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Characterisation of lowland streamsusing a single‐station diurnal curveanalysis model with continuousmonitoring data for dissolved oxygenand temperatureRobert J. Wilcock a , John W. Nagels a , Graham B. McBride a ,Kevin J. Collier a , Brent T. Wilson b & Beat A. Huser ba National Institute of Water & Atmospheric Research Ltd ,P.O.Box 11 115, Hamilton, New Zealandb Environment Waikato , P.O. Box 4010, Hamilton East, NewZealandPublished online: 30 Mar 2010.

To cite this article: Robert J. Wilcock , John W. Nagels , Graham B. McBride , Kevin J. Collier ,Brent T. Wilson & Beat A. Huser (1998) Characterisation of lowland streams using a single‐stationdiurnal curve analysis model with continuous monitoring data for dissolved oxygen andtemperature, New Zealand Journal of Marine and Freshwater Research, 32:1, 67-79, DOI:10.1080/00288330.1998.9516806

To link to this article: http://dx.doi.org/10.1080/00288330.1998.9516806

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New Zealand Journal of Marine and Freshwater Research, 1998, Vol. 32: 67-790028-8330/98/3201-0067 $7.00/0 © The Royal Society of New Zealand 1998

67

Characterisation of lowland streamsusing a single-station diurnal curve analysis modelwith continuous monitoring data for dissolved oxygenand temperature

ROBERT J. WILCOCKJOHN W. NAGELSGRAHAM B. McBRIDEKEVIN J. COLLIER

National Institute of Water & AtmosphericResearch Ltd

P.O.Box 11 115Hamilton, New Zealand

BRENT T. WILSON

BEATA. HUSER

Environment WaikatoP.O. Box 4010Hamilton East, New Zealand

Abstract Twenty-three lowland streamsthroughout the Waikato region, New Zealand, incatchments having a wide range of land uses andintensities, were monitored continuously over 3 -4-day periods for changes in dissolved oxygen (DO)and temperature. A single-station diurnal curvemodel, DOFLO (Dissolved Oxygen at Low Flow),was used to produce reach-averaged values for:K2(20), the reaeration coefficient at 20°C; Pmax., themaximum daily rate of photosynthetic productionof oxygen; R20, the daily respiration rate at 20°C;and Q10, the ratio of respiration rates 10°C apart.In addition, 24-h average values for the ratio P/Rwere calculated and maximum and minimum valuesof DO and temperature tabulated for each site.Values of K2(20) (0.05-40 d-1, median 6.0 d-1) werein broad agreement with values calculated using amodified form of the O'Connor-Dobbins equation.

M97023Received 26 June 1997; accepted 18 September 1997

Values of gross primary production in daylight (0.5-29.2 g m - 2 d -1) calculated from Pmax. (1.75-86.5 g m -3 d-1) were similar to data reported forother streams in agriculturally developed catch-ments in New Zealand and North America.Respiration rates (3.50-55.0 g m - 3 d-1) weregenerally larger than values reported in theliterature, and P/R ratios were mostly well below1.0, indicative of heterotrophic respirationassociated with decaying vegetation and otherorganic inputs and consistent with diurnal DOexcursions of 40 to 120% saturation beingcommonly observed. Maximum daily temperaturesup to 25.7°C (median 20.5°C) were weaklynegatively correlated with DO minima. Fivegroupings of streams were identified from DOFLOparameter values, with K2(20) being most critical inregulating average DO deficits.

Keywords dissolved oxygen; temperature;lowland stream; agriculture; diurnal curve analysis

INTRODUCTION

Lowland streams in agriculturally developedcatchments in New Zealand have been describedas being in "poor condition" (Smith et al. 1993).The streams typically have reduced biodiversity,taxonomic richness, and numbers of sensitiveinvertebrate species, by comparison with streamswith < 30% of their catchments converted toimproved pasture (Quinn & Hickey 1990). Lowlandstreams are important migratory pathways for manynative fish species so that the presence of stressfulconditions (large diurnal variations in temperaturewith high daily maximum values, highconcentrations of ammoniacal nitrogen, and lowdissolved oxygen (DO) concentrations) and the lossof suitable habitat and food resources has animportant bearing on their survival (McDowall1990; Boubee et al. 1991; Richardson et al. 1994).Where streams pass through heavily stocked areas,

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68 New Zealand Journal of Marine and Freshwater Research, 1997, Vol. 32

such as dairy farms, there is often obvious damageto stream banks caused by trampling (Williamsonet al. 1992). The presence of adventive aquatic plantspecies has caused radical changes in streamhabitats and, in the instance of submerged weeds(e.g., Lagarosiphon major, Egeria densa), oftenexacerbated diurnal fluctuations in pH and DO(Edwards 1968). Riparian vegetation may providesignificant organic loadings to lowland streams byintercepting nutrients in agricultural run-off andthen decaying in stream channels (Wilcock et al.1995). Some lowland streams have been excavated,or periodically cleared of aquatic plants using adragline, to improve drainage in low-lying areas(ARWB 1987; McColl & Ward 1987).

