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Epilithon and dissolved oxygen depletion in the Manawatu River, New Zealand: Simple models and management implications
Wat. Res. Vol. 23, No. 7, pp. 825-832, 1989 Printed in Great Britain.All rights reserved
0043-1354/89$3.00+ 0.00 Copyright © 1989MaxwellPergamonMacmillanplc
EPILITHON A N D DISSOLVED OXYGEN DEPLETION IN THE MANAWATU RIVER, NEW ZEALAND: SIMPLE MODELS A N D MANAGEMENT IMPLICATIONS J. M. QUINN1and P. N. MCFARLANE2 IWater Quality Centre, Department of Scientific and Industrial Research, P.O. Box 11-115, Hillcrest, Hamilton, New Zealand and 2NewZealand Forest Research Institute, Private Bag, Rotorua, New Zealand (First received February 1988; accepted in revised form January 1989)
Abstract--The relationship between epilithic biomass and oxygen depletion was studied above and below wastewater discharges to the Manawatu River in an attempt to guide fiver management by defining the maximum respiration rates and the corresponding epilithic biomass consistent with maintenance of the legal minimum DO concentration of 5 g m -3. A computer model, which simulated the fiver DO under lowflow conditions, predicted that the maximum allowable respiration rate giving a dawn DO concentration of 5 g m -3 ranged from 20 g 02 m -2 d-i at 21°C to 24.5 g 0 2m -2 d -I at 12°C. A multiple regression model developed from chamber measurements made in situ showed that the total biomass density and water temperature accounted for 73% of the variation in the respiration rate observed. When adapted to allow for respiration occurring in the water column, the model gave reasonably accurate predictions of respiration when checked against the whole river respiration rate measurements. Biomass densities, at which DO levels reach 5 gm -3 predicted using these models, ranged from 34 g ash free dry wt m -2 at 21°C to 143 g ash free dry wt m -2 at 12°C. Analyses of historical hydrological data and the results of epilithon growth measurements under a variety of water quality conditions in the Manawatu River indicate that the present management strategy of limiting instream BOD5 to below 5 g m -3 but not limiting nutrient (N and P) levels will not prevent the occurrence in summer of the predicted nuisance biomasses and hence unacceptable deoxygenation. Key words~dissolved oxygen, river respiration, models, epilithon, algae, sewage fungus, wastewaters, in situ chambers
INTRODUCTION
The Manawatu River receives wastewaters from Palmerston North (pop. 68,000) and a dairy factory and slaughterhouse nearby (Fig. 1) that together have resulted in the river's long history of "sewage fungus" and algal proliferations (Quinn, 1985). The metabolism of these growths on the river's relatively shallow (mean depth = 1 m at flow of 20 m 3 s-l), cobble bed results in marked diurnal dissolved oxygen (DO) fluctuations. Fish kills have occurred. Dawn dissolved oxygen (DO) levels below 5 g m -3 have been common at sites within a few kilometres of the discharges when the flow drops below 20 m s -~ (cf. median = 55m3s -~) during summer when daytime temperatures can reach 23°C (Quinn, 1985). This DO concentration is generally accepted as the minimum acceptable for freshwaters (U.S.EPA, 1976) and is the legal minimum for the Manawatu River. The nighttime DO depletion rate greatly exceeds that predicted by the classical Streeter-Phelps technique, which uses information on wastewater BOD decay kinetics (kl) and river reaeration rate (k2) to predict DO depletion (e.g. McBride, 1982). Similar high respiration rates have been observed in other sewage fungus infested rivers in New Zealand (Hickey, 1988) and the United Kingdom (Boyle and Scott, 1984).
The Manawatu Regional Water Board (MRWB) has attempted to control the sewage fungus growth and DO depletion to acceptable levels by issuing water fights which restrict the organic material inputs by the wastewater dischargers to a level at which the in-river 5 day biochemical oxygen demand (BODs) is calculated to be less than 5 g m -3 at the end of a defined waste mixing zone (site D, Fig. I). This control measure was based on the observation that at BOD 5 levels below 5 g m -3 sewage fungus outbreaks were usually only "light to moderate" in rivers in the United Kingdom (Curtis and Harrington, 1971). It has been effective in controlling sewage fungus when primary sewage or meatworks wasteworks comprised most of the BOD5. However, when the dairy wastewater comprised approx. 5 0 0 or more of the BOD5 sewage fungus was abundant at BOD5 concentrations above 3 g m -3. These different effects were attributed to the differences in the contributions of low molecular weight organics to the BOD5 of the wastewaters (Quinn and McFarlane, 1988). The present study attempted to guide the management of DO in the fiver by: describing night-time respiration over reaches above and below the discharges during summer of 1983-1984; developing models to predict allowable respiration rates and corresponding epilithic biomass densities; and using
825
J. M. QUINN and P. N. MCFARLANE
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Fig. 1. Location map showing study sites (A-F) and wastewater discharges.
these models, the results o f epilithic growth experiments, and hydrological data, to evaluate whether further wastewater discharge control measures were required. METHODS Respiration and biomass measurements (i) Whole river measurements. Community respiration rates were calculated from continuous DO and temperature measurements made for periods of approx. 24 h using Yellow Springs Instrument (YSI) (Model 56) recorders with their probes mounted on stakes in the main river flow. At sites below the discharges submersible stirrers (YSI Model 5695) were attached to the probes to prevent microbial overgrowth of their membranes, which was found to cause artifically low DO values. This was not necessary at site A where fouling of DO membranes did not occur. The probes were calibrated in moist air and in the two station studies the probes were also checked against each other in air saturated water before and after each run. The Odum (1956) methods for single stations (upstream of the discharges) and two stations (downstream of the discharges) were used to calculate the net rate of change in DO at hourly intervals, and the night-time measurements were used to calculate the mean reach respiration rate. The influences of the two minor tributary inflows between the city and dairy discharges were neglected from the respiration calculations due to their relatively low summer flows (0.02-0.10m3s -1) (Currie, 1977) and the similarity between DO values in the tributaries and the river. However, the direct deoxygenating effects of the near-anaerobic wastewater discharges were allowed for. The reach travel times were calculated by interpolating between values measured in dye studies at flows of 15.6, 20.0 and 26.3m3s -~ (Rutherford and Currie, 1979; Wilcock, 1984a) or, when the flow was outside this range, by using the measured travel times at given flows in the equation relating travel time to flow of Rutherford and Currie (1979), derived from the Manning equation. The river flow was recorded continuously near site A (Fig. 1) by the Manawatu Regional Water Board. Reaeration rates were measured (Wilcock, 1984a), using the methyl chloride tracer technique (Wilcock,
1984b), between sites C and F (Fig. 1) at a flow of 15.6m3s -~. As found in four other New Zealand rivers (Wilcock, 1984b), the k 2 values measured were on average 40% greater than those calculated using the equation of O'Connor and Dobbins (1958). Thus, measured k 2 values were used for flows of 15-20 m 3 s- ~but for reaches upstream of site C and for flows in excess of 20 m 3s- ~, k 2 values were calculated by multiplying the values given by the O'Connor and Dobbins (1958) equation by 1.4. Respiration rates calculated using the procedure were on average 8% greater than those calculated using hydraulic radius data in the equations of O'Connor and Dobbins (1958). Values of k 2 were adjusted for temperature using the equation of Elmore and West (1961). Because wind can markedly increase reaeration (Elder and Gloyna, 1969), diurnal curve measurements were only used to calculate respiration on occasions when there was little or no wind or when the wind was from the directions which did not generate surface waves on the river. (ii) Chamber measurements made in situ. Epilithon respiration rates were measured using either a 2.661. volume, cylindrical, chamber built by Boyle and Scott (1984) (modified by using a 6 V, geared motor) or 5.51. volume, unidirectional-flow, chambers built by Freeman (1983). These chambers gave results within 6% of each other (Quinn, 1985). A single layer of surface stones or concrete substrates with attached epilithon was placed within the chamber, which had previously been filled with river water from the site. The propeller or pump speeds were adjusted to produce a similar level of disturbance of the epilithon (assessed by eye) to that observed in the river. Typical river velocities were 0.3-0.4 m s-1 (measured 5 cm above the bed). The chamber was then covered with a butyl rubber, blackout sheet and the change in DO with time measured for 20-40 min with a YSI probe (Model 5739) inserted in the chamber and attached to a YSI Model 56 recorder. Following these measurements the epilithon was removed from the stones with rubber gloves and a stiff brush and stored on ice in the dark for transport to the laboratory (<3 h). Total biomass was measured immediately as ash free dry weight (AFDW) (APHA, 1980) and triplicate subsamples for analysis of phaeophyton-corrected, chlorophyll a (APHA, 1980) content were frozen for up to 1 month. These data
Epilithon and DO depletion were used to calculate the autotrophic index (AI) values of the epilithon (APHA, 1980). The stone volume was measured by displacement to allow calculation, by difference, of the chamber volume. The epilithon respiration rate (g O2m -2 d -~ ) was calculated from the initial slope of the DO versus time curve knowing the volume of water in the chamber and the surface area of its base occupied by the stones. River water samples were collected in 21. acid-cleaned polyethylene bottles at the the same time as the chamber was filled prior to the respiration measurements, These were stored on ice, in the dark, and analysis for BOD 5 (without nitrification inhibition) (APHA, 1980) was commenced within 4 h of collection. The respiration of biomass suspended in the water column was measured on different occasions in the chambers in the absence of epilithon or using a stirred DO probe (YSI model 5720) to measure the initial and final DO concentrations of river water incubated in unstirred BOD bottles for 3-4 h in the dark in the river temperature.
