Impact of Land-Use Change on Nutrient Loads from Diffuse Sources (Proceedings of IUGG 99 Symposium HS3, Birmingham, July 1999). IAHS Publ. no. 257, 1999. 87

Nutrient concentrations and fluxes in tributaries to the Swan-Canning estuary,

NORMAN E. PETERS US Geological Survey, 3039 Amwiler Road, Suite 130, Atlanta, Georgia 30360, USA e-mail: [email protected]

ROBERT DONOHUE Waters and Rivers Commission, Hyatt Centre, East , Western Australia 6004

Abstract In Western Australia, catchment nutrient availability on an areal basis is primarily controlled by the disposal of animal waste and the type and rate of fertilizer application, particularly in coastal areas. The coastal areas receive notably higher rainfall and have more intense horticulture and animal production than inland areas, and are undergoing rapid urbanization, particularly adjacent to the estuary. Also, the surficial aquifers on the coastal plain are generally sandy having a low nutrient retention capacity and rapidly transmit soluble and colloidal material through the subsurface. In the Swan- Canning basin, high air and soil temperatures and seasonally arid conditions cause rapid mineralization of nitrogen and phosphorus. The nutrients are subsequently available for transport during the onset of seasonal wet weather, which typically begins during the period from late April to June. In addition to the rapid mobility of nutrients in streamwater from agricultural areas during the wet season, drains in urban areas, which typically have high nutrient concentrations, also are an important source of nutrients as the drains flow directly to the estuary throughout the year.

INTRODUCTION

Freshwater nutrient concentrations, particularly N species, are increasing globally (Heathwaite et al, 1996). Most of the increase is attributed to the increasing rates of fertilizer application stimulated by decreasing costs of fertilizer and increasing demand for agricultural products (e.g. for the USA, Puckett, 1995). Even in relatively pristine catchments, the transport of fossil fuel emissions and other airborne emissions cause increasing atmospheric nitrogen (N) deposition. Solid and liquid waste treatment and disposal also affects the amount and speciation of nutrients. The result of the increasing application of nutrients to the landscape has been a general increase in eutrophication of freshwaters, which are contributing to the eutrophication of estuarine and coastal waters (Heathwaite et al, 1996). In more developed countries, freshwater eutrophication occurs during urban development due to the lack of effective waste treatment and disposal. In particular, most freshwater systems are phosphorus (P) limited and increases in P concentrations, due to the disposal of wastewater that contains high P concentrations from detergents and human or animal waste, cause eutrophication (Correll, 1998). Also, P readily adsorbs to Fe and Al oxides and hydroxides in soils. However, soil erosion, which is typically higher in agricultural settings, where fertilizer use increases soil P, than in forest or rural areas, can transport 88 Norman E. Peters & Robert Donohue

P through aquatic systems. Even for forest soils, particulate P dominates P export. Although the timescale of P movement in soils may be very long, depending on the soil adsorption characteristics, mobilization will eventually occur and is controlled by P content, which in turn, is controlled by the application of P (Smith et ah, 1995). The purpose of this paper is to investigate the concentrations and fluxes of nitrogen (N) and phosphorus (P) species in the tributaries to the Swan-Canning estuary.

BACKGROUND

The Swan-Canning estuary is located in Western Australia and has a drainage area of about 121 000 ha. The estuary is becoming increasingly eutrophic, marked by increased annual phytoplankton blooms in the upper estuary of the Swan and Canning Rivers (Thompson & Hosja, 1996). Monitored tributaries draining to the estuary consist of one main stream, the Avon River (-119 000 ha), and 14 additional tributaries (Fig. 1). The 14 tributaries consist of 10 smaller streams, of which the largest is (660 ha), and four urban drains around Perth, which is on the

