Near-field loading dynamics of total phosphorus and short-term water quality variations at a rainbow trout cage farm in Lake Huron

Abstract

Aquatic total phosphorus (Tot-P) is measured at fish-cages in Lake Huron for environmental regulatory compliance. An improved understanding of how Tot-P is manifested in the near-field (≤30 m) water column relative to reference sites within the same water body, current movement and farm structures is required to help interpret sample results and refine monitoring protocols where necessary. The primary aim of this study is to investigate spatial and temporal concentrations of Tot-P at a commercial fish farm. A secondary aim investigates the utility of automated dissolved oxygen (DO) and pH monitoring to assist with Tot-P monitoring. Near-field TP at a commercial rainbow trout (Oncorhynchus mykiss) cage-farm in the North Channel of Lake Huron was intensively sampled for six multi-day periods of differing environmental and fish production scenarios. Current profiles were measured by acoustical Doppler current profilers for five periods, and multi-probes continuously measured temperature, pH and dissolved oxygen, at the farm centre and 30 m north/south for two sampling periods. Near-field Tot-P data was non-normally distributed. Differences between parametric and non-parametric measures were minor. Phosphorus depth profiles appeared to be influenced by the locations of cages, manure collectors and the lake bottom. Near-field lateral Tot-P concentrations were elevated above background only in down-current locations except during one period of high production and slow current velocity; suggesting adequately flushed cages will have background concentrations at up-current locations. Variation of DO, pH and Tot-P and the correlations among these parameters, increased from the up-current, down-current and the site centre locations, respectively. These relationships suggest some limited utility for the use multi-probes deployed around fish cages to determine nutrient flow direction, thereby inferring short-term trends of Tot-P concentrations adjacent to the farm. Implications for present monitoring practices are discussed.