The Effects of Flood History and Environmental Cues on Microcrustaceans Hatching from Floodplain Sediment and the Predictions of the Impact of Water Resource Development and Climate Change on Microcrustacean Hatching Response

Abstract

<p>Microcrustaceans are an important component of the dryland river systems. They play a crucial role in maintaining trophic links between microbial, primary producers and higher consumers, primarily larval and juvenile fish. Due to the high variability in dryland river systems, microcrustaceans produce dormant eggs to survive prolonged dry periods, and hatch when the subsequent rewetting in the waterbody stimulates the diapausing eggs to break their dormancy. There are many factors, including physical, chemical, biological, and environmental cues that influence hatching patterns, active or adult communities in the waterbody and subsequently the dormant egg banks. Flood history is an important hydrological factor which has the potential to influence microcrustaceans’ egg bank composition in the dryland river soil which has not been investigated much in Australian dryland river systems. In addition to these factors, dryland river systems are also subject to anthropogenic alteration because of water resource development (WRD), and climate change which changes hydrological and temperature regimes in floodplain wetlands. However, the predicted influences of WRD and climate change on microcrustacean communities has not been explored widely. </p> <p>The aim of this PhD research project is to examine ecosystem responses to hydrological events. Specifically, the research examines: how a flood pulse event stimulates the hatching and emergence of microcrustaceans that persist through dry periods as desiccation resistant eggs. This research has four main objectives. These are to determine: a. how inundation frequency and duration influence abundance and community structure of emerging microcrustaceans; b. how water temperature and water depth act as potential environmental cues acting to influence the abundance and community structure of emerging microcrustaceans; c. how flood history and environmental cues might interact to further influence emergence patterns; d. how changes to inundation frequency and duration, temperature and depth arising from water resource development and climate may have influenced and may continue to influence emergence patterns and hence extant microcrustaceans assemblages in these systems. </p> <p>This study has been conducted in the Macintyre River which drains the Border Rivers catchment in south-eastern Australia and a typical of Australian allogenic lowland, dryland river system.</p> <p>This study determined that flood history variables such as annual return interval (ARI) and time since last flood can influence egg bank composition, with study also found more frequent flooding having a negative effect on the number of microcrustaceans hatching from the egg bank. However, the duration of inundation was found to be the most crucial factor in determining the abundance and composition of the hatched microcrustacean assemblage from egg bank.</p> <p>With regard to environmental cues for hatching, temperature was found to have a significant effect on the number and assemblage composition of microcrustacean hatching from sediment, with warmer temperatures resulting in a higher number of hatchlings. Importantly, temperature may have a greater effect on the hatching numbers of nonostracods compared to ostracods. However, there does not appear to be any significant effect of water depth on microcrustacean hatching.</p> <p>The composition of the egg bank and the hatching response of microcrustaceans are influenced by various factors, which interact to drive spatial variation in egg banks across the floodplain. The variation in egg bank composition is affected by both predictable spatial variation between shallow and deep areas within sites and unexplained spatial variation among sites. The interaction between factors influencing egg bank composition and microcrustacean hatching response is also influenced by temperature, and this interaction mainly affects the assemblage composition of non-ostracod microcrustaceans. Therefore, any changes in temperature and inundation duration can affect microcrustacean community composition and abundance.</p> <p>The flood pulse exerts a major influence on the structure and function of floodplain river systems However, the flood pulse has been, and continues to be, altered by water resource development (WRD) and climate change. This study predicts that WRD has greater individual impact on microcrustaceans hatching than climate change. However, the combination of WRD and climate change has the strongest impact on microcrustacean hatching. This prediction was made based on the relationships between the duration and temperature of flooding and microcrustacean hatching established in laboratory trials. In conclusion, this study highlights the critical influence of flood duration and temperature on hatching patterns of microcrustaceans from inundated sediments. Thus, changes to temperature during inundation periods arising from global climate change or river regulation are likely to influence the abundance and composition of microcrustacean assemblages, especially among non-ostracods, which will influence food availability for larval and juvenile native fish and hence recruitment. Notably, this study helped us to predict how the changes in temperature and inundation period due to WRD and climate change may influence microcrustacean communities and abundance hence the ecological integrity of dryland river systems.</p>