115°45'E 116°E

Fig. 1 Map of the Swan-Canning estuary showing the monitored tributaries and urban drains for the drainage area adjacent to the estuary. Nutrient concentrations and fluxes to the Swan-Canning estuary, Western Australia 89 northern shore of the estuary. These 15 sites account for more than 99% of the drainage area of the Swan-Canning estuary (-121 000 km2) leaving only 100 km2 of ungauged area (Donohue et ai, 1994). The climate is a Mediterranean type with hot, dry summers and mild, wet winters. The mean daily air temperature ranges from 13 to 24°C, the mean daily maximum ranges from 17 to 30°C in January and February and the mean daily minimum ranges from 9 to 18°C in July and August. Annual rainfall decreases from a maximum of about 1200 mm adjacent to the to 300 mm in the upper Avon River on the Darling Plateau. The spatial variability in rainfall affects streamflow. For example, 80% of the flow in the Avon River is contributed by only 10% of the drainage area, which is in the western part toward the downstream end (Harris, 1996). Furthermore, rainfall is highly seasonal with more than 90% occurring from April through October. Consequently, many of the streams are ephemeral with channel areas varying from recharge to discharge zones depending on timing and landscape position. Most of Avon River drainage is on the Darling Plateau. Soils on the Darling Plateau are old deep and are clay rich. Several of the tributaries drain the coastal plain, which is west of the Darling Scarp (Fig. 1) and where streamflow is controlled predominantly by groundwater discharge. The coastal plain soils have an extremely high sand content (>90%) and these soils generally overlay and inter-finger with a silty and clayey unit, the Guildford clay, at the base of the Darling Scarp (Davidson, 1995). However, the headwater for other tributaries, such as Jane Brook, Southern River, Canning River and , is on the Darling Plateau, where during the wet season, some of the streamflow is generated from runoff on the plateau and increases from groundwater discharge downstream, particularly on the coastal plain. The dominant land use in the drainage basin is agriculture and horticulture, but urbanization is important and is increasing adjacent to the estuary. Land clearing commenced at the end of the 1800s and the rate of clearing increased markedly after 1945 with more than 75% of the land cleared for agriculture. The clearing occurred not only in upland areas, but in riparian zones as well. The result is that farm animals generally have unlimited access to the stream channels, at least until recently. The increasing rate of application of low cost fertilizer for agriculture in the drainage basin is a primary concern, because fertilizer is the main source of nutrients in other aquatic systems in southwestern Australia (Hodgkin & Hamilton, 1993).

METHODS

Grab samples were collected at each of 11 tributaries and four urban drains (Fig. 1) during the winter wet period (which typically begins in May or June and ends at the beginning of the summer dry period in November), from 1987 to 1992. Since 1993, samples have been collected throughout the year provided the tributary or drain was flowing. Samples were collected in 1-litre linear polyethylene sample bottles. The bottles were pre-rinsed with water from the sampling site prior to sample collection. A sample aliquot was filtered on site through a 0.45 pm cellulose nitrate filter for the determination of reactive phosphorus (FRP). All samples were chilled on site and were either refrigerated or stored frozen prior to analysis. The raw samples were analysed for 90 Norman E. Peters & Robert Donohue

nitrate (NO3-N), ammonium (NH4-N), total nitrogen (TN) and total phosphorus (TP). Total inorganic nitrogen (TIN) was calculated as the sum of ammonium and nitrate and organic N (Org-N) was calculated as the difference between TN and TIN. Streamflow was monitored continuously at 14 of the 15 tributaries and drains using standard gauging techniques from 1987 to 1992. At one site, Blackadder Creek, runoff was estimated from the runoff at a nearby catchment, Yule Brook. Since 1992, several of the streamflow stations were decommissioned. Daily fluxes (mass per unit area) were calculated for 1987-1992, as the product of a daily concentration and daily runoff. The flux calculation assumes that the concentration was relatively invariant on any given day and that the daily nutrient concentration for non-sampling days could be estimated by a linear interpolation between successive measurements. The average annual flux was determined from the sum of the daily fluxes.

RESULTS AND DISCUSSION

The average tributary nutrient concentrations are listed in Table 1 and the P and N species fluxes are listed in Tables 2 and 3, respectively. Urban drains typically have

Table 1 Average annual runoff with percentage of total gauged runoff in parentheses, and volume- weighted nutrient concentrations of tributaries to the Swan-Canning estuary. The values for FRP, PSP, and TP in bold italics are those greater than 0.10 mg 1"'.

