production and major nutrient composition of three grass species on the magela floodplain, northern...

Aquatic Botany, 41 ( 1991 ) 263-280 263 Elsevier Science Publishers B.V., Amste rdam

Production and major nutrient composition of three grass species on the Magela floodplain,

Northern Territory, Australia

C.M. Finlayson ~ Alligator Rivers Region Research Institute, Office of the Supervising Scientist. Post Office, Jabiru,

NT 0886, Australia

(Accepted 12 March 1991

ABSTRACT

Finlayson, C.M., 1991. Production and major nutrient composition of three grass species on the Ma- gela floodplain, Northern Territory, Australia. Aquat. Bot., 41: 263-280.

Seasonal changes in dry weights and nutrient concentrations in three aquatic grasses on the seasonally inundated Magela floodplain in northern Australia were investigated over an 18 month period. The dry weight of the aquatic grass species Pseudoraphis spinescens ( R. Br. ) Vick., Hymenachne acutigluma (Steudel) Gilliland and O0~za meridionalis Ng varied with water depth on the floodplain. Maximum dry, weights ( 1.67+_0.21 kg m -z , 1.41 _+0.10 kg m -2, 0.51 _+0.10 kg m -2, respectively) occurred at the end of the wet season when water depth was decreasing. The perennial species P. spinescens and H. acutigluma had two growth periods and an annual productivity of 1.91 _+ 0.26 kg m -2 and 2.09_+0.38 kg m -2, respectively, compared with 0.51 _+0.10 kg m -2 for the annual O. mer,- dionalis. Relative to other aquatic and wetland plant species, the former two grasses have high production rates. The major nutrients nitrogen, phosphorus, sulphur, chloride, magnesium, calcium, sodium and potassium were generally present in low concentrations compared with concentrations in other plant species. The annual O. meridionalis had distinct seasonal changes in nutrient concentrations, whereas for P. spinescens and H. acutigluma the changes were not as obvious. An assessment of the nutrient loads that could potentially be turned-over in the detritus originating from these grasses was made. Compared with the annual input to the floodplain of major nutrients in creekwater thv grass debris/detri tus contained substantially more nitrogen, phosphorus and potassium, less sodium and chloride, and similar amounts of sulphur, magnesium and calcium. Thus, the grasses are an important potential component in the chemical turnover pathways on the floodplain.

I N T R O D U C T I O N

Pseudoraphis spinescens (R. Br.) Vick., Hymenachne acutigiuma (Steu- del) Gilliland and Oryza meridionalis Ng on the Magela Creek floodplain ir northern Australia were sampled to determine biomass dry weights and nutrient concentrations over a seasonal cycle. These grasses are major compo-

• ~ Present address: IWRB, S| imbridge, GL2 7BX, UK.

264 C.M. FINLAYSON

nents of the floodplain vegetation (Finlayson et al., 1989 ) and, as such, could be important in the turnover of nutrients in the seasonally wet-dry conditions that characterise the Magela Creek ecosystem. Interest in chemical turnover by the floodplain vegetation arose from investigations designed to assist in determining an acceptable water release policy for the Ranger uranium mine

MAG CRE

[ ] B I L L A B O I 5 L A N D

M U D G I N B E R R I B I L L A B O N G

k m

Fig. 1. Magela Creek floodplain showing the sample sites ( 1, Pseudoraphis spinescens; 2, Hy- menachne acutigluma; 3, Oryza meridionalis). Inset maps show the location of Magela Creek in the Alligator Rivers Region (ARR) in northern Australia.

THREE GRASS SPECIES ON THE MAGELA FLOODPLAIN, AUSTRALIA 265

at Jabiru. The mine is located in the catchment of Magela Creek and upstream of the area referred to as the Magela floodplain (Fig. 1 ). The large unexploited Jabiluka uranium deposit is also contained in the catchment of the Magela.

Mining and milling of uranium has been underway at the Ranger site since 1980. At commencement of mining, a policy of on-site retention of water was adopted, although it was recognised that future management procedures might need to include controlled release of water to Magela Creek (Fox et al., 1977 ). During the time this policy has been in place the tasks of collecting and as- sessing ecological information from the Magela Creek ecosystem and of pre- dicting the fate of potential pollutants have been addressed (e.g. Alligator Rivers Region Research Institute, 1985 ).

The floodplain ecosystem is a highly variable one, particularly in relation to the seasonal presence or absence of surface water which affects the growth/ decay cycle of the plant species present. The three grasses selected for this study each occupy a different region of the floodplain and have different growth forms and growth strategies (Finlayson et al., 1989). Pseudoraphis spinescens is a perennial species with two distinct growth forms and a C4 photosynthetic pathway. Hymenachne acutigluma is also perennial, but with a C3 photosynthetic pathway, while O. meridionalis is an annual C3 species. The growth pattern, as expressed by aboveground biomass (as dry weight) in relation to climate, and nutrient concentrations of these grasses were investigated over an 18 month period, encompassing the 1983-1984 wet season, the 1984 dry season and the start of the 1984-1985 wet season (i.e. October 1983- February 1985 ).

