OCEANOGRAPHIC PROCESSES OF CORAL REEFS: Physical and Biological Links in the Great Barrier Reef - Chapter 3

3 Landcover and Water Quality in River Catchments of the Great Barrier Reef Marine Park Andrew K.L. Johnson, Robert G.V. Bramley, and Christian H. Roth CONTENTS Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Methods and Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 The Herbert River Catchment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Landcover. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Surface Water Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Contemporary Broadscale Landcover Change in GBRMP Catchments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Landcover Change in the Herbert River Catchment . . . . . . . . . . . . . . . . . . . . 26 Water Quality in the Herbert River Catchment . . . . . . . . . . . . . . . . . . . . . . . . 27 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 INTRODUCTION The Great Barrier Reef Marine Park (GBRMP) covers an area of approximately 350,000 km2 and spans almost 2,000 km of the east coast of Queensland, Australia. The GBRMP is a marine ecosystem that is recognised internationally for its unique biological and physical features. Fifteen river catchments, covering an area of approximately 375,000 km2, drain directly into the GBRMP (Figure 1). Land use in thesecatchmentsisdominatedin areal terms by grazing. Cropping, particularly sugar-cane production, is a major user of land resources in a number of catchments and is predominantly located on fertile coastal floodplains immediately adjacent to GBRMP waters (Table 1). 19 © 2001 by CRC Press LLC 20 Oceanographic Processes of Coral Reefs TABLE 1 Approximate Area of Major Land Uses in Catchments Adjoining the GBRMP 1996 Catchment Catchment Percentage of Catchment Area Name NE Caped Daintree Mossman Barron Mulgrave Russell Johnstone Tully Murray Herbert Black Haughton Burdekin Don Proserpine O’Connell Pioneer Shoalwater Bay-Sarinae Fitzroy Curtis Coastg Area (km2) 43,300 2,130 490 2,180 2,020 2,330 1,690 1,140 10,130 1,080 3,650 129,860 3,890 2,490 2,440 1,490 11,270 152,640 9,225 Foresta 4.3 37.7 30.4 36.4 16.9 25.3 62.5 32.9 9.5 18.0 0.8 1.0 0.2 9.6 7.6 22.7 1.3 6.7 12.2 Pristineb 33.9 31.7 11.0 2.0 25.1 12.8 2.1 27.3 9.7 9.3 10.8 1.3 2.6 4.0 4.4 6.1 41.6f 2.3 11.3 Grazingc 61.7 26.7 44.6 47.7 38.9 41.6 20.7 29.6 71.1 67.4 74.0 94.8 91.3 74.6 70.5 48.5 44.1 87.5 68.9 Crops Urban 0.05 0.05 1.9 2.0 10.1 3.9 6.8 6.9 13.3 5.8 15.9 4.4 11.1 3.7 7.0 3.3 7.0 2.7 1.1 4.2 10.9 3.5 1.0 2.0 2.8 3.1 7.5 4.3 11.1 6.5 17.9 4.7 10.3 2.7 3.3 0.2 0.57 6.7 Total area Total (%) 369,480 28,007 39,830 7.6 10.8 284,056 76.9 13,597 3,990 3.7 1.1 aComprises state forests and timber reserves. bComprises national parks and other reserves. cComprises unimproved and improved pastures. dComprises Jacky Jacky Creek, Olive-Pascoe, Lockhart, Stewart, Jeannie, Normanby, and Endeavour catchments. eComprises Plane Creek, Styx, Shoalwater Creek, and Water Park Creek catchments. fApproximately 65% of this area occupied by the Shoalwater Bay Field Training Area of the Australian Defence Forces. gComprises Calliope, Boyne, and Baffle Creek catchments. Source: QDPI, 1993; EPA, 1999; Johnson et al., 1999. Current environmental trends suggest a decline in coastal terrestrial and riverine systems, and on the adjacent GBRMP marine environment (Anonymous, 1993; Arthington et al., 1997). The vegetation of many of the river catchments adjoining the GBRMP has been extensively cleared (Russell & Hales, 1996) since the mid-19th century. Freshwater wetlands and riparian forests once covered large areas of coastal floodplains which are now used for agriculture (Tait, 1994; Johnson et al., 1999). © 2001 by CRC Press LLC Landcover and Water Quality in River Catchments 21 Prior to clearing, these wetlands would have provided extensive buffer strips and freshwater habitats adjacent to coastal river systems, estuaries, and shorelines. Clearing, notably for sugarcane cultivation, has left only remnants of these ecosys-tems (Russell et al., 1996). Present-day wetlands and riparian forests in many catch-ments are frequently narrow and sparsely vegetated and have been invaded by exotic weeds (Johns et al., 1997). It is likely those wetlands and riparian forests in such poor condition have suffered a corresponding degradation of their intrinsic