the microcosms. To some extent this may reflect the fact that the floating pair 

 measurements and dye patches respond to the horizontal turbulent field and 

 the gypsum and gas exchange are also influenced by vertical motion. The 

 rotating plastic paddles appeared to add a lot of horizontal mixing energy to 

 the tanks, but the vertical eddy diffusivity in the microcosms was lower than 

 Hess (1976) calculated for Narragansett Bay (Table 25-1). 



While none of these measurements allows us to make a very convincing 

 absolute comparison of turbulent energy in the microcosms with that of the 

 bay, it does seem clear that the full paddle, half paddle, no paddle 

 configuration provided quite different turbulent water regimes in the 

 microcosms. Since the input of turbulent energy to Narragansett Bay must Vary 

 considerably during the tidal cycle and from day-to-day according to the 

 winds, it seems reasonable that the natural pelagic community may well 

 experience all of the turbulent conditions used in the microcosms. For 

 comparative purposes, it is interesting to note that all of the methods used for 

 measuring turbulence except the determination of neighbor diffusivity and 

 energy flux (e) indicated that even the full paddle configuration was low 

 relative to the bay. 



Response of the Plankton 



The first turbulence experiment was carried out during the month of April 

 when water temperatures in the microcosms ranged from 8 to 12^C. The 

 standing crop of phytoplankton as indicated by chl a increased dramatically in 

 the one paddle and half paddle treatments compared with the unstirred tanks 

 (Figure 25-1). A number of cursory analyses of water samples did not indicate 

 that there were any major shifts in species composition in the different tanks. 

 However, there were also marked and significant differences (Perez et al 1977) 

 among treatments in the numbers oi Acartia clausi, the dominant zoo plankton 

 in the microcosms and in the bay (Figure 25-2). While the rapid increase in 

 phytoplankton in the one paddle tanks began almost immediately, Acartia 

 nauplii did not really start to decHne until after 10 days. In fact, a portion of 

 the decUne in nauplii between 10 and 16 days was simply due to growth of the 

 animals into juveniles (Figure 25-2). An analysis of covariance was performed 

 to estabhsh whether the changes in phytoplankton density could be attributed 

 to changes in zooplankton density the covariate, total grazers was found to be 

 non-significant. This meant that the inverse relationship expressed by 

 zooplankton and phytoplankton to water turbulence was due to a direct 

 pattern than the indirect effect of water turbulence. In fact, an analysis of 

 covariance on the mean algal standing crop during the experiment indicated 

 that interactions with the total numbers of grazers in the microcosms (the 

 covariate) was not significant (Perez et al 1977). It is possible, however, that 

 the zooplankton present did not feed as effectively in the more turbulent 



395 