The Waikato region of New Zealand isintensively farmed having c. 1.4 x 106 dairy cattle,0.65 x 106 beef cattle, 0.24 x 106 pigs, deer, andgoats, and 1.9 x 106 sheep on a total farmed area of8.33 x 105 ha (Statistics New Zealand 1996). As aconsequence, lowland streams in the region areoften impacted by wastewater discharges, waterabstraction during droughts, removal of riparianshading, and diffuse pollution. Increasingly inrecent times, land previously used for beef cattleand sheep farming now supports dairy herds, so thatmany faster flowing streams in moderately steeply-sloping catchments are potentially at risk of stressmore commonly associated with "typical" lowlandstreams located in flood plains.

Dissolved oxygen depletion and high watertemperatures are known to affect stream faunaadversely (USEPA 1986; Amour 1991 ; Richardsonet al. 1994). Key parameters that affect DO may,therefore, be used to categorise streams accordingto their abilities to withstand inputs of oxygen-demanding materials, and to maintain DO levelsthat can support diverse ecosystems. Diurnaltemperature extremes are a consequence of therelative amounts of incident radiation fallingdirectly on stream surfaces, and shading providedby riparian vegetation (Westlake 1966; Collier etal. 1995). Pastoral streams typically have lessshading and are exposed to greater proportions ofsolar radiation, than streams having more riparianvegetation (Quinn et al. 1994; Collier et al. 1995).

In this study lowland streams of an intensivelygrazed area of New Zealand were monitored forDO and temperature during summer and the resultsanalysed to produce values of descriptiveparameters for classifying streams according tophotosynthetic productivity, respiration, andreaeration.

METHODS

Data collectionTwenty-three lowland streams in the Waikato region(Fig. 1, Table 1) were monitored at a total of 25sites all < 200 m above sea level (a.s.l.), except forOraka Stream (220 m a.s.l.) in order to define therange of conditions they exhibit during summer.Some stream sites (Piako, Topehaehae, and Waitoaat Mellon Road) were monitored twice, so that atotal of 28 data sets were collected. Water qualitydataloggers (DataSonde 3, Hydrolab Corp., Austin,Texas, United States) were used to monitor DO,temperature, and in some instances, pH,conductivity, and (nephelometric) turbidity. Statedaccuracies of temperature and DO by themanufacturer are ± 0.1 °C and ± 0.2 g m~3,respectively (Hydrolab technical note 204,Hydrolab Corp., Austin, Texas, United States).Dataloggers were calibrated before and after eachdeployment in accordance with the manufacturer'srecommendations by ensuring that temperatureswere within ± 0.1 °C of a calibrated referencethermometer, and DO agreed to within + 0.1 g irr3

with a saturated-air calibrated DO meter and probe(YSI models 58 and YSI 5739, respectively). Inaddition, field measurements of temperature andDO were made at the beginning and end of eachdeployment and compared with datalogger values.Agreement between datalogger and fieldmeasurements of temperature and DO was within± 0.2°C and ± 5%, respectively, on all occasions.Dataloggers were deployed for ~i-A days in periodsof protracted fine weather and steady flow duringthe summer months of December-March, 1992-95 (with one exception in September) and recordeddata at 15-min intervals. Datasets from field-deployed instruments were transferred to a personalcomputer for analysis and production of graphicalrecords of DO (g irr3) and temperature (°C)variation with (New Zealand Standard) time.

Single-station diurnal curve analysisDissolved oxygen and temperature records duringperiods of stable weather and stream flow exhibitedreproducible diurnal variations. A 24-h subset ofeach dataset from the middle of each monitoringperiod and beginning at midnight was subjected toanalysis by the single-station diurnal curve method(Odum 1956; Hornberger & Kelly 1975) using acomputational model, DOFLO (Dissolved Oxygenat Low Flow) (McBride 1995; Wilcock et al. 1995).Maximum temperatures and minimum DO levels

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Wilcock et al.—Characterisation of lowland streams 69Fig. 1 Location of stream sites.Site numbers relate to streamslisted in Table 1.

I170» E

- 40° SN

D DataSonde Deployment Sites

were also recorded as being useful indicators ofstress in lowland streams, given that stream faunaare sensitive to extreme values (USEPA 1986;Boubee et al. 1991 ; Richardson et al. 1994; Collier1995). Diurnal curve analysis using DOFLO yieldsparameter values that describe existing streamconditions at the time of monitoring, and alsoquantify the stream's ability to withstand largeswings in diurnal DO. Monitoring data andsubsequent data analysis were carried out at twodifferent times for three river sites, to examine theconsistency of the derived parameters and streamproperties.

