Phosphorus and Nitrogen Loads in Waters ... - CSIRO Publishing

Aust. J . Mar. Freshwater Res., 1982. 33, 223-43

Phosphorus and Nitrogen Loads in Waters Associated with the River Murray near Albury-Wodonga, and their Effects on Phytoplankton Populations K. F walkerA and T. J. ~ i l l r n a n ~ Department of Zoology, University of Adelaide, G.P.O. Box 498, Adelaide, S.A. 5001 Peter Till Environmental Laboratory, Albury-Wodonga Development Corporation, Albury, N.S.W. 2640.

A

Abstract This is part of a survey carried out to assess the likely impact on the River Murray of urban development at Albury-Wodonga. The study area included two impoundments, Lake Hume and Lake Mulwala, the tributary Mitta Mitta, Kiewa and Ovens Rivers, and the River Murray. Phosphorus and nitrogen loads were examined over 4 years, incorporating a high flow period, 1974-75, and a low flow period, 1976-77. Loads varied directly with stream flow, although loadings to the two impoundments were generally high and above levels considered indicative of eutrophy elsewhere. Phytoplankton biomasses did not reflect this situation, howeyer, and averaged below 2000 mm3 m - ? Phytoplankton growth, and hence nutrient assimilation, probably were restricted by low light penetration resulting from high turbidities. This is supported by preliminary laboratory experiments involving nutrient enrichments over a range of turbidities. Under low flow conditions, nutrient inputs to the impoundments were reduced and underwater light penetration increased, due to settling of abiogenic suspended material. Nutrient enrichment experiments in Lake Mulwala supported the proposition that availability of nutrients, particularly phosphorus, could limit phytoplankton biomass under those conditions. The results generally indicate that pro rata inputs from Albury-Wodonga need to be reduced as the urban population grows, if the risk of nuisance algal blooms during low flow periods is to be minimized. Extra keyword: eutrophication

Introduction

In 1973, the Australian Government and the Governments of New South Wales and Victoria agreed to promote urban development in the region of Albury and Wodonga, near the headwaters of the Kiver Murray in south-eastern Australia. The initial planning study (Anon. 1974a) included an ecological survey of local river and floodplain environments, and recommended additional survey work that was completed in 1976 (Walker and Hillman 1977). The survey, and results obtained since, have provided a general appreciation of the ecology of the local river and associated waters, and are relevant particularly for planning and management. They are also of scientific interest because the ecology of the River Murray is little known despite the river's importance as a national water resource. Some reports are published (Croome et al. 1976; Walker et al. 1978; Walker 1979, 1980; Croome 1980), and others are in preparation. The Albury-Wodonga Development Corporation has been actively associated with these investigations since their inception, and now maintains a continuing program of monitoring and research at the Peter Till Environmental Laboratory, Albury. An important task of the survey was to determine the likelihood that troublesome plant growths would occur in local waters as a result of increased nutrient inputs

K. F. Walker and 7. J. Hillman

associated with urban development. Accordingly, phosphorus and nitrogen concentrations were monitored regularly at several sampling stations, to provide information about nutrients in relation to variable river flows, and to allow an assessment of the trophic status of the two local river impoundments, Lake Hume and Lake Mulwala. This information was also related to changes in phytoplankton populations in the two impoundments, and the relationships were further explored in field and laboratory nutrient-enrichment experiments. A description of these studies is the basis of the present paper. The paper draws on data from the survey reports (Anon. 1974a, ch. 8; Walker and Hillman 1977) covering the years 1974-76, and also provides data for 1977.

Fig. 1. Location of sampling sites.

Study Area

The study area (Fig. 1 ) included 350 km of the River Murray's course from above Lake Hume to below Lake Mulwala, and extended to the lower reaches of the Mitta Mitta, Kiewa and Ovens Rivers. Fifteen sampling stations were established, in locations determined by recognizable 'environmental units' (Walker and Hillman 1977). Lake Hume is impounded by Hume Dam and Weir, upstream from Albury-Wodonga. The lake impounds winter and spring flows from the upper Murray and Mitta Mitta Rivers, and releases water to downstream irrigation areas in summer and autumn. Discharge to the Murray may be through valves in the dam wall or through the turbines of a small power station. Surplus water may be released via spillway gates. Lake Mulwala, about 200 river-km downstream from Lake Hume, is impounded by Yarrawonga Weir and provides a gravity diversion for two large irrigation canals. Unlike Lake Hume, Lake Mulwala generally is shallow and includes an extensive area of drowned red-gum forest. The comparative morphometry of the two lakes is summarized below: x Volume (MI) Surface area ( A , km2) Maximum depth (Z, m) Mean depth ( T , m)

