because of theirpotential to function as sinks or sources of N and P andin turn alter the amount, form and timing of nutrientexports to downstream ecosystems. A first step inaddressing this knowledge gap is to quantify nutrientstorage across a range of river systems which can thenprovide greater context for estimates of inputs to, andoutputs from, large river systems.

Most of what is known about nutrient storage inrivers and streams comes from studies of temperate,forested headwater streams where both N and P havebeen found to be associated predominately with fine andcoarse organic material (Dodds et al. 2002; Hall et al.2002; Mulholland et al. 1985; Newbold et al. 1983a).Importantly, the uptake of dissolved nutrients has beenlinked to the mass of this material (Mulholland et al.1985, 2000; Newbold et al. 1983a), indicating animportant role for instream biomass in riverine nutrientretention. Furthermore, nutrients held in biomass maybecome a source to downstream ecosystems duringflow events (Golladay et al. 1992). The extent to whichthe results from relatively pristine forested headwaterstreams can be applied to more disturbed systems or tolarger river systems is unclear.

There is likely to be differences in the relativeimportance of detrital and photoautotrophic nutrientstorage in degraded versus pristine headwater streams.The riparian vegetation provides both a source ofnutrients to the riverbed via leaf litter inputs and alsohas an inhibitory effect on primary production due tothe shading provided by canopy cover (Sabater et al.2000). Furthermore, the River Continuum Conceptproposed by Vannote et al. (1980) emphasises agreater abundance of leaf litter in smaller headwaterstreams relative to larger higher-order rivers wheremacrophytes and phytoplankton dominate. Anotherpotentially important nutrient pool in river systemsare sediments, particularly in catchments which havebeen degraded by heavy grazing. Sediments havebeen identified as playing a major role in P storage incanals (Diaz et al. 2006) and marshes (Noe et al.2002) of the Florida Everglades National Park, USA.Reddy et al. (1996) found that most of the P loaded towetlands and streams of the Lake OkeechobeeWatershed was retained in soils and sediments whilein the Canning River, Australia, Vincent (2001)reported that the majority of N and P was held inthe bed sediments.

The capacity to improve conceptual and predictivemodels of N and P fluxes at the catchment scale islimited by a lack of published data on instreamnutrient storage. Some of the key questions whichremain unanswered relate to the extent to whichnutrient partitioning varies within individual rivernetworks, the relative size of instream storage andannual river exports, and the extent to whichimportant storage pools are transported downstreamduring flow events. The primary objective of thisstudy is to quantify nutrient (N and P) storage in a drytropical river system and compare this with N and Pexports during a wet season. In addition, the studywill compare nutrient partitioning across reaches ofvarying geomorphology and examine the affect of aflow event on standing biomass stocks of N and P.

1.1 The Upper Brisbane River Catchment

The study was conducted in the upper Brisbane River(UBR) catchment in southeast Queensland, Australia,approximately 80 km northwest of the city ofBrisbane. Land use in the upper Brisbane Rivercatchment is dominated by cattle grazing (69%), withproduction forestry accounting for 13% of total landuse and intensive animal uses such as dairy farmingmaking up approximately 4% of catchment land use.The upper Brisbane River flows into southeastQueensland’s largest potable water storage. Like mostof tropical and sub-tropical eastern Australia, theregion is influenced by El Niño and La Niña eventswhich cause substantial inter-annual variability inrainfall (Davis and Koop 2006; Hamilton and Gehrke2005), and the highest monthly rainfall occurs in thesummer months. The flow regime in the upperBrisbane River is therefore characterised by a highdegree of inter-annual and intra-annual variation.

2 Methods

2.1 Location of Study Sites and Flow ConditionsDuring the Study Period

The study sites consisted of five reaches (∼100 m) in theUBR catchment. The sites included three in the UBR,one in the upper catchment (UBR17), one in the mid tolower catchment (UBRA) and one in the lowercatchment, upstream of a large potable water storage,

524 Water Air Soil Pollut (2011) 217:523–534

Lake Wivenhoe (UBRG). There were also two siteslocated in the lower reaches of the Emu and CressbrookCreek tributaries (Fig. 1). Reach geomorphology wascharacterised at each site and standing biomass,sediment and water column samples were collected.Sediment and biomass samples were collected fromeach reach between the 24th of November and the 4thof December 2005. Water samples were collected atfortnightly intervals from 24th of November 2005 tothe 28th of February 2006.

In the period leading up to the study (01 Jun 2005–24Nov 2005) and in the period following (Mar–Jun 2006),there was little or no discharge in either the UBR ortributaries. During the study period, the first flow eventof the wet season in late November 2005 produced apeak discharge above the 75th percentile of historicalmean daily flows at the most downstream site (UBRG).The highest discharge of the wet season occurred inearly December 2005 and produced a peak flow greaterthan the 95th percentile of historic daily flows atgauging stations in the UBR and Emu Ck. Dischargewas above long-term median values for most of thestudy period. At Cressbrook Creek, peak discharge was

much lower than the other sites, and for the majority ofthe study, there was very little flow at this site. Althoughthere are no discharge data for UBR17, there was noevidence of any rain event occurring immediately priorto sampling. Biomass and sediment samples werecollected prior to the first flow events at UBRA,UBR17 and Cressbrook Creek while at UBRG andEmu Creek samples were collected shortly after the firstflows of the wet season (29 Nov 2005 and 4 Dec 2005).At UBRA, biomass sampling was repeated after the firstflow event.