The concentrations of a number of minor nutrients and non-essential nutrients were also determined but are not considered in this paper (see Alliga- tor Rivers Region Research Institute, 1985, for a summary of these results).

MATERIALS AND METHODS

Three sites (Fig. 1 ), each dominated by one of the grass species, were selected on the basis of having a uniform plant cover, a flat topography and year-round access by boat and /o r motor vehicle. At each site, a transect line at right angles to the line of the edge of the floodplain was marked out with steel pickets. Sampling was carried out using five quadrats positioned 5 m apart along a line at right angles to this transect. At each successive occasion of sampling, the line was positioned a further 10 m along the transect. Sam- ples were collected at 4-week intervals from October 1983 to February 1985 at the P. spinescens and H. acutigluma sites, and from November 1983 to June 1984 and January to February 1985 at the O. meridionalis site. The minimum size of quadrat required to obtain a representative weight of aboveground biomass (based on a comparison of mean values and standard errors

266 C.M. FINLAYSON

obtained by comparing different sized quadrats) was 0.5625 m 2 for P. spinescens and 0.25 m 2 for O. meridionalis and H. acutigluma.

Plant samples were harvested by clipping at ground level all the vegetation growing in each quadrat. During the wet season, the sites were covered with water and it was necessary to use SCUBA equipment to enable sampling to be carried out. Plant material from below ground was also collected from each quadrat. Initially this was done by taking all of the sediment from each quadrat to a depth of 10 cm. However, difficulties encountered in sampling under- water in this way during the wet season resulted in a change to use of cores: two cores, 10 cm in diameter and 15 cm deep, were collected from each quadrat. Each sample was placed in a separate box. Sampling was stopped for safety reasons in February 1985 because of the presence of large saltwater crocodiles ( Crocodylus porosus Schneider) near the sample sites.

The samples were returned to the laboratory and washed to remove loosely bound epiphytes and sediment. They were then dried at 65°C for 24 h and then at 105°C for 72 h before being weighed and ground to a powder (in mechanical grinders with stainless steel and plastic components) prior to chemical analysis for nitrogen, phosphorus, sulphur, chloride, magnesium, calcium, sodium and potassium (Table 1 ). The analytical accuracy, as determined using "standard" plant material previously analysed at a number of laboratories, was better than 5% relative error for all analyses. Chloride analyses were performed after extraction with a glacial and nitric acid mixture (6.4 ml of concentrated nitric acid and 100 ml of acetic acid mixed in 900 ml of distilled water) whilst all other analyses, except for nitrogen, were done on samples digested with concentrated nitric acid. Nitrogen was determined col- orimetrically (ammonium-sal icyla te /sodium nitroprusside complex) after Kjeldahl digestion with concentrated sulphuric acid.

TABLE 1

Methods used for laboratory analysis of plant material

Element Method Detection level Precision (mgg - l )

Silver-ion titration 0.01 Colorimetric 0.2

Chloride Nitrogen Phosphorus Sulphur 'Magnesium Caicium Sodium i~otassium

Incipiently conducted plasma

10%at 3 mgClg -~ 10% at 0.2 mg N g-~ 10%at 2 mg Pg-~ 10%at 2 m g S g -~ 10% at 2 mg Mg g- 10%at 2 mgCag - t 10%at 2 m g N a g -~ 10%at 2 nag Kg - t

T H R E E GRASS SPECIES ON THE MAGELA FLOODPLAIN, AUSTRALIA 267

RESULTS

Aboveground biomass

All plant weights are expressed in kilograms dry weight per square metre. The dry weight of each plant per unit area was variably correlated (but not necessarily in the same way) with water depth, which is a function of the rainfall and surface water flow in Magela Creek (Fig. 2). While there is an association between unit area dry weight and water depth, the "correlation" appears (broadly) positive for two species, but negative for H. acutiglurna.

A comprehensive analysis of the relationships between water flow, depth and rainfall is not available. Over the period of this study, these relationships can be illustrated by comparing rainfall recorded at Jabiru and water flow and depth recorded (Fig, 3 ) at the Northern Territory Government flow gauging

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Fig. 2. Aboveground biomass (dry weight) and water depths at sampling sites on the Magela floodplain. Error bars are least significant differences ( P = 0 . 0 5 ) for the biomass.