Hume 3070 202 5 41.5 15.2

Mulwala 116 44.6 13.7 2.6

Circumference (S, km) Shoreline development (St2 Basin form (f/Z) Catchment area (km2)

m)

Hume 150

Mulwala 48

4.95 0.37 15275

2.03 0.19 27 300

Nutrients in the River Murray

The study area is near the foothills of the Great Dividing Range, and includes a wide diversity of physical environments. Although the climate is Iocally influenced by orographic factors, the general pattern is for relatively cool, wet winters and hot, dry summers (Anon. 19743). Average temperatures at Albury range from 8OC in July to 24OC in January. Annual rainfall averages 679 mm, with monthly extremes in June (82 mm) and January (29 mm) or February (32 mm). Evaporation tends to an annual average near 2000 mm, with extremes in January (325 mm) and June or July (40 mm). The climate, however, is prone to secular as well as seasonal variations; these may markedly affect river flows because about 20-25% of the River Murray's total annual discharge (c. 12 x lo6 M1) originates in the catchment above Lake Mulwala. Annual river flows varied considerably during the study period. Methods Observations during 1974-77 The frequencies of sampling and analysis varied between years. Until March 1975. all stations were sampled monthly, but subsequently stations 5-10 were sampled six times monthly and stations 1 4 and 11-15 four times monthly. Analyses were at similar frequencies, with the exceptions of total Kjeldahl nitrogen (TKN) and algal biomass which were measured monthly. Biomass determinations were made twice monthly during the low flow periods of 1976 and 1977. Weekly mean flow data were obtained from the River Murray Commission, Canberra. Filterable reactive phosphorus (FRP) in water samples was determined by the stannous chloride spectrophotometric method, after filtration through a 0.45-km membrane. Total phosphorus (TP. including FRP) in unfiltered samples was determined by the same method, after acid persulfate digestion and p H adjustment. Analyses were according to Anon. (1975). Nitrogen was determined as organic (total Kjeldahl) nitrogen and as nitrate. in unfiltered samples (comparative analyses of filtered and unfiltered waters showed no significant differences). The Kjeldahl digestion method converts biogenic, organically bound nitrogen to ammonia which is determined by the phenate method (Anon. 1975). Nitrate measurements approximate total inorganic nitrogen because ammonium and nitrate compounds are unlikely to be significant in well-oxygenated waters. Some inaccuracies may exist in the data becauseuntil January 1976 the brucinemethod (Anon. 1975) was used; this has a lower detection limit near 20 mg m-3, and samples frequently fell below this level. Subsequently, the cadmium-copper reduction method (Anon. 1975) was used, providing sensitivity that was ten times greater. Phytoplankton samples were obtained by lowering 4 m of 25-mm diam. plastic tubing into the water, closing the uppermost end, raising the tube and pouring the contents (c. 2 litres) into a bucket. A subsample was taken for 'live' examination, and another for counting and biomass measurement, following staining and settling with Lugol's iodine solution. Biomass estimations (in mm3 m-3) were based on counts and cellvolume approximations for the abundant species. Nutrient Enrichment Experiments Various preliminary experiments were devised to explore the interactions between phytoplankton growth, nutrient supply and turbidity. These were necessarily of an exploratory nature. because time and facilities were limited. The results, however, offer some preliminary guidelines for future study. The first experiment examined whether high turbidities, such as prevailed during 1974-75. could override the stimulatory effects of high nutrient levels. Three 50-litre plastic bins were painted black and filled with river water. Three turbidity levels (monitored by Hach Turbidimeter) were established in the respective bins, using fine river silt kept in suspension by aeration (at a late stage it was necessary to restore the turbidities in two bins by further addition ofsilt). Four I-litre glass bottles were fixed by suctioncups to the bottom ofeach bin, and each bottle was fitted with a rubber stopper and plastic 'breather' tube to the surface. Each set of bottles contained Lake Hume water with an adjusted nitrate concentration 8f 0 . 2 mg 1-'; and adjusted T P concentrations of O.01,O. 02,O. 05 and 0 . 2 mg 1-I, respectively. The bottles were shaken and rotated in the bins daily for 1 month (31 days preceding 12 November 1976). The bins were exposed to the open air and hence water temperatures varied considerably (12.5-23, S°C). Samples were withdrawn periodically from the bottles for determinations of nutrients and algal biomass. The second experiment concerned the suggestion (Anon. 1974a, ch. 8) that nutrients from urban development would probably have noticeable adverse effects on Lake Mulwala during low flow per~ods

