Water Quality of Rivers in the Inglis – Flowerdale Catchment PART 4 Authors: Christopher Bobbi David Andrews & Mark Bantich Environmental & Resource Analysis, Hydro Tasmania December 2003 Copyright Notice: Material contained in the report provided is subject to Australian copyright law. Other than in accordance with the Copyright Act 1968 of the Commonwealth Parliament, no part of this report may, in any form or by any means, be reproduced, transmitted or used. This report cannot be redistributed for any commercial purpose whatsoever, or distributed to a third party for such purpose, without prior written permission being sought from the Department of Primary Industries, Water and Environment, on behalf of the Crown in Right of the State of Tasmania. Disclaimer: Whilst DPIWE has made every attempt to ensure the accuracy and reliability of the information and data provided, it is the responsibility of the data user to make their own decisions about the accuracy, currency, reliability and correctness of information provided. The Department of Primary Industries, Water and Environment, its employees and agents, and the Crown in the Right of the State of Tasmania do not accept any liability for any damage caused by, or economic loss arising from, reliance on this information. Preferred Citation: DPIWE (2003) State of the River Report for the Inglis-Flowerdale Catchment. Water Assessment and Planning Branch, Department of Primary Industries, Water and Environment, Hobart. Technical Report No. WAP 03/12 ISSN: 1449-5996 The Department of Primary Industries, Water and Environment The Department of Primary Industries, Water and Environment provides leadership in the sustainable management and development of Tasmania’s resources. The Mission of the Department is to advance Tasmania’s prosperity through the sustainable development of our natural resources and the conservation of our natural and cultural heritage for the future. The Water Resources Division provides a focus for water management and water development in Tasmania through a diverse range of functions including the design of policy and regulatory frameworks to ensure sustainable use of the surface water and groundwater resources; monitoring, assessment and reporting on the condition of the State’s freshwater resources; facilitation of infrastructure development projects to ensure the efficient and sustainable supply of water; and implementation of the Water Management Act 1999, related legislation and the State Water Development Plan. ii 3.6 Diurnal Water Quality Variations Temporary monitoring equipment was deployed at two sites in the catchment for a 2-day period in early January 2000 to assess the scale of diurnal changes in temperature, dissolved oxygen and pH. The sites investigated were: IF13 – Calder River at Takone Rd; IF16 – Flowerdale River at Preolenna Rd In addition to these deployments, monitoring equipment that had been installed at IF4 in the lower Inglis River to record river level simultaneously collected data on the diurnal changes in water temperature in this reach of the river for comparison. The following discussion presents the data collected during these logging events and discusses the time series in relation to site characteristics and the river management practices being utilised at the time of the investigation. Site IF13 is located in the upper reaches of the Calder River, where the river flows through low gradient, pasture land. This reach of the Calder River is characterised by a near-total lack of riparian vegetation (Figure 3.34), and this along with the added input of nutrients from stock access, has encouraged extensive invasion of nuisance aquatic weeds (Typha spp) and blooms of green algae during the summer months. Figure 3.34: Photo of the Calder River upstream from site IF13 The monthly monitoring data presented in Section 3.1.5 showed that oxygen levels in this reach of the river were wildly variable, and the nutrient data from the snapshot surveys showed that there was some enrichment of this site relative to other headwater sites located in nearby rivers. A short-term water quality monitoring event was carried out in the middle of January 2000 when conditions in the catchment were dry and warm. The data from this event (Figures 3.35 and 3.36) show that peak water temperature at this site reached an extremely high 30 oC, which is likely to also have been the maximum air temperature for the period. 