2.2 Characterisation of Reach Geomorphology

Reach-scale geomorphic characteristics were measuredin conjunction with biomass and sediment collection ateach site over the period of 24 Nov 2005 to 4 Dec 2005.Within each reach, pool, run and riffle habitats wereidentified visually based on characteristics outlined bythe US Geological Survey (Fitzpatrick et al. 1998).Stream substrate, water depth and the occurrence ofwoody debris were determined at regularly spacedsampling points along transects. The distance between

Fig. 1 Map of the upperBrisbane River (UBR)catchment showing studysites

Water Air Soil Pollut (2011) 217:523–534 525

sampling points along each transect was proportional tothe width of the stream at each transect (i.e. 10% oftransect width). Percentage stream substrate compositionwas determined visually and with the use of calliperswithin a 0.27-m2 area of each sampling point along eachtransect and classified based on the Wentworth particlescale (Walters et al. 2007) as boulder, cobble or gravel.Particles less than 2 mm were further sorted in thelaboratory where approximately 200 g of dried sedimentwas shaken and passed through a set of sieves ofdecreasing screen size (Gordon et al. 1992). The percentcover of woody debris was determined visually withineach study reach and was defined as logs and branchesgreater than 10 cm in diameter and greater than 1 m inlength (Parsons et al. 2002). The percent of the streambed area shaded by riparian vegetation was estimatedvisually along the length of each reach (Parsons et al.2002).

2.3 Water Column Sampling and Analysis

Water samples were collected twice per month fromthe 24th of November 2005 through to the 28th ofFebruary 2006. Triplicate samples were collectedfrom mid-stream surface waters for analysis ofnutrients and total suspended solids. Samples fornutrient analysis were collected into pre-washed (5%HCl and deionised water) 30-ml vials for analysis oftotal N and P. Samples for dissolved N and P werefiltered onsite through 0.45-μm membrane filters(Millipore, USA), stored on ice and then frozen at−30ºC in the laboratory prior to analysis.

Filterable reactive phosphorus (FRP) was deter-mined based on ascorbic acid reduction of phospho-molybdate (APHA 2005). Ammonia (NH3) wasdetermined based on the production of the indophenolblue colour complex, and nitrate and nitrite weredetermined together as total oxidisable nitrogen(TON) using the cadmium reduction method (APHA2005). All dissolved nutrient samples were analysedsimultaneously using an automated LACHAT8000QC flow injection system. Samples for totalphosphorus (TP), total dissolved phosphorus, totalnitrogen (TN) and total dissolved nitrogen weredigested using a modified, simultaneous persulphatedigestion (APHA 2005; Hosomi and Sudo 1986).Where samples were turbid, a Kjeldahl digest wasused. FRP and TON were then determined using themethods already described.

2.4 Sampling and Analysis of Macrophyte, LeafLitter, Periphyton and Sediment

Macrophyte and leaf litter samples were collected usinga stratified random sampling approach (Hamilton et al.2001; Merriam et al. 2002). Each reach was firstseparated into either pool or riffle/run habitats, and threeto six random transects were taken within each habitattype. Depending on the stream width at each transect,two to five samples were collected from each transect.All plant components (roots, stems and leaves) wereremoved from 0.27-m−2 quadrats by hand and storedon ice prior to being frozen at −30ºC in the laboratory.Prior to analysis, all macrophyte and leaf litter compo-nents were washed with deionised water to removeattached sediment particles. Periphyton was collectedfrom two to five randomly selected cobbles, along eachtransect where a cobble substrate was present to give atotal of 15 samples per reach. Cobbles were taken backto the lab and the attached biomass removed using awire brush (Hamilton et al. 2001; Merriam et al. 2002).Sediment samples were collected using a stratifiedrandom sampling approach based on replicate coresalong each transect. A PVC core, approximately 10 cmin diameter, was used, and sediment was extrudedfrom each core on site. Three samples of the top 2 cmof sediment were extruded and homogenised fromeach point to obtain a composite sample. Between sixand 11 homogenised composite samples were collectedfrom each reach. Sediment samples were sealed inziplock bags and stored on ice prior to being frozen inthe laboratory.

Macrophyte and leaf litter samples were dried at60ºC in a drying oven until weight was constant todetermine dry weight per unit area. Sub-samples werethen combusted in a furnace at 500ºC for 4 h andreweighed to determine ash-free dry mass throughsubtraction of the ashed mass from the dry weightmass (Hamilton et al. 2001; Merriam et al. 2002).Periphyton biomass was measured in a similarmanner by drying, combusting and weighing eachsample. Sub-samples of sediment from each corewere dried at 60ºC to determine percent dry weight,and a known volume of wet sediment was weighed todetermine sediment density (gml−1). All biomass andsediment samples were ground using a Retsch MM200mixer mill (Haan, Germany). Ground sediment waspassed through a 2-mm mesh to ensure sufficienthomogenisation of samples. The %N and %C of


biomass and sediment samples were determined using amass spectrophotometer (GV Isoprime, Manchester,UK). TPwas determined using a Kjeldahl acid digestionaccording to Lachat Instruments, QuikCemMethod 13-115-01-1-B (LACHAT 1996), followed by colorimetricdetermination using the ascorbic acid method (APHA2005). Absorbance was measured using a ShimadzuUV-601 spectrophotometer (Sydney, Australia).

