128 20.5 Effects of temperature on ecological values 20.5.1 Temperature thresholds Water temperature is important for a wide range of aquatic ecological functions, and alterations to temperature can have a range of different effects. Effects can occur over short to long periods of time so duration is also important to consider. Many species have specific temperature thresholds for different stages of their life cycles and are adapted to temperature regimes. In addition, changes to temperature can result in modified biotic (e.g., competition/predation with introduced species) and food-web interactions. A range of methods and approaches has been used to assess the thermal tolerance of aquatic organisms, ranging from knowledge of field distribution of species, particularly at temperature extremes to controlled laboratory experiments which provide the most defensible estimates of the thermal tolerance of aquatic species. In a review of thermal tolerance in aquatic biota, ARC (2012) outlines acute and chronic temperature thresholds for native fish and aquatic macroinvertebrates, including critical maximum temperatures, temperature preferences, and upper limits of behavioural and development effects. Based on the criteria, the ARC (2012) assessment of temperature thresholds conclude that: • Maximum temperatures in upland streams that are less than 20°C should protect even the most sensitive native taxa. • The most sensitive native taxa in lowland streams should be protected as long as maximum temperatures are less than 25°C. Thermal tolerance of a range of fish species is shown in Table 39 and for fish and macroinvertebrates in Figure 30. The effects of the proposed reduced environmental flows on water temperature will be influenced by several key factors including (but not limited to): • The extent of reduction in wetted area and depth of the receiving watercourse. • The extent of shading available to the surface waters of the watercourse. • The temperature of the source of water for the environmental flows. We note that the maximum temperature thresholds distinguish upland and lowland streams. The Ohinemuri River at Waihi is some distance upstream from the coastline but retains a more lowland character with flow and gradient, notably within the Waihi Basin. 20.5.2 Potential temperature attribute thresholds There are no attribute tables for temperature within the NOF. However, NIWA (2013) proposed potential thresholds for temperature for the NOF and we comment briefly on these recommendations here. We emphasise that such limits have no regulatory standing but capture the results of analysis of temperature for NZ waters. The comprehensive review of temperature criteria for New Zealand native fauna by ARC (2012) as discussed above provided the basis for proposing tentative temperature limits in NIWA (2013). An Attribute band A/B (‘no effect’) threshold of 18⁰C was proposed and an Attribute band C/D (‘bottom line’) limit of 24⁰C was proposed. The recommendation allowed that the natural thermal regime of streams in eastern dry regions may be hotter than in more maritime climates, with a
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