The DOFLO model assumes that diurnal DO isaffected by three fundamental processes: reaeration,plant and bacterial respiration, and photosynthesis,as described by Equation 1 :

where: C is the DO at time t, Cs is the saturationvalue for DO calculated as a function oftemperature (Benson & Krause 1984), K2 is thereaeration coefficient (d"1), and P¡ and R¡ are theinstantaneous rates of photosynthetic productionand respiration (g m~3 d"1), respectively. Theeffects of longitudinal spatial gradients may beignored, for a given site, provided that a uniformdistribution of plants extends upstream for adistance of at least:

ÍÜ

where jj is the mean stream velocity (m s-1) in thereach (Chapra & Di Toro 1991). Details of themodel equations and calibration procedure are givenelsewhere (McBride 1995; Wilcock et al. 1995).Parameter values from DOFLO were analysedusing established ordination techniques (Datadeskver. 6.0, Data Description Inc., Ithaca, N.Y.,

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70 New Zealand Journal of Marine and Freshwater Research, 1997, Vol. 32

United States; StatSoft, Tulsa, Oklahoma, UnitedStates).

Reaeration, the process in which oxygen transferacross the air-water interface is facilitated byturbulence, is characterised by the value of thereaeration coefficient at 20°C, A"2(20) (d"1 )• As A (20)values increase, stream DO is increasinglydominated by reaeration and exhibits smallerdiurnal variation. Community respiratory oxygenuptake, expressed by R20 (the respiration rate at20°C, g O2 m"3 d"1), is assumed to only vary withtemperature over a 24-h period. gio> the ratio ofrespiration rates 10°C apart, is also used incalibrating DOFLO. The magnitude of R20determines the extent of the DO minimum, or "sag",and is offset by photosynthesis and reaeration.Photosynthetic activity by plants during daylighthours, characterised by Pmax, (the maximum dailyphotosynthesis rate occurring at solar noon, g O2m~3 d"1 ) has a bearing on DO maxima—particularlywhen supersaturation occurs (Edwards 1968). The

model yields values of^2(20), ^max., 20> Q\o> andP/R, the average ratio of photosynthetic productionto respiration. Chapra & Di Toro (1991) haveestablished that the average daily deficit, D (thedifference between saturation DO at ambienttemperature and the actual DO), is related to K2and to 24-h average values of R and P by:

(2)

Thus, when P/R = 1, the daily average deficit iszero. When P/R > 1 average DO is supersaturated,whereas when P/R < 1 average DO is belowsaturation and may approach zero when Ki is small(e.g., < 0.1 d"1).

The calibration procedure provided for a uniqueset of calibrated parameter values, guaranteed bythe result that reaeration alone governs the phaselag between the time of DO maximum and solarnoon. This has been demonstrated for the modelwithout diurnal temperature variation (Chapra &

Table 1 Streams monitored and principal land uses of catchments.

Stream

AwaroaKaniwhaniwhaKaniwhaniwhaKomakorauMangaoneMangaorongaMangaotamaMangawaraMatahuruOhinemuriOpitonuiOpuatiaOrakaPiakoPiakonuiToenepiTopehaehaeWaiauWaihekauWaihouWaitoaWaitoaWhakapipiWhangamarinoWharekawa

Site

Moseley RoadWright RoadLimeworks Loop RoadHenry RoadAnnebrooke RoadFarm BridgeWhatawhata M8Olds RoadWaiterimuState Highway 25Awaroa StreamBrien RoadWaione RoadKiwitahiPaku RoadTahuroa RoadBell RoadFordCurtains BridgeScorpion RoadWaharoaMellon RoadState Highway 22Jefferies RoadAdams Farm

Site no.f(Fig. 1)

62728162632181511917

3117222123

214302413584

Catchmentarea (km2)

28.0671258704721

204105

17292747

105271627154823

123357499647

Grazing intensity(total no. cows,total s.u. ha^')t

862, 8.41811,8.70,7.18543, 15.99589, 15.312073, 22.5216,7.715551, 11.61957, 14.8945, 7.40,2.80,7.00,014042§, 10.91240,5.63816, 17.2735, 8.00,02920, 6.50,01010,4.653150§, 11.8260, 6.85274, 10.30,3.5

fSite numbers 3, 10, 12, 19, 20, 25, 29 not used in this study. Total streams = 23; total sites = 25.JOne stock unit (s.u.) is equivalent to a 55 kg ewe. Adult cattle are each equivalent to 6.0 s.u.

(Metherell & Morrison 1984).§From Williams (1993).

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Table 2 Results of single-station diurnalat one site.)

