ECOSYSTEM RESPONSES TO STRESS 109 



affect ecosystem stability (e.g., Pomeroy, 1975; Webster, Waide, and 

 Patten, 1975). They suggest that increased nutrient availabihty 

 would increase the constancy and resistance of ecosystems. As 

 mentioned previously, through intuitive syntheses, Pomeroy pre- 

 dicted increased resilience, v^hereas Webster, Waide, and Patten 

 postulated an inverse relationship between resistance and resilience 

 on the basis of mathematical simulations. I have evaluated these 

 hypotheses experimentally with aquatic microcosms (Leffler, manu- 

 script in preparation). 



Four-liter, open, flow-through systems were used in the study. 

 All microcosms initially contained identical chemical mediums, and 

 each was inoculated from the same stocks. Cross-seeding improved 

 replication. Three levels of nutrient subsidy and three levels of 

 energy subsidy were used. Nutrients were added in the form of 

 Taub's #36 microcosm medium (Taub and Dollar, 1964). Concentra- 

 tions of the inflow solutions were set at one-tenth, normal, and ten 

 times the normal strength of this medium. Turnover time for each 

 system was 7 days. Energy subsidies were provided by Teflon stir 

 bars and magnetic stirrers, which operated constantly, for 2 hr on 

 alternating days, and not at all. Each combination of nutrients 

 and energy was replicated four times, for a total of 36 microcosms. 

 After inoculation, all systems experienced a 90-day succession and 

 were then monitored at steady state for an additional 90 days. A 

 thermal stress was induced by increasing the temperature from 22 to 

 40° C for 48 hr and then returning it to normal. Stress response was 

 monitored for 75 days. At the end of this period, new normal 

 operating ranges were described for each system by further observa- 

 tion for 70 days. Another thermal stress of the same magnitude but 

 of 7-day s duration was induced. The microcosms' responses were 

 followed for 50 days. 



A variety of parameters characterized ecosystem dynamics— net 

 day production, night respiration, P/R ratio, chlorophyll a, particu- 

 late matter, heterotrophic bacterial numbers, and inflow-to-outflow 

 ratios of ammonia, nitrate, phosphorus, and magnesium. The 

 responses of the microcosms to the thermal stresses were analyzed as 

 described for the diversity experiment. Nonparametric tests were 

 used to determine the significance of the ordering of nutrient 

 treatments, energy treatments, and nutrient— energy combinations 

 for the five measures of stability for each parameter. Few consistent 

 generalizations could be drawn from this analysis. Effects of energy 

 subsidies were seldom significant for any parameter or stability 

 measure. Constancy stability was enhanced for all chemical inflow- 

 to-outflow ratios by increasing nutrient subsidies and by increasing 

 the level of nutrient— energy combinations. 