Benthic biomass During the whole river respiration measurements the reach benthic biomass was assessed visually and sampled quantitatively (in triplicate) at 1-3 sites per reach of 0.3-0.4 m depth and average current velocity (0.3-0.5 m s -~) using the methods of Quinn and McFarlane (1985), Biomass sampling involved: isolating a column of river water and a 0.049 m 2 area of riverbed using an open-ended cylinder; dislodging the attached growths by stirring the surficial sediments; and collecting a 21. subsample of the cylinder water once the growths were uniformly dispersed. The total volume of water in the cylinder was calculated from the depth and known cross-sectional area. Biomass was measured as AFDW (APHA, 1980). On 3 occasions the visual observations were used to assess the biomass as AFDW by comparison with calibrated observations on other occasions.
Modelling of allowable respiration rates The effects on the dawn DO concentration of night-time respiration rate, sunset DO concentration and temperature were investigated using a numerical computer simulation model (Appendix 1) based on equation (l) DOt = DO,_ ~- R + k2(20 ) 1.0241(r-2°)(C~(z~ - DO,_ i) (1) where t = time (rain) DOt, DOt- i = DO at times t and t - 1 R = respiration rate (g O2 m -3 min-i), assumed constant k2(r) = reaeration rate (min -2) Cs(:~ = saturation DO concentration (g m -3) T = temperature, assumed to decrease by 2°C during the night (typical for Manawatu River) from the initial specified value. DO was specified as the typical dusk value at site A of 9 . 8 g m -3. The model then calculated the DO at l min intervals throughout a 10h night using a k2(20) value of
Reach/site (Fig. 1) A Sewage-B Dairy~C Dairy-D D--E D--EF
827
3.93 d -~ measured for the reach C to F (Fig. 1) at a flow of 1 5 . 6 m 3 s - ~by Wilcock (1984a) for different values of respiration rate and dusk river temperature.
Epilithon growth intervals Flows in excess of approx. 100 m 3 s -I scour epilithon in the Manawatu River (Freeman, 1983; Quinn, 1985), whereas at flows below approx. 40 m 3 s - ' the physical conditions are favourable for epilithon development during summer. Thus, the durations of "epilithon growth intervals", when the conditions were suitable for epilithon development between summer spates, were calculated from the Manawatu Regional Water Board records of daily river flow near site A (Fig. 1) during 1972-1985 by summing the number of days between November and March when the flow was below 40 m 3 s -~ between consecutive flows in excess of 100 m 3 s -t.
Modelling of factors affecting benthic respiration The Minitab (1982) statistical package was used to investigate the relationships between benthic respiration rates, measured in situ, and environmental factors. RESULTS AND DISCUSSION
(i) River dissolved oxygen dynamics A t site A (Fig. 1) respiration rates d u r i n g the s u m m e r of 1983-1984 were low (1.9-5.3 g m - 2 d -1) due to low levels o f B O D 5 (x + S D = 0.7 __. 0.4 g m -3, n = 5) a n d dissolved reactive p h o s p h o r u s ( D R P ) (x +__SD = 4 . 8 + 2 . 7 g m -3, n = 4 ) (Quinn, 1985). Higher respiration rates ( 1 - 1 8 g m - 2 d -1) were recorded at site A d u r i n g the s u m m e r of 1982-1983 when the reach average biomass of Cladophora glomerata was up to 68 g dry wt m -2 ( F r e e m a n , 1983). T h e respiration rates m e a s u r e d during b o t h s u m m e r s were within the range of 0.2-21.5 g 0 2 m -2 d - l reported for studies of 56 u n p o l l u t e d streams in the U.S.A. a n d C a n a d a summarised by Bott et al. (1985). Below the wastewater discharges prolific growths of sewage fungus a n d / o r filamentous green algae occurred a n d the m e a s u r e d respiration rates (12.537.6 g O l m 2 d - l ) were higher t h a n those m e a s u r e d at site A. The m a x i m u m rate (37.6 g 02 m -2 d - l ) is similar to values reported elsewhere for streams receiving organic discharges by O d u m (1956) (53 g O 2 m - 2 d - l ) . Jeppesen (1982) ( 3 9 . 3 g O 2 m - 2 d - l ) , C o o p e r (1984) (44.8 g 02 m -2 d -1 ) a n d Hickey (1988) ( 7 7 . 2 g O 2 m - 2 d -1) a n d for a sewage c h a n n e l containing m a c r o p h y t e a n d sewage fungus growths by Curtis (1972) ( 5 0 . 4 g O 2 m 2d-1). The c o m m o n occurrence o f low D O values ( < 5 g m -3) d o w n s t r e a m of the discharges (Table 1)
Table 1. Results of diurnal DO measurements during summer 1983-1984 Daily mean River Minimum Benthic Community temperature flow DO hiomass respiration (range) (range) (range) (range) (range) Number of (°C) (m3s-I) (gm 3) (gAFDWm-2) (gO2m-2d 1) observations 17-21 14.~19.4 7.6~8.2 <20 1.9-5.3 5 17-21 16.3-16.8 5.1-6.7 83-100 20.7-26.1 2 20.7 16.8 3.4 150 36.0 1 17--17.5 17-18 5.0-5.1 120 19.3-20.6 2 18.2-20.3 17-30 2.3-7.4 60-120 17.0-33.1 3 15.5-19.7 14.8-58 1.2-8.1 2(~150 12.5-37.6 7
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J.M. QUINNand P. N. MCFARLANE 26
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Fig. 2. Night-time changes in DO concentration of water passing the sewage discharge (Fig. 1) at sunset (time = 0 h; D O = 9 . 8 g m -3) and reaching site E (Fig. 1) at dawn (time = 10h) predicted for respiration rates of 10-35g 02 m -2 d -I by a computer model (Appendix) simulating Manawatu River lowflow conditions (temperature= 22-20°C; k2(20) = 3.93 d-~; depth = 1 m). showed that the assimilative capacity of the river was often exceeded during summer. To aid in the management of these river reaches, the maximum respiration rate permissible to maintain at least 5 g m -3 DO was calculated. The epilithic biomass associated with the respiration rate, termed the nuisance biomass level, was also estimated. The results of these studies are presented in the following sections. (ii) M a x i m u m permissible respiration rate to maintain 5 g i n -3 D O in the river
The effects on night-time DO levels of various respiration rates were investigated using a computer simulation model (Appendix 1). To obtain data representing adverse conditions, the model used the mean reaeration coefficient [kEt20) = 3.93d -l] measured at the 94% low flow, 15.6m3s -l (Wilcock, 1984a), and the maximum night-time water temperatures of 22-20°C observed during the summer of 1984. Mean depth at low flow is 1 m. Under these conditions the model predicts that respiration rates in the range of 10-35 g 02 m -2 d -1 will reduce the DO concentration of water entering the waste discharge zone (Fig. 1) from a typical sunset DO of 9.8 to between 7 and 2 g m -3, respectively, at dawn (Fig. 2). This water would travel approx. 11 km from the Table 2. Comparison of biomass specific respiration rate of algal and sewage fungus epilithic communities Community type Dominant organisms Autotrophic index Biomass (g AFDW m 2) Temperature (°C) Initial DO (g m-3) Initial BOD5 (g m -J) Specific respiration rate (g 02 g A F D W - i d- l )
Algal
Cladophora glomerata 114 55.3 21.2 7.0 1.7 0.26
Sewage fungus
Sphaerotilus natans 664 54.6 20.2 5.3 7.3 0.39
I 12
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18
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Average night-time temperature (°C) Fig. 3. Maximum respiration rates permissiblewhilst maintaining dawn DO ~>5 g m -3 predicted by computer simulation model vs nightly average temperature [k2(20) = 3.93 d - I d = lm].
sewage discharge at sunset to reach site E (Fig. 1) at dawn, thus remaining within the stony bedded area of the river prone to epilithic proliferations [which extends downstream to site F (Fig. 1)]. Under these conditions, the model predicts that the maximum allowable respiration rate, giving the minimum permitted dawn DO value of 5 g m -3, is 20 g 02 m -2 d - 1 (Fig. 2). As the average night-time temperature decreases below the maximum observed 21°C, the increase in the saturation DO concentration raises the atmospheric reaeration rate and thus raises the respiration rate allowable in order to maintain a dawn DO concentration of 5 g m -3 (by 0.5 g 02 m -2 d - ] per 1°C crop in average temperature) (Fig. 3).
