Each microcosm also contained an opaque plastic box (167 cirr) of intact 
bay sediment and associated benthos. This size produced the same sediment 
surface to water volume ratio as found in the bay. Water from the pelagic phase 
of the microcosm was moved through the box and over the sediment by 
vacuum pump so as not to damage the plankton. The inside walls of the tanks 
were cleaned regularly to prevent fouling, and organic matter settling on the 
bottoms of the tanks was collected and placed in the sediment 6oxes. In all of 
the turbulence experiments, it is important to note that the artificial nature of 
the “bottom” community isolated it from the turbulent energy of the water, 
except as the benthos might respond to changes in the plankton. However, 
even in tanks that were unstirred, the benthic box pumps provided some very 
gentle circulation for the pelagic community, since the flow rate used was 
capable of putting 150 liters of water through the box about three times each 
day. Additional mixing was contributed by the approximately daily wall 
cleaning and by the addition of 10 liters of bay water to each tank three times 
each week. The latter was maintained so that the microcosms functioned as 
open systems with a flushing rate similar to that of Narragansett Bay. 
Different turbulence levels were imposed on the microcosms by leaving 
them unstirred except for the benthic pump and cleaning operations or by 
mixing them with plastic mesh paddles of 0.14 or 0.07 m area. The 
opening size of the plastic grid in the paddles was 1.2 cm x 1.2 cm. The paddles 
were driven at 32 rpm by an electric motor connected to all the paddle shafts 
by a chain, thus producing identical rotation rates in all of the tanks. Each 
paddle was rotated in one direction for 30 sec., then stopped for 6 sec., then 
reversed for 30 sec. in a continuous cycle. 
Turbulence Measurements 
Vertical Eddy Diffusivity 
We attempted to obtain a variety of both relative and absolute 
measurements of turbulent mixing in the microcosms and in Narragansett Bay. 
In some cases, such as the estimation ol a vertical eddy diffusivity, the 
techniques used were conventional. A small amount (~2 ml) of Rhodamine-WT 
dye was dissolved in sea water (1:100) and released “instantaneously” at 
mid-depth in the microcosms. Near-surface and near-bottom water samples 
were then collected at short intervals (2-5 min.) and the concentration of dye 
determined fluorometrically. The rate-of-change in concentration in both sets 
of samples was virtually identical, indicating that the tanks were mixed 
uniformly up and down, and that the vertical eddy diffusivity could be 
estimated by 
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