Estimating the effects of changing land use patterns on Connecticut lakes

Changes in land use of 30 Connecticut lake watersheds between 1934, 1970, and 1990 were quantified using aerial photographs. Results were used with existing land use models to estimate changes in concentrations of total phosphorus (TP) and total nitrogen (TN) over the 56-yr period. On average, the watersheds have increased in urban-residential land cover from 2% in 1934 to 16% in 1990, and decreased in agricultural land from 20 to 7% during the same time period. The mean percentage of forested land has remained relatively constant. Based on the land use models of Norvell et al. (1979) and Frink (1991), the mean estimated total phosphorus concentration (eTP) increased from 15 mu g L(-1) in 1934 to 25 mu g L(-1) in 1990. The eTP concentrations increased in 26 of the 30 study lakes. In contrast, the mean estimated concentration of total nitrogen (eTN) increased only 20% from 374 to 450 mu g L(-1). Principal component analysis (PCA) was used to score each study lake according to its current trophic and ionic condition using chemical data from 1991 to 1993, and the results regressed against the 1990 land use types. Trophic scores were most highly correlated with forest cover, while ionic scores were most highly correlated with forest cover and the degree of urban-residential land cover. The effect of water retention time is discussed. Land use models provide useful tools in the management of lakes.