Site Area Runoff FRP PSP TP NH3 N03 TIN Org-N TN 2 1 (km ) (mm) (mgl"•' ) (mgl•' ) (mgr1) (mgr ) (mgl" ') (mgl"') (mgl"1) (mgl"1) Drains Bayswater 262 46 (2.4) 0.02 0.26 0.28 0.33 0.42 0.75 0.75 1.50 Claise Brook 16 337 0.02 0.07 0.09 0.15 0.63 0.78 0.54 1.31 (1.0) Mill Street 12 425 0.10 0.12 0.22 0.43 0.51 0.94 1.10 2.04 (1.1) South 10 308 0.07 0.10 0.17 0.17 0.30 0.47 0.73 1.20 Belmont (0.6) Rivers Avon River 119 035 3(71) 0.01 0.04 0.06 0.07 0.22 0.29 0.84 1.12 Bannister Creek 23 400(1.8) 0.08 0.05 0.13 0.12 0.36 0.48 1.15 1.63 Bennet Brook 99 102(2.0) 0.04 0.05 0.09 0.04 0.30 0.34 0.96 1.30 Blackadder Creek 13 238 (0.6) 0.03 0.03 0.07 0.03 0.80 0.83 0.67 1.50 Canning River 163 104(3.3) 0.02 0.03 0.05 0.04 0.76 0.80 0.44 1.24 Ellen Brook 664 56(7.3) 0.51 0.19 0.70 0.11 0.14 0.24 1.84 2.08 Helena River 161 62 (2.0) 0.02 0.02 0.04 0.06 0.39 0.45 0.48 0.93 Jane Brook 135 17(0.4) 0.01 0.02 0.03 0.04 0.39 0.43 0.30 0.74 Southern River 149 141 (4.1) 0.17 0.10 0.27 0.08 0.06 0.14 1.07 1.20 Susannah Brook 19 21 (0.1) 0.03 0.03 0.05 0.05 0.75 0.80 0.70 1.50 Yule Brook 53 245 (2.5) 0.03 0.05 0.08 0.06 0.47 0.53 0.70 1.23 Subtotals Drain 300 84 (0.5) 0.04 0.17 0.21 0.29 0.47 0.76 0.77 1.53 Tributary 120 500 4(95.) 0.06 0.05 0.12 0.07 0.25 0.31 0.90 1.22 Total 120 800 4.2 0.06 0.06 0.12 0.08 0.26 0.34 0.90 1.23 Nutrient concentrations andfluxes to the Swan-Canning estuary, Western Australia 91 higher TP concentrations than the rivers, with the exception of Banister Creek, Ellen Brook and the Southern River (Table 1). However, these three tributaries drain sandy coastal plain soils and the extremely high P concentrations (both FRP and TP) are attributed to the rapid mobility of superphosphate fertilizer applied to pastures. In addition, FRP is a smaller percentage of TP than that associated with particulates for all urban drains and most of the tributaries except the three noted above (Table 1). The sandy soils of the coastal plain are particularly vulnerable to nutrient leaching and transport, particularly for phosphorus (P). Because the sandy soil is nutrient deficient and subject to rapid leaching, the soils are fertilized. The P typically is added to the soils in a highly soluble form, superphosphate, which is rapidly dissolved and leached during the wet season. Consequently, the timing and rate of application of superphosphate fertilizer to the pastures of the sandy coastal plain in the Ellen Brook catchment are probably the main factors controlling the high streamwater P concentrations of Ellen Brook. Despite the high TP concentration and load in Ellen Brook, the P transport is small (<0.5 kg ha"1 year"1) compared to fertilizer application rate of P to pastures (10-20 kg ha"1 year"1) (Gerritse & Adeney, 1992). The fertilized- pasture area is much less than 10% of the total catchment area. Even small amounts of clay, silt and Fe or Al hydroxides result in weakly adsorbing sandy soils that are sufficient to delay leaching of P. An intensive chemical analysis of soils at 7950 sites in catchments draining to the south coast of Western Australia, approximately 300 km south of Perth, indicates accumulation of P in soils increases with the amount of oxalate extractable Fe (Weaver & Reed, 1998). Because of the rapid transport through

Table 2 Average annual phosphorus fluxes of tributaries to the Swan-Canning estuary, 1987-1992. The percentage of annual runoff and fluxes of FRP, PSP and TP reflect the contribution of a tributary to the total flux to the estuary.