K . F. Walker and T . J . Hillman

when turbidities are minimal. These conditions prevailed during mid-1976, and provided an opportunity to undertake a series of field and laboratory experiments. The laboratory experiments were to determine the nutrient element(s) in shortest supply relative to the needs of the phytoplankton in Lake Mulwala. Trials involved enrichment of lake water with various nutrients. and monitoring of algal growth. Lake water was collected on four occasions. Each treatment involved a flask containing 300 ml lake water; supplemented by 2 mg I - ' of the given nutrient (N, P, Mo, Si), or without supplement in the case of controls. Molybdenum was included because local soils are deficient in molybdenum, and Cyanobacteria have a relatively high requirement for the nutrient. Biomass and community composition were determined after prolonged exposure to daylight on a laboratory shelf. The field experiments were to determine whether increased concentrations of the potentially limiting nutrients do stimulate algal growth under lake conditions. One litre of lake water was placed in each of six (later eight) dialysis sacs, and to each was added a smaller dialysis sac containing concentrated nutrient solution or, in the case of controls, distilled water. The sacs were then suspended at 1 m depth in the lake, each separated by about 20 m to avoid possible contamination. After 2 weeks, the sacs were recovered and the contents analysed for nutrients, total organic carbon (Oceanographic TOC Analyser) and algal biomass. This basic design was repeated three times.

Results and Discussion

River Flows The four years of observation included two 'wet' years, 1974-75, and two 'dry' years, 1976-77. Thus, annual rainfalls at Wodonga were 1127 mm (1974), 821 mm (1975), 488 mm (1976) and 472 mm (1977), compared with the long-term median (1898-1969) of 707 mm (Commonwealth Bureau of Meteorology records). Although rainfall in 1975 was 16% above the median (not a great variation by Australian standards), the volume of runoff was disproportionately large due to consistently high rainfall in the previous year. Stream flow data for several stations are shown in Fig. 2. Flows leaving the study area (cf. stn 14) in the wet years were three times greater than those in the dry years. Still greater variations occurred in tributary flows, but their influence was moderated by operations at Hume Dam. During 1974-75 an average of c. 1400 M1 local runoff per day was added to flows in the Murray between Albury-Wodonga and Lake Mulwala, but in 1976-77 the supplement was considerably smaller. Irrigation diversions from Lake Mulwala were 7.5% of the lake outflow in 1974-75 and 44% in 1976-77. Water renewal rates (discharge/volume) for each lake show clearly the differences in river flows over the 4-year period. Lake Mulwala had a 2-year average renewal rate of 125 year- over 1974-75 and 40.1 year-' over 1976-77. The corresponding values for Lake Hume were 3.09 and 1 .26 year- .

'

'

Phosphorus Concentrations of F R P and TP at the various lake stations are shown in Fig. 3. Most of the phosphorus present was associated with particulate matter, as would be expected in waters of high seston content and moderate to high algal productivity (see later). At lake stations, F R P levels usually were near the limit of analytical detection (4 mg rnp3). At river stations, where algal uptake was less significant, F R P accounted for about one quarter of the T P present. The data suggest that suspended solids are an important vehicle for downstream transport of phosphorus (cf. also Buckney 1979). Phosphorus concentrations at river stations, of course, varied with flows. It is more informative to consider trends in terms of mass transport (loads), as this permits ready identification of inputs and outputs. Loads for any given period were calculated (as kg P day-') by multiplying the measured TP concentration by the corresponding mean flow rate. Comparisons showed that loads calculated on this basis were similar to those


calculated with daily flow records. Comparisons were made also to determine how load estimates were influenced by different sampling frequencies at the various stations. It was concluded that differences between stations of 50 kg day-' or less are unlikely to be significant, and that differences of 100 kg day-' or more probably are significant (Anon. 1974a, ch. 8). Station 1 (Murray)

__-L --L Station 2 (Mitta Mitta)

6 Station 6 (Murray)

r l l 1 l l m ) m Volume of Lake Hume

Station 7 (Kiewa)

Station 1 2 (Murray)

Station 1 4 (Murray) Volume of Lake Mulwala

Station 15 (Ovens)

Fig. 2. River flows at selected sampling stations, 1974-77. Data by courtesy of River Murray Commission.

