Thermal challenges for stream invertebrate assemblages and species - insights into impacts of extreme wildfires

Brendan G. Cox

doi:10.25918/thesis.376

The frequency and intensity of extreme wildfire events have increased globally. The impact of extreme wildfires on stream temperatures and subsequent stream invertebrate survival is not well understood and very few studies have focused on temperature spikes during the combustion phase of these wildfire. Stream invertebrates are essential for maintaining the overall health and functions of stream ecosystems. However, as ectotherms, stream invertebrates are vulnerable to increasing temperatures, dying from heat stress when lethal thresholds are exceeded. This thesis investigates the immediate response of stream invertebrates to acute short-term stream temperature spikes, specifically those resembling spikes caused during the combustion phase of extreme wildfire events. The research examined stream invertebrate mortality, population changes, and overall ecosystem resilience during and immediately after simulated acute short-term extreme temperature events. The first component of this research used highly controlled laboratory mesocosm temperature disturbance experiments to explore the effects of 45°C and 55°C acute short-term temperature spikes on natural stream invertebrate assemblage. The results showed that a 55°C temperature spike caused a 95.9% reduction in abundance and functional feeding group diversity was reduced to scrapers and shredders. Conversely, the 45°C temperature spike had no significant impact on either abundance or richness of functional feeding groups. Individual families responded differently to the temperature spike, with some families being very sensitive to the increase in temperature and two being tolerant of the spike and the resulting reduction in abundance and diversity will influence post fire recovery. The second component of this thesis investigated, for the first time, the specific Upper Lethal Temperature (ULT) (LT50) of five Australian subtropical stream invertebrate taxa (four species and one genus). These taxa were chosen based on their identified sensitivity or tolerance to higher temperatures observed during the mesocosm experiments. The aim was to gain insights into the lethal thresholds of these taxa in response to temperature extremes. The ULTs ranged from 35.2°C for the shrimp Paratya spinosa to 37.2°C for the adult beetle Macrogyrus oblongus. The ULTs were all below the temperature spikes of 55°C and 45°C investigated in the mesocosm experiments. The results highlight the importance of refugia areas in stream ecosystems. The availability and maintenance of thermal refugia, such as the hyporheic zone, play a fundamental role in providing shelter and buffering against extreme water temperatures for stream invertebrates. Invertebrates inhabiting subtropical streams appear to be living closer to their ULT than those identified in previous studies in temperate zones, making them more susceptible to extreme heat events. Finally, the thesis explored the potential for the thermal plasticity of the shrimp Paratya spp. as an adaptive mechanism in response to changing thermal conditions. Results showed warm-water acclimated shrimp had a significantly higher LT50 of 36.1°C than winter acclimated shrimp at 34.6°C. Paratya spp. had a potential critical temperature (LT90) of ~37°C, beyond which the shrimp could not survive. This critical limit underscores the vulnerability of Paratya spp. to extreme heat events and highlights the need for more understanding of the impacts of increasing stream temperatures. The research completed in this thesis helps to fill gaps in knowledge by evaluating stream invertebrate assemblage dynamics and providing insights into lethal temperature thresholds for different taxa. It also highlights the potential for adaptive responses and resilience in stream ecosystems. This research thesis improves and enhances our knowledge of the vulnerability of stream invertebrates to extreme water temperature spikes and future thermal extreme events.

Thermal challenges for stream invertebrate assemblages and species - insights into impacts of extreme wildfires

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Thermal challenges for stream invertebrate assemblages and species - insights into impacts of extreme wildfires

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