(iii) Maximum permissible benthic biomass with respect to river DO
The chamber benthic respiration measurements made in situ were used to investigate the relationship between benthic biomass levels and respiration rates so that maximum allowable biomass levels could be predicted from the maximum respiration rates calculated above. A comparison of the specific respiration rates of an algal dominated and a sewage fungus dominated community (Table 2) showed that under similar conditions of biomass and temperature the sewage fungus had a specific respiration rate approx. 1.5 times that of the algae. Epilithic communities below the discharge range over the phototrophic/heterotrophic continuum. Algae were always dominant between the sewage discharge and site B and usually dominant below site D, whereas sewage fungus was usually dominant between the dairy company discharge and site D (Fig. 1). Thus, 23 measurements of respiration rates were made in situ using a variety of
829
Epilithon and DO depletion Table 3. Conditions during respiratory measurements in situ used for regression model development
oJ
Parameter
Range
'E
Temperature (T) BOD 5 Initial DO Biomass (BM) Chlorophyll a Autotrophic index Benthic respiration (BR)
11-23.2°C 0.7-7.0 g m 3 5.3-12.7 g m -3 4.8-106 g AFDW m 2 9.5-601 mg m -2 85-3637 1.5-30.7 g 02 m -2 d -I
o~
40
/
:50 ¢.9
community types and river conditions. The range of values investigated is summarised in Table 3. The following parsimonious linear regression equation for benthic respiration (BR), including all statistically significant predictors (at the 95 % confidence level), was developed from these data: BR = - 18.9 + 1.41T + 0.158 BM (g 02 m -2 d-~).
(2) The standard deviation of the data about the regression equation was 3 . 6 g O 2 m - 2 d - L Biomass and temperature account for 73% of the variation in benthic respiration (r2= 0.73). The absence of any significant effect due to the BOD5 probably results from: (i) the large algal component of many of the communities studied (algal respiration is not expected to be affected by BODs) and (ii) the variable response of sewage fungus respiration to short-term change in BOD 5 depending upon the recent BOD5 conditions (Quinn, 1985). Equation (2) predicted only the respiration occurring at the riverbed and to it must be added the respiration occurring in the water column in order to predict the total river respiration rates. The respiration rates of river water measured on 12 occasions in quiescent BOD bottles ranged from < 0 . 7 4 . 3 g O 2 m - 3 d - L However because higher rates would be expected under the turbulent conditions existing in the river than in the quiescent BOD bottles a rate of 4 g O 2 m - 3 d -~ was assumed typical for water column respiration. Thus, 4 g 02 m -2 d - l (for the 1 m deep river) was added to equation (2) to give the following adapted model: RR = - 14.9 + 1.4IT + 0.158 BM ( g O 2 m - 2 d - l ) .
(3) To test the validity of the use of this model to investigate the maximum permissible benthic biomass in the Manawatu River the respiration rates calculated from the whole river respiration measurements over reaches below the wastewater discharges were compared with those predicted by equation (3) for the measured conditions of biomass and temperature during the whole river measurements. This comparison (Fig. 4) shows that equation (3) provides a reasonably accurate model of respiration rates in the Manawatu River. The predicted rates were generally within approx. 20% of those observed and the average difference between measured and
20
i o
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i
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i
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Predicted totat respiration rate (gO2 m'2d -1)
Fig. 4. Respiration rates measured in the whole river studies below the discharges vs rates predicted for the biomass and temperature conditions by the adapted regression model [equation (3)]. predicted rates was 0 . 1 _ _ 5 . 2 g m - 2 d -1 (x__+SD, n = 17). The discrepancies between the measured and predicted respiration rates were greatest for sewage fungus dominated reaches under unusually low and high wastewater loading conditions. Equation (3) overestimated the measured respiration rate for the reach below the dairy company by 9.4 g O 2 m - 2 d -~ (49%) when this discharge was only sufficient to increase the river daily mean BOD 5 by 1.3 g m -3 on mixing (personal communication B. Gilliland, Manawatu Regional Water Board) whereas the respiration rate of a sewage fungus dominated reach between sites D and E (Fig. 1) was underestimated by l l . 4 g O 2 m - 2 d -1 (35%) when both the dairy and meatworks wastewater loadings were high and resuited in a daily mean measured BOD s at site D of 8.9 g m -3 (Quinn, 1985). Nevertheless, the comparison of the measured river respiration rates with those predicted by equation (3) (Fig. 4) shows that this model does provide a reasonably satisfactory method of predicting the river respiration rates in the Manawatu River. Solving equation (3) for the epilithic biomass densities resulting in the predicted maximum allowable respiration rates at average temperatures of 12-21 °C, given in Fig. 3, allows estimation of the maximum allowable epilithic biomass to maintain river DO above 5 g m -3 (Fig. 5). This analysis indicates that communities composed of sewage fungus and algae with biomass densities above these values constitute a nuisance with respect to DO depletion under low flow conditions in the Manawatu River. The predicted 4.3-fold decrease in the nuisance epilithic biomass densities for a 10°C temperature change (Fig. 5) was greater than expected. Microbial cellular reaction and growth rates typically increase 2 to 3-fold for each 10°C temperature rise between their minimum and maximum temperature tolerances (Mandelstam and McQuillen, 1973; Gaudy and Gaudy 1981). The reduction in allowable river respiration rate for a 10°C increase in temperature, due to
J. M.
830
QUINN
and P. N. MCFARLANE Table 4. Intervals required for epilithon to reach the predicted nuisance biomass at 21°C (34 g A F D W m -2) at 3 Manawatu River sites under 1983-1984 conditions (data from Quinn and McFarlane, 1985) and frequency of these intervals under summer conditions
E 140
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P ( m g m -3)
TN ( m g m 3)
BOD 5 (gm-3)
Intervals (days)~
3* 70? 130t
354 ~ 700 ~800
0.3 1.9-3.4 4-8
> 100 23 10
Frequency (yr-I)~ 0.1 1.6 2.8
*DRP; tTP; :~Epilithic biomasses on the flat upper surfaces of concrete substrate (100 x 150 mm) multiplied by 1.63 to allow for higher exposed surface area on cobble bed than on flat substrata (Quinn, 1985); §From Manawatu Regional Water Board flow data the months N o v e m b e ~ M a r c h for 1972-1985.
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River temperature (°C)
Fig. 5. Predicted maximum benthic biomass resulting in acceptable respiration rates which do not reduce river DO to < 5 g m -3 vs nightly average river temperature [derived from Fig. 3 and equation (3)]. reduced reaeration, accounts for an additional 0.25fold reduction in nuisance epilithic biomass density for a 10°C temperature increase (Fig. 3). Other factors which probably contributed to the apparent temperature effect were: the additional stimulatory effect on the respiration of the algal epilithon of high solar radiation levels (Darley, 1982), (which coincide with high water temperatures); and a bias towards low BOD 5 levels (1.2-1.4 g m 3) at the time of the respiration rate measurements made at low temperatures (11-14°C). However, the close agreement between the observed river respiration rates and those predicted by equation (3) (Fig. 4) suggests that the predicted nuisance biomass densities are realistic, particularly at summer temperatures of 17-21°C under which most of the data were collected. At the maximum average night-time temperature observed during this study of 21°C the predicted nuisance biomass level is 34 g A F D W m -2 (Fig. 5). This value is low compared with the biomass values observed in this study (Table 1) suggesting that unacceptable DO conditions (DO < 5 g m -a) will recur unless the wastewater discharges are further restricted during summer. Table 4 summarises data on the intervals of stable flows required for the predicted nuisance biomass at 21 °C to develop under similar physical conditions but different water qualities at sites A, B and C (Fig. 1) and the historical frequencies of the required stable flow intervals. Under the conditions at site A, where the BOD5 was very low (0.3 g m -3) and the average dissolved reactive phosphorus (DRP) below the
5 mg m -3 level believed to limit Cladophoraglomerata in the Manawatu River (Freeman, 1986), the predicted nuisance epilithic biomass is never likely to develop. By contrast, under conditions at site C of abundant BOD5 and nutrients an average of 2.8 intervals during the predicted 21°C nuisance biomass develops are expected each summer. The results at site B indicate that modest reductions in the BOD5 level without significant reduction in nutrients (N and P) will allow the predicted 21°C nuisance epilithic biomass to develop during an average of 1.6 intervals per summer. However, reducing the BOD 5 is also expected to increase the nuisance biomass density at a given temperature by reducing the heterotrophic content, and hence the biomass specific respiration, of the epilithon (Table 2). Assuming that nuisance biomass density for filamentous algae in the Manawatu River is approx. 50 g A F D W m -2 at 21°C (compared with 34g A F D W m -2 predicted for mixed algal sewage fungus communities), the development of algal dominated epilithon under conditions of relatively low organic material but ample nutrients at site B (Quinn and McFarlane, 1985) indicates that this biomass density would be achieved in an interval of approx. 36 days of stable flows. This interval had a recurrence of 0.7yr ~ during 1972-1985. This suggests that reduction of BODs, but not nutrient concentrations, will reduce the frequency of unacceptable deoxygenation. However, if the assumed nuisance biomass for algal epilithon of 50 g A F D W m-3 is correct, the results suggest that limiting the BOD 5 to < 5 g m -3, but not limiting nutrient levels, will result in unacceptable deoxygenation occurring in the Manawatu River in two out of every three summers. Further investigation of the relationship between the biomass, temperature and respiration of algal epilithon is required to test this. The nuisance levels described here for the Manawatu River are not directly applicable elsewhere. These will depend upon the nature of the epilithon (heterotroph content) and the physical factors that determine the allowable night-time respiration rate [i.e. reaeration rate (k2), mean depth, night-length, temperature and minimum acceptable DO]. Our model for prediction of acceptable night-time respiration rate (Appendix) can be applied to any river by
Epilithon and DO depletion specifying appropriate parameter values. However, due to the expected differences between the respiration rates of different types of epilithon (e.g. Table 2), equations (2) and (3) are not expected to be generally applicable. Thus, until more generally applicable models of epilithon respiration are available, site specific studies, similar to those described in this paper, will be required to model the relationship between epilithon biomass, temperature and respiration rates in a given fiver. Nevertheless, the approach of employing simple modelling techniques to determine m a x i m u m levels of epilithic biomass relative to the river D O levels appears to have considerable potential for use in directing the management of epilithon to maintain adequate D O levels in relatively shallow rivers. Conventional Streeter-Phelps B O D / D O models are inappropriate in shallow rivers because the respiration of the benthic community is the predominant D O removal mechanism (Hickey, 1988). By combining the information gained on the epilithic biomass density at which nuisance effects on river D O depletion occur with data on epilithic growth rates under different water quality conditions and the length of "growth intervals" between epilithon-scouring spates, the fiver manager can gain guidance on the management measures require to maintain adequate D O conditions in rivers while allowing their use for waste assimilation. Acknowledgements--This work formed part of a PhD study in the Biotechnology Department of Massey University. It was funded by a NWASCA research contract and a State Services Commission Study Award to J. M. Quinn. Thanks are also due to Dr Richard Archer of the Biotechnology Department for assistance with development of the DO simulation model; to Dr Mike Freeman of the Biotechnology Department and Mr John Boyle of the University of Exeter, for the use of their respiratory chambers; to Drs Kit Rutherford, Mike Freeman and Chris Hickey for reviewing the draft manuscript; and to Mrs Mary Stokes for her typing.