Site Area Annual runoff: FRP: PSP: TP: 1 (km2) (mm) (%) (kg ha') (%) (kg ha') (%) (kg ha ) (%) Drains Bayswater 262 46 2.4 0.008 0.7 0.12 10.0 0.13 5.3 Claise Brook 16 337 1.0 0.062 0.3 0.25 1.3 0.31 0.8 Mill Street 12 425 1.1 0.434 1.6 0.52 1.9 0.95 1.8 South Belmont 10 308 0.6 0.205 0.7 0.31 1.0 0.51 0.8 Rivers Avon River 119 035 3 71.0 O.000 16.0 <0.01 48.0 <0.01 32.0 Bannister Creek 23 400 1.8 0.304 2.3 0.22 1.6 0.52 1.9 Bennet Brook 99 102 2.0 0.041 1.3 0.05 1.6 0.09 1.5 Blackadder Creek 13 238 0.6 0.079 0.3 0.08 0.3 0.16 0.3 Canning River 163 104 3.3 0.018 1.0 0.03 1.6 0.05 1.3 Ellen Brook 664 56 7.3 0.286 62.0 0.11 22.0 0.39 42.0 Helena River 161 62 2.0 0.012 0.7 0.01 0.6 0.02 0.6 Jane Brook 135 17 0.4 0.002 0.1 <0.01 0.1 0.01 0.1 Southern River 149 141 4.1 0.235 11.0 0.15 7.1 0.38 9.2 Susannah Brook 19 21 0.1 0.005 <0.1 0.01 <0.1 0.01 <0.1 Yule Brook 53 245 2.5 0.075 1.3 0.11 1.9 0.19 1.6 Subtotals Drain 300 84 0.5 0.002 3.3 <0.01 14.2 <0.01 8.8 River 120 500 4 95.0 0.033 96.7 0.15 85.8 0.18 91.2 Total 120 800 4.2 0.003 0.003 0.005 92 Norman E. Peters & Robert Donohue

the sandy coastal plain soils, the application of slower release P fertilizer may reduce the P transport and supply P to the vegetation more uniformly through the growing season. The P concentrations in Ellen Brook are not only high, but the P fluxes are extremely high as well when compared to the other sites (Table 2). Although the Ellen Brook drainage is only about 0.6% of the total catchment area and runoff contributes about 7% of the total annual runoff, the FRP and TP fluxes are 62% and 42% of the total fluxes to the estuary from all tributaries. An assessment of sediment deposition in the Swan-Canning Estuary indicates a 2 to 3 fold increase since 1940 and that Ellen Brook is the largest contributor of any of the tributaries (Gerritse et al, 1998). The concentrations of the N species are affected by land use, particularly with respect to fertilizer application, but also with respect to the soil characteristics. For example, Ellen Brook had the highest average organic N and total N concentrations and basin soil conditions favour denitrification which reduces inorganic N concentrations. Also, for the inorganic N species that are not denitrified and are flushed from the soil, the high FRP concentrations favour rapid uptake of N by aquatic biota, which also may account for the high organic N concentrations and fluxes of Ellen Brook (Correll, 1998). Most freshwater systems are P limited. However, Ellen Brook has extremely high P concentrations associated with the superphosphate fertilizer application and the rapid transport of the P and DOC through sandy soils. Furthermore, NH/ concentrations, which generally are extremely low in surface water, were higher in the urban drains and in two of the coastal plain tributaries, Ellen Brook and

Table 3 Average annual nitrogen fluxes of tributaries to the Swan-Canning estuary, 1987-1992. The percentage of annual runoff and fluxes of NH4, N03, TIN, Org-N and TN reflect the contribution of a tributary to the total flux to the estuary.

Site Area Annual runoff NH4 N03 TIN Org-N TN (km2) (mm) (%) (kg ha') (%) (kgha') (%) (kg ha") (%) (kgha') (%) (kgha') (%) Drains Bayswater 262 46 2.4 0.15 9.8 0.19 3.8 0.34 5.2 0.34 2.0 0.69 2.9 Claise Brook 16 337 1.0 0.50 2.0 2.12 2.6 2.62 2.4 1.81 0.6 4.43 1.1 Mill Street 12 425 1.1 1.82 5.2 2.17 1.9 3.99 2.7 4.69 1.2 8.68 1.6 South Belmont 10 308 0.6 0.51 1.2 0.92 0.7 1.44 0.8 2.26 0.5 3.69 0.6 Rivers Avon River 119 035 3 71.0 <0.01 58.9 0.01 61.1 0.01 60.6 0.03 66.2 0.03 64.7 Bannister 23 400 1.8 0.48 2.7 1.43 2.5 1.91 2.6 4.61 2.3 6.52 2.4 Creek Bennet Brook 99 102 2 0.04 1.0 0.3 2.3 0.35 2.0 0.97 2.1 1.32 2.1 Blackadder 13 238 0.6 0.08 0.2 1.91 1.8 1.99 1.5 1.59 0.4 3.58 0.7 Creek Canning River 163 104 3.3 0.04 1.5 0.8 9.9 0.84 7.9 0.45 1.6 1.29 3.3 Ellen Brook 664 56 7.3 0.06 9.8 0.08 3.8 0.14 5.2 1.02 14.9 1.16 12.2 Helena River 161 62 2 0.04 1.5 0.24 3.0 0.28 2.6 0.3 1.0 0.58 1.5 Jane Brook 135 17 0.4 0.01 0.2 0.07 0.7 0.07 0.6 0.05 0.2 0.13 0.3 Southern River 149 141 4.1 0.11 3.9 0.09 1.0 0.19 1.7 1.50 4.9 1.7 4.0 Susannah 19 21 0.1 0.01 0.0 0.16 0.2 0.17 0.2 0.15 0.1 0.32 0.1 Brook Yule Brook 53 245 2.5 0.15 2.0 1.15 4.7 1.3 4.0 1.72 2.0 3.02 2.5 Subtotals Drain 300 84 0.5 0.003 18.2 0.010 9.0 0.013 11.2 0.036 4.3 0.049 6.2 River 120 500 4 95.0 0.247 81.8 0.394 91.0 0.641 88.8 0.651 95.7 1.293 93.8 Total 120 800 4.2 0.003 0.011 0.014 0.038 0.052 Nutrient concentrations and fluxes to the Swan-Canning estuary, Western Australia 93