2.5 Estimation of C, N and P Storage in Biomass,Sediment and Water Column Pools

The total mass of N and P per unit area in leaf litter,periphyton and individual macrophyte species wasquantified by multiplying the standing biomass perunit area of each storage compartment by the mean Nand P concentration of replicate sub-samples (n=5).The standing biomass of macrophyte (separated intoindividual species) and leaf litter was estimated ateach reach from the mean biomass per unit area ofreplicate quadrats (n=10–25) within run/riffle or poolhabitats and the proportion (by area) of each habitattype within the study reach. For Azolla, the reachbiomass was estimated from the mean biomass ofreplicate quadrats (n=10) of 100% cover and anestimate of the proportion of stream surface coveredby Azolla at each reach. Periphyton standing biomasswas determined by tracing a piece of paper around theshape of each rock, cutting around the traced area andweighing the paper. The weight was then converted toarea using a conversion factor obtained from previouslyweighing the same paper and measuring its area. Meanperiphyton biomass per unit area of cobble at each reachwas then multiplied by the estimated proportion ofstreambed area with cobble or boulder substrate. Thetotal N and P stocks (mgm−2) in each biomasscompartment were then estimated by multiplying thestanding biomass (gm−2 dry wt.) of each compartmentby their mean N and P concentrations.

Total sediment N and P stocks were calculated asmilligramme per square metre of surface sediment (0–2 cm) by converting concentration in milligramme perkilogramme to milligramme per litre using the sedimentdensity of each sample. Estimates of sediment N and Pwere made on the top 2 cm of sediment. It was thoughtthat the top 2 cm of sediment was more likely to beactive in exchange with the overlying water column andalso most susceptible to transport downstream. Watercolumn N and P stocks were calculated based on the

mean concentration of N and P at each site and theestimated volume of water in each reach. The volumeof water in each reach was estimated based on themean channel width and depth of each habitat typeand the proportion of riffle/run and pool habitats ineach reach. Proportions of N and P in phytoplanktonwere not estimated separately but are included in theestimate of water column particulate total N and Pstocks.

2.6 Estimation of N and P Exports During the StudyPeriod

N and P loads were estimated at UBRG using therating curve method. A logarithmic relationshipbetween paired load and discharge measurements isused to estimate load on those occasions whenconcentration is not measured (Cooper and Watts2002). Daily discharge data were obtained from theQueensland Department of Natural Resources andWater. N and P concentrations were obtained fromregular fortnightly sampling during the study period andthe mean daily nutrient concentrations during eventflows from event monitoring data provided by theSoutheast Queensland Water Corporation (Seqwater).The regression equation derived from these plots wasused to estimate the concentration of non-sampledperiods from the flow data. Total wet season load wasthe sum of the estimated daily loads from the period 24Nov 2005 to 28 Feb 2006.

3 Results and Discussion

3.1 The Effect of Reach-Scale Geomorphologyon Nutrient Partitioning

The study reaches were variable in terms of theirgeomorphological characteristics (Table 1). TheUBRA and UBRG reaches had a relatively widestream width and no riparian canopy cover. TheUBR17, Emu Creek and Cressbrook Creek reacheshad relatively narrow channels which were shaded bythe adjacent riparian vegetation. The dominant habitattype was riffle/run at UBR17 and Emu Creek andpool at the other three sites. The substrate at UBRA,Emu Creek and Cressbrook Creek was a mix ofcobble, sand and fine sediment. At UBRG, thesubstrate was a mix of sand, fine sediment and gravel


Tab

le1

Geomorphologicalandphysiochem

icalpropertiesof

five

studyreacheswith

intheupperBrisbaneRivercatchm

ent(geom

orphologicalcharacteristicsweremeasuredbetween24

Nov

2005

and4Dec

2005,physiochemicalpropertiesmeasuredfortnightly

from

24Nov

2005

to28

Feb2006

andarepresentedas

theminim

umandmaxim

umvalues

forthatperiod)

UBR17

UBRA

Emucreek

UBRG

Cressbroo

kcreek

Catchmentarea

(km

2)

119

2,06

491

53,86

644

7

Dom

inantriparian

vegetatio

nCasua

rina

s,Callistemon

Pasture

grasses

andshrubs

Callistemon

,Melaleuca

andCasua

rina

Pasture

grasses

Cinna

mom

um,Callistemon

andMelaleuca

Dom

inanthabitat

Riffle/run

Poo

lRiffle/run

Poo

lPoo

l

Meanchannelwidth

(m)

422

545

5

Meandepth(cm)

1825

4498

19

Total

steam

bedarea

(m2)

505

2,18

046

24,97

846

9

Total

volume(m

3)

8854

520

34,85

789

Canop

ycover(%

)60

038

050

Dom

inantsubstrate

Bolder/cobb

leCob

ble/sand

andfines

Cob

ble/sand

andfines

Sandandfines/gravel

Cob

ble/sand

andfines

Woo

dydebris

Abu

ndant

Infrequent

Abu

ndant

Infrequent

Sparse

DO

(%sat)

52–1

4475–8

52–82

36–7

110

–60

pH7.70

–8.32

7.79–7

.95

7.25–7

.80

7.89–8

.37

6.87

–7.32

Con

ductivity

670–

919

240–57

856

5–71

855

7–60

330

8–68

7

Tem

p.(°C)

24.6–3

0.1

26.9–3

0.4

23.0–2

6.4

25.5–2

9.7

24.8–26.0


while at UBR17 the substrate was boulder and cobble.There was little woody debris in the UBRA andUBRG reaches unlike the sites at UBR17 and EmuCreek where there was a relatively high amount ofwoody debris.