Stream site

AwaroafKaniwhaniwha (W)Kaniwhaniwha (L)KomakorauMangaoneMangaorongaMangaotamaMangawaraMatahuruOhinemuriOpitonuiOpuatiaOrakaPiako/1Piako/2PiakonuiToenepiTopehaehae/1Topehaehae/2WaiauWaihekauWaihouWaitoa (W)Waitoa/1Waitoa/2WhakapipiWhangamarinoWharekawa

Name

Moseley RoadWright RoadLimeworks Loop RoadHenry RoadAnnebrooke RoadFarm BridgeWhatawhata M8Olds RoadWaiterimuState Highway 25Awaroa StreamBrien RoadWaione RoadKiwitahiKiwitahiPaku RoadTahuroa RoadBell Road BridgeBell Road BridgeFordCurtains BridgeScorpion RoadWaharoaMellon RoadMellon RoadState Highway 22Jefferies RoadAdams Farm

curve analysis

Date

31 Jan 199515 Feb 199515 Feb 199511 Feb 19952 Mar 19955 Mar 19955 Dec 1994

17 Dec 19944 Feb 19951 Jan 1994

6 Dec 19946 Dec 19949 Mar 1994

19 Mar 199429Sep l99522 Feb 199418 Jan 1995

18 Mar 19949 Dec 19941 Feb 1992

17 Dec 199419 Dec 199416 Dec 199414 Mar 199422 Mar 199410 Dec 199418 Dec 199410 Jan 1994

. ((W) and (L) are site codes

Flow(litres s"1)

107732300

-400402278

-332314280

747122808304028

10585

183302378360570890230

-316

Depth(m)

1.70.790.20-0.660.460.79-0.540.540.24-0.570.531.010.230.230.800.710.390.500.450.530.650.840.52-0.44

^2(20)

(d"1)

0.052.50

40.03.008.508.509.000.052.75

11.021.04.109.502.256.009.501.556.005.754.759.006.00

13.010.50.106.750.051.50

where there are two sites on one river;

p' max.

(g m-3 d"1)

22.518.840.012.85.50

38.537.532.517.020.015.512.54.00

31.010.58.506.00

30.048.3

3.5035.0

1.7586.544.020.013.532.04.25

^20(gnr 3 d- ' )

8.103.50

55.012.535.027.047.510.98.50

15.037.510.022.520.012.810.810.038.328.5

4.048.5

5.7540.039.0

7.226.010.83.75

Ö10

1.252.002.002.002.001.252.002.002.001.001.252.002.002.001.501.502.002.001.002.001.002.001.001.251.251.252.001.00

P/R

0.941.870.330.340.070.470.450.980.720.450.150.460.110.540.340.310.250.290.650.360.280.200.830.410.900.210.970.42

site/1 and 12 are from repeat

DOmin.(g nr 3 )

3.79.27.65.46.26.16.34.27.17.98.96.39.13.78.58.84.34.04.48.53.89.85.95.84.25.74.36.8

DOmax.(g n r 3 )

7.812.09.76.87.09.5

10.29.99.69.59.79.0

10.18.29.79.65.57.3

10.49.36.9

10.112.29.57.47.1

10.37.4

1 min.

C )

22.518.315.418.915.518.216.121.019.017.711.115.29.5

17.813.015.218.017.319.315.518.212.517.718.516.418.021.122.3

surveys

*max.

(°C)

25.021.920.223.217.222.421.323.421.320.212.924.712.819.515.317.219.219.123.220.724.013.822.518.917.819.623.625.7

cock et I

Ihai

CO

o

1Q"oo*

3Cu

3

fSingle-station curve analysis is strictly not applicable to this data because of non-compliance with the condition that stream plants be uniformly distributedfor a sufficiently long distance i>1UIKi) upstream of the monitored site (Chapra & Di Toro 1991).

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72 New Zealand Journal of Marine and Freshwater Research, 1997, Vol. 32

Di Toro 1991). Also, the Q\Q value alone affectsthe slope of the DO profile from midnight to dawn.Model fits are sensitive to changes in parameters(McBride 1995).

Stream characteristicsValues of A 2(20) from DOFLO were compared withvalues calculated from a version of the O'Connor-Dobbins (1958) equation modified for applicationin New Zealand streams (Equation 3) (Himmelblau1964; Wilcock 1984). Average depths (H) andchannel widths were measured over 50-100 mreaches above each site (Collier et al. 1998) and,together with average flow, used to calculate valuesof U.

K2(20) (3)

Values of/ 2(20) were calculated using velocitiesand depths measured when dataloggers weredeployed, or were derived from velocities anddepths measured at known flows at other times andthen adjusted to the same flows as the modelledresults. To do this we assumed that for many streamsU and H vary with flow (Q, litre s"1) in such away that A^o) is approximately proportional tog"0-3 (Park 1977; Wilcock 1988).

