Each microcosm also contained an opaque plastic box (1 67 cm ) 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 1 50 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 1 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 bentliic pump and cleaning operations or by 

 mixing them with plastic mesh paddles of 0.14 m 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 of 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. Tlie 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 



389 



