At present there are no accepted criteria by which it is possible to establish 
whether a microcosm behaves in a way similar to the natural system it is 
designed to mimic, or to judge whether its behaviour is satisfactory for use as 
an experimental tool. A major concern should be with replicability, but one 
difficulty here is that nature herself is highly variable, and it is not easy to 
properly frame the tests to be applied. 
In general, we suggest that if the enclosed ecosystems maintain similar 
species composition and diversity, if the metabolic rates in the systems and the 
major chemical fluxes and transformations are within the range of variability of 
the natural systems, and if the statistical behaviour of the systems is similar to 
that of the natural system, then it is reasonable to conclude that the major 
biological activities are carried on in similar ways. If so, one may have some 
confidence that the enclosed ecosystems are useful experimental tools. 
In this paper we analyse a portion of the data obtained during the first four 
months of running the microcosms at the Marine Ecosystems Research 
Laboratory, to examine their replicability with respect to each other and to 
Narragansett Bay. 
FACILITY AND PROCEDURES 
Narragansett Bay 
Since the microcosms to be described were designed in part to act as a 
model of Narragansett Bay, a brief introductory description of this bay is 
presented here. 
Narragansett Bay is about 40 km long by 18 km wide, oriented N-S with 
the mouth opening into Rhode Island Sound (Figure 24-1). The presence of 
islands causes a complex tidal current regime and some isolation of regions. 
Small fresh water inputs result in a weak salinity gradient from the mouth (31 
o/oo) to the northern end (V20 o/oo). The water column is generally well 
mixed, although slight stratification occurs at times. The annual temperature 
range is from -1°C to about 25°C. Sediments are generally a mixture of silt and 
clay, although sand is found in some locations. Tidal currents resuspend 
flocculent bottom sediments in the bay, which has an average depth of about 8 
m. The turnover time of the bay, based on a hydraulic model (USACE 1959) 
and a numerical hydrodynamic model (6) (7) is about 30 days. 
Phytoplankton populations in Narragansett Bay are characterized by a 
winter-spring diatom bloom, followed by multiple blooms of flagellates, 
diatoms, and micro-flagellates in the summer. There is considerable 
year-to-year variation in the occurrence and timing of the various blooms. 
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