Mean monthly loads of TP at the principal river stations are shown in Fig. 4. The figure conveys an impression of the relative magnitudes of tributary and mainstream loads, and the influence of variations in flow. At each station, loads were markedly greater in 1974 and 1975, the two years of high flows, than in 1976 and 1977, when flows were comparatively low. To demonstrate more clearly the differences between stations and to provide 'balance sheets', summaries of load data are shown in Table 1 . The mean TP loads for each period are shown also in schematic form. The following discussion mainly concerns the T P data, but F R P loads are tabulated for information. The data for the wet and dry years differ both in magnitude and pattern. Throughout the study area daily loads during 1974-75 were two to three times higher than those in 1976-77. In 1974-75, runoff from land between Albury-Wodonga and Lake Mulwala increased the TP load in the River Murray by 49%; this came principally from agricultural sources, as the small towns along that reach contribute little or no direct sewage input. During this period, phosphorus inputs from agricultural runoff dwarfed the potential inputs associated with urban development: the maximum weekly

K. F. Walker and T. J. Hillman

mean load at station 12 (13 376 kg P day-') corresponds to the phosphorus content of raw sewage from a population of c. 5 million (assuming 2 . 5 kg per person per day; see Anon. 1974~).The annual use of agricultural fertilizers in the Albury-Wodonga district exceeds 40 x lo6 kg superphosphate and 7 x lo5 kg nitrogenous fertilizers, Table 1. Loads of total phosphorus (TP) and filterable reactive phosphorus (FRP) at sampling points Mean T P loads are also shown in schematic form-upper numerals, wet years (1974-75); lower numerals, dry years (1976-77) --

Station

-

TP load (kg day-') Min. Max. Mean

FRP load (kg day-l) Min. Max. Mean

WeightedB mean concn

AMulwala and Yarrawonga Irrigation Canals. BTotal loadltotal flow (mg m-3).

and approaches 4 x lo6 kg mixed fertilizers, including nitrogen, phosphorus and potassium (Anon. 1974a, ch. 8). In 1976-77, the relative proportions of loads in the tributaries and the River Murray near the respective confluences mirrored changes in flow rather than changes in phosphorus concentration (see below). Along the Murray between Albury-Wodonga and Lake Mulwala the load increment was less substantial (84 kg P day-', or 17%),


indicating less runoff and perhaps less bank erosion. The difference between the load below Albury-Wodonga and the sum of loads at stations 6 and 7 (River Murray plus Kiewa River) was similar in wet and dry years. Despite load variations, the corrected mean concentrations of TP (load/flow for the period) remained largely unaffected by changes in flow (Table 1). The relatively low

L

Station 4

c

Station 5

z

e

,...,'..

.... ,'.._,,"

:,,,

, a

.

;-.*

I I II

,

%

.....'.-......,, I.

r

I I 2

I I

II

,, , ,,.

,

____.-... .. ,','l\ ,: %

..I

.,

0

Fig. 3. Concentrations of filterable reactive phosphorus (-) (----) at lake stations, 1974-77.

and total phosphorus

records for station 1 in 1976 may have resulted from an increased diversion from the Snowy Mountains Hydroelectric Scheme to supplement irrigation needs along the Murray. Anon. (1974a, ch. 8) cited preliminary observations of the stormwater channels Bungambrawatha Creek (Albury) and House Creek (Wodonga) to show that local stormwaters may carry significant loads of TP during heavy rainfall. For example, a rainstorm on 19 February 1974 brought estimated flows of 2500 M1 over 24 h in Bungambrawatha Creek; the average T P concentration (300 mg m-3) indicated a T P load of 750 kg day-'. Although these periodic inputs may be concealed here by the calculation of longer-term averages, they deserve close consideration in future studies, particularly in regard to the smaller tributaries.


m

0

n-n

- 0

n

N

O

m

- 0

in

n

0

m


Total phosphorus budgets for Lakes Hume and Mulwala Total phosphorus budgets for the two lakes, based on load estimates, are shown in Table 2. The data for 'retention', calculated as differences between major inputs and outputs, incorporate assumed minor influences including rainfall on the lake surface, groundwater seepage, small creeks and local runoff, losses via organisms, retention in sediments and differences in total nutrient content of each lake over the budget period (see below). The phosphorus retained in Lake Hume was 39% of the inflow load in 1974-75 and 20% in 1976-77, a difference contrary to that suggested by water renewal times (3.09 and 1.26 year-' respectively). The discrepancy is accounted for by the respective differences in lake volume: at the end of 1975, Lake Hume was near full capacity (3.05 x lo6 M1) but at the end of 1977 it was down to 68% of capacity (2.08 x lo6 Ml). The phosphorus concentrations were similar (c. 9 mg rnp3), so that the total quantity of phosphorus in the lake system in late 1975 was approximately two-thirds that in late 1977. Table 2. Total phosphorus budgets for Lakes Hume and Mulwala in 1974-75 (high flow) and 1976-77 (low flow) Lake