42 The data for dissolved oxygen shows that the daily change in saturation levels was also extreme, with daytime levels peaking at about 120% and nighttime levels plummeting to about 50%. While this has also influenced changes in pH at this site to some degree (Figure 3.36), it is the magnitude of the changes in dissolved oxygen that are most significant. The scale of this daily variation is indicative of a highly modified and out-of-balance ecosystem, and is likely to cause acute stress to any aquatic organisms inhabiting this stretch of the Calder River. 35 140 30 120 25 100 20 80 15 60 Dissolved Oxygen (%Sat) Dissolved Water Tempaerature (DegC) Tempaerature Water 10 40 5 20 Temperature DO% 0 0 10/01/2000 12:00 11/01/2000 12:00 12/01/2000 12:00 Date/Time Figure 3.35: Short-term time series showing diurnal changes in water temperature and dissolved oxygen (% saturation) at IF13 (Calder River u/s Takone Rd) in January 2000. 35 7.5 Temperature Field pH 7.4 30 7.3 25 7.2 7.1 20 7 Field pH Field 15 6.9 Water Tempaerature (DegC) Tempaerature Water 6.8 10 6.7 5 6.6 0 6.5 10/01/2000 12:00 11/01/2000 12:00 12/01/2000 12:00 Date/Time Figure 3.36: Short-term time series showing diurnal changes in water temperature and pH at IF13 (Calder River u/s Takone Rd) in January 2000. 43 Simultaneous short-term monitoring was also carried out at IF16 on the lower Flowerdale River, along with continuous monitoring of water temperature at site IF6 on the Inglis River. At the time of the logging event (January 2000), both these reaches of the Flowerdale and lower Inglis Rivers were totally enclosed by dense infestations of Crack Willow (Salix fragilis), and this has a marked influence on the scale of diurnal changes in water quality at these two sites. As can be clearly seen from the data from IF16 (Figure 3.37 & 3.38), the magnitude of the daily change in water temperature, dissolved oxygen and pH was much less than was recorded in the upper Calder River over the same time period. The daily change in temperature at both IF16 and IF6 was about 3 oC, with a midday maximum of only 20 oC. This was much less than the overall change of 15 oC and peak temperature of 30 oC that was recorded at IF13, and reflects the huge influence shading by willows (or other dense riparian vegetation) can have on water temperature in rivers. The scale of daily change in dissolved oxygen at IF16 (Figure 3.37) was also significantly dampened by the presence of willows, which through the provision of shade reduced the level of instream algal and microbiological activity. The presence of extensive willow roots on the banks and in the bed of the river is also likely to have reduced near-bed flows and increased the depositional characteristics of the river, leading to greater deposition and decomposition and an increased demand for oxygen. This is the most likely explanation as to why saturation levels vary within a much narrower range at this site, and do not exceed 80% during the daylight hours. 25 100 95 20 90 85 15 80 75 10 70 DIssolved Oxygen (%Sat) DIssolved Water Temperature (DegC) Temperature Water 65 5 60 Temperature 55 DO% 0 50 10/01/2000 12:00 11/01/2000 12:00 12/01/2000 12:00 Date/Time Figure 3.37: Short-term time series showing diurnal changes in water temperature and dissolved oxygen (% saturation) at IF16 (Flowerdale River at Preolenna Rd) in January 2000. The more restricted scale of change in dissolved oxygen concentration at this site also means that pH (Figure 3.38) is much more stable. In addition, the ionic characteristics of the water in the lower Flowerdale River are such that, while still being fairly dilute, it has some ability to buffer against changes in pH that might be caused by changes in the concentration of dissolved oxygen. As a result of these two factors, the pH at IF16 is much more stable than was measured at IF13, varying by less than 0.1 over the period of the logging event. 44 25 7.5 24 7.4 23 7.3 22 7.2 21 7.1 20 7 Field pH 19 6.9 Water Temperature (DegC) Temperature Water 18 6.8 17 6.7 16 Temperature 6.6 Field pH 15 6.5 10/01/2000 12:00 11/01/2000 12:00 12/01/2000 12:00 Date/Time Figure 3.38: Short-term time series showing diurnal changes in water temperature and pH at IF16 (Flowerdale River at Preolenna Rd) in January 2000. Following the short-term monitoring event conducted in January 2000, extensive willow removal was undertaken throughout the lower regions of the Inglis-Flowerdale catchment. While any remnant native riparian vegetation still living was left standing, the removal of all willows along large stretches of the river system meant that long reaches of rivers and streams were left totally exposed to the sun. The huge increase in light penetration that resulted is likely to have dramatically altered the energy dynamics of the aquatic ecosystem of these streams, the most obvious being higher daytime water temperatures and massive increases in algal productivity.