REEVE and BAKER: PRODUCTION OF A CHAETOGNATH AND CTENOPHORE 



differences in plankton biomass are accompanied 

 by differences in biomass for both Sagitta and 

 Mnemiopsis. In surface net tows from the 200-yum 

 mesh, the biomass of the chaetognath in Biscayne 

 Bay is 2.4 times that in Card Sound. The 

 ctenophore is totally absent from Card Sound (ex- 

 cept for rare isolated individuals). Baker (1973), 

 relating stations with low plankton standing stock 

 to low ctenophore levels in Biscayne Bay, sug- 

 gested that the Card Sound plankton could not 

 support a ctenophore population. 



Since the waters of Card Sound are contiguous 

 with those of Biscayne Bay to the north, and neri- 

 tic waters to the east, where ctenophores are often 

 abundant, the phenomenon of their exclusion from 

 Card Sound can hardly be a physical one. A pos- 

 