Bannister Creek, than in the other tributaries. Also, the urban drains, which drain less than 0.3% of the total catchment of the Swan-Canning Estuary, contributed 18% of the total NH.4+ flux. High NH41" concentrations are typical in urban runoff and are attributed to inadequate waste treatment and the transport of domestic animal wastes. The Bannister Creek catchment is highly urbanized, but Ellen Brook is not. But, until 1995, livestock in the Ellen Brook catchment had unlimited access to the stream channel. Although unit runoff for the Avon River is extremely low compared to other sites, the contribution of nutrients from the Avon River to the estuary is relatively large (Tables 2 and 3). The nutrient concentrations for the Avon River are intermediate with respect to the other tributaries (Table 1). Consequently, the extremely large drainage area of the Avon River compensates for the low unit runoff in controlling the nutrient fluxes. Natural riparian-zone vegetation, including wetlands, can be effective in removing nutrients, particularly for N (Kadlec, 1994). Restoring natural vegetation in the main channel and the tributaries removes nutrients, particularly N which is lost to the atmosphere through denitrification. The unrestricted livestock access to catchment streams currently controls the stream vegetation and the livestock also provide nutrients directly to the stream. Furthermore, these coastal catchments provide proportionately higher dissolved organic carbon (DOC) than inland catchments along and above the Darling scarp. The DOC inhibits light penetration and consequently controls phytoplankton growth. The effect of DOC supply from the catchment on particle formation and settling in the estuary is not known. Also, the catchment source and supply rate of the DOC is not known.

CONCLUSIONS

In Western Australia, catchment nutrient availability is primarily controlled by the disposal of animal waste and the type and rate of fertilizer application, particularly in coastal areas. Nutrient concentrations and fluxes in 15 tributaries to the Swan-Canning Estuary, associated with more than 99.9% of the drainage area, were investigated from 1986 to 1996 and from 1986 to 1992, respectively, The Avon River is the largest tributary drainage and above the monitoring site drains 98.5% of the 121 000 km2 Swan-Canning catchment area. The Avon contributes most of the nutrients, although the nutrient fluxes per area are much less than other tributaries closer to the estuary. The coastal areas receive notably higher rainfall and have more intense horticulture and animal production than inland areas, and are undergoing rapid urbanization, particularly adjacent to the estuary. Annual rainfall near the coast is 1200 mm and in the eastern most areas in the headwaters of the Avon River is less than 300 mm. Also, the surficial aquifers on the coastal plain are generally sandy having a low nutrient retention capacity and rapidly transmit soluble and colloidal material through the subsurface. Most notably, Ellen Brook which drains less than 0.6% of the catchment contributes 62% and 42% of the total catchment flux of FRP and TP to estuary, respectively. In the Swan-Canning basin, high air and soil temperatures and seasonally arid conditions cause rapid mineralization of nitrogen and phosphorus. The nutrients are subsequently available for transport during the onset of seasonal wet weather, 94 Norman E. Peters & Robert Donohue which typically begins during the period from late April to June. In addition to the rapid mobility of nutrients in tributaries draining agricultural areas during the wet season, urban drainage, which typically has high nutrient concentrations, also is an important source of nutrients. For example, the urban drains, which drain less than 0.3% of the total catchment of the Swan-Canning Estuary, contributed 18% of the total + NH4 flux. The effect of timing of nutrient delivery for the tributaries may be an important control on estuarine eutrophication.

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