DISCUSSION 



These initial observations provide us with insight into the problems of 

 running microcosms in such a way that they are analogous to some natural 

 system. The natural system itself is higlily variable and difficult to define, 

 except within broad Hmits. Generally, for most of the variables measured, the 

 values from the MERL microcosms fell within the ranges observed for adjacent 

 Narragansett Bay. We have no evidence that the major features of 

 phytoplankton and nutrient dynamics were different from Narragansett Bay. 

 This lends support to the hope that the MERL microcosms will be useful 

 experimental systems in which investigations wiU produce results transferable 

 to comparable open, natural systems. 



An exception to the generahzations above was the zooplankton abundance. 

 The biomass of zooplankton in the MERL microcosms (Figure 24-9) was 

 somewhat less than in the bay, especially towards the end of the time period 

 considered. This factor is responsible for the microcosms lying somewhat 

 outside the fields for the bay data shown in Figures 24-6 and 24-8, because the 

 1972-73 bay survey also returned somewhat higher zooplankton biomass 

 concentrations than were found in the tanks. We believe that the tendency 

 towards low zooplankton biomass was due to an artifact associated with the 

 deUvery of water to the tanks and that this problem will be rectified by 

 subsequent changes to the plumbing. It is therefore premature now to dwell on 

 the nature of the differences in zooplankton. 



The general behaviour of the nutrient and the phytoplankton data sets 

 (and, to a lesser extent, that of the zooplankton) was most reassuring. No wild 

 excursions occurred. The variability in the microcosms was generally similar to 

 that in mid-bay stations, and the species abundances were generally similar, 

 taking the data as a whole. On the other hand, the quantitative variability of 

 the tanks between themselves (as may be inferred by examination of the ranges 

 shown in Figure 24-3) violates our usual perception of the way in which 

 experimental systems should behave. We expect them to replicate well, so that 

 experiments can be performed and the results have good statistical vaUdity. 

 The difficulty with repUcation of nature is that nature herself is highly variable. 

 Working with such systems requires that large data sets be obtained, and that 

 multivariate statistical techniques be applied to reduce these correlated data 

 sets to manageable formats for analysis. 



A possible way of assessing microcosm and natural system behaviour and 

 developing a criterion for comparison is to calculate the generalized distance 

 between data sets (Blackith and Reyment 1971). Tlie assumptions of 

 homogeneity, multivariate normality and linear correlation between variables 

 must be met for such a technique to be rigorously apphed but, as with all 



379 