Flows were obtained in three ways: from flowrecords for monitored sites; from instantaneous flowgaugings made at each site; and by area-scaling ofknown flows recorded at upstream or downstreamsites. Thus, using flows from an upstream site:

-Q«A

(4)

where A and Au are the catchment areas for the studysite and the upstream site with known flow, Qu,respectively.

Land use intensityFarming intensity in each of the catchments wasexpressed in terms of the number of equivalent stockunits (s.u.)* per ha (Metherell & Morrison 1984;Nguyen & Goh 1994). The numbers of cows in eachof the site catchments were obtained either directlyfrom Environment Waikato (Dairy FarmMonitoring Programme), or from Williams (1993).Other stock numbers were obtained from Statistics

*One stock unit (s.u.) is equivalent to a 55 kg ewe. Adultcattle are approximately equivalent to 6.0 s.u.(Metherell & Morrison 1984).

New Zealand ( 1996) by assuming that the densitiesof beef cattle, sheep, and other stock (pigs, deer,and goats) in each stream catchment were the sameas for the respective territorial local authorityregions (Statistics New Zealand 1996).

RESULTS

Locations of stream sites and times whendataloggers were installed are indicated in Fig. 1and Tables 1 and 2. Further details are givenelsewhere (Wilcock et al. 1996). Streams variedwidely in catchment area, land-use intensity,channel size, and flow. Catchment areas were 15-357 km2 and grazing densities ranged from 0 s.u.ha"1 in afforested catchments to 22.5 s.u. ha"1 inareas of intensive dairy farming, with the medianbeing 8.2 s.u. ha"1 for grazed catchments. Streamchannel dimensions and proportion of bed coveredby macrophytes measured during summer and atflows similar (79-230%) to this study are given byCollier et al. (1998). Stream flows (28-890 litre s"1,median 302 litre s"1), mean depths at each site, DO,and temperature maxima and minima, and derivedvalues of DOFLO parameters (Q \0, and P/R) are listed in Table 2.

DISCUSSION

Maximum and minimum temperature and DODissolved oxygen maxima were often 10-12 g m"3,or c. 120-145% of saturation values, whereasminima were commonly c. 4 g m~3, or 40% (Table2). Although little has been published on thepreferences and tolerance levels of native fish andinvertebrate species for DO, it is known that someexotic species will be adversely affected when DOfalls below 50% of saturation for prolonged periods.The effects are exacerbated by elevatedtemperatures (Scott 1982). More extreme variationsin DO have been observed in another New Zealandlowland stream (Wilcock et al. 1995) and mightoccur more commonly than has been observed inthis study.

Temperature maxima were commonly above23°C and occasionally above 25°C. These maximawere below those known to be lethal to native fishbut exceeded preferred temperatures derived fromchoice experiments, for several species,(Richardson et al. 1994). Stream temperaturemaxima exhibited were negatively correlated withDO minima (R = - 0.55, P < 0.01), the three coolest

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Wilcock et al.—Characterisation of lowland streams

2673

DOFLO analysisResults using DOFLO in conjunction with streamDO and temperature data are given in Table 2,showing the start date for each 24-h period modelledand values of the key stream parameters (A (20>Anax.> 20> a n d Qio)- Diurnal DO curves generatedby DOFLO, and selected experimental dataloggervalues, for the Mangawara and MangaorongaStreams and the Waihou River (Fig. 3) exemplifythe diversity of stream types in the study. TheMangawara Stream site, in a heavily grazedcatchment (11.6 s.u. ha~') had a low reaerationcoefficient (0.05 d"1), causing the DO maximumto occur 6 h after solar noon (Chapra & Di Toro1991). The midnight-to-dawn DO minimum was4 g m 3 and would have been lower if the P/R hadbeen less than found (0.98). By contrast, the WaihouRiver, situated in plantation forest, had reducedlevels of photosynthesis (Pmax. = 1.75 g m~3 d"1)and respiration (^?20= 5.75 g m~3 d~') and a muchhigherA"2(20) value (6.0 d~') with the result that DOwas close to 10 g m~3 d"1 (95% of saturation atstream temperatures) throughout the 24-h period.The Mangaoronga Stream, also in an intensivelygrazed catchment (22.5 s.u. ha"1) had a DO rangethat was intermediate between the other two

12

Fig. 2 Comparison of diurnal minimum dissolved •«oxygen (DO) concentrations and maximum temperatures jg(Tmax.) measured in streams. S

streams with highest DO values being located eitherin established forest catchments (Waihou andOraka) or in a catchment undergoing conversionfrom pastoral farming to plantation forestry(Opitonui) (Fig. 2).

