STOICHIOMETRIC CONSTRAINTS ON FOOD-WEB DYNAMICS - A WHOLE-LAKE EXPERIMENT ON THE CANADIAN SHIELD

A whole-lake manipulation of food-web structure (introduction of a top predator, northern pike, to a minnow-dominated lake) was performed in a Canadian Shield lake (L110) to examine the stoichiometric consequen ces of changes in planktonic community structure generated by altered food-web structure. Minnow abundance, zooplankton biomass and communit y composition, microconsumer abundance, and concentration and carbon-p hosphorus (C:P) ratio of suspended particulate matter were monitored i n L110 and unmanipulated L240 before (1992) and after (1993-95) pike i ntroduction. Algal biomass in L110 determined from microscopic examina tion for postmanipulation and premanipulation periods was also compare d with dynamics in a suite of unmanipulated reference lakes from longt erm monitoring records. Pike were added in spring in 1993 and 1994 in sufficient quantity to raise pike biomass to levels of around 22 kg ha (-1) by 1994. Minnow populations in L110 responded dramatically, decre asing to levels 30% (1993), 10% (1994), and less than 1% (1995) of pre manipulation values. However, most components lower in the food web di d not respond in a manner consistent with predictions of existing food -web theory, such as the idea of cascading trophic interactions (CTI). While Daphnia biomass increased in L110 in the first year following m anipulation, consistent with CTI, this effect was temporary and Daphni a collapsed in 1995, the year of lowest minnow abundance. Total zoopla nkton biomass in both lakes declined during the study period and, cont rary to CTI, this decline appeared somewhat stronger in L110 than in L 240. Dominant microconsumers (heterotrophic microflagellates) did not differ among years in either lake and did not appear to respond to foo d-web manipulation. At the bottom of the food web, no changes in bacte rial biomass occurred in either lake. However, total concentrations of particulate matter appeared to increase in L110 after manipulation (c ontrary to expectations based on the theory of CTI) while algal biomas s did not change in the manipulated lake relative to reference systems . Finally, particulate C:P increased in both L110 and L240 during the study period. The lack of strong response of Daphnia, the lack of resp onse of the microbial food web, decreases in zooplankton biomass and i ncreases in particulate biomass following reduction of minnow populati ons after piscivore introduction are at odds with expectations from ex isting food-web theory, such as the idea of CTI as currently formulate d. However, the extremely high C:P ratios in particulate matter at the base of the food webs in these lakes, the coincidence of zooplankton declines and increases in particulate C:P ratios, and the results of s mall-scale mesocosm food-quality experiments are consistent with a hyp othesis of a stoichiometric constraint operating on food-web dynamics in this and similar ecosystems.