Period

x Input (kg year-')

x Output (kg year- I )

x Retention (kg year-')

Hume

1974-75 1976-77

359 111

219 89

140 (39%) 22 (20%)

1.78 0.56

Mulwala

1974-75 1976-77

630 209

583 193

47 (7%) 16 (8%)

14.21 4.69

Surface loading [g (m2 year)-']

In Lake Mulwala, phosphorus retention did not vary significantly (7-8%) over the 4-year period, despite a fourfold difference in water renewal rates (125 and 40.1 year-', respectively). This suggests that low river flows pass rapidly through the lake, along the former river channel, not mixing with water in the extensive shallow marginal areas. Sediments in the uppermost reaches of the lake contain significantly less phosphorus than in the lower reaches (Hart et al. 1976), perhaps associated with progressive deposition of suspensoids along the length of the lake. Trophic status of Lakes Hume and Mulwala It is possible to represent the trophic status (potential fertility) of lakes in terms of the annual phosphorus input pro rata of lake surface area. The 'surface loading' [g P (m2 year)-'], when related to the lake mean depth ( F ) and water renewal rate ( t ) , correlates with the trophic status of certain Northern Hemisphere lakes (e.g. Vollenweider 1975, 1976). The annual surface loadings to Lakes Hume and Mulwala are plotted against the appropriate values of ( F . t ) in Fig. 5. Data for each year are plotted separately, but it is clear that conditions in 1974-75 and 1976-77 were distinctive. The broken lines in the figure indicate 'permissible' and 'dangerous' limits, corresponding to steady-state T P concentrations of 10 and 20 mg m P 3 respectively. These levels are conventionally regarded as the thresholds of mesotrophy and eutrophy, respectively (e.g. Vollenweider 1968). According to these criteria, both lakes exceed the 'dangerous' limit; indeed, Fig. 3 shows that the two lakes frequently have

K . F. Walker and T. J. Hillman

TP concentrations exceeding 20 mg mP3. If the logarithmic scale of Fig. 5 is considered. it appears that Lake Mulwala is farther beyond the 'dangerous' limit than is Lake Hume. The log scale also conceals the greater interseasonal variability of Lake Mulwala, an important consideration in prediction of the effects of urban development. Further, the fact that data for each lake tend to a line parallel to the limit line, suggests that loads vary directly with flows, while concentrations remain constant. A similar observation was made earlier, in reference to Table 1. This may simply reflect the use of calculated averages, as there is clear evidence (e.g. Cullen et al. 1978; Anon. 1981) that stormwater nutrient concentrations are flow-dependent over short periods.

dangerous

EUTROPHIC

perm~sa~ble

Fig. 5. Total phosphorus regimes in Lakes Hume and Mulwala. cl Annual loading. 2-year mean annual loading (wet years v. dry years). 0 4-year mean annual loading.

OLIGOTROPHIC

77

Dillon (1975) used an alternative form of Vollenweider's (1975, 1976) model to take direct account of phosphorus retention within a lake and to adjust surface loadings for lake volume changes. Dillon's equation is [PI

=

L(l -R)/'t,

where [PI is total phosphorus concentration (g P m-2), L is surface loading [g P (m2 year)-'], R is retention coefficient (the fraction of the inflow load not lost in outflow), and t is water renewal rate (annual dischargellake volume, year-'). Values of [PI for Lakes Hume and Mulwala are readily calculated from the morphometric data and Table 2. Knowing the mean depths of the lakes, and assuming no change in the retention coefficients, it is also possible to specify values corresponding to 'permissible' and 'dangerous' TP concentrations (Table 3). For Lake Hume, the value of [PI remained similar in wet and dry years. For Lake Mulwala, however, the value during low river flows (1976-77) was somewhat lower than during high flows (1974-75). This recalls the previous note that phosphorus retention in Lake Mulwala did not increase during low flow periods (see Table 2), possibly because of channelled flows through the impoundment. The implications of these factors for other shallow impoundments might be noted. The results concur with the earlier conclusion that both lakes, and particularly Lake Mulwala, are significantly beyond the 'dangerous' limit. This implies that each lake environment favours the development of troublesome algal growths. It is stressed.