The relative importance of leaf litter and macro-phyte storage was related to the geomorphologicalcharacteristics of each reach. Specifically, in reacheswith a relatively narrow channel width and intactriparian vegetation (UBR17, Emu Ck and CressbrookCk) most of the standing biomass N and P was storedin leaf litter, while in the wider unshaded reaches(UBRA and UBRG), macrophyte biomass was thedominant storage (Table 2). The P stored in leaf litterbiomass at UBR17, Emu Ck and Cressbrook Ck (94–153 mgm−2) was similar to values reported byNewbold et al. (1983a) at Walker Branch (77 mgm−2) but much smaller than the total organic P storedin leaf and twig litter in another small deciduousforested stream in the USA (1.3 gm−2) (Meyer andLikens 1979). N storage in leaf litter biomass at these

sites ranged from 2.0 to 3.5 gm−2 which was similarto N storage in coarse benthic organic matter (leavesand wood >1 mm) in several forested streams in theUSA (Hamilton et al. 2001; Mulholland et al. 2000;Triska et al. 1984) but greater than N storage in leafand wood litter in a small tropical stream in PuertoRico (Merriam et al. 2002).

The UBRG and UBRA reaches were distinctlydifferent from the other three sites as the biomasscomprised exclusively of macrophyte biota. Totalmacrophyte biomass at UBRA (455 gm−2 DW) andUBRG (119 gm−2 DW) was similar to maximumbiomass in the Canning River, Western Australia(316 gm−2 DW) (Vincent 2001), a shaded reach inNew Zealand (190–380 gm−2 DW) (Wilcock et al.2002) and in the backwaters of the lower FitzroyRiver, Queensland (0–225 gm−2 DW) (Houston andDuivenvoorden 2002). The macrophyte N and Pstores in the mid to lower reaches of the UBR couldtherefore be seen to be representative of a range oflarger river systems while at the UBR17, Emu Creek

Table 2 Biomass (gm−2 dry wt. ±1 SD), N and P content (% of dry biomass), molar C/N/P ratios of macrophyte, leaf litter andperiphyton at five study reaches

Site Biomass pool Biomass (gm2 DW) %N %P mg Nm2 DW mg Pm2 DW C/N N/P

UBR17 Azolla 30 (4) 2.5 0.22 750 (180) 66 (9) 18 24

Ceratophyllum 15 (9) 2.3 0.27 343 (176) 40 (24) 20 18

FGA 3 (1) 2.7 0.33 81 (10) 10 (3) 18 18

Macrophyte (other) 10 (2) 2.3 0.21 230 (146) 21 (4) 20 23

Periphyton 30 (15) 1.2 0.25 360 (188) 75 (37) 8 10

Leaf litter 218 (110) 1.6 0.07 3,488 (1,238) 153 (52) 33 49

UBRA Azolla 67 (18) 2.0 0.18 1,340 (367) 121 (32) 25 24

Ceratophyllum 341 (210) 2.3 0.26 7,843 (2,472) 887 (546) 20 19

FGA 0.4 2.7 0.30 11 (2) 1 (0.2) 18 19

Nymphaea 47 (17) 3.8 0.29 1,786 (646) 136 (49) 14 28

Emu creek Ceratophyllum 5 (3) 2.3 0.27 115 (72) 14 (8) 20 18

Macrophyte(other) 7 (3) 3.9 0.23 273 (106) 16 (6) 11 36

Vallisneria 12 (3) 3.5 0.33 420 (113) 40 (10) 13 23

Leaf litter 236 (121) 0.9 0.04 2,124 (1,089) 94 (54) 63 48

UBRG Azolla 1 (0.3) 2 0.19 20 (6) 2 (0.6) 25 22

Ceratophyllum 74 (45) 2.3 0.27 1,702 (1,045) 200 (121) 20 18

Vallisneria 44 (11) 2.3 0.28 1,012 (262) 123 (31) 19 18

Cressbrook creek Azolla 5 (1) 2 0.20 100 (27) 10 (2) 25 21

Ceratophyllum 12 (7) 2.3 0.27 276 (169) 32 (19) 20 18

Vallisneria 4 (1) 4.1 0.28 164 (43) 11 (3) 12 31

Periphyton 5 (2) 1.2 0.29 60 (25) 14 (6) 8 9

Leaf litter 173 (101) 1.2 0.06 2,076 (1,216) 104 (57) 44 43


and Cressbrook Creek sites the N and P stored in leaflitter biomass were comparable with smaller forestedstreams in the USA. Because of the substantialheterogeneity in terms of instream nutrient storage,there is likely also to be substantial spatial variabilityin terms of instream nutrient transformations.

The relative importance of heterotrophic and autotro-phic uptake in river systems will depend largely on thedominant biomass instream. In the UBR catchment, therewas a clear shift from mixed leaf litter and macrophytebiomass in the smaller shaded reaches to macrophytebiomass in the mid to lower reaches. This may haveimportant implications because it suggests that the way inwhich nutrients are processed instream may changeconsiderably from the upper to lower reaches. Whilethe nutrient uptake rate per unit of biomass will varysubstantially between communities, it is often the arealcoverage of a particular biotic community that determinesits relative importance in controlling nutrient uptakelengths (Mulholland et al. 1985; Newbold et al. 1983a,b). Results from the UBR and tributaries suggest that ashift from mixed heterotrophic and autotrophic uptakein narrow shaded reaches to predominately autotrophicnutrient uptake in wider, lower-order reaches may occur.It should be noted that, despite substantial differences inriparian shading, there was not a clear gradient inchlorophyll a concentrations from upstream to down-stream sites (median chlorophyll a 2.6–7.7 μgl−1).Therefore, while the relative importance of detritalversus macrophyte uptake and turnover is likely tochange substantially from upstream to downstream, theturnover of N and P by algae is likely to be relativelyconstant across spatial scales.

