Primary production and its interactions with nutrients and light transmission

Paul Lake, Wisconsin (control), showed no trends that suggest regional changes in determinants of primary production. Relations among nutrient, phytoplankton and zooplankton varieties in Paul Lake were weak. The one exception was the relation between alkaline phosphatase activity and primary production, which suggests that production is P-limited and that periods of high production were followed by periods of P-limitation. In Peter Lake, changes in cladoceran size following fish manipulations had several significant effects. Inreased cladoceran size caused decreases in chlorophyll and primary production, and increases in P-limitation of algae, irradiance at the thermocline, and thermocline depth. In Tuesday Lake, establishment of large cladocerans in 1985 caused significant decreases in chlorophyll, primary production and N-limitation of algae, and significant increases in P-limitation of algae. These shifts were large relative to long-term mean conditions in the lake, and were sustained from 1985-1987. Because Tueday Lake has stained water, changes in algal biomass had no effect on physical properties of the water column such as irradiance at the thermocline and thermocline depth. Removal of large daphnids in 1988 had weaker effects than establishment of large daphnids in 1985. Nevertheless, significant increases in chlorophyll concentration and primary production followed the decline of large daphnids. This asymmetric response of Tuesday Lake to opposite food web manipulations is an example of ecosystem hysteresis. -from Authors