however, that this would apply only in the absence of any overriding effects, such as limitation of plant growth by light availability or by the availability of nutrients other than phosphorus. This is considered further shortly. Note that the plot of phosphorus loadings and calculated values of the Dillon equation provided by Walker and Hillman (1977, pp. 51-2) were in error, although the general conclusions of that report are unaffected. Table 3. Total phosphorus concentration and concentrations corresponding to levels, calculated from Dillon's 'permissible' (10 mg m-3) and 'dangerous' (20 mg (1975) equation Lake

Period Calculated

Total phosphorus (g w 2 ) 'Permissible' 'Dangerous'

Hume

1974475 1976-77

0.35 0.36

0.09 0 12

0.19 0.26

Mulwala

1974-75 1976-77

0.104 0.079

0.024 0.023

0.048 0,047

Nitrogen Concentrations of nitrate and T K N at lake stations are shown in Fig. 6. Variations were erratic, without apparent seasonal or annual bases, but there were clear correspondences between stations on Lake Hume, and between Lake Hume and Lake Mulwala. Nitrate was a variable, but generally small, proportion of the total nitrogen present, suggesting that most nitrogen was associated with organic material. As with phosphorus, nitrogen data for river stations are more meaningfully considered in terms of loads. These were calculated in similar ways for nitrate and TKN. The effects of different sampling frequencies were such that differences of 65 kg day-' or less are unlikely to be significant, whereas differences of more than 100 kg day-' do warrant attention (Anon. 1974a, ch. 8). Mean monthly loads of nitrate at the principal river stations are shown in Fig. 7. The diagram shows the relative magnitudes of loads at tributary and mainstream stations, and also the effects of different flow rates. Loads were considerably higher in 1974-75 when flows were high, than in 1976-77 when flows were low. Nitrate and T K N load data are shown in Table 4. As with TP, T K N loads significantly increased along the River Murray between Lakes Hume and Mulwala. The same pattern was evident under high and low flow conditions, although the respective magnitudes of the loads differed markedly. Nitrate loads showed a comparable pattern, but with a relatively small downstream increment. Stormwater nitrogen loads are likely to be of considerable significance, although this is obscured here by the use of calculated averages. Observations on Bungambrawatha Creek in Albury, reported by Anon. (1974a, ch. 8). indicate that nitrate-nitrogen loads of 1250 kg day-' may occur during heavy rainfall. Again, there is a need for more intensive studies of local stormwater nutrient inputs. The concentration of particular nitrogen species in water at any time is a net result of processes of biological assimilation, mineralization, nitrification and denitrification (e.g. Keeney 1973). In a crude input-output budget, therefore, it is unrealistic to expect close balances. In fact, the nitrate data do not balance closely, whereas those for T K N


show quite close, perhaps fortuitous, correspondences, suggesting that a similar net amount of T K N was preserved despite biological transformations. It is likely, for example, that denitrification was a significant component of the nitrogen budget of Lake Hume, which typically has a deoxygenated hypolimnion in summer (Croome 1980).

looO

F

., ,,

Station 3

Station 5

Station 13

Fig. 6. Concentrations of nitrate and total Kjeldahl nitrogen at lake stations, 1974-77.

Nitrogen budgets for Lakes Hume and Mulwala The biological lability of nitrogen means that budget calculations of the kind applied to phosphorus data would have little real value. Currently there is no satisfactory means of relating nitrogen loads to 'algal nuisance' criteria, although nitrate concentration data do have some significance in assessment of trophic status. Nitrate and T K N budgets for the two lakes are shown in Table 5. The data for each lake show that the amount of nitrogen associated with organic material is five to ten times greater than that present as nitrate. The nitrate-nitrogen loads flowing from Lake Hume exceeded the input loads, particularly in 1974-75, but in Lake Mulwala the opposite was true. For total nitrogen, the effects of flow variations were apparent in that output exceeded input during high flows (1974-75), and input exceeded output during low flows (1976-77). It appears that denitrification was significant in both lakes during the low flow period, but there is



insufficient information for detailed analysis. The fact that both lakes behaved similarly in respect to total nitrogen, but not nitrate, recalls the earlier comment about the conservation of T K N despite nitrogen transformations.

