Phosphorus cycling in the Lake Mendota ecosystem: Internal versus external nutrient supply

In most north temperate lakes such as Lake Mendota, phosphorus (P) often limits primary productivity. The largest source of P to the lake is the watershed. When P loads to the lake are large, blue-green algae (cyanobacteria) dominate and form nuisance blooms. My research objectives were to: (1) develop a spatially explicit model to quantify the effect of land use change on annual P loading; (2) compare the relative contribution of internal and external P sources to Lake Mendota's P-budget during summer; and (3) determine the predictability of blue-green algal blooms in Lake Mendota.; In collaboration with others, I developed a new simple model to estimate nonpoint-source P loading from land to water using geographic information system (GIS) databases. The model has only three parameters and accounts for the distribution of land uses with distance from open water. The greatest contribution to loading was found to come from a heterogeneous riparian corridor that surrounds the lake and streams. The width of this corridor varies among years depending on annual precipitation. This concept of a "variable source area" has rarely been applied to P loading in large watersheds. However, because internal sources of P in lakes can be large, I compared internal and external loads during two summers. Internal loading from sediment release made up a large part of the P budget during both years, and was about 10 times larger than external loading during the average loading year. The two sources of P were about equal in the higher than average loading year. Because algae respond to environmental fluctuations at time lags ranging from 1-15 days, I examined the response of blue-green algae to internal and external P supply and environmental variables at the daily scale in Lake Mendota during summer 1993. Surface blooms occurred every 20-30 days, and lasted from 1-3 days. Surprisingly, water column chlorophyll and surface blooms were not strongly related. This is important because water column chlorophyll is commonly measured and predicted by water quality models, but it is surface blooms that are perceived by the public. My results show that blue-green algal dynamics are not predictable at the daily scale.