REFERENCES
APHA (1980) Standard Methods for the Examination o f Water and Wastewater, 15th edition. American Public Health Association, Washington D.C. Bott T. L., Brock J. T., Dunn C. S., Naiman R. J., Ovink R. W. and Petersen R. C. (1985) Benthic community metabolism in four temperate stream systems: an interbiome comparison and evaluation of the river continuum concept. Hydrobiologia 123, 3~,5. Boyle J. D. and Scott J. A. (1984) The role of benthic films in the oxygen balance in an East Devon river. Wat. Res. 18, 1089-1099. Cooper A. B. (1984) Activities of benthic nitrifiers in streams and their role in oxygen consumption. Microb. Ecol. 10, 317-334. Currie K. J. (1977) Water quality management report-Manawatu River. Manawatu Regional Water Board, Palmerston North, New Zealand. Curtis E. J. C. (1972) Sewage fungus in rivers in the United Kingdom. Wat. Pollut. Control 1972, 673-685.
831
Curtis E. J. C. and Harrington D. W. (1971) The occurrence of sewage fungus in rivers in the United Kingdom. War. Res. 5, 281-290. Darley W. (1982) Algal Biology: a Physiological Approach. Blackwell, Oxford. Elder J. M. and Gloyna E. F. (1969) Oxygen production and loss in a model river. Technical Report No. 1 to the office of Water Resources Research Centre for Research in Water Resources, Environmental Health, Engineering Research Laboratory, Civil Engineering Dept., University of Texas, Austin. Elmore H. L. and West W. F. (1961) Effect of water temperature on stream reaeration. J. sanit. Engng Div. Am. Soc. cir. Engrs 87, 59-71. Freeman M. C. (1983) Periphyton and water quality in the Manawatu River, New Zealand. PhD thesis, Massey University Library, Palmerston North, New Zealand. Freeman M. C. (1986) The role of nitrogen and phosphorus in the development of Cladophora glomerata (L.) Kutzing in the Manawatu River, New Zealand. Hydrobiologia 133, 23-30. Gaudy A. F. and Gaudy E. T. (1981) Microbiology for Environmental Scientists and Engineers. McGraw-Hill, London. Hickey C. W. (1988) Benthic chamber for use in rivers: testing against oxygen mass balance. J. envir. Engng Am. Soc. cir. Engrs 114, 828-845. Jeppesen T. (1962) A comparison between four oxygen balance models for small organic polluted streams with many macrophytes. Progress in Ecological Engineering and Management by Mathematical Modelling; Proceedings of the Second International Conference on Sate-ofthe-Art in Ecological Modelling, April 1980, Leige, Belgium. Mandelstam J. and McQuillen K. (1973) Biochemistry of Bacterial Growth. Blackwell, Oxford. McBride G. B. (1982) Nomographs for rapid solutions for the Streeter-Phelps equations. J. Wat. Pollut. Control Fed. 54, 378-384. O'Connor D. J. and Dobbins W. E. (1958) Mechanisms of reaeration in natural streams. Trans. Am. Soc. cir. Engrs 123, 641--684. Odum H. T. (1956) primary production in flowing waters. Limnol. Oceangr. 1, 102-117. Quinn J. M. (1985) Wastewater effects on epilithon, particularly sewage fungus, and water quafity in the Manawatu River, New Zealand. PhD thesis, Massey University, Palmerston North, New Zealand. Quinn J. M~ and McFarlane P. N. (1985) Sewage fungus as a monitor of water quality. In Biological Monitoring in Freshwaters: Proceedings o f a Seminar (Edited by Pridmore R. D. and Cooper A. B.), pp. 143-162. Water and Soil Misc. Publ. No. 82, Water and Soil Directorate, MWD, Wellington. Quinn J. M. and McFarlane P. N. (1988) Control of sewage fungus to enhance recreational use of the Manawatu River New Zealand. Verh. int. Verein. Limnol. 23, 1572-1577. Rutherford J. C. and Currie K. J. (1979) Investigatwns of mechanisms affecting BOD concentrations in the Manawatu River near Palmerston North. A joint report of MWD, Hamilton Science Centre and Manawatu Regional Water Board. U.S.EPA (1976) Quality Criteria for Water. United States Environmental Protection Agency, Washington, D.C, Wilcock R. J. (1984a) Dye studies and field measurements of reaeration coeefficients of the Manawatu River: February-March 1983. Internal Report 84/12, Water Quality Centre, Ministry of Works and Development, Hamilton, New Zealand. Wilcock R. J. (1984b) Methyl chloride as a gas-tracer for measuring stream reaeration coefficients--II. Stream studies. Wat. Res. 18, 53-57.