268 C.M. FINLAYSON

"~600 Q

~400'

~ 200

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WATER FLOW

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Fig. 3. Flow rate and water depth at Gauge Station 821009 in Magela Creek and rainfall at Jabiru airport, October 1983-February 1985.

station 8210009 in Magela Creek, situated upstream of the floodplain. Dur- ing the 1983-1984 wet season, water depth reached its maximum of about 200 cm at the floodplain sample sites (Fig. 2) and about 300 cm in the creek (Fig. 3 ) following consistent rainfall in January and February.

Dissipation of the surface water on the floodplain in 1983-1984 took 28 weeks at the P. spinescens site and 12 weeks at the shallower (i.e. more ele- vated) O. meridionalis site. The H. acutigluma site retained surface water throughout the study period. Inflow to the floodplain along Magela Creek ceased in April 1984, at the end of the wet season.

At the start of the wet season, P. spinescens underwent a change in growth form, from a short-stemmed (turf-like) habit, developing elongated culms that grew up through the water as depth increased. The rate of growth in length did not, however, keep up with the rate of increase in water depth that occurred during February. The maximum dry weight of 1.67 _+ 0.21 kg m -2 was


TABLE 2

Plant species, in addition to Oryza meridionalis, found within the O<vza meridionalis sample quadrats

Date Other species Aboveground biomass (kg dry weight m -2)

15 Nov. 1983 Coldenia procumbens L. 0.10 _+ 0.02 Phyla nodiflora ~ (L.) E. Greene

13 Dec. 1983 Coldenia procumbens L. 0.17 +_ 0.02 Digitaria sp. Hygrochloa aquatica Lazarides Phyla nodiflora ~ (L.) E. Greene

10 Jan. 1984 Digitaria sp. 0.22 _+ 0.05 Hygrochloa aquatica Lazarides Phyla nodifiora I (L.) E. Greene

7 Feb. 1984 Digitaria sp. 0.35 ± 0.07 14vgrochloa aquat&a Lazarides Phyla nod~flora i ( L. ) E. Greene Pseudoraphis spinescens ( R. Br. ) Vick.

7 Mar. 1984 Eh'ocharis sp. 0.34 _+ 0.08 Hygrochloa aquatica Lazarides Hymenachne acutigluma (Steudel) Gilliland Phyla nod~flora i ( L. ) E. Greene Pseudoraphis spinescens (R. Br. ) Vick.

2 Apr. 1984 Eleocharis sp. 0.13 _+ 0.03 Phyla nod(flora ~ ( L. ) E. Greene

1 May 1984 Commelina lanceolata R. Br. 0.07 + 0.02 Eleocharis sp. Hygrochloa aquatica Lazarides lsoetes sp. Nymphoides sp. Phyla nod(flora ~ (L.) E. Greene Pseudoraphis spinescens ( R. Br. ) Vick. Utricularia spp.

29 May 1984 Eleocharis sp. 0.11 _+ 0.07 Phyla nod(flora ( L. ) E. Greene Pseudoraphis spinescens ~ ( R. Br. ) Vick.

18 Jan. 1985 Eh'ocharis sp. 0.14 + 0.02 Hygrochloa aquatica Lazarides Phyla nod(flora (L.) E. Greene Pseudoraphis spinescens ~ (R. Br. ) Vick.

12 Feb. 1985 Blyxa sp. 0.17_+0.03 Eleocharis sp. Hygrochloa aquatica Lazarides lsoetes sp. Maidenia rubra W. Fitzg. ex Rendle Phyla nodiflora (L.) E. Greene Pseudoraphis spinescens ~ (R. Br.) Vick.

Dominant species.

270 C.M. FINLAYSON

recorded during May when the water level was falling. The culms had reached the surface at this stage and the plants were either in flower or fruiting. Over the next 8 weeks the culms senesced and the habit of the plant reverted to a turf-like one, the plants surviving in this form throughout the dry season, de- spite the absence of free surface water.

The 11. acutigluma site was located in a large swamp (termed either Magela Plain or Western Plain) that continuously had surface water present throughout the period of this study. Changes in water level were similar to those recorded at the P. spinescens site with the maximum level being recorded in February (Fig. 2 ) following heavy rainfall in the catchment (Fig. 3 ).

During the dry season H. acutigluma has a semi-erect/creeping habit with short internodes, and horizontal culms that are sheathed by leaves and an- chored to the substrate by roots that develop at the nodes. It responded to the early wet season rain by an increase in dry weight per square metre, but showed a reduction in dry weight per square metre following the large increase in water level during February. When submerged by flood waters, the plant con- sists of vertical culms with long internodes and few leaves. The decrease in dry weight from December to March was from 0.78 +0.10 to 0.23_+0.03 kg m -2. A maximum dry weight, 1.41 _+ 0.10 kg m -2, was recorded at the end of the 1983-1984 wet season when the water level was falling (Fig. 2). Flower- ing of H. acutigluma culms was first noted in March and seeding in April, lasting until July. As with the P. spinescens site other plant species were found in the vicinity of the H. acutigluma site, but not in the quadrats.