3.2 Comparison of Total Nitrogen and PhosphorusStorage in Reaches of the Upper Brisbane Riverand Tributaries

The majority of total P stocks in four of the five siteswere in the surface sediments (Fig. 2). P storage in thetop 2 cm of sediments in the UBR catchment (4.2–8.8 gm−2) was similar to a freshwater impoundmentof the Canning River, Western Australia (3–7 g Pm−2) (Vincent 2001), and for sub-tropical streamsediments in a heavily grazed catchment in Florida,USA (116 gm−2 at 0–30 cm∼7.7 gm−2 at 0–2 cm)(Reddy et al. 1996). Like P, the majority of N storagewas also associated with sediments at UBRG andCressbrook Creek with approximately 88% and 92%,

respectively (Fig. 3), and this was also similar toresults reported for the Canning River (94%) (Vincent2001). The contribution of sediment to total N stocksat Emu Creek and UBRAwas equal to 50% and 57%,respectively, which was substantially lower than for Pat these sites. At UBR17, the majority of N and P wasin the leaf litter (66% and 42%) and macrophytebiomass (35% and 25%), with a small contributionfrom periphyton (7% and 14.5%) and water column(1% and 6%) pools.

Total N and P storage was substantially higher at theUBRA, UBRG and Cressbrook Creek sites relative toUBR17 and Emu Creek (Figs. 2 and 3). The differencesin total N and P storage between reaches can be largelyattributed to differences in substrate type. There was asignificant (P<0.05) correlation between sedimentparticle size and TN (R2=0.57) and TP (R2=0.43)across all sites (excluding UBR17). Despite having asubstrate dominated by sediments, the sediment at EmuCreek was coarser than the other three sites, and as aresult sediment N and P were considerably lower inconcentration (Table 3) and total sediment stock.Discharge plays an important role in sediment deposi-tion, and the results from Emu Creek are likely due tosubstantially lower fine sediment deposition at this sitedue to greater flow velocities through the comparativelynarrow channel (Wood and Armitage 1997). This inturn appears to have a major effect on total nutrientstorage and N and P stoichiometry at this site.

At the UBRA and Emu Creek sites, the partitioningof N showed a different pattern than for P. Thedifferences in the partitioning of P relative to N reflectdifferences in the elemental ratios of N and P within

Fig. 2 Total reach P (gm−2) and relative percent contributionof water column, leaf litter, macrophyte, periphyton andsediment storages in each study reach


sediment and biomass components. At UBRA, thesediment concentration (Table 3) was similar to UBRGand Cressbrook Creek. However, the substantiallyhigher macrophyte biomass at this site had a dispro-portionate affect on N distribution (43%) relative to P(13%). This can be explained by the high N/P ratios ofmacrophytes (Table 2) relative to sediments (Table 3)resulting in a more substantial influence on Npartitioning relative to P in the site with comparativelyhigh macrophyte biomass (UBRA). At Emu Creek, thelower proportion of N in bed sediments (∼50%)relative to P (∼96%) reflected the particularly low N/Pmolar ratio of the bed sediments at this site relative toothers (Table 3). The results suggest that differences inthe N/P ratios of the standing biomass and thesediments as well as variation between sediments canexplain the substantial divergence in the storage of Nand P in parts of the river system. The divergence of Nand P distribution within parts of the river systempresent a significant challenge in terms of hownutrients are managed at the catchment scale.

There is an increasing recognition that N and P, ratherthan one supposedly limiting nutrient, should beconsidered when developing management strategies toreduce nutrient inputs to waters (Heathwaite et al. 2000;Davis and Koop 2006; Alexander et al. 2008). Thisstudy suggests that, in river systems with substantialmacrophyte biomass and low N/P sediments, there arelikely to be substantial differences between the twonutrients in terms of instream storage. Therefore, it isreasonable to suggest that management strategiestargeting soil erosion alone will have a greater impacton P fluxes downstream relative to N, while manage-ment actions targeting macrophyte biomass will have a

greater influence on N fluxes downstream relative to P.While the need to better target critical areas of N and Pexport has been identified as an important componentof integrated nutrient management (Heathwaite et al.2000), our results suggest that differences in instreamstorage should also be considered. Divergence of Nand P storage instream has the potential to alter theratio of river N and P export. This in turn may alternutrient stoichiometry downstream and therefore effectecosystem function (Arbuckle and Downing 2001).

3.3 The Effect of a Flow Event on MacrophyteBiomass in the UBR (UBRA)

Prior to a flow event on the 24th November 2005, thenarrow sections of the UBRA reach were dry, and thereach was stagnant and disconnected from down-stream reaches. After the flow event (25 Nov 2005),the reach including the previously dry portion wasflowing. The magnitude of flow was relatively smallin the context of historic peak flows (75th–95thpercentile of historic flows). For Ceratophyllum andNymphaea, there were no significant differences inbiomass before and after the flow event (P>0.05).The biomass of Azolla however was significantlylower after the rain event (P<0.05) (Fig. 4). A total of0.24 kg of P and 2.7 kg of N was exported fromUBRA in association with Azolla during the flowevent. Furthermore, in the biomass samples takenfrom UBRG post-event, Azolla biomass was small.During a previous visit (August 2005) to this site,Azolla was visually estimated to have covered about70% of the water surface. The low Azolla at UBRGwas therefore most likely the result of export duringthe first flow event of the wet season 4 days prior tomacrophyte sampling. Together with the data fromUBRA, this suggests that Azolla biomass is a

Fig. 3 Total reach N (gm−2) and relative percent contributionof water column, leaf litter, macrophyte, periphyton andsediment storages in each study reach

Table 3 Mean (±1 SD) total nitrogen and phosphorus (mgkg−1

dry wt.) in riverbed sediment (n=11 at UBRA and UBRG, n=6at Emu and Cressbrook Ck)

TP TN C/Nmolar

N/Pmolar

UBRA 807 (134) 1,482 (564) 8.8 4.1

Cressbrookcreek

734 (239) 2,667 (1,611) 13.3 7.7

Emu ck 279 (73) 200 (100) 9.9 1.6

UBRG 574 (188) 1,767 (1,233) 12.6 63


relatively mobile nutrient store that is transporteddownstream during flow events.