J. M. QUINN and P. N. McFARLANE
832 APPENDIX
FORTRAN Program used in Computer Oxygen Modelling Studies C H A R A C T E R * 4 0 F 1,F2 D I M E N S I O N 0(601) Write(l,'(///)') C A L L C O U A ( ' D A T A FILE:-') R E A D (I,'(A40)') F1 C A L L C O U A ( ' O U T P U T FILE: ~-') R E A D (1,'(A40)') F2 O P E N (5,FILE = FI) O P E N (6,File = F2) T R e a d (5,*) AI,A2,A3,A4,A5 ¢- specifies conditions 0 ( 1 ) = A1 N=0 D O 100 1 = 2,601 T = A2-A3* R E A L ( 1) ¢-- calculates temperature C = 13.55-.225"T ,-- calculates saturation D O X2T = A5*(1.0241**(T.20)) ~ adjusts k 2 for temperature 0(I) = 0(I-1)-A4 + X2T*(C-0(I-1)) ~ calculates D O N=N+I
IF (N.GE.60) T H E N C A L L C O U A ('.') W R I T E (6,200) O(I), T N=0 200 FORMAT(2F10.3) ENDIF I00 C O N T I N U E CLOSE(5) CLOSE(6) STOP END where T = C= X2T = O = At = A2 = Aa=
temperature (°C) saturation dissolved oxygen concentration (g m -3) k 2 at the river temperature (min -m) dissolved oxygen concentration (g m -3) initial dissolved oxygen at sunset ( g m -3) initial temperature (°C) temperature decay factor (0.0033 gives 2°C decline in 10 h) A 4 = respiration rate (g 02 m -3 min-1 ) A5 = reaeration k2(20) value (min -1 )
0043-1354/89$3.00+ 0.00 Copyright © 1989MaxwellPergamonMacmillanplc
EPILITHON A N D DISSOLVED OXYGEN DEPLETION IN THE MANAWATU RIVER, NEW ZEALAND: SIMPLE MODELS A N D MANAGEMENT IMPLICATIONS J. M. QUINN1and P. N. MCFARLANE2 IWater Quality Centre, Department of Scientific and Industrial Research, P.O. Box 11-115, Hillcrest, Hamilton, New Zealand and 2NewZealand Forest Research Institute, Private Bag, Rotorua, New Zealand (First received February 1988; accepted in revised form January 1989)
Abstract--The relationship between epilithic biomass and oxygen depletion was studied above and below wastewater discharges to the Manawatu River in an attempt to guide fiver management by defining the maximum respiration rates and the corresponding epilithic biomass consistent with maintenance of the legal minimum DO concentration of 5 g m -3. A computer model, which simulated the fiver DO under lowflow conditions, predicted that the maximum allowable respiration rate giving a dawn DO concentration of 5 g m -3 ranged from 20 g 02 m -2 d-i at 21°C to 24.5 g 0 2m -2 d -I at 12°C. A multiple regression model developed from chamber measurements made in situ showed that the total biomass density and water temperature accounted for 73% of the variation in the respiration rate observed. When adapted to allow for respiration occurring in the water column, the model gave reasonably accurate predictions of respiration when checked against the whole river respiration rate measurements. Biomass densities, at which DO levels reach 5 gm -3 predicted using these models, ranged from 34 g ash free dry wt m -2 at 21°C to 143 g ash free dry wt m -2 at 12°C. Analyses of historical hydrological data and the results of epilithon growth measurements under a variety of water quality conditions in the Manawatu River indicate that the present management strategy of limiting instream BOD5 to below 5 g m -3 but not limiting nutrient (N and P) levels will not prevent the occurrence in summer of the predicted nuisance biomasses and hence unacceptable deoxygenation. Key words~dissolved oxygen, river respiration, models, epilithon, algae, sewage fungus, wastewaters, in situ chambers
INTRODUCTION
The Manawatu River receives wastewaters from Palmerston North (pop. 68,000) and a dairy factory and slaughterhouse nearby (Fig. 1) that together have resulted in the river's long history of "sewage fungus" and algal proliferations (Quinn, 1985). The metabolism of these growths on the river's relatively shallow (mean depth = 1 m at flow of 20 m 3 s-l), cobble bed results in marked diurnal dissolved oxygen (DO) fluctuations. Fish kills have occurred. Dawn dissolved oxygen (DO) levels below 5 g m -3 have been common at sites within a few kilometres of the discharges when the flow drops below 20 m s -~ (cf. median = 55m3s -~) during summer when daytime temperatures can reach 23°C (Quinn, 1985). This DO concentration is generally accepted as the minimum acceptable for freshwaters (U.S.EPA, 1976) and is the legal minimum for the Manawatu River. The nighttime DO depletion rate greatly exceeds that predicted by the classical Streeter-Phelps technique, which uses information on wastewater BOD decay kinetics (kl) and river reaeration rate (k2) to predict DO depletion (e.g. McBride, 1982). Similar high respiration rates have been observed in other sewage fungus infested rivers in New Zealand (Hickey, 1988) and the United Kingdom (Boyle and Scott, 1984).
The Manawatu Regional Water Board (MRWB) has attempted to control the sewage fungus growth and DO depletion to acceptable levels by issuing water fights which restrict the organic material inputs by the wastewater dischargers to a level at which the in-river 5 day biochemical oxygen demand (BODs) is calculated to be less than 5 g m -3 at the end of a defined waste mixing zone (site D, Fig. I). This control measure was based on the observation that at BOD 5 levels below 5 g m -3 sewage fungus outbreaks were usually only "light to moderate" in rivers in the United Kingdom (Curtis and Harrington, 1971). It has been effective in controlling sewage fungus when primary sewage or meatworks wasteworks comprised most of the BOD5. However, when the dairy wastewater comprised approx. 5 0 0 or more of the BOD5 sewage fungus was abundant at BOD5 concentrations above 3 g m -3. These different effects were attributed to the differences in the contributions of low molecular weight organics to the BOD5 of the wastewaters (Quinn and McFarlane, 1988). The present study attempted to guide the management of DO in the fiver by: describing night-time respiration over reaches above and below the discharges during summer of 1983-1984; developing models to predict allowable respiration rates and corresponding epilithic biomass densities; and using
825
J. M. QUINN and P. N. MCFARLANE
826 i
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~ ISLAND
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Meatworks Discharge
Opiki ,~
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Fig. 1. Location map showing study sites (A-F) and wastewater discharges.
these models, the results o f epilithic growth experiments, and hydrological data, to evaluate whether further wastewater discharge control measures were required. METHODS Respiration and biomass measurements (i) Whole river measurements. Community respiration rates were calculated from continuous DO and temperature measurements made for periods of approx. 24 h using Yellow Springs Instrument (YSI) (Model 56) recorders with their probes mounted on stakes in the main river flow. At sites below the discharges submersible stirrers (YSI Model 5695) were attached to the probes to prevent microbial overgrowth of their membranes, which was found to cause artifically low DO values. This was not necessary at site A where fouling of DO membranes did not occur. The probes were calibrated in moist air and in the two station studies the probes were also checked against each other in air saturated water before and after each run. The Odum (1956) methods for single stations (upstream of the discharges) and two stations (downstream of the discharges) were used to calculate the net rate of change in DO at hourly intervals, and the night-time measurements were used to calculate the mean reach respiration rate. The influences of the two minor tributary inflows between the city and dairy discharges were neglected from the respiration calculations due to their relatively low summer flows (0.02-0.10m3s -1) (Currie, 1977) and the similarity between DO values in the tributaries and the river. However, the direct deoxygenating effects of the near-anaerobic wastewater discharges were allowed for. The reach travel times were calculated by interpolating between values measured in dye studies at flows of 15.6, 20.0 and 26.3m3s -~ (Rutherford and Currie, 1979; Wilcock, 1984a) or, when the flow was outside this range, by using the measured travel times at given flows in the equation relating travel time to flow of Rutherford and Currie (1979), derived from the Manning equation. The river flow was recorded continuously near site A (Fig. 1) by the Manawatu Regional Water Board. Reaeration rates were measured (Wilcock, 1984a), using the methyl chloride tracer technique (Wilcock,
1984b), between sites C and F (Fig. 1) at a flow of 15.6m3s -~. As found in four other New Zealand rivers (Wilcock, 1984b), the k 2 values measured were on average 40% greater than those calculated using the equation of O'Connor and Dobbins (1958). Thus, measured k 2 values were used for flows of 15-20 m 3 s- ~but for reaches upstream of site C and for flows in excess of 20 m 3s- ~, k 2 values were calculated by multiplying the values given by the O'Connor and Dobbins (1958) equation by 1.4. Respiration rates calculated using the procedure were on average 8% greater than those calculated using hydraulic radius data in the equations of O'Connor and Dobbins (1958). Values of k 2 were adjusted for temperature using the equation of Elmore and West (1961). Because wind can markedly increase reaeration (Elder and Gloyna, 1969), diurnal curve measurements were only used to calculate respiration on occasions when there was little or no wind or when the wind was from the directions which did not generate surface waves on the river. (ii) Chamber measurements made in situ. Epilithon respiration rates were measured using either a 2.661. volume, cylindrical, chamber built by Boyle and Scott (1984) (modified by using a 6 V, geared motor) or 5.51. volume, unidirectional-flow, chambers built by Freeman (1983). These chambers gave results within 6% of each other (Quinn, 1985). A single layer of surface stones or concrete substrates with attached epilithon was placed within the chamber, which had previously been filled with river water from the site. The propeller or pump speeds were adjusted to produce a similar level of disturbance of the epilithon (assessed by eye) to that observed in the river. Typical river velocities were 0.3-0.4 m s-1 (measured 5 cm above the bed). The chamber was then covered with a butyl rubber, blackout sheet and the change in DO with time measured for 20-40 min with a YSI probe (Model 5739) inserted in the chamber and attached to a YSI Model 56 recorder. Following these measurements the epilithon was removed from the stones with rubber gloves and a stiff brush and stored on ice in the dark for transport to the laboratory (<3 h). Total biomass was measured immediately as ash free dry weight (AFDW) (APHA, 1980) and triplicate subsamples for analysis of phaeophyton-corrected, chlorophyll a (APHA, 1980) content were frozen for up to 1 month. These data
Epilithon and DO depletion were used to calculate the autotrophic index (AI) values of the epilithon (APHA, 1980). The stone volume was measured by displacement to allow calculation, by difference, of the chamber volume. The epilithon respiration rate (g O2m -2 d -~ ) was calculated from the initial slope of the DO versus time curve knowing the volume of water in the chamber and the surface area of its base occupied by the stones. River water samples were collected in 21. acid-cleaned polyethylene bottles at the the same time as the chamber was filled prior to the respiration measurements, These were stored on ice, in the dark, and analysis for BOD 5 (without nitrification inhibition) (APHA, 1980) was commenced within 4 h of collection. The respiration of biomass suspended in the water column was measured on different occasions in the chambers in the absence of epilithon or using a stirred DO probe (YSI model 5720) to measure the initial and final DO concentrations of river water incubated in unstirred BOD bottles for 3-4 h in the dark in the river temperature.