Mangawara

Waihou

i . . . i

Mangaoronga

8 12 1C 20

Hours since midnight24

Fig. 3 Dissolved oxygen (DO) profiles generated byDOFLO (—) fitted to experimental data recorded withdataloggers (•). Mangawara Stream: K2(20) = 0.05 d"1,Pmax. = 32.5gm3d"1,^20=10.9gm3d-',e10 = 2.0andP/R = 0.98; Waihou River: K2(2Q) = 6.0 (H, />max = 1.75g m3 d"1, #20 = 5.75 g m3 cH, Qio = 2.0 and P/R = 0.20;and Mangaoronga Stream: A (20) = 8.50 d"1, Pmax =38.5g m3 d"1, R20 = 27.0 g m3 d"1, Ql0 = 1.25, and P/R =0.47.

streams. The moderately high reaeration coefficient(8.5 d~') prevented significant depletion of DOduring midnight-to-dawn. However, the baselinelevel of DO at 6 g m"3 (cf. 10 g m~3 for the WaihouRiver) indicated substantial removal of DO fromthe water by respiration (./?20 = 27 g mr3 d"1),counteracting the input from reaeration. The highrate of photosynthetic production during the day(•Pmax. = 38.5 g n r 3 d"1) ameliorated the dawn-to-dusk DO profile.

Calibration parametersValues of A (20) ranged from 0.05 to 40 d~' (median6.0 d"1) and were similar to those measured in small

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74 New Zealand Journal of Marine and Freshwater Research, 1997, Vol. 32

lowland streams elswhere (NCASI 1985; Thyssen& Erlandsen 1987). Comparison with calculatedvalues of A (20) (Equation 3) yielded a linearrelationship (R2 = 0.78, n = 22; slope ± 95% C.L. =0.90 ± 0.22). The modified O'Connor-Dobbinsexpression for £2(20) (Equation 3) was derived fromgas tracer measurements made in New Zealandrivers having flows of 0.3—36 m3 s"1 in whichnatural turbulence causes on average a 40% greaterrate of reaeration than is predicted by theory(Wilcock 1984). Many of the streams in the presentstudy had substantial nocturnal respiration oxygendemands, characterised by if 20 (Table 2), and wouldhave recorded DO values approaching zero were itnot for turbulent flow causing significant reaerationto occur. A similar result was found for theWhangamaire Stream (Wilcock et al. 1995).

The dependence of 2(20) on velocity and depth(Equation 3) is shown by the very low values(<0.10 d ') that occurred when velocities were nearzero, such as for tributaries flowing into the muchlarger Waikato River (Awaroa, Mangawara,Whangamarino), and for deeper streams (e.g.,Waitoa/2) at lower than usual flows.

Photosynthetic production rates are mostcommonly expressed as gross primary productionduring daylight (GPP). Average photosyntheticproduction rate (P) over 24 h is related to .Pmax. byEquation 5 (Chapra & Di Toro 1991):

Qpp _n

(5)

(6)

where/is the photoperiod (h) and t is 24 h. Valuesof Pmax. (1.75-86.5 g m"3 d"1, median 19.4 g m"3

d"1) converted to GPP (Equation 6) gave valuesranging from 0.5 g O2 m~2 d"1 for Waihou (forestr ang ing f ro m0.5 g O2 m~2 d"1 for Waihou ( fo(pasture catchment, 4.6 s.u. ha"1), and a median of6.8 g O2 m~2 d"1 for all sites. These compare withGPP values of < 0.3-9.6 g O2 n r 2 d"1 measured ina longitudinal survey of the Taieri River (SouthIsland, New Zealand), during summer-autumn(Young & Huryn 1996), 0.1-1.7 g O2 m"2 d"1 forWalker Branch (a first-order forest stream in easternTennessee) (Marzolf et al. 1994), and representativemean summer values of < 0.1—44.2 g O2 m~2 d"1

for the Vermilion River (an agriculturally developedprairie river sytem in Illinois) (Wiley et al. 1990).Stream channels in the present study were generallynarrow (c. 6 m) and incised, with an average

macrophyte cover of 26% during December (Collieret al. 1998). Channel shading by banks and riparianvegetation may limit GPP in these streams, bycomparison with broad shallow streams with littleshading.

Values of ^20 (3.50-55.0 g nr^d" 1 , median13.9 g m~3 d"1) multiplied by H gave arealrespiration rates ranging from 1.6 g m~2 d"1 forWaiau (forest) to 37.5 g m~2 d~l for Mangaotama(pasture, 7.7 s.u. ha"1), with a median of10.8 g m~2 d"1. These data compare with 0.7—9.8 g n r 2 d"1 (Taieri River), 1.06-2.48 g n r 2 d"1

(Walker Branch), and 6.2-41.6 g m"3 d"1

(Vermillion River), indicating that many Waikatolowland streams may have low nocturnal DObecause of high respiration rates.