Oryza meridionalis was sampled at a seasonally inundated site to the north of Western Plain (Fig. 1 ). It germinated after the first rainfalls of the wet season. The site was dominated at that t ime by terrestrial herbs (e.g. Coldenia procumbens L. and Phyla nodiflora (L.) E. Greene) and the aquatic grass Hygrochloa aquatica Lazarides (Table 2 ). Oryza meridionalis did not reach its maximum dry weight per square metre until after the February flood when it attained a weight of 0.51 _+0.10 kg m -2 (Fig. 2). Flowering and seeding, followed by senescence, occurred over the next 8 weeks as the site dried out.

Primary production and productivity

Production is defined here as an increase in aboveground plant dry weight per unit area over an arbitrary period, whereas productivity is the increase expressed per unit time, i.e. the production rate (Westlake, 1963). Above- ground production in aquatic plants is often estimated by taking the difference between the maximum and min imum dry weights (Bradbury and Grace, 1983 ). This method, however, is recognised as usually underestimating productivity as it fails to take into account material lost (e.g. by grazing) before the maximum weight is attained, and for production following, or even during, a period of overall senescence. Methods that account for mortality and


production throughout the year involve determining the dry and/or wet weights of both living and dead plant material (Linthurst and Reimold, 1978; Groenendijk, 1984). Owing to periodic difficulties in quantitatively sam- piing in the Magela Creek environment (because of e.g. physical problems associated with large and sudden changes in water levels) and the particular growth characteristics of the perennial grasses (e.g. changes in habit and sloughing-off of leaves and leaf sheaths) the dry weight of dead plant material was not determined. In view of this limitation, the aboveground production was described by the maximum-minimum difference method and is therefore an underestimate.

Oryza meridionalis, an annual species, had a maximum dry weight of 0.51 + 0.10 kg m-2 which, as there was no evidence of loss from leaf mortality, nor an extensive degree of grazing, or further production, after reaching a maximum, can be taken as representative of the aboveground productivity for this species. Pseudoraphis spinescens had two growth periods; November- May with a dry matter production of 1.06 + 0.23 kg m -2 and July-January with 0.85 _+ 0.03 kg m- : . Hymenachne acutigluma similarly had two growth periods; November-January with 0.90_+ 0.26 kg m -2 and March-June with 1.19 + 0 .12 kg m -2. As these values occurred within the 1983-1984 wet-dry cycle the total annual production of dry weight for P. spinescens was at least 1.91 +_ 0.26 kg m -2 year- l and for H. acutigluma, 2.09 _+ 0.38 kg m -2 year- i. The latter is the greater underestimate as it does not take into account evident losses due to leaf and leaf-sheath sloughing and grazing by insects.

Concentrations of major nutrients

Seasonal changes in the dry weight concentrations of the major nutrients nitrogen, phosphorus, sulphur, chloride, magnesium, calcium, sodium and potassium in the aboveground and belowground material of the grasses are graphically presented in Figs. 4 and 5, and a summary of mean values and range of concentrations is presented in Table 3. Each element is considered separately in relation to dry weight, water depth and rainfall information presented in Figs. 2 and 3.

Nitrogen Aboveground material of each species had higher mean dry weight nitrogen

concentrations than had the belowground material. Oryza meridionalis showed the highest concentration of nitrogen (33.9 + 3.10 mg N g- 1 ) of all plants, in seedlings at the start of the 1983-1984 wet season. Pseudoraphis spinescens and H. acutigluma had relatively low nitrogen concentrations in the belowground material during the dry season when the dry weight of the aboveground material of former species was low, and for the latter species was decreasing.

272 C.M. FINLAYSON

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Fig. 4. Nitrogen, phosphorus, sulphur and chloride concentrations in aboveground and belowground material of Pseudoraphis spinescens, Hymenachne acutigluma and Oryza meridion- a/is. Error bars are least significant differences (P= 0.05 ).

T H R E E G R A S S S P E C I E S O N T H E M A G E L A F L O O D P L A I N , A U S T R A L I A 273

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Fig. 5. Calcium, magnesium, sodium and potassium concentrations in aboveground and belowground material of Pseudoraphis spinescens, ttymenachne acutigluma and Oryza meridionalis. Error bars are least significant differences (P=0.05) .

