Temporary versus constant flows: a flat-lined pulse
We compared floodplain sediments dry for 11, 4 and 1 year. We found that densities of microinvertebrates were extremely high, but that this food resource declined with loss of flooding. Further down the food web, loss of flooding also depleted organic matter and caused an earlier shift to oxygen production via photosynthesis of algae and a reduction in respiration (the breakdown of organic matter). Levels of organic carbon, phosphorous and nitrogen in the sediments and the water declined as the time without flooding extended. Microbial community composition involved in the breakdown of organic matter and the dominant algae both changed with loss of flooding. These observed changes indicate that after long periods without flooding, the lower food web will be less productive and diverse when flooding eventually occurs. There will be less food to support waterbirds and fish. As we finalise analysis of this study we are developing thresholds of concern to trigger management action when densities and diversities of microinvertebrates or concentrations of nutrients fall to these critical levels.