|dc.description.abstract||Agricultural development has resulted in the degradation of freshwater ecosystems worldwide. Disentangling the individual and combined effects of agricultural stressors is crucial for future land-use management and restoration efforts. Two key stressors impacting streams and rivers draining agricultural catchments are deposited fine sediment and water abstraction. The effects of fine sediment grain size combined with water abstraction on benthic invertebrate communities have not yet been investigated. I addressed this knowledge gap by conducting a manipulative experiment in 60 outdoor stream mesocosms using four fine sediment treatments (no added sediment, silt: 0-0.125mm, fine sand: 0.125-0.250mm, coarse sand: 1-2mm) combined with three flow velocity treatments, simulating varying levels of water abstraction (fast: 26.5cm/s; medium: 13.9cm/s; slow: 0.0cm/s).
In Chapter 2, I determine the effects of fine sediment and reduced flow velocity on the benthic, drifting, and emerging stream macroinvertebrate communities (55 invertebrate response variables). There were 24 significant responses to sediment (44% of all variables; 83% negative responses) and 27 to flow velocity (49%; 85% negative), and 3 complex interactions (5%; and 20% additive multiple-stressor responses). My results imply that deposited fine sediment (of varying sizes) and reduced flow velocity can have pervasive detrimental impacts on stream invertebrate communities, and that the combined effects of these two stressors may be mainly additive. Effects of fine sediment were often negative regardless of grain size, especially for sediment-sensitive invertebrate taxa (e.g. benthic EPT taxon richness). The negative effects of fine sediment were also often worse at the smaller two grain sizes (e.g. benthic taxon richness and evenness). Finer sediments may reduce benthic habitat, food, and oxygen availability to a greater degree than larger sediments, and cause more damage to sensitive individuals through abrasion, smothering, or burial.
In Chapter 3, I use biological traits to determine how stream invertebrate communities responded to the two stressors, by analysing the responses of 8 biological traits (consisting of 33 trait modalities). Biological trait modalities showed 25 significant responses to sediment (76% of all variables; 60% negative responses) and 20 to flow velocity (61%; 60% negative). There were 12 complex interactions between stressors (36%; and also 12 additive multiplestressor responses). Results indicated that fine sediment (of varying grain sizes) and reduced flow velocity often have pervasive negative effects on the functional composition of benthic invertebrate communities, individually and in a multiple-stressor context. My results also provide insights into the mechanisms driving stressor responses (e.g. prevalence of Crawlers decreased as fine sediment size decreased, likely due to reduced interstitial habitat availability).
My key findings are that regardless of grain size, fine sediment can negatively affect benthic freshwater invertebrate communities, and the effects of fine sediment may be augmented by reduced flow velocities. Furthermore, sediment grain size matters for some invertebrate taxa and biological traits, and the severity of sediment effects can increase as sediment particle size decreases. Management decisions should seek to avoid fine sediment inputs, and minimize water abstraction, and strategies must be informed by knowledge of the individual and interactive effects of multiple agricultural stressors.||