EFFECTS OF LAKE SIZE ON NUTRIENT AVAILABILITY IN THE MIXED-LAYER DURING SUMMER STRATIFICATION

Fluxes to the summer mixed layer of N, Si, and P were estimated in a s ize series of northwestern Ontario Canadian Shield lakes. Increasing t urbulence caused upward fluxes through the thermocline to increase wit h lake size for nutrients that increased in concentration below the th ermocline (soluble reactive Si, total inorganic N, and NO3-; but not t otal N or any form of P). Precipitation and terrestrial runoff were eq ually important sources of N and P in all but very small lakes (<100 h a), where precipitation supplied much more than runoff. Runoff was the only important source of Si, except in very large lakes (>100 000 ha) where mixing through the thermocline was important. N-2-fixation was unimportant except in intermediate-sized lakes (700-2000 ha). Si fluxe s nearly equaled phytoplankton requirements, but N and P were below re quirements, and recycling within the mixed layer must be the most impo rtant source of these nutrients. N and P deficits increased progressiv ely with lake size, implying that the efficiency of nutrient recycling increased with take size; mixed layers in large lakes are more turbul ent and thicker than in small lakes and these processes increase the p robability of nutrient regeneration within the mixed layer.