274 C.M. FINLAYSON

TABLE 3

Mean, standard error and range (in parentheses) of concentrations (mg g ~ ) of major nutrients in aboveground and belowground material of Pseudoraphis spinescens, I4vmenachne acutigluma and Oryza meridionalis

Pseudoraphis spinescens Hyrnenachne acutigluma O0'za meridionalis

Above Below Above Below Above Below

Nitrogen 12.43-+0.25 11.71 _+0.32 11.03_+0.23 (0.90-1.54) (0.58-1.48) (0.88-1.61

Phosphorus 0.78_+0.03 0.53+0.02 2.21 +0.09 (0.37-1.23) (0.38-0.64) (1.44-3.84

Sulphur 2.71 +_0.09 2.76+0.10 3.65_+0.13 (1.70-4.25) (1.79-3.83) (2.11-5.08)

Chloride 2.20-+0.13 0.95_+0.05 11.48_+0.36 (0.86-5.35) (0.29-1.70) (6.77-17.51

Magnesium 1.54+0.05 1.17_+0.04 2.65_+0.07 (0.67-2.55) (0.53-1.66) (1.21-3.91

Calcium 1.44_+0.11 1.16+0.06 1.64_+0.06 (0.74-2.80) (0.52-2.08) (0.78-2.83

Sodium 1.10_+0.07 0.51 _+0.04 1.18_+0.07 (0.37-2.49) (0.28-1.42) (0.14-2.26

Potassium 5.72_+0.36 2.37_+0.12 23.87_+0.97 (2.87-t5.57) (1.42-4.60) (13.10-42.50

9.62_+0.39 14.37_+ 1 .25 9.64_+0.24 (0.35-1.47) (0.60-3.39) (0.86-1.14)

0.85_+0.05 1.09+0.07 0.66_+0.04 (0.41-1.59) (0.65-1.69) (0.44-0.79)

2.79+0.11 1.60+0.11 2.14+0.10 (1.33-3.98) (0.57-2.55) (1.72-2.86)

1.12_+0.12 9 .43_+0.59 2.54_+0.24 (0.15-2.38) (4.48-13.97) (0.98-3.52)

1.91_+0.06 1.91+0.10 2.01_+0.10 (1.35-3.54) (1.20-2.88) (1.55-2.49)

2.45-+0.08 1 .65_+0.08 1.67-+0.08 (1.88-4.70) (1.08-2.85) (1.47-2.25)

0.85_+0.07 2.76+0.28 1.81 _+0.21 (0.3-1.78) (0.38-5.60) (0.47-2.38)

3.67_+0.36 18.38_+ 1 .04 6.58_+0.68 (1.11-8.88) (12.00-27.ll) (3.14-10.95)

Phosphorus Aboveground material ofH. acutigluma had higher phosphorus concentra-

tions than that of the other species with a dry weight maximum of 3.84 _+ 0.27 mg P g - l early (October) in the 1983-1984 wet season. The maximum concentration present at the start of the 1984-1985 wet season was, however, much lower. Lower concentrations were recorded during the dry season when the standing vegetation dry weight reached its maximum. Pseudoraphis spinescens and O. meridionalis had an aboveground phosphorus concentration peak before the 1983-1984 wet season peak in biomass. The former species phosphorus concentration also peaked again during the dry season when the biomass was at its minimum. All species had similar or higher phosphorus concentrations in the aboveground material than in that from belowground, except for one occasion in the case ofP. spinescens.

Sulphur Sulphur concentrations in aboveground and belowground material of each

species underwent similar patterns of change. Pseudoraphis spinescens showed a decline in concentration at the start of both wet seasons and a peak in the aboveground material at the end of the dry season. Hymenachne acutigluma had higher sulphur concentrations during the wet season than during the dry


season, whereas in O. meridionalis, sulphur concentrations were higher in the aboveground material (seedlings) than in the mature plants, i.e. concentrations decreased over the wet season as the standing material aged.

Chloride Chloride concentrations in aboveground material of P. spinescens were

much lower than in the other species, and peaked before increases in dry weights were recorded. Hymenachne acutigluma had a maximum value in aboveground material of 17.51 _+ 1.53 mg C1 g- t at the start of the 1983-1984 wet season with much lower values during the dry season. Chloride values for O. meridionalis values were variable.

Magnesium Magnesium concentration in aboveground material of P. spinescens reached

its max imum at the end of the dry season, whereas H. acutigluma had its max imum value early in the 1984-1985 wet season. The dry weights for both species were relatively low at this t ime compared with earlier values. Oryza meridionalis seedlings contained higher magnesium concentrations than did the older plants.

Calcium Calcium concentrations recorded in the H. acutigluma belowground mate-

rial reached a maximum during the wet season. The aboveground material for this species had relatively high concentrations at the start of the 1984- 1985 wet season. Pseudoraphis spinescens material, in general, had higher calcium concentrations during the wet seasons than during the dry season. Oryza meridionalis aboveground material contained higher calcium concentrations at the seedling stage, decreasing as the plants aged.

