Estimating the contribution of nitrogen and phosphorus to waterbodies by colonial nesting waterbirds

Fecal deposition by colonial nesting waterbirds is a potential source of nutrient enrichment and pollution of nearby waterways. Excess concentrations of nitrogen (N) and phosphorus (P) can cause water quality concerns or impairment. We estimated concentrations of N and P deposited to waterbodies by birds nesting in four colonies in east-central Texas during 2011-2013, and developed an age-structured compartment model to estimate the amount of fecal N and P material deposited by birds during the entire breeding season. There was a seasonal variation in the accumulation of N and P in waterbodies where heronries were located with a significant increase from June to July at the peak of the breeding season. Also, there was a significant positive correlation in concentrations of N (P=0.023) in fecal material and water samples from one of the colonies (Murphy Park), suggesting a significant contribution of N from feces to water. Concentrations of N in water near the colonies were also significantly correlated (P=0.004, R²=0.22) with the average amount of precipitation recorded for each region. The simulation model results indicated that total N and P deposition increased proportionally with heronry size, and daily deposition rates varied due to the shifting daily activity budgets of the birds as the breeding season progressed. The total estimated loads from the model reached a maximum of 2170kg N and 240kg P for a given colony, with daily deposition estimated at 22.8kg N, and 2.5kg P. Based on the model, one bird can release about 1mg of N and 0.115mg of P on a daily basis to the soil substrate or directly over water. Results from this study could be useful to estimate the contribution of N and P from wildlife to waterbodies, and for watershed management plans.