Microbial Response to Urbanisation and Earthquake Damage in New Zealand Streams

Abstract

Microbial organisms play a pivotal role in stream nutrient cycling by taking up nutrients from organic and inorganic sources; this supports higher trophic levels and prevents downstream nutrient export. Nutrient loading to urban streams is likely to alter the identity and magnitude of limiting nutrients, this is important to understand if we are to manage nutrient pollution effectively by placing controls on the limiting nutrient. The aim of this study was to determine how urbanisation influences microbial nutrient limitation in New Zealand streams. Nutrient limitation was assessed in Auckland and Christchurch over Spring and Summer using three commonly used methodologies: 1) in situ organic and inorganic nutrient diffusing substrates (NDS), 2) sediment extracellular enzyme activity (EEA), and 3) water chemistry ratios. Nutrient diffusers experimentally tested nutrient limitation, and nutrient limitation was inferred from ratios of EEA and water chemistry. Biofilms demonstrated a clear switch in the identity of limiting nutrients from nitrogen (N) limited in native sites to phosphorus (P) limited in urban and agricultural sites, with urban sites demonstrating N-saturation at 30% land-use. Demonstrating rough agreement with the Redfield ratio, organic biofilms demonstrated a switch in nutrient limitation from N limited at <19N:1P to P limited at >15N:1P. Earthquake damage also produced noticeable effects including N and P suppression on biofilms and water column Nconcentrations up to seven-fold lower. Limitation predicted by sediment EEA suggested a predominance of N limitation and did not align with limitation inferred from water chemistry which suggested P limitation; however enzyme activity did vary with urbanisation impact suggesting this could be a promising bio-assessment tool. Sediment EEA was also not coherent when compared to limitation from organic biofilms; but EEA on organic biofilms from nutrient diffusers was more accurate. Results demonstrate the complexity of nutrient limitation between stream compartments as microbial organisms are reliant on the water column for N and P to different degrees. Additionally, nutrient limitation according to nutrient diffusers was often colimited or secondarily limited, suggesting that controls need to be placed on both N and P. Future NDS studies should incorporate organic substrates as these may be a valuable tool for consistently gauging microbial response to land-use change. Keywords: Nutrient limitation, stream biofilms, urbanisation, microbial enzymes, earthquake.