Values of P/R were all, with the exception ofKaniwhaniwha (W) (Table 2), < 1.0 (median 0.42).Edwards ( 1968) has estimated that over a diurnalcycle, consumption of oxygen by macrophytes isc. 75% of the production, so that in the absence ofalgae and heterotrophic respiration P/R valueswould be expected to be > 1.0. The low P/R valuescalculated for the Waikato lowland streams may bea result of heterotrophic respiration, associated withdecaying plant material and other organic inputs.This is consistent with diurnal DO ranges typicallyof 50-130% Cs, characteristic of lowland streamshaving a high density of plant material on days ofbright sunlight (Edwards 1968). The extent and rateof benthic respiration may also affect the dynamics(amplitude and timing) of diurnal changes in DO.

Cluster analysisThe DOFLO model provides for a unique set ofparameter values when calibrated with datacovering a 24-h period. The values of the parametersreflect the significance of the processes that affectDO in streams. Fig. 4 is a dendogram of the resultsof complete linkage cluster analysis of the threeDOFLO calibration parameters (A"2(20)> Pmax.., 20)with observations of temperature and DO variation.Cluster analysis provided a method of groupingstreams according to the similarity of processesaffecting their DO. Mean values of A"2(2o> Pma\.,R20, average maximum temperature, minimumDO, diurnal range ( ADO ), and calculated 24-h-average DO (Equation 2) are listed for each cluster(Table 3).

Cluster 1 streams characteristically had very lowvalues of^2(20) (average 1.1 d"1), with the resultthat DO was especially sensitive to P and R

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Temp,range(°C)

DOrange(gm-3) River/stream

t Whangamarinot Mangawara

*Piako/l*Awaroa

t Waitoa/2Kaniwhaniwha(W)

Matahurut Toenepi

WharekawaWaiau

WaihouPiako/2

KomakorauOpuatia

* Topehaehae/1Mangaoronga

t Topehaehae/2•Waihekau

MangaotamaWaitoa/1Piakonui

OhinemuriWhakapipi

WhakaMangaone

OpitonuiWaitoa(W)

Kaniwhaniwha(L)

max. » R 20

Low P max.and Ä 20

BELOW AVERAGEJST 2(20) AND /?20

Pmax./?20

K 2(20) AND R 20> AVERAGE

' max. £ R 20 (except for Ohinemuri)

HIGH K 2(20),max. AND R 20

small = 2 small = 1 t DO minima < 5 g nr3

large =>5 arge = £5 * DOminima<4gm-3Fig. 4 Dendogram of results of cluster analysis (complete linkage) for A (20)> Anax >and .R20 calibration parameters. Observed diurnal ranges for temperature and dissolvedoxygen (DO) are also shown.

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76 New Zealand Journal of Marine and Freshwater Research, 1997, Vol. 32

(Equation 2). These streams were generally warmerthan average and had wide ranges of diurnal DO,with nocturnal minima of 3.7^4.3 g m~3 and daytimemaxima at or near saturation because of highphotosynthetic production rates (17.0-32.5 gm~3 d"1).Diurnal excursions of DO may be even greaterduring low flow conditions in summer, when A^o)may also be lower than usual. Such streams mayfrom time to time exhibit very low DO valuesbrought about by small changes in flow that causereductions in K2(20)> increased respiration, ordecreased photosynthetic production. Low DOvalues may also result from increases intemperature, nutrients and organic loadings causingPIR to decrease. Because of their sensitivity to suchchanges we have categorised these streams ashaving a high risk of DO deficit stress.

Dissolved oxygen minima of streams groupedin cluster 2, with the exception of Toenepi (4.3 gnT3), were above 5 g m~3. Diurnal ranges tended tobe low ( ADO = 1.2 g m~3). Values of AT2(20) were1.50-6.00 (average 3.8) d~', but Pmax, and R20 werebelow average. Thus, although PIR values were 0.2-0.46 there was sufficient reaeration to maintain DOat levels able to support a diversity of streamorganisms (USEPA 1986). These streams areconsidered to constitute a low (DO) stress riskgroup. Changes in riparian management causing areduction in shade and increased productivity andrespiration, coupled with low A^o) values duringdroughts, could reduce night-time DO levels.

Cluster 3 streams characteristically hadmoderate A^o) values (5.75-10.5 d"1) and higherthan average levels of photosynthetic activity {Pm&x.= 30^8.3 g m"3 d"1) and respiration (R20 = 30.0-48.3 g m~3 d~ ' ) . Diurnal ranges of DO andtemperature were highly variable for these streamsbut values of DOmjn. were the lowest of anygrouping. Temporal variations in DO and

temperature in cluster 3 streams were similar tothose in cluster 1, which had lower values of A()/"max.* and/?20- Streams in cluster 3, because of theirhigher than average A^po) values, are categorisedas having a moderate risk of DO stress.