Benthic biomass During the whole river respiration measurements the reach benthic biomass was assessed visually and sampled quantitatively (in triplicate) at 1-3 sites per reach of 0.3-0.4 m depth and average current velocity (0.3-0.5 m s -~) using the methods of Quinn and McFarlane (1985), Biomass sampling involved: isolating a column of river water and a 0.049 m 2 area of riverbed using an open-ended cylinder; dislodging the attached growths by stirring the surficial sediments; and collecting a 21. subsample of the cylinder water once the growths were uniformly dispersed. The total volume of water in the cylinder was calculated from the depth and known cross-sectional area. Biomass was measured as AFDW (APHA, 1980). On 3 occasions the visual observations were used to assess the biomass as AFDW by comparison with calibrated observations on other occasions.
Modelling of allowable respiration rates The effects on the dawn DO concentration of night-time respiration rate, sunset DO concentration and temperature were investigated using a numerical computer simulation model (Appendix 1) based on equation (l) DOt = DO,_ ~- R + k2(20 ) 1.0241(r-2°)(C~(z~ - DO,_ i) (1) where t = time (rain) DOt, DOt- i = DO at times t and t - 1 R = respiration rate (g O2 m -3 min-i), assumed constant k2(r) = reaeration rate (min -2) Cs(:~ = saturation DO concentration (g m -3) T = temperature, assumed to decrease by 2°C during the night (typical for Manawatu River) from the initial specified value. DO was specified as the typical dusk value at site A of 9 . 8 g m -3. The model then calculated the DO at l min intervals throughout a 10h night using a k2(20) value of
Reach/site (Fig. 1) A Sewage-B Dairy~C Dairy-D D--E D--EF
827
3.93 d -~ measured for the reach C to F (Fig. 1) at a flow of 1 5 . 6 m 3 s - ~by Wilcock (1984a) for different values of respiration rate and dusk river temperature.
Epilithon growth intervals Flows in excess of approx. 100 m 3 s -I scour epilithon in the Manawatu River (Freeman, 1983; Quinn, 1985), whereas at flows below approx. 40 m 3 s - ' the physical conditions are favourable for epilithon development during summer. Thus, the durations of "epilithon growth intervals", when the conditions were suitable for epilithon development between summer spates, were calculated from the Manawatu Regional Water Board records of daily river flow near site A (Fig. 1) during 1972-1985 by summing the number of days between November and March when the flow was below 40 m 3 s -~ between consecutive flows in excess of 100 m 3 s -t.
Modelling of factors affecting benthic respiration The Minitab (1982) statistical package was used to investigate the relationships between benthic respiration rates, measured in situ, and environmental factors. RESULTS AND DISCUSSION
(i) River dissolved oxygen dynamics A t site A (Fig. 1) respiration rates d u r i n g the s u m m e r of 1983-1984 were low (1.9-5.3 g m - 2 d -1) due to low levels o f B O D 5 (x + S D = 0.7 __. 0.4 g m -3, n = 5) a n d dissolved reactive p h o s p h o r u s ( D R P ) (x +__SD = 4 . 8 + 2 . 7 g m -3, n = 4 ) (Quinn, 1985). Higher respiration rates ( 1 - 1 8 g m - 2 d -1) were recorded at site A d u r i n g the s u m m e r of 1982-1983 when the reach average biomass of Cladophora glomerata was up to 68 g dry wt m -2 ( F r e e m a n , 1983). T h e respiration rates m e a s u r e d during b o t h s u m m e r s were within the range of 0.2-21.5 g 0 2 m -2 d - l reported for studies of 56 u n p o l l u t e d streams in the U.S.A. a n d C a n a d a summarised by Bott et al. (1985). Below the wastewater discharges prolific growths of sewage fungus a n d / o r filamentous green algae occurred a n d the m e a s u r e d respiration rates (12.537.6 g O l m 2 d - l ) were higher t h a n those m e a s u r e d at site A. The m a x i m u m rate (37.6 g 02 m -2 d - l ) is similar to values reported elsewhere for streams receiving organic discharges by O d u m (1956) (53 g O 2 m - 2 d - l ) . Jeppesen (1982) ( 3 9 . 3 g O 2 m - 2 d - l ) , C o o p e r (1984) (44.8 g 02 m -2 d -1 ) a n d Hickey (1988) ( 7 7 . 2 g O 2 m - 2 d -1) a n d for a sewage c h a n n e l containing m a c r o p h y t e a n d sewage fungus growths by Curtis (1972) ( 5 0 . 4 g O 2 m 2d-1). The c o m m o n occurrence o f low D O values ( < 5 g m -3) d o w n s t r e a m of the discharges (Table 1)
Table 1. Results of diurnal DO measurements during summer 1983-1984 Daily mean River Minimum Benthic Community temperature flow DO hiomass respiration (range) (range) (range) (range) (range) Number of (°C) (m3s-I) (gm 3) (gAFDWm-2) (gO2m-2d 1) observations 17-21 14.~19.4 7.6~8.2 <20 1.9-5.3 5 17-21 16.3-16.8 5.1-6.7 83-100 20.7-26.1 2 20.7 16.8 3.4 150 36.0 1 17--17.5 17-18 5.0-5.1 120 19.3-20.6 2 18.2-20.3 17-30 2.3-7.4 60-120 17.0-33.1 3 15.5-19.7 14.8-58 1.2-8.1 2(~150 12.5-37.6 7
828
J.M. QUINNand P. N. MCFARLANE 26
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20
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Respirationr
~
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I 2
I 4 Time
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from
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16
Fig. 2. Night-time changes in DO concentration of water passing the sewage discharge (Fig. 1) at sunset (time = 0 h; D O = 9 . 8 g m -3) and reaching site E (Fig. 1) at dawn (time = 10h) predicted for respiration rates of 10-35g 02 m -2 d -I by a computer model (Appendix) simulating Manawatu River lowflow conditions (temperature= 22-20°C; k2(20) = 3.93 d-~; depth = 1 m). showed that the assimilative capacity of the river was often exceeded during summer. To aid in the management of these river reaches, the maximum respiration rate permissible to maintain at least 5 g m -3 DO was calculated. The epilithic biomass associated with the respiration rate, termed the nuisance biomass level, was also estimated. The results of these studies are presented in the following sections. (ii) M a x i m u m permissible respiration rate to maintain 5 g i n -3 D O in the river
The effects on night-time DO levels of various respiration rates were investigated using a computer simulation model (Appendix 1). To obtain data representing adverse conditions, the model used the mean reaeration coefficient [kEt20) = 3.93d -l] measured at the 94% low flow, 15.6m3s -l (Wilcock, 1984a), and the maximum night-time water temperatures of 22-20°C observed during the summer of 1984. Mean depth at low flow is 1 m. Under these conditions the model predicts that respiration rates in the range of 10-35 g 02 m -2 d -1 will reduce the DO concentration of water entering the waste discharge zone (Fig. 1) from a typical sunset DO of 9.8 to between 7 and 2 g m -3, respectively, at dawn (Fig. 2). This water would travel approx. 11 km from the Table 2. Comparison of biomass specific respiration rate of algal and sewage fungus epilithic communities Community type Dominant organisms Autotrophic index Biomass (g AFDW m 2) Temperature (°C) Initial DO (g m-3) Initial BOD5 (g m -J) Specific respiration rate (g 02 g A F D W - i d- l )
Algal
Cladophora glomerata 114 55.3 21.2 7.0 1.7 0.26
Sewage fungus
Sphaerotilus natans 664 54.6 20.2 5.3 7.3 0.39
I 12
I
14
I
i
16
i
|
18
20
22
Average night-time temperature (°C) Fig. 3. Maximum respiration rates permissiblewhilst maintaining dawn DO ~>5 g m -3 predicted by computer simulation model vs nightly average temperature [k2(20) = 3.93 d - I d = lm].
sewage discharge at sunset to reach site E (Fig. 1) at dawn, thus remaining within the stony bedded area of the river prone to epilithic proliferations [which extends downstream to site F (Fig. 1)]. Under these conditions, the model predicts that the maximum allowable respiration rate, giving the minimum permitted dawn DO value of 5 g m -3, is 20 g 02 m -2 d - 1 (Fig. 2). As the average night-time temperature decreases below the maximum observed 21°C, the increase in the saturation DO concentration raises the atmospheric reaeration rate and thus raises the respiration rate allowable in order to maintain a dawn DO concentration of 5 g m -3 (by 0.5 g 02 m -2 d - ] per 1°C crop in average temperature) (Fig. 3).