The loss of Azolla following the first flow event ofthe wet season has several important implicationswithin the river system and downstream. Firstly, thisrepresents a substantial loss of organic N and P fromthe nutrient pool at UBRA, which would otherwise beturned over quickly relative to leaf litter pools(McClain et al. 1998). In addition, the loss of Azollarepresents the loss of a potentially important uptakepathway for dissolved N and P (Greenway and Woolley1999) at a time when dissolved nutrient concentrationsare elevated as a result of a rain event (Kerr et al. 2010).The transport of Azolla from upstream may alsoincrease the total nutrient stocks in the receiving watersand increase the rate of decomposition and mineralisa-tion in downstream bed sediments (den Heyer and Kalff1998; Findlay et al. 1986; Vichkovitten and Holmer2004). This in turn may provide a substantial source ofbioavailable N and P to the water column viamineralisation or a reduction of dissolved oxygen inthe bottom sediments, leading to increased nutrientrelease via redox processes (Wetzel 1999).

3.4 Comparison of Wet Season N and P Loadswith Instream N and P Storage

Pre-flow sediment and biomass data from UBRAwere used to give an estimate of nutrient storage priorto the onset of wet season flows in the mid to lowerreaches of the UBR. The estimated P stock in the midto lower reaches of the UBR was 18.1 T with themajority of this as sediment P (15.8 T) (Fig. 5). Pstorage in the top 2 cm of bed sediment wasapproximately five times greater than the total wetseason P load (2.9 T). The macrophyte P stock was

similar in magnitude to total P export with anestimated P pool of 2.3 T. The majority of this wasCeratophyllum (1.8 T), with Azolla and Nymphaeamaking up the remaining 0.2 and 0.3 T. The estimatedN storage was 50.3 T and was almost five timeshigher than the wet season N load (11.0 T). Unlike P,macrophyte N was greater than N export with 21.8 Tof N in macrophyte biomass (Ceratophyllum 15.6,Azolla 2.7 and Nymphaea 3.5 T). Sediment N storagewas 29.1 T which was more than double the wetseason N load. The N/P ratio of the wet seasonnutrient load (8) was well below the Redfield ratio of16:1 and substantially different than the sediment (4)and macrophyte (21) pools (Fig. 5).

Macrophyte transport during peak flow events maybe an important component of total N and P export. Theestimated N and P content of Azolla in the mid to lowerreaches of the UBR was approximately 9% and 24%of the total P and N export over the study period. Thisis a substantial amount considering that Azolla wasshown to be highly mobile during the first flow eventof the wet season at UBRA. This type of output is notbeing quantified in most catchment export budgets,and this may be an important omission given thepotential implications downstream. There is a risk thatnutrient loads are substantially underestimated whenbudgets are calculated based on water column fluxesalone. While this would appear to be the case in theUBR catchment, the extent to which this may occur inother systems is difficult to ascertain due to a lack ofpublished data on instream storage and biomasstransport during flow events in other systems. Whatcan be inferred from our results is that catchment N

Fig. 5 Comparison of the estimated pre-flow nitrogen andphosphorus storage (T) in the mid to lower upper BrisbaneRiver and total nutrient export (T) during the wet season (24Nov 2005 to 1 Mar 2006)

Fig. 4 Comparison of macrophyte biomass (gm−2±1 SD) inthe upper Brisbane River at UBRA before and after the firstflow event of the wet season of 2005/2006


and P fluxes will likely be substantially underestimatedif a certain set of conditions apply. In river systemswhere flow is highly seasonal and free-floatingmacrophyte biomass is high, our results would indicatethat event flows early in the wet season will transportmuch of this material from the river system. Theintermittent flow patterns in the UBR are similar toother unregulated river systems in the sub-tropics andtropics (Kennard et al. 2009), and the macrophytebiomass is also within the range reported elsewhere(Vincent 2001; Houston and Duivenvoorden 2002;Wilcock et al. 2002). Therefore, because macrophytebiomass fluxes are rarely quantified, it is likely thatcatchment N and P budgets for many sub-tropical andtropical river systems are currently underestimatingnutrient loads.

4 Conclusion

Results from the UBR highlighted important issuesbrought about by the lack of published data on nutrientstorage in large river systems. A major issue in the UBRwas the transport of macrophyte biomass during peakflow events. In other river systems with highly seasonalflows and high free-floating macrophyte biomass,nutrient loads are likely to be substantially under-estimated. Furthermore, the decomposition of riverinebiomass in downstream ecosystems following peakflow events may be more important than previouslythought. Another major finding from our study was thesubstantial divergence in N/P ratios between majornutrient pools and between reaches. In river systemswith free-floating macrophyte biomass, draining low N/Psoils, management actions such as erosion control,macrophyte weed control and riparian restoration arelikely to have a different effect on N versus P fluxes. Thismay have important implications for ecosystem functiondownstream. Therefore, further work focusing on howthese actions might alter N and P stoichiometry instream,and in downstream ecosystems, is needed. Finally, ourresults identified the importance of riparian shading as adriver of instream nutrient storage. There is currentlyinsufficient detail in most catchment models to accountfor variation in biomass storage and therefore uptake andturnover of nutrients within a single river system. Ameasure of riparian cover across a river system mayprovide a simple measure of nutrient processing toaddress this deficiency.