Sodium Sodium concentrations in aboveground material of O. meridionalis in-

creased over the wet season with a max imum of 5.60 + 0.24 mg Na g- l just prior to senescence. Pseudoraphis spinescens aboveground material had a maximum sodium concentration preceding an increase in dry weight, whereas in H. acutigluma there appeared to be higher concentrations when weights were low or decreasing.

Potassium Potassium concentrations in P. spinescens were considerably lower than in

the other species, although like them, the mean aboveground concentration was higher than in the belowground concentration. All species had aboveground potassium concentration peaks prior to increases in dry weights. The

276 C.M. FINLAYSON

two perennial species, especially H. acutigluma, had lower potassium concentrations during the dry than during the wet seasons.

DISCUSSION

Productivity

Reviews of rates of productivity of aquatic and/or wetland plant species are available (e.g. Westlake, 1963; Bradbury and Grace, 1983) but most of the information is not directly comparable with that collected from the Ma- gela. Much of the information collected in Australia is for submerged and floating aquatic plants and for large emergent species with different habits to the grasses considered in this study (i.e. they are flooded during the growing season and then have to cope with dry, or almost dry conditions for the rest of the year). In general, emergent plants are more productive than submerged plants, while tropical and sub-tropical plants are not necessarily more productive than temperate plants (Howard-Williams and Gaudet, 1985 ).

Whilst the production rates for the grasses on the Magela floodplain are not as high as those reported for some of the large emergent aquatic plant species growing elsewhere, productivity is relatively high when compared with the range for wetland species, 0.12-2.59 kg m - 2 year- l, determined by Bradbury and Grace ( 1983 ). This particularly applies to H. acutigluma with 2.09 + 0.38 k g m - 2 year - l and P. spinescens with 1.91 +0.26 kg m -2 year -1, based on data for a single year. As the three grasses in this study occupy 35% of the floodplain area of 22 000 ha, and the floodplain supports about 220 more plant species that qualitatively appear to have high production rates (Finlay- son et al., 1989) the Magela ecosystem can be described as highly productive.

Nutrient concentrations

Dry weight concentrations of nitrogen, phosphorus, sulphur, calcium, magnesium and chloride in the aquatic grasses on the Magela were lower than the generalised ranges presented by Mengel and Kirkby (1979) for these nutrients in terrestrial plants. Furthermore, nitrogen, phosphorus, calcium and sodium concentrations in the three grasses and chloride and potassium in P. spinescens only, were lower than the mean concentrations reported by Hutchinson (1975) for aquatic plants. In general, therefore, the concentrations of major nutrients in the three aquatic grasses from the Magela floodplain are low relative to other plants. Direct comparisons with other grasses of a similar habit are not known. Morley ( 1981 ) presented the results of analysis of a number of aquatic plant species from the Magela; magnesium, calcium, sodium and potassium concentrations in P. spinescens and H. acutigluma were similar to those reported in this study, although higher sodium and


calcium concentrations were found in other species. The nitrogen and phosphorus concentrations in the three grass species from the Magela are not low when compared with values for other Australian aquatic and wetland plant species (Hocking, 1981; Finlayson et al., 1984; Room and Thomas, 1986). Comparative information from Australian species for the other nutrients considered in this study is generally not available.

Nitrogen, phosphorus and potassium concentrations are generally found in higher concentrations in young plant material, whereas calcium, magnesium and chloride are more concentrated in more mature material (Mengel and Kirkby, 1979). Aboveground material of the annual O. meridionalis had higher concentrations of nitrogen, sulphur, calcium and magnesium in seedlings than in mature plants. In contrast, sodium was more concentrated in the mature tissue, while the maximum concentrations of phosphorus and potassium occurred prior to the peak in dry weight. This change suggests that the requirement for the first three nutrients by the plants was greater than the rate of absorption as the culms of the plants grew larger, and as flowering and seeding eventually occurred, whereas phosphorus and potassium were being continually absorbed in sufficient quantities whilst the stems were elongating, but not during flowering and seeding. Sodium was continually absorbed at a rate that exceeded the rate of utilisation, perhaps reflecting the less important nature of this element in plant metabolism (Mengel and Kirkby, 1979). In- formation on retranslocation of nutrients during plant growth was not collected in this study. In general though, mobile elements such as nitrogen, phosphorus and potassium are retranslocated as plants mature and senesce, whereas calcium, magnesium and chloride remain in the older tissue and are not retranslocated (Mengel and Kirkby, 1979).