(iii) Maximum permissible benthic biomass with respect to river DO
The chamber benthic respiration measurements made in situ were used to investigate the relationship between benthic biomass levels and respiration rates so that maximum allowable biomass levels could be predicted from the maximum respiration rates calculated above. A comparison of the specific respiration rates of an algal dominated and a sewage fungus dominated community (Table 2) showed that under similar conditions of biomass and temperature the sewage fungus had a specific respiration rate approx. 1.5 times that of the algae. Epilithic communities below the discharge range over the phototrophic/heterotrophic continuum. Algae were always dominant between the sewage discharge and site B and usually dominant below site D, whereas sewage fungus was usually dominant between the dairy company discharge and site D (Fig. 1). Thus, 23 measurements of respiration rates were made in situ using a variety of
829
Epilithon and DO depletion Table 3. Conditions during respiratory measurements in situ used for regression model development
oJ
Parameter
Range
'E
Temperature (T) BOD 5 Initial DO Biomass (BM) Chlorophyll a Autotrophic index Benthic respiration (BR)
11-23.2°C 0.7-7.0 g m 3 5.3-12.7 g m -3 4.8-106 g AFDW m 2 9.5-601 mg m -2 85-3637 1.5-30.7 g 02 m -2 d -I
o~
40
/
:50 ¢.9
community types and river conditions. The range of values investigated is summarised in Table 3. The following parsimonious linear regression equation for benthic respiration (BR), including all statistically significant predictors (at the 95 % confidence level), was developed from these data: BR = - 18.9 + 1.41T + 0.158 BM (g 02 m -2 d-~).
(2) The standard deviation of the data about the regression equation was 3 . 6 g O 2 m - 2 d - L Biomass and temperature account for 73% of the variation in benthic respiration (r2= 0.73). The absence of any significant effect due to the BOD5 probably results from: (i) the large algal component of many of the communities studied (algal respiration is not expected to be affected by BODs) and (ii) the variable response of sewage fungus respiration to short-term change in BOD 5 depending upon the recent BOD5 conditions (Quinn, 1985). Equation (2) predicted only the respiration occurring at the riverbed and to it must be added the respiration occurring in the water column in order to predict the total river respiration rates. The respiration rates of river water measured on 12 occasions in quiescent BOD bottles ranged from < 0 . 7 4 . 3 g O 2 m - 3 d - L However because higher rates would be expected under the turbulent conditions existing in the river than in the quiescent BOD bottles a rate of 4 g O 2 m - 3 d -~ was assumed typical for water column respiration. Thus, 4 g 02 m -2 d - l (for the 1 m deep river) was added to equation (2) to give the following adapted model: RR = - 14.9 + 1.4IT + 0.158 BM ( g O 2 m - 2 d - l ) .
(3) To test the validity of the use of this model to investigate the maximum permissible benthic biomass in the Manawatu River the respiration rates calculated from the whole river respiration measurements over reaches below the wastewater discharges were compared with those predicted by equation (3) for the measured conditions of biomass and temperature during the whole river measurements. This comparison (Fig. 4) shows that equation (3) provides a reasonably accurate model of respiration rates in the Manawatu River. The predicted rates were generally within approx. 20% of those observed and the average difference between measured and
20
i o
I 10
i
I 20
i
I :~0
i
I 40
Predicted totat respiration rate (gO2 m'2d -1)
Fig. 4. Respiration rates measured in the whole river studies below the discharges vs rates predicted for the biomass and temperature conditions by the adapted regression model [equation (3)]. predicted rates was 0 . 1 _ _ 5 . 2 g m - 2 d -1 (x__+SD, n = 17). The discrepancies between the measured and predicted respiration rates were greatest for sewage fungus dominated reaches under unusually low and high wastewater loading conditions. Equation (3) overestimated the measured respiration rate for the reach below the dairy company by 9.4 g O 2 m - 2 d -~ (49%) when this discharge was only sufficient to increase the river daily mean BOD 5 by 1.3 g m -3 on mixing (personal communication B. Gilliland, Manawatu Regional Water Board) whereas the respiration rate of a sewage fungus dominated reach between sites D and E (Fig. 1) was underestimated by l l . 4 g O 2 m - 2 d -1 (35%) when both the dairy and meatworks wastewater loadings were high and resuited in a daily mean measured BOD s at site D of 8.9 g m -3 (Quinn, 1985). Nevertheless, the comparison of the measured river respiration rates with those predicted by equation (3) (Fig. 4) shows that this model does provide a reasonably satisfactory method of predicting the river respiration rates in the Manawatu River. Solving equation (3) for the epilithic biomass densities resulting in the predicted maximum allowable respiration rates at average temperatures of 12-21 °C, given in Fig. 3, allows estimation of the maximum allowable epilithic biomass to maintain river DO above 5 g m -3 (Fig. 5). This analysis indicates that communities composed of sewage fungus and algae with biomass densities above these values constitute a nuisance with respect to DO depletion under low flow conditions in the Manawatu River. The predicted 4.3-fold decrease in the nuisance epilithic biomass densities for a 10°C temperature change (Fig. 5) was greater than expected. Microbial cellular reaction and growth rates typically increase 2 to 3-fold for each 10°C temperature rise between their minimum and maximum temperature tolerances (Mandelstam and McQuillen, 1973; Gaudy and Gaudy 1981). The reduction in allowable river respiration rate for a 10°C increase in temperature, due to
J. M.
830
QUINN
and P. N. MCFARLANE Table 4. Intervals required for epilithon to reach the predicted nuisance biomass at 21°C (34 g A F D W m -2) at 3 Manawatu River sites under 1983-1984 conditions (data from Quinn and McFarlane, 1985) and frequency of these intervals under summer conditions
E 140
Site
== o
E
o .a ¢.) .w r-
A B C
120
P ( m g m -3)
TN ( m g m 3)
BOD 5 (gm-3)
Intervals (days)~
3* 70? 130t
354 ~ 700 ~800
0.3 1.9-3.4 4-8
> 100 23 10
Frequency (yr-I)~ 0.1 1.6 2.8
*DRP; tTP; :~Epilithic biomasses on the flat upper surfaces of concrete substrate (100 x 150 mm) multiplied by 1.63 to allow for higher exposed surface area on cobble bed than on flat substrata (Quinn, 1985); §From Manawatu Regional Water Board flow data the months N o v e m b e ~ M a r c h for 1972-1985.
