FERTILIZATION OF AN OLIGOTROPHIC LAKE WITH A DEEP CHLOROPHYLL MAXIMUM- PREDICTING THE EFFECT ON PRIMARY PRODUCTIVITY

We investigated how epilimnetic fertilization would affect chlorophyll levels and light penetration of oligotrophic sockeye salmon (Oncorhyn chus nerka) lakes and how the resulting self-shading would affect prim ary production of the prominent deep chlorophyll maxima (DCM) of the l akes. Epilimnetic nutrient additions to large mesocosms (330 m(3)) in Redfish Lake, Idaho, increased levels of primary productivity and chlo rophyll a but decreased Secchi depths and light available in the metal imnion and hypolimnion. Redfish Lake and other Sawtooth Valley (Idaho) lakes had DCM in which the mean chlorophyll a peaks were 240-1000% of mean epilimnetic chlorophyll a concentrations. The DCM existed at low ;light levels and accounted for 36-72% of the lakes' primary productio n. Simulations using photosynthesis-irradiance (P-I) curves demonstrat ed that fertilization would increase predicted water column primary pr oduction by 75-101%. Most of this increase occurred in the epilimnion, with only a slight decrease occurring in the DCM as the result of inc reased shading.