PREDICTIVE INDEXES OF ECOSYSTEM RESILIENCE IN MODELS OF NORTH TEMPERATE LAKES

Predictive indicators of the rate of ecosystem recovery after a pertur bation (resilience) could be valuable tools for ecological studies tha t need to anticipate system response to perturbation. We used a simula tion model of summer phosphorus cycling in north temperate lakes to (1 ) evaluate whether resilience decreases with increasing food web lengt h and increases with increasing nutrient loading and (2) test the corr espondence between two potential indicators of resilience (nutrient tu rnover rate, NTR, and the dominant eigenvalue, lambda(max)) and postpe rturbation measurements of recovery rate. We determined lambda(max) an d NTR for reference simulations of planktivore- and piscivore-dominate d food webs at three baseline phosphorus loading rates (0.1, 1.0, and 2.0 mu g.L(-1).d(-1)), then measured recovery rates from simulated pul ses of available phosphorus. The planktivore-dominated (short) food we b was more resilient at baseline phosphorus loading rates of 0.1 and 1 .0 mu g.L(-1).d(-1). However, the piscivore-dominated (long) food web was more resilient at the highest baseline phosphorus loading rate (2. 0 mu g.L(-1).d(-1)), apparently because the additional nutrients were incorporated into biota more rapidly. Recovery rates predicted from NT R (but not from lambda(max)) were consistent with measured recovery ra tes. NTR appears to be a useful predictor of lake ecosystem recovery f rom nutrient pulses, particularly because it can be estimated relative ly easily. Field tests comparing preperturbation estimates of NTR and postperturbation recovery rates are necessary to further validate this approach.