Seasonal changes in nutrient concentrations in the perennial P. spinescens and H. acutigluma were not as clear cut as in O. meridionalis. Pseudoraphis spinescens had definite peaks in phosphorus, chloride, calcium, sodium and potassium concentrations preceding the maximum dry weight at the onset of flowering and seeding. These findings could indicate that absorption of these nutrients was exceeded by the metabolic requirements for these elements or, in the case of the cations, that uptake of potassium competitively excluded uptake of other cations. Hymenachne acutigluma had high concentrations of nitrogen, phosphorus, chloride and potassium early in the 1983-1984 wet season when dry weight was initially increasing, but later declined as the dry weight first declined and then increased again. Nitrogen, phosphorus and potassium are usually more concentrated in younger than in older tissue, whereas the opposite is usually the case for chloride.

Nutrient turnover

In terms of nutrient turnover, plants on the Magela floodplain have the potential to transfer nutrients between the sediment soil and the water (Wetzel,

278 C.M. FINLAYSON

1983; Pomogyi et al., 1984). An estimate of the dry weights of major nutrients present in the aboveground material of the grasses on the Magela and later added to the detrital "pool" on the floodplain can be obtained by mul- tiplying the dry weight by the concentration of each nutrient. In short, the periods of decline in dry weight are assumed to represent loss of plant material and nutrients which enter the detrital pool. For P. spinescens, aboveground biomass fell from 1.67 + 0.21 to 0.21 + 0,02 kg m -2 during May-July; H. acutigluma decreased from 1.29 _+ 0.22 to 0.23 _+ 0.03 kg m -2 during Jan- uary-March and again from 1.41 + 0.10 to 0.55 _+ 0.09 kg m-2 during June- November; O. meridionalis decreased from 0.51 + 0.10 kg m - 2 during April- May. By subtracting the nutrient loads in the aboveground plant dry weight at the end of these periods from that at the start, an estimate of the detrital (and perhaps debris) nutrient load contributed by these plants can be determined. For the purpose of this exercise, the actual fate of nutrients contained in the senescing and decaying plant material has not been considered, although it is recognised that this is an important aspect of nutrient turnover and some of the nutrients will be lost to solution or translocated to the soil during senescence.

The maximum nutrient loads held and then lost by the grasses are substantial when compared with the loads added to the floodplain by creekwater inflow (Table 4). The values represent the maximum amounts turned over on the area occupied by each grass communi ty on the floodplain: namely 3050 ha ofP. spinescens, 1930 ha ofH. acutigluma and 2730 ha ofO. meridionalis (Finlayson et al., 1989 ). As the areas occupied by the grass communit ies were not calculated with error factors, it is not possible to present a calculated error on the detrital values in Table 4. However, by assuming a 10% error in the mapping areas, errors of + 3 t, + 2 t and + 3 t can be assigned to the P. spines-

TABLE4

Nutrient loads in debris/detri tus attributed to the aquatic grasses Pseudoraphis spinescens, Hymen- achne acutigluma and Oryza meridionalis, and in creekwater entering the floodplain during the 1982- 1983 wet season (from Hart et al., 1987). All values are tonnes with errors in the creekwater loads shown in parentheses

Pseudoraphis Hymenachne Oryza Magela Creek spinescens acutigluma meridionalis water

Nitrogen 560 390 90 20 (10) Phosphorus 40 80 15 3 ( 3 ) Sulphur 100 260 10 370 (80) Chloride 90 460 60 710 (300) Magnesium 80 90 20 170 (46) Calcium 70 60 20 120 (150) Sodium 30 15 30 370 (90) Potassium 250 1040 150 120 (50)


cens, H. acutigluma and O. meridionalis detrital values respectively. A 50% error in the mapping areas would increase these errors to + 15 t, _+ 10 t and + 3 t, respectively. Errors of this magnitude, however, do not affect the general interpretation of the results when compared with the nutrient loads in the creekwater (Table 4).

The grass debris/detri tus had a substantially higher content of nitrogen, phosphorus and potassium, less sodium and chloride and similar amounts of sulphur, magnesium and calcium as the loads of each in the creekwater (Ta- ble 4). Thus, in terms of the overall budget of major nutrients on the floodplain, the grasses are potentially an important component of the turnover mechanisms that occur. The nature and extent of transfer of nutrients between the sediment and water via the grasses is not known, and would be dependent on factors such as the physico-chemical conditions that occur.

The material budget for the Magela determined by Hart et al. (1987) con- tains an indication that there is a net gain of nitrogen and phosphorus on the floodplain resulting from runoff from the catchment. The magnitude of the net gain, 25+_48 t nitrogen and 5_ + 10 t phosphorus, however, is not large compared with the amounts in the grass debris/detritus. Thus, the trophic status of the floodplain (and hence indirectly the level of production ) is prob- ably not greatly affected by current levels of nitrogen and phosphorus input from the catchment. In other words, the relatively high rates of primary production on the floodplain are not entirely dependent on nitrogen and phosphorus inputs from the catchment. In fact, the plants maintain a high rate of production with relatively low tissue nutrient concentrations. The likely source of nutrients for the plants is the floodplain sediment.

