A CLOSED ARTIFICIAL STREAM FOR CONDUCTING EXPERIMENTS REQUIRING A CONTROLLED SPECIES POOL

Experimental testing of ecological theory in the field is often diffic ult because of the number of factors influencing the test system. Adeq uate control of relevant variables may require the use of laboratory m icrocosms for the performance of proper experimental tests. Our effort s to study the effects of species invasion sequence on community struc ture and ecosystem properties of the developing system, requires that the experimental systems be closed. To meet our demanding experimental requirements, we have designed a closed artificial stream that permit s careful control over algal species introductions while minimizing th e risks of contamination by unwanted species. In assessing the perform ance of these streams, we examined the variability among experimental replicates in water chemistry parameters and biomass accumulation. Tho se water chemistry parameters that were not greatly influenced by biol ogical processes in the streams (Ca, Mg, bicarbonate, and sulfate) had average coefficients of variation less than 10%. Those parameters tha t were clearly influenced by biological processes (P, N, Fe, and Si) h ad average coefficients of variation ranging from 13.5%-40.4%. Ash fre e dry mass had an average coefficient of variation of 23.3%, but chlor ophyll a was much more variable at 183%. This pattern of variation ind icates that the artificial streams performed well in terms of replicab ility, but some experimental goals may require carefully conceived des igns that adequately partition sources of variation in biologically me diated Variables to obtain satisfactory results.