Model for the Hypolimnetic Oxygen Deficit in Hamilton Harbour

Abstract

<p>The goal of this research work is to examine the capability of alternative biochemical models for explaining quantitatively the rapid depletion of hypolimnetic dissolved oxygen in Hamilton Harbour during summer stratification. The technique used in the construction of mechanistic mass-balance models in which physical transport mechanisms and biochemical reactions are the building blocks. Three energy cources for oxygen consumption (land-based inputs of BOD and ammonia in situ formation of organics) are developed into four biochemical models. Each model is functionably controlled by land-based inputs of alternative materials (BOD, ammonia, phosphorus, or phosphorus plus nitrogen). Each model is potentially capable of explaining the oxygen deficient. Limnological information collected on Hamilton Harbour in 1975 to 1977 serve as the observational data base for discriminating the merits of each of these oxygen models.</p> <p>The BOD oxygen model was found to explain only 5% of the total dissolved oxygen depletion in the hypolimnion of Hamilton Harbour. The nitrogen-oxygen model and the phosphorus oxygen model explained 40% and 20% respectively. The phytoplankton-oxygen model using oxygen sinks of plankton decay, nitrification and sediment oxygen demand explains the dissolved oxygen (DO) decline in a reasonable fashion and is considered to be verified for predictive purpose. Analysis of water quality management alternatives suggest that a significant improvement will occur ( a minimum DO of 3.5 mg/ℓ) in the long term upon significant reduction in phosphorus and ammonia discharges. An immediate improvement will not be observed due to the high rate of sediment oxygen demand.</p> <p>Further work to improve the model should be directed in three main areas. Estimates of vertical exchange across the thermocline and Lake-Harbor exchange flows on a weekly rather than monthly or seasonal basis would significantly improve the agreement between model predictions and observations. A sediment model is required capable of predicting the rate of change of sediment oxygen demand due to control of land-based inputs. Winter-time data is required for checking and improving the phytoplankton-oxygen model. Good agreement between predictions for and observations in chlorophyll a cannot be expected until the effects of variations in scales of turbulence upon phytoplankton growth are expected.</p>