The nutrient status of the floodplain would, based on the same argument, be more affected by inputs of sodium, chlorine, sulphur, magnesium than of nitrogen and phosphorus via creekwater inflow. Hart et al. ( 1987 ), however, suggest that these nutrients are exported from the floodplain by outflowing water. It is possible that decomposit ion of grasses during the wet season (e.g. H. acutigluma during January-March 1984 ) and prior to the cessation of out- flow could contribute to this loss of material. Detrital material that is not exported could either be incorporated into the sediment, consumed by detri- tivores, or lost to the atmosphere as a result of dry season fires. The P. spinescens and H. acutigluma communit ies considered in this study occupy areas of the floodplain that generally remain moist or retain surface water and hence are not prone to fire. Extensive fires can occur in areas of the floodplain occupied by the annual O. meridionalis and grass-sedgeland to the north of the study sites (Finlayson et al., 1989).

A C K N O W L E D G E M E N T S

Bruce Bailey and Ian Cowie made substantial contributions to the field work and laboratory preparation of samples for analysis. Barry Noller provided

280 C.M. FINLAYSON

advice on analytical procedures and precision and accuracy levels, while Colin Mackintosh provided assistance in the computer storage, analysis and graph- ing of the results.

REFERENCES

Alligator Rivers Region Research Institute, 1985. Annual Research Report 1984-1985. Super- vising Scientist for the Alligator Rivers Region, AGPS, Canberra, 155 pp.

Bradbury, J.K. and Grace, J., 1983. Primary production in wetlands. In: A.J.P. Gore (Editor), Ecosystems of the World, 4A Mires: Swamp, Bog, Fen and Moor. Elsevier, Amsterdam, pp. 285-310.

Finlayson, C.M., Farrell, T.P. and Griffiths, D.J., 1984. Studies of the hydrobiology of a tropical lake in north-western Queensland. III. Growth, chemical composition and potential for har- vesting of the aquatic vegetation. Aust. J. Mar. Freshwater Res., 35: 525-536.

Finlayson, C.M., Bailey, B.J. and Cowie, I.D., 1989. The macrophyte vegetation of Magela Creek floodplain, Alligator Rivers Region, Northern Territory. Supervising Scientist for the Alli- gator Rivers Region, Res. Rep. 5, 38 pp.

Fox, R.W., Kelleher, C.G. and Kerr, C.B., 1977. Ranger uranium environmental enquiry, 2nd Rep., AGPS, Canberra, 415 pp.

Groenendijk, A.M., 1984. Primary production of four dominant salt-marsh angiosperms in the SW Netherlands. Vegetatio, 57:143-152.

Hart, B.T., Ottaway, E.M. and Noller, B.N., 1987. The Magela Creek system, northern Aus- tralia. II. Material budget for the floodplain. Aust. J. Mar. Freshwater Res., 38:261-288.

Hocking, P.J., 1981. Response of Typha domingensis to salinity and high levels of manganese in the rooting medium. Aust. J. Mar. Freshwater Res., 32:907-919.

Howard-Williams, C. and Gaudet, J.J., 1985. The structures and functioning of African swamps. In: P. Denny (Editor), The Ecology and Management of African Wetland Vegetation. W. Junk, Dordrecht, pp. 153-175.

Hutchinson, G.E., 1975. A Treatise on Limnology, Vol. 3. Limnological Botany. John Wiley, New York, 660 pp.

Linthurst, R.A. and Reimold, R.J., 1978. An evaluation of methods of estimating the net aerial primary productivity of estuarine angiosperms. J. Appl. Ecol., 15:919-931..

Mengel, K. and Kirkby, E.A., 1979. Principles of Plant Nutrition, 2nd edn. International Potash Institute, Bern, 593 pp.

Morley, A.W. (Editor), 1981. A Review of Jabiluka Environment Studies, Vol. 4. Pancontinen- tal Mining, Sydney.

Pomogyi, P., Best, E.P.H. Dassen, J.H.A. and Boon, J.J., 1984. On the relation between age, plant composition and nutrient release from living and killed Ceratophyllum plants. Aquat. Bot., 19: 243-250.

Room, P.M. and Thomas, P.A., 1986. Nitrogen, phosphorus and potassium in Salvinia molesta Mitchell in the field: Effects of weather, insect damage, fertilisers and age. Aquat. Bot., 24: 213-232.

Westlake, D.F., 1963. Comparison of plant productivity. Biol. Rev., 38: 385-425. Wetzel, R.G., 1983. Limnology, 2nd edn. CBS College Publishing, New York, 858 pp.

production and major nutrient composition of three grass species on the magela floodplain, northern...

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