EXPERIMENTAL AND EMPIRICAL ANALYSIS OF THE ROLE OF WATER TEMPERATURE ON INTERNAL PHOSPHORUS LOADING IN A EUTROPHIC LAKE

Excess phosphorus is a leading cause of lake eutrophication worldwide, and identifying the source of P is essential for remediation. A small eutrophic lake in the northern Appalachian region of North America (Lake Carmi, Vermont, USA) has experienced toxic summer algae blooms for decades, and despite extensive research and management initiatives that have reduced surface water sources of P, lake conditions have not improved. In fact, they appear to be worsening, suggesting that a significant source of P has not been identified and addressed. This study reports on an integrated assessment of P sources including (1) experimental and empirical analysis of the effect of lake temperature on internal loading, (2) quantifying groundwater P transport empirically (monitoring wells) and by groundwater flow modeling (MODFLOW), and (3) published data on surface water and atmospheric P inputs. Phosphorus in lake water increases annually from ~600 kg to 1200 kg in the transition from spring to summer, and empirical data shows a strong correlation between lake temperature and P in lake water. Experimental analysis with temperature as the only manipulated variable demonstrates its key role in internal P loading -- dissolved P in experimental lake water after 7 days at 20 °C was double dissolved P in lake water after 7 days at 5 °C; after 7 days at 35 ^oC, lake water contained 2.5 to 3 times more P than 5 ^oC lake water. The upper 10-cm of Lake Carmi sediments contain ~ 700 ppm P, 40 % of which is labile, providing a source of internal P loading totaling about 6.1 kg/day in the annual transition from spring to summer. Results indicate that, lacking remediation to limit internal P release, eutrophication is likely to persist and perhaps worsen in the face of projected warming trends and the high amounts of environmentally available P stored in Lake Carmi sediments.