Man’s activities can greatly speed up the natural process of lake aging. Urban development and agriculture will release quantities of nutrients – nitrates and phosphates – into lake waters, where they will nourish vegetation and especially microscopic algae. This will tend to turn the water turbid and eventually will alter the fish populations. Calculations by Biological Station scientists indicate that septic tank systems are releasing up to three tons of nutrients each year on some medium-sized lakes. These data deal only with the nutrients which flow into the lakes each year – not with the amounts already there (to be reported in future Lakeland Reports). Although the nutrient ratios do not measure the present water quality, they warn about possible future changes in water quality – waht we call lake "sensitivity."