ecological val-ues (Arthington et al., 1997). The status of freshwater wetlands and riparian forests in river catchments adja-cent to the GBRMP has been reviewed superficially by a number of authors (Arthington & Hegerl, 1988; Anonymous, 1993; Blackman et al., 1996). Accounts have increasingly confirmed their very high biological richness, diversity, geograph-ical extent, importance as habitat for a similarly rich and diverse biota, and funda-mental role in ensuring the health of key GBRMP ecosystems. Of the 19 Queensland wetlands identified as having national importance (Blackman et al., 1996), 8 are located in areas immediately adjacent to or within the GBRMP. While the present sta-tus of these ecosystems is known, there have been no detailed assessments of histor-ical changes in coastal wetlands and riparian forests in GBRMP catchments. Similarly, while the current extent of landcover in river catchments adjoining the GBRMP is generally known, the spatial and temporal distribution of landcover since European settlement is poorly understood. The aim of this chapter is to describe broad-scale changes in landcover in GBRMP catchments and to examine in detail changes that have occurred using a case study undertaken in the lower Herbert River catchment. We also describe the likely impact that these changes have had on the water quality of the Herbert River. While the focus of the chapter is not on the impacts of these changes per se, we discuss sig-nificant issues that are central to the maintenance and function of estuarine and marine ecosystems in the GBRMP. METHODS AND MATERIALS THE HERBERT RIVER CATCHMENT The Herbert River catchment drains an area of approximately 10,000 km2 to the Coral Sea and is the largest of the river systems located in Australia’s sub-humid to humid tropical northeast (latitude 15 to 19°S, longitude 145 to 146°E) (Figure 2). Average annual rainfall is approximately 2500 mm. Mean annual runoff for the catchment is 4991 109 m3 or 493 mm, and the rainfall-to-runoff ratio approximately 37% (Hausler, 1991). Natural vegetation consists predominantly of open Eucalyptus and Melaleuca woodlands, with areas of open grassy plains and dense Melaleuca wetlands. Rainforest patterns occur on the creek and river levees and on some of the northern ranges. Large areas of the upper catchment remain under natural vegetation, although much of the lower catchment has been cleared for crop production or exotic pas-tures. Agricultural and pastoral activities are the largest users of land (in area) in the © 2001 by CRC Press LLC 22 Oceanographic Processes of Coral Reefs catchment. The catchment has a population of approximately 18,000 (1993 Census), of which 75% are located in the lower catchment. LANDCOVER A desktop study was conducted to collect data from a range of published and unpub-lished sources on landcover in catchments adjoining the GBRMP. This activity drew heavily on work undertaken by the Queensland Statewide Landcover and Tree Study (SLATS) (QDNR, 1999a and 1999b). The study utilised Landsat Thematic Mapper (TM) imagery (spatial accuracy 30 m) and ground surveys to map changes in woody vegetation cover (where woody vegetation was defined as approximately 12% foliage projective cover or greater) between 1988, 1991, 1995, and 1997. The study attempted to map vegetation change for all perennial woody plants of sizes that could be distinguished by Landsat TM imagery. Accuracy of areal interpretation for the whole state was reported as 8% at a 95% confidence interval. Error data associated with misclassification were not reported, although incidences of misclassification in areas of pasture and in highly fragmented landscapes (e.g., narrow riparian zones in coastal areas) were acknowledged. Anecdotal evidence from field-workers also suggests the existence of substantive misclassification in grazing lands (A. Ash, personal communication). QDNR (1999a and 1999b) describes the method used in more detail. Landcover in the Herbert River catchment was visually interpreted from scanned and rectified 1:25,000 aerial photography acquired in 1943, 1961, 1970, 1977, 1988, and 1992 (spatial accuracy 7 m) and 1:10,000 digital orthophotography acquired in 1993, 1994, and 1995 (spatial accuracy 1 m). An unsupervised classification of SPOT Panchromatic and MSS imagery was used to map landcover in 1996 (spatial accuracy 10 m). Landcover