Nutrient Residence Times in Relation to the Trophic Condition of Lakes
This thesis is concerned with defining systematic relationships between nutrient flow rates in aquatic ecosystems and their overall biological productivity. The most significant finding was that phosphorus residence time produced a consistent pattern relative to the trophic condition of freshwater lakes and reservoirs which allows both characterization of trophic state and more precise calculation of the concentration response to loadings as commonly applied in lake management.
Nutrient residence times were examined from the perspective of two data types. The data types included time series information on nine individual lake cases and single year cross-sectional information on 220 waterbodies of the OECD Programme on Eutrophication.
The nine case histories gave insight into the applicability of cross-sectionally derived models. These cases showed that sedimentation rates of phosphorus in a given lake varied tremendously from year to year and therefore models based on the assumption of a constant rate are in most instances insufficient. Case histories also showed that phosphorus relative residence times followed a remarkably consistent pattern of increase with an increase in trophic status regardless of the number or types of underlying mechanisms in control of nutrient flows. This increase is related to an acceleration of phosphorus recycling mechanism rather than a decline in its gross deposition rate. The net decelleration of phosphorus flow rates creates a shift in the relative importance of elimination pathways, from deposition in the sediments to removal via flushing (i.e. δ /ρ <1), with increasing eutrophication. Lastly, case histories demonstrated that concentration ratios of lake/inflow gave reasonable estimates of nutrient relative residence times as calculated from budget information. Transposing this, the distinctly different dynamics according to trophic category reflected in the cross-sectional concentration ratio data may be interpreted as a functional pattern.
The cross-sectional data indicated that in the case of phosphorus, relative residence time was best described by a different multiple regression equation for each trophic category whereas there was no difference in nitrogen relative residence times according to trophic category. Practical application of this result is in refinement of the calculation of critical loads when the trophic status of a lake is known.
Nutrient limitation judged on the basis of most rapid flow rate implies that phosphorus limitation is far more common than nitrogen limitation, even in eutrophic waterbodies. The relative flow rates given by the ratio (τn/τp) is always greater than 1 in oligotrophic and mesotrophic waterbodies, and may be greater than 1 in eutrophic waterbodies. This ratio only drops below 1 in some eutrophic waterbodies. Therefore values less that 1 are not necessarily a consequence of eutrophication.
Phosphorus relative residence time was found to be a simple but highly integrative measure capable of characterizing trophic state and its temporal evolution.