Reach-scale biogeochemistry within agricultural streams: interactions with riparian vegetation, channel geomorphology and hydrology

Abstract

Biogeochemical processes represent the mass-balance and cycling of key elements (carbon, nitrogen and phosphorus) in aquatic systems, providing basal resources to aquatic foodwebs, as well as being an important indicator of ecosystem function. The rate at which autotrophic and heterotrophic processes occur and the carbon:nutrient stoichiometry within reaches can be influenced by abiotic parameters linked to channel geomorphology and hydrology. In agricultural streams, the removal of riparian vegetation has led to a decline in nutrient retention through reduced geomorphic complexity. Stream restoration projects commonly use riparian revegetation techniques at a reach scale to reinstate structural complexity with the aim of restoring biogeochemical processes such as organic matter and nutrient retention. However, there has been little research into the success and outcomes of these projects, particularly at spatial scales relevant to restoration. This thesis examines in stream biogeochemical processes in vegetated and non-vegetated reaches within intermittent streams in an agricultural setting, located within the Gwydir River catchment, NSW, Australia. I predicted that there would be an increase in geomorphic complexity at vegetated reaches compared to non-vegetated reaches, which would lead to an increase in nutrient and organic matter retention within the vegetated reaches. I also predicted that enhanced nutrient and organic matter retention would be linked to increased rates of ecosystem respiration at vegetated reaches compared to non-vegetated reaches. To explore the interaction of hydrology on biogeochemical processes in intermittent streams, the study took place during four different magnitudes of discharge including a cease-to-flow period.