boundaries were mapped onto a geo-referenced digital base (spatial accuracy 10 m) in ARCINFO GIS. The classification methodology (Johnson et al., 1999 and 2000) drew heavily on previous vegetation (Tracey, 1982; Blackman et al., 1992; Perry, 1995) and soil (Wilson & Baker, 1990) surveys in the region. Validation of mapping units and mapped boundaries was conducted in 1996 by vehicle and foot traverses. Approximately 150 sites were visited. Classification of units and bound-aries not inspected in 1996 was undertaken by extrapolation from equivalent photo-graphic units. In addition to mapping observed landcover, an estimate of landcover prior to European settlement (circa 1860s) in the Herbert was developed from a simple rule base that related remaining stands of native vegetation and the known distribution of soils, topography, relief, hydrology, and rainfall. A time series was developed to elu-cidate spatial and temporal change in landcover (Johnson et al., 1999). SURFACE WATER QUALITY A number of sites were selected to reflect the major landcover classes, soil types (Wilson & Baker, 1990; Wood, 1984, 1985, and 1988), and sub-catchments in the lower Herbert floodplain (Figure 3) on the basis that water sampled at any given site reflected the biophysical characteristics of the land upstream of that site. © 2001 by CRC Press LLC Landcover and Water Quality in River Catchments 23 Beginning in October 1992, surface grab samples of river water were taken at each of these sites at monthly intervals and also in response to rainfall events of inten-sity greater than 50 mm d1. The samples were collected either by lowering a bucket from a bridge at a point above the centre of the flowing part of the channel, or more directly by wading into the stream. The samples were collected in acid-washed poly-ethylene bottles and were stored in a portable refrigerator for transfer back to the lab-oratory. On each sampling occasion, the distance between the surface of the water and a fixed arbitrary point such as a bridge rail was also measured for later estima-tion of actual water depth and then discharge. The laboratory procedures used in this study have been detailed by Muller et al. (1995). Total concentrations of nitrogen (N) and phosphorus (P) were determined according to USEPA (1984) methodology. Total suspended solids (TSS) were deter-mined by gravimetric measurement of the amount of particulate material retained on 0.45 m cellulose acetate filter papers. For the analysis of land use impact on water quality, the landcover classification (Figure 3) was simplified into land under sugarcane, grazing (i.e., improved grazing or Eucalyptus-dominated patterns), and forestry (i.e., plantation forestry or natural rainforest). This was done to simplify the attribution of water quality differences, given that for the majority of sites, several land uses exist upstream of those sites (i.e., water quality measurements made at a particular site may integrate the effects of more than one landcover class). This simplification of landcover categories is also consistent with the results of Hunter and Walton (1997), who found that in the Johnstone catchment, whilst it was possible to discriminate between the effects of intensive and non-intensive land uses on water quality, it was not possible to dis-criminate within these broad groupings. For the purposes of the present study, time of sampling was treated as an inde-pendent variable because although several authors (e.g., Hunter et al., 1996 and ref-erences therein; Mitchell et al., 1996 and 1997) have demonstrated the strong seasonality of riverine discharge and water quality in north Queensland rivers and their links to the strongly seasonal climate, our purpose here was to examine the effects of landcover on water quality. RESULTS CONTEMPORARY BROADSCALE LANDCOVER CHANGE IN GBRMP CATCHMENTS Tables 2 and 3 show contemporary woody vegetation changes in GBRMP catchments for the period 1991 to 1997. They show: • Large areas of woody vegetation converted to pasture in the Fitzroy, Burdekin, Normanby, Don, Proserpine, and Baffle Creek catchments, implying a change from extensive grazing woodlands to more intensive forms of grazing on improved pastures • Large areas converted to crops in the Herbert, Murray, Haughton, Plane Creek, and Fitzroy catchments © 2001 by CRC Press LLC ... - tailieumienphi.vn