Streams having very high K2(20) values hadcomparatively small DO excursions and did notdeviate greatly from saturation. Streams withmoderate-high R20 values (cluster 4) had narrowdiurnal DO ranges (0.8-1.6 g m~3) and minima of5.7-9.1 (mean 7.8) g m~3. As A^o) increases, DOis increasingly dominated by reaeration and less byP and R and concentrations tends towards saturationvalues. Cluster 4 streams were cooler and lessproductive than the other streams, because ofgenerally shadier environments, whereas the twostreams in cluster 5 had very high rates ofphotosynthetic production and respiration offset byvery high reaeration rates. Streams in clusters 4 and5 have a low risk of incurring large DO deficits.

That repeat monitoring and analysis of streamsites yielded different sets of parameter valuesindicates the temporal variability of Q (and henceA"2(20))> 20. a nd Pmax.m these streams and indicatesthe need for care in choosing the time when datamonitoring is carried out. Management decisionmaking might require that this be carried out underspecified low flow conditions during late summerin order to describe the most stressful conditions.The use of other ordination techniques includingunweighted pair-group average and unweightedpair-group centroid methods, respectively, andpartition methods (K-means clustering) producedsimilar groupings.

CONCLUSIONS

Diurnal changes in stream DO and temperatureduring summer were monitored continuously for

Table 3 Mean parameter values from dissolved oxygen_(DO) at low flow (DOFLO), maximumtemperature, DO minimum ( DO min ) and diurnal range ( ADO), and calculated 24-h-average values(24-h DO) calculated from Equation 2, for cluster grouping of streams.

Cluster no.

12345

All streams

•^2(20)(d"1)

1.13.88.1

11.026.5

7.2

(g m"3 d"1)

24.87.3

38.911.263.3

23.3

^20(g m -3 d-')

9.98.4

38.124.547.5

21.4

T

(°C)

21.820.421.416.721.4

21.4

DO m i n

(gm"3)

5.27.15.17.86.8

6.3

ADO(gm-3)

4.11.23.91.24.2

2.7

24 h DO(g nr3 , % sat.)

8.4 (93)7.9 (84)6.2 (68)8.1(81)8.4 (90)

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Wilcock et al.—Characterisation of lowland streams 77

3—4-day periods on 23 rural lowland streams (somewith two sites and some sites being monitoredtwice) in catchments having stock densities of upto 22.5 s.u. ha~'. The application of a single-stationdiurnal curve model (DOFLO) provided a set ofvalues for AT2(20). ^max., ^20, 2io, and P/Rcharacterising the relative influences of reaeration,photosynthetic production, and respiration on DOfor reaches extending upstream of each site.Minimum DO was < 4 g m~3 on five (of 28)occasions, and < 5 g m"3 nine times. Maximumdaily temperatures up to 25.7°C (mean 21.4°C)were recorded and were weakly (inversely)correlated with diurnal DO minimum values.Streams in shaded forest catchments tended to becooler and have smaller DO deviations fromsaturation compared with streams in openintensively grazed catchments.

Reaeration coefficients (0.05—40 d"1) were inbroad agreement with values calculated using aform of the O'Connor-Dobbins surface renewalmodel, previously modified for use in New Zealandrivers. Maximum rates of photosynthetic production(1.75-86.5 g m"3 d"1) converted to rates of grossprimary production in daylight (GPP) were 0.5-29.2g O2 m~2 d"1, similar to values reported elsewherefor streams in agriculturally developed catchments.Respiration rates at 20°C (3.50-55.0 g m"3 d"1)were generally larger than values reported in theliterature, and P/R ratios were mostly well below1.0, indicative of heterotrophic respirationassociated with decaying vegetation and otherorganic inputs.

Synoptic surveys of this type permit streams tobe classified using both experimental variables,such as DOmin and Tmax , and parameters derivedfrom diurnal curve analysis. Cluster analysis of^2(20)> Pmax., and R20 identified five groupings.Streams in cluster 1 (average K2QO)

= 11 d~') hadlarge diurnal DO ranges, whereas streams in clusters2-5 had increasingly greater A^o) values andmostly smaller DO ranges. Cluster 2 streams hadlow reaeration rates and low values of Pmax. andR20, whereas cluster 4 streams had high reaerationcoefficients and low productivity and respirationrates. Streams in clusters 3 and 5 had highrespiration rates balanced by moderate to highreaeration coefficients.

ACKNOWLEDGMENTS

Our thanks to Murray Boardman and Kathy Speed forhelping with field work and the analysis of stream data.

Financial assistance provided by Auckland RegionalCouncil for the development of the DOFLO model isgratefully acknowledged. This research was carried outunder contract CO 1403 from the Foundation forResearch, Science and Technology (New Zealand).

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