Acknowledgements We wish to thank the Australian ResearchCouncil, the Southeast Queensland Healthy Waterways Partnershipand the Seqwater for their financial support. We wish to thankSeqwater, the Queensland Department of Natural Resources andWater and the Southeast Queensland Healthy Waterways Partner-ship for providing important background data related to the project.We would also like to thank Rene Diocares and Scott Byrnes fortheir help in the laboratory and Queensland Health ScientificServices for analysing water column nutrients.

References

Alexander, R. B., Smith, R. A., Schwarz, G. E., Boyer, E. W.,Nolan, J. V., & Brakebill, J. W. (2008). Differences inphosphorus and nitrogen delivery to the Gulf of Mexicofrom the Mississippi River basin. Environmental Science& Technology, 42, 822–830.

APHA. (2005). Standard methods for the examination of waterand waste water. Washington: American Public HealthAssociation, American Water Works Association, WaterEnvironment Federation.

Arbuckle, K. E., & Downing, J. A. (2001). The influence ofwatershed land use on lake N:P in a predominantlyagricultural landscape. Limnology and Oceanography,46, 970–975.

Cooper, D. M., &Watts, C. D. (2002). A comparison of river loadestimation techniques: Application to dissolved organiccarbon. Environmetrics, 13, 733–750.

Davis, R. J., & Koop, K. (2006). Eutrophication in Australianrivers, reservoirs and estuaries—A southern hemisphereperspective on the science and its implications.Hydrobiologia,559, 23–76.

den Heyer, C., & Kalff, J. (1998). Organic matter mineraliza-tion rates in sediments: A within- and among-lake study.Limnology and Oceanography, 43, 695–705.

Diaz, O. A., Daroub, S. H., Stuck, J. D., Clark, M. W., Lang, T.A., & Reddy, K. R. (2006). Sediment inventory andphosphorus fractions for water conservation area canals inthe Everglades. Soil Science Society of America Journal,70, 863–871.

Dodds, W. K., Lopez, A. J., Bowden, W. B., Gregory, S., Grimm,N. B., Hamilton, S. K., et al. (2002). N uptake as a function ofconcentration in streams. Journal of the North AmericanBenthological Society, 21, 206–220.

Findlay, S., Smith, P. J., & Meyer, J. L. (1986). Effect ofdetritus addition on metabolism of river sediment. Hydro-biologia, 137, 257–263.

Fitzpatrick, F. A., Waite, I. R., D’Arconte, P. J., Meador, M. R.,Maupin, M. A., & Gurtz, M. E. (1998). Revised methods forcharacterizing stream habitat in the national water-qualityassessment program. US Geological Survey Water-ResourcesInvestigations Report. Raleigh: US Geological Survey.

Golladay, S. W., Webster, J. R., Benfield, E. F., & Swank, W. T.(1992). Changes in stream stability following forestclearing as indicated by storm nutrient budgets. Archivfür Hydrobiologie, 90, 1–33.

Gordon, N. D., McMahon, T. A., & Finlayson, B. L. (1992). Streamhydrology: An introduction for ecologists. Chichester: Wiley.


Greenway, M., & Woolley, A. (1999). Constructed wetlands inQueensland: Performance efficiency and nutrient bioaccu-mulation. Ecological Engineering, 12, 39–55.

Hall, R. O. J., Bernhardt, E. S., & Likens, G. E. (2002). Relatingnutrient uptake with transient storage in forested mountainstreams. Limnology and Oceanography, 47, 255–265.

Hamilton, S. K., & Gehrke, P. C. (2005). Australia’s tropicalriver systems: Current scientific understanding and criticalknowledge gaps for sustainable management. Marine andFreshwater Research, 56, 243–252.

Hamilton, S. K., Tank, J. L., Raikow, D. F., Wollheim, W. M.,Peterson, B. J., & Webster, J. R. (2001). Nitrogen uptakeand transformation in a Midwestern U.S stream: A stableisotope enrichment study. Biogeochemistry, 54, 297–340.

Heathwaite, L., Sharpley, A., & Gburek, W. (2000). A conceptualapproach for integrating phosphorus and nitrogenmanagementat watershed scales. Journal of Environmental Quality, 29,158–166.

Hosomi, M., & Sudo, R. (1986). Simultaneous determination oftotal nitrogen and total phosphorus in freshwater samplesusing persulfate digestion. International Journal of Envi-ronmental Studies, 27, 267–275.

Houston, W. A., & Duivenvoorden, L. J. (2002). Replacementof littoral native vegetation with the ponded pasture grassHymenachne amplexicaulis: Effects on plant, macroinver-tebrate and fish biodiversity of backwaters in the FitzroyRiver, Central Queensland, Australia. Marine and Fresh-water Research, 53, 1235–1244.

Kennard, M., Pusey, B., Olden, J., Mackay, S., Stein, J., & Marsh,N. (2009). Classification of natural flow regimes in Australiato support environmental flow management. FreshwaterBiology, 55, 171–193.

Kerr, J. G., Burford, M., Olley, J., & Udy, J. (2010). Phosphorussorption in soils and sediments: implications for phosphatesupply to a subtropical river in southeast Queensland,Australia. Biogeochemistry. doi:10.1007/s10533-010-9422-9.

