Urbanisation and earthquake disturbance influence microbial nutrient limitation in streams

Summary Nutrient loading as a consequence of urbanisation is a common phenomenon that can influence microbial nutrient limitation in streams. How nutrient loading and nutrient limitation relate to increasing urban intensity, and whether there is any coherence in patterns across cities, is not well known. Nutrient limitation may also be influenced by natural disturbances that alter urban infrastructure and processes in urban streams. To examine this, we measured microbial nutrient limitation across urbanisation gradients in two cities in New Zealand. We took advantage of recent large earthquakes in one of the cities to determine how natural disturbance influences processes in an urban system. Biofilm responses to nitrogen (N) and phosphorus (P) amendment were tested using nutrient diffusing substrata ( NDS ) in 24 streams, 12 in each of New Zealand's two largest cities (Auckland and Christchurch). Biofilm response was measured across a gradient of land‐use intensity in each city. Regression models were used to quantify the relationships among water chemistry, land use and degree of biofilm N and P limitation. Water column N:P ( DIN : SRP ) was a good predictor of response of community respiration on organic substrata, but not of response of chl a on inorganic substrata. Autotrophic response was often not nutrient‐limited and was not likely to have been limited by light, suggesting other factors governed the development of autotrophs. We found a switch in response of biofilm respiration on organic substrata from N to P limitation in Auckland, with N limitation increasing below a N:P ratio of 20.8 and P limitation increasing above a ratio of 14.5, roughly in line with the Redfield ratio. Similarly, in Christchurch microbes showed little response to nutrient enrichment at sites with elevated nutrient concentrations. In Auckland, increasing urban intensity was associated with increased nutrient concentrations, but N increased faster than P. This translated to a nonlinear response of biofilm nutrient limitation to increasing urbanisation indicated as population density. There was a breakpoint from N to P limitation at a relatively low population density of >97 people per km 2 . Patterns of limitation with urbanisation were not as strong in Christchurch; however, responses were similar within the range of overlapping population densities in the two cities. Earthquake disturbance had noticeable effects on streams, including sevenfold lower DIN concentrations and inhibition by both N and P of community respiration on organic substrata. Urbanisation is a key driver of microbial nutrient limitation. Between cities, nutrient loading associated with urbanisation consistently affected the capacity of biofilms to process excess nutrients. Earthquake effects further influenced microbial limitation patterns with natural disturbance dramatically altering how urbanisation effects are manifested.