100
J~ 80 .Q o
o o
60
E E
40
E
2O
0
fl_
12
I
I
I
I
I
14
16
18
20
22
River temperature (°C)
Fig. 5. Predicted maximum benthic biomass resulting in acceptable respiration rates which do not reduce river DO to < 5 g m -3 vs nightly average river temperature [derived from Fig. 3 and equation (3)]. reduced reaeration, accounts for an additional 0.25fold reduction in nuisance epilithic biomass density for a 10°C temperature increase (Fig. 3). Other factors which probably contributed to the apparent temperature effect were: the additional stimulatory effect on the respiration of the algal epilithon of high solar radiation levels (Darley, 1982), (which coincide with high water temperatures); and a bias towards low BOD 5 levels (1.2-1.4 g m 3) at the time of the respiration rate measurements made at low temperatures (11-14°C). However, the close agreement between the observed river respiration rates and those predicted by equation (3) (Fig. 4) suggests that the predicted nuisance biomass densities are realistic, particularly at summer temperatures of 17-21°C under which most of the data were collected. At the maximum average night-time temperature observed during this study of 21°C the predicted nuisance biomass level is 34 g A F D W m -2 (Fig. 5). This value is low compared with the biomass values observed in this study (Table 1) suggesting that unacceptable DO conditions (DO < 5 g m -a) will recur unless the wastewater discharges are further restricted during summer. Table 4 summarises data on the intervals of stable flows required for the predicted nuisance biomass at 21 °C to develop under similar physical conditions but different water qualities at sites A, B and C (Fig. 1) and the historical frequencies of the required stable flow intervals. Under the conditions at site A, where the BOD5 was very low (0.3 g m -3) and the average dissolved reactive phosphorus (DRP) below the
5 mg m -3 level believed to limit Cladophoraglomerata in the Manawatu River (Freeman, 1986), the predicted nuisance epilithic biomass is never likely to develop. By contrast, under conditions at site C of abundant BOD5 and nutrients an average of 2.8 intervals during the predicted 21°C nuisance biomass develops are expected each summer. The results at site B indicate that modest reductions in the BOD5 level without significant reduction in nutrients (N and P) will allow the predicted 21°C nuisance epilithic biomass to develop during an average of 1.6 intervals per summer. However, reducing the BOD 5 is also expected to increase the nuisance biomass density at a given temperature by reducing the heterotrophic content, and hence the biomass specific respiration, of the epilithon (Table 2). Assuming that nuisance biomass density for filamentous algae in the Manawatu River is approx. 50 g A F D W m -2 at 21°C (compared with 34g A F D W m -2 predicted for mixed algal sewage fungus communities), the development of algal dominated epilithon under conditions of relatively low organic material but ample nutrients at site B (Quinn and McFarlane, 1985) indicates that this biomass density would be achieved in an interval of approx. 36 days of stable flows. This interval had a recurrence of 0.7yr ~ during 1972-1985. This suggests that reduction of BODs, but not nutrient concentrations, will reduce the frequency of unacceptable deoxygenation. However, if the assumed nuisance biomass for algal epilithon of 50 g A F D W m-3 is correct, the results suggest that limiting the BOD 5 to < 5 g m -3, but not limiting nutrient levels, will result in unacceptable deoxygenation occurring in the Manawatu River in two out of every three summers. Further investigation of the relationship between the biomass, temperature and respiration of algal epilithon is required to test this. The nuisance levels described here for the Manawatu River are not directly applicable elsewhere. These will depend upon the nature of the epilithon (heterotroph content) and the physical factors that determine the allowable night-time respiration rate [i.e. reaeration rate (k2), mean depth, night-length, temperature and minimum acceptable DO]. Our model for prediction of acceptable night-time respiration rate (Appendix) can be applied to any river by
Epilithon and DO depletion specifying appropriate parameter values. However, due to the expected differences between the respiration rates of different types of epilithon (e.g. Table 2), equations (2) and (3) are not expected to be generally applicable. Thus, until more generally applicable models of epilithon respiration are available, site specific studies, similar to those described in this paper, will be required to model the relationship between epilithon biomass, temperature and respiration rates in a given fiver. Nevertheless, the approach of employing simple modelling techniques to determine m a x i m u m levels of epilithic biomass relative to the river D O levels appears to have considerable potential for use in directing the management of epilithon to maintain adequate D O levels in relatively shallow rivers. Conventional Streeter-Phelps B O D / D O models are inappropriate in shallow rivers because the respiration of the benthic community is the predominant D O removal mechanism (Hickey, 1988). By combining the information gained on the epilithic biomass density at which nuisance effects on river D O depletion occur with data on epilithic growth rates under different water quality conditions and the length of "growth intervals" between epilithon-scouring spates, the fiver manager can gain guidance on the management measures require to maintain adequate D O conditions in rivers while allowing their use for waste assimilation. Acknowledgements--This work formed part of a PhD study in the Biotechnology Department of Massey University. It was funded by a NWASCA research contract and a State Services Commission Study Award to J. M. Quinn. Thanks are also due to Dr Richard Archer of the Biotechnology Department for assistance with development of the DO simulation model; to Dr Mike Freeman of the Biotechnology Department and Mr John Boyle of the University of Exeter, for the use of their respiratory chambers; to Drs Kit Rutherford, Mike Freeman and Chris Hickey for reviewing the draft manuscript; and to Mrs Mary Stokes for her typing.
REFERENCES
APHA (1980) Standard Methods for the Examination o f Water and Wastewater, 15th edition. American Public Health Association, Washington D.C. Bott T. L., Brock J. T., Dunn C. S., Naiman R. J., Ovink R. W. and Petersen R. C. (1985) Benthic community metabolism in four temperate stream systems: an interbiome comparison and evaluation of the river continuum concept. Hydrobiologia 123, 3~,5. Boyle J. D. and Scott J. A. (1984) The role of benthic films in the oxygen balance in an East Devon river. Wat. Res. 18, 1089-1099. Cooper A. B. (1984) Activities of benthic nitrifiers in streams and their role in oxygen consumption. Microb. Ecol. 10, 317-334. Currie K. J. (1977) Water quality management report-Manawatu River. Manawatu Regional Water Board, Palmerston North, New Zealand. Curtis E. J. C. (1972) Sewage fungus in rivers in the United Kingdom. Wat. Pollut. Control 1972, 673-685.
831
Curtis E. J. C. and Harrington D. W. (1971) The occurrence of sewage fungus in rivers in the United Kingdom. War. Res. 5, 281-290. Darley W. (1982) Algal Biology: a Physiological Approach. Blackwell, Oxford. Elder J. M. and Gloyna E. F. (1969) Oxygen production and loss in a model river. Technical Report No. 1 to the office of Water Resources Research Centre for Research in Water Resources, Environmental Health, Engineering Research Laboratory, Civil Engineering Dept., University of Texas, Austin. Elmore H. L. and West W. F. (1961) Effect of water temperature on stream reaeration. J. sanit. Engng Div. Am. Soc. cir. Engrs 87, 59-71. Freeman M. C. (1983) Periphyton and water quality in the Manawatu River, New Zealand. PhD thesis, Massey University Library, Palmerston North, New Zealand. Freeman M. C. (1986) The role of nitrogen and phosphorus in the development of Cladophora glomerata (L.) Kutzing in the Manawatu River, New Zealand. Hydrobiologia 133, 23-30. Gaudy A. F. and Gaudy E. T. (1981) Microbiology for Environmental Scientists and Engineers. McGraw-Hill, London. Hickey C. W. (1988) Benthic chamber for use in rivers: testing against oxygen mass balance. J. envir. Engng Am. Soc. cir. Engrs 114, 828-845. Jeppesen T. (1962) A comparison between four oxygen balance models for small organic polluted streams with many macrophytes. Progress in Ecological Engineering and Management by Mathematical Modelling; Proceedings of the Second International Conference on Sate-ofthe-Art in Ecological Modelling, April 1980, Leige, Belgium. Mandelstam J. and McQuillen K. (1973) Biochemistry of Bacterial Growth. Blackwell, Oxford. McBride G. B. (1982) Nomographs for rapid solutions for the Streeter-Phelps equations. J. Wat. Pollut. Control Fed. 54, 378-384. O'Connor D. J. and Dobbins W. E. (1958) Mechanisms of reaeration in natural streams. Trans. Am. Soc. cir. Engrs 123, 641--684. Odum H. T. (1956) primary production in flowing waters. Limnol. Oceangr. 1, 102-117. Quinn J. M. (1985) Wastewater effects on epilithon, particularly sewage fungus, and water quafity in the Manawatu River, New Zealand. PhD thesis, Massey University, Palmerston North, New Zealand. Quinn J. M~ and McFarlane P. N. (1985) Sewage fungus as a monitor of water quality. In Biological Monitoring in Freshwaters: Proceedings o f a Seminar (Edited by Pridmore R. D. and Cooper A. B.), pp. 143-162. Water and Soil Misc. Publ. No. 82, Water and Soil Directorate, MWD, Wellington. Quinn J. M. and McFarlane P. N. (1988) Control of sewage fungus to enhance recreational use of the Manawatu River New Zealand. Verh. int. Verein. Limnol. 23, 1572-1577. Rutherford J. C. and Currie K. J. (1979) Investigatwns of mechanisms affecting BOD concentrations in the Manawatu River near Palmerston North. A joint report of MWD, Hamilton Science Centre and Manawatu Regional Water Board. U.S.EPA (1976) Quality Criteria for Water. United States Environmental Protection Agency, Washington, D.C, Wilcock R. J. (1984a) Dye studies and field measurements of reaeration coeefficients of the Manawatu River: February-March 1983. Internal Report 84/12, Water Quality Centre, Ministry of Works and Development, Hamilton, New Zealand. Wilcock R. J. (1984b) Methyl chloride as a gas-tracer for measuring stream reaeration coefficients--II. Stream studies. Wat. Res. 18, 53-57.
J. M. QUINN and P. N. McFARLANE
832 APPENDIX
FORTRAN Program used in Computer Oxygen Modelling Studies C H A R A C T E R * 4 0 F 1,F2 D I M E N S I O N 0(601) Write(l,'(///)') C A L L C O U A ( ' D A T A FILE:-') R E A D (I,'(A40)') F1 C A L L C O U A ( ' O U T P U T FILE: ~-') R E A D (1,'(A40)') F2 O P E N (5,FILE = FI) O P E N (6,File = F2) T R e a d (5,*) AI,A2,A3,A4,A5 ¢- specifies conditions 0 ( 1 ) = A1 N=0 D O 100 1 = 2,601 T = A2-A3* R E A L ( 1) ¢-- calculates temperature C = 13.55-.225"T ,-- calculates saturation D O X2T = A5*(1.0241**(T.20)) ~ adjusts k 2 for temperature 0(I) = 0(I-1)-A4 + X2T*(C-0(I-1)) ~ calculates D O N=N+I
IF (N.GE.60) T H E N C A L L C O U A ('.') W R I T E (6,200) O(I), T N=0 200 FORMAT(2F10.3) ENDIF I00 C O N T I N U E CLOSE(5) CLOSE(6) STOP END where T = C= X2T = O = At = A2 = Aa=
temperature (°C) saturation dissolved oxygen concentration (g m -3) k 2 at the river temperature (min -m) dissolved oxygen concentration (g m -3) initial dissolved oxygen at sunset ( g m -3) initial temperature (°C) temperature decay factor (0.0033 gives 2°C decline in 10 h) A 4 = respiration rate (g 02 m -3 min-1 ) A5 = reaeration k2(20) value (min -1 )