Table 4. Loads of total Kjeldahl nitrogen (TKN) and nitrate at sampling points Mean nitrate loads are also shown in schematic form-upper numerals, wet years (1974-75); lower numerals, dry years (1976-77)

Station

TKN load (kg day-l) Min. Max. Mean

Nitrate load (kg N day-') Min. Max. Mean

AMulwala and Yarrawonga Irrigation Canals. 904

Phytoplankton Phytoplankton was sampled at least monthly at the four lake stations (3, 4, 5 and 13). Mean monthly algal biomass estimates (mm3 mP3) are shown in Fig. 8. One-hundred-and-thirty-seven algal species were recorded (Walker and Hillman 1977), with the dominant species generally being diatoms of the genus Melosira (M.


distans, M . granulata and, in 1974 only, M . varians). The majority of recorded species was shared between the lakes, not surprisingly, as Lake Mulwala receives most of its inflow from Lake Hume. Melosira species are dominant phytoplankters along the length of the River Murray (cf. Falter 1978). Table 5. Nitrogen budgets for Lakes Hume and Mulwala in 1974-75 (high flow) and 1976-77 (low flow)

Lake

Perlod

Nitrate-nltrogen (kg year-') x lo-; x x Input Output Retention

10-3 x Input

TKN (kg year-') x x Output Retention

Hume

1974-75 1976-77

472 123

772 154

- 301 -31

4339 896

4494 732

-155 164

Mulwala

1974-75 1976-77

1422 308

962 73

460 235

7664 1481

8164 1350

- 500 131

Similar species were 'subdominant' at all stations in any one year, but there were marked differences between years. In 1974, the principal subdominant species were the cyanobacterium Anabaena spiroides and the chrysophytes Cryptomonas and Synura.

;L,,,"

0

10

n,

m -

Fig. 8. Mean monthly phytoplankton biomass at lake stations, 1974-77.

V)

E

.o n

0

In 1975, the principal subdominants were colonial green algae and chrysophytes, and Cyanobacteria were insignificant. In 1976, there was a marked increase in the relative abundance of species other than Melosira, notably of colonial green algae, the


euglenophyte Trachelomonas, the diatom Asterionella formosa and the Cyanobacteria Anabaena spiroides and Anacystis cyanea. In 1977, these same groups shared subdominance with Melosira, with the addition of thecolonial green alga Volvox which reached large numbers in both lakes during early spring.

Fig. 9. Secchi disc depths at lake stations, 1974-77

Thus, as a general pattern, species of Melosira virtually dominated the algal community during wet years and, though still abundant (especially during summer), were subdominant during dry years. It is not possible to ascribe simple cause-andeffect relationships to these observations as many environmental factors were correlated with seasonal river-flow changes. Differences in turbidity and nutrient supply are likely to have been important. Phytoplankton community structure is discussed further by Walker and Hillman (1977). Seasonal trends of biomass varied somewhat between stations and years. In 1974, there was a well-defined summer-autumn peak in biomass at stations 3 and 4 in Lake Hume. There was minor growth at station 5 over this period but the principal peak there occurred in winter, declining through spring. In Lake Mulwala there was a summer maximum at station 13, declining through autumn and winter. The same broad pattern was evident in 1975, except that the seasonal peak at station 3 was in spring (rather than summer-autumn), and winter growth at station 13 was interrupted when Lake Mulwala was drained for maintenance at Yarrawonga Weir. In 1976-77, conditions were affected by low river flows. At station 3 there was little significant


development of biomass throughout 1976, and although a summer-autumn peak occurred at station 4 it was of lesser magnitude than in 1974-75. Station 5 showed a summer-autumn maximum twice that of 1974 or 1975. In Lake Mulwala the population developed through spring to attain an autumn peak. The pattern at station 5 was consistent in that population maxima occurred regularly in autumn and winter, a few months after the corresponding peak at station 4. The absence of summer growth at station 5 would have been partly a result of nutrient depletion of surface waters during stratification (e.g. Croome 1980). Average biomasses were similar between stations 4 and 13, and between stations 3 and 5, the former being about twice the latter. Table 6. Biomass and filterable reactive phosphorus (FRP) levels during turbidity-phosphorus trials, October-November 1976 NTU, nephelometric turbidity units Initial F R P (mg I-')

Biomass as % of initial population after: 6 days 15 days 25 days 31 days

F R P concn (mg I-') after: 6 days 15 days 25 days 31 days

Low turbidity, mean 5 . 0 NTU 96 0,008 139 258 20 1 554 142 0.077 -A 227 4u8 0,014 225 389 1931 0,158 Medium turbidity, mean 32.5 NTU 263 101 104 0.012 59 0,012 109 62 -6

168

94

112

0.100

High turbidity, mean 64.4 KTU 44 0.012 165 67 238 0,016 299 333 B

32 Astopper leak?

BStopper displaced.