LACHAT. (1996). Total phosphorus in Kjeldahl digests QuikChemmethod 13-115-01-1-B. Milwaukee: Lachat Instruments.

Mainstone, C. P., & Parr, W. (2002). Phosphorus in rivers—Ecology and management. The Science of the TotalEnvironment, 282–283, 25–47.

McClain, M. E., Bilby, R. E., & Triska, F. J. (1998). Chapter 14:Nutrient cycling and responses to disturbance. In R. Naimen& R. Bilby (Eds.), River ecology and management: Lessonsfrom the Pacific coastal ecoregion (pp. 347–372). NewYork: Springer.

Merriam, J. L., McDowell, W. H., Tank, J. L., Wollheim, W. M.,Crenshaw, C. L., & Johnson, S. L. (2002). Characterizingnitrogen dynamics, retention and transport in a tropicalrainforest stream using an in situ 15N addition. FreshwaterBiology, 47, 143–160.

Meyer, J. L., & Likens, G. E. (1979). Transport andtransformation of phosphorus in a forest stream ecosystem.Ecology, 60, 1255–1269.

Mulholland, P. J., Newbold, J. D., Elwood, J. W., Ferren, L. A., &Webster, J. R. (1985). Phosphorus spiralling in a woodlandstream: Seasonal variations. Ecology, 66, 1012–1023.

Mulholland, P. J., Tank, J. L., Sanzone, D. M., Wollheim, W.M., Peterson, B. J., Webster, J. R., et al. (2000). Nitrogencycling in a forest stream determined by a 15 N traceraddition. Ecological Monographs, 70, 471–493.

Newbold, J. D. (1996). Cycles and spirals of nutrients. In P. Calow&G. E. Petts (Eds.), River flows and channel forms. Oxford:Blackwell.

Newbold, J. D., Elwood, J. W., O’Neill, R. V., & Sheldon, A. L.(1983). Phosphorus dynamics in a woodland stream ecosys-tem: A study of nutrient spiralling. Ecology, 64, 1249–1265.

Newbold, J. D., Elwood, J. W., Schulze, M. S., Stark, R. W., &Barmeier, J. C. (1983). Continuous ammonium enrichmentof a woodland stream: Uptake kinetics, leaf decomposi-tions, and nitrification. Freshwater Biology, 13, 193–204.

Noe, G. B., Childers, D. L., Edwards, A. L., Gaiser, E.,Jayachandran, K., Lee, D., et al. (2002). Short-termchanges in phosphorus storage in an oligotrophic Ever-glades wetland ecosystem receiving experimental nutrientenrichment. Biogeochemistry, 59, 239–267.

Parsons, M., Thoms, M., & Norris, R. (2002). Australian RiverAssessment System: AusRivAS Physical Assessment Pro-tocol. Canberra: University of Canberra.

Reddy, K. R., Flaig, E. G., & Graetz, D. A. (1996). Phosphorusstorage capacity of uplands, wetlands and streams of theLake Okeechobee Watershed, Florida. Agriculture, Eco-systems & Environment, 59, 203–216.

Reddy, K. R., Kadlec, R. H., Flaig, E., & Gale, P. M. (1999).Phosphorus retention in streams and wetlands: A review.Critical Reviews in Environmental Science and Technology,29, 83–146.

Sabater, F., Butturini, A., Marti, E., Munoz, I., Romani, A., Wray,J., et al. (2000). Effects of riparian vegetation removal onnutrient retention in a Mediterranean stream. Journal of theNorth American Benthological Society, 19, 609–620.

Triska, F. J., Sedell, J. R., Cromack Junior, K., Gregory, S. V., &McCorison, F. M. (1984). Nitrogen budget for a smallconiferous forest stream.Ecological Monographs, 1, 119–140.

Vannote, R. L., Minshall, G. W., Cummins, K. W., Sedell, J. R., &Cushing, C. E. (1980). The river continuum concept.CanadianJournal of Fisheries and Aquatic Sciences, 37, 130–137.

Vichkovitten, T., & Holmer, M. (2004). Contribution of plantcarbohydrates to sedimentary carbon mineralization. OrganicGeochemistry, 35, 1053–1066.

Vincent, W. J. (2001). Nutrient partitioning in the upperCanning River, Western Australia, and implications forthe control of cyanobacterial blooms using salinity.Ecological Engineering, 16, 359–371.

Walters, D. M., Fritz, K. M., & Phillips, D. L. (2007). Reach-scalegeomorphology affects organic matter and consumer δ13C ina forested Piedmont stream. Freshwater Biology, 52, 1105–1119.

Wetzel, R. G. (1999). Organic phosphorus mineralization insoils and sediments. In K. R. Reddy, G. A. O’Conner, &C. L. Schelske (Eds.), Phosphorus biogeochemistry insubtropical ecosystems (pp. 225–241). Boca Raton: Lewis.

Wilcock, R. J., Scarsbrook, M. R., Costley, K. J., & Nagels, J.W. (2002). Controlled release experiments to determinethe effects of shade and plants on nutrient retention in alowland stream. Hydrobiologia, 485, 153–162.

Wood, P. J., & Armitage, P. D. (1997). Biological effects of finesediment in the lotic environment. Environmental Man-agement, 21, 203–217.

Young, W. J., Marston, F. M., & Davis, J. R. (1996). Nutrientexports and land use in Australian catchments. Journal ofEnvironmental Management, 47, 165–183.


http://dx.doi.org/10.1007/s10533-010-9422-9

Documents

Nitrogen and Phosphorus Storage in Contrasting Reaches of a Sub-tropical River System