112C

1329C

0.140

CBottle floated.

Underwater Light Penetration The relatively high turbidity of the River Murray waters, including Lakes Hume and Mulwala, is undoubtedly an important factor governing plant growth. Secchi disc measurements for the four lake stations are shown in Fig. 9. From comparative measurements with an underwater light meter (Whitney LMT-8A, spectral range 400-700 nm), it was established that these Secchi disc data approximate the depth of 10% surface light penetration, and that the depth of 1% light penetration (Dl%,m) could be estimated from the Secchi depth (D,, m) using the regression equation (Anon. 1974a, ch. 8):

The depth of 1% surface light penetration is a conventional indication 6f the extent of the photosynthetic zone. Fig. 9 shows that Secchi depths were somewhat greater in 1976-77 than in 1974-75, in accord with decreased river flows and turbidities. This is most apparent for station


13 in Lake Mulwala, where in 1974-75 the photosynthetic zone was restricted to the uppermost 1-2 m but in 1976-77 extended to 2-6 m. At stations 3 and 4 in Lake Hume the photosynthetic zone was rarely deeper than 3-4 m, and at station 5, where turbidities tended to be lower because of settling, the zone extended to about 6 m. From these data alone it seems that little phytoplankton growth could have occurred below about 4 m. This was confirmed, at least in 1974, by carbon-14 productivity measurements (Walker and Hillman 1977; Croome 1980). The data also suggest that light availability under some conditions is likely to override nutrient supply as a factor limiting algal growth. Table 7. Biomass and composition of phytoplankton in laboratory enrichment trials Treatment

Algal biomass (mm3 m - 9

% contribution to total phytoplankton Diatoms Green algae Cyanobacteria

18 June-28 July 1976 Control + 2 mg 1-I P + 2 mg 1 - I N

294 820 1498

Control + 2 mg I-! P + 2 mg I - ) N

1408

68 64 46

24 J u n e 2 August 1976 67

B

21 July-5 August 1976 Control f 0 . 2 mg I-' Mo + 2 mg 1 - I Si + 2 mg 1-I P + 2 mg I - ' N

2540 2703 243 1 9289 1 1 540

Control +2mg1-I P + 2 mg 1-' N + 2 mg 1-I P + N

10013 37326 15333 70064

63 65 68 88 92

2 August-17 August 1976

*Present at levels

t 1%.

10 2 19 8

BNo data; flask lost.

Nutrient Enrichment The high river flows of 1974-75 were accompanied by high turbidities. Light penetration in Lakes Hume and Mulwala was minimal, and the respective phytoplankton communities maintained only moderate biomasses despite the substantial imported nutrient loads. The first experiment was intended to discover whether phytoplankton growth was suppressed by similar high turbidities, in the presence of abundant nitrogen and different phosphorus levels. Progressive changes in F R P concentration and algal biomass are shown in Table 6. At low turbidities the biomass increased by a factor of 19 over the course of the experiment, although the increase was less sustained at the lowest phosphorus concentration. At high turbidities phytoplankton growth was variable but noticeably less than at low turbidities. There was a marked increase in


biomass when, as a result of a mishap, the bottle with an FRP concentration of 0 . 2 mg I-' in the high turbidity treatment floated to the surface. Results of the second series of (laboratory) experiments are shown in Table 7. Increased silicon and molybdenum concentrations did not significantly increase algal growth, but both nitrogen and phosphorus did (nitrogen showed a substantial effect early, and phosphorus became progressively more significant). Nitrogen and phosphorus caused responses in different elements of the phytoplankton community, Table 8. Field enrichment experiments: algal growth in samples from Lake Mulwala maintained in situ Blank space. no analysis performed

Treatment

Final nutrient content (mg I-') NitrateFRPA TPB nitrogen

Biological indices TOC Algal biomass (mg I-') (mm3 m-3)

18 June-24 June 1976 Control. 0 m +P, Om + N , Om Control, 1 m +P, I m +N. I m

Control, 0 m +P. O m +N, Om Control. I m +P, 1 m +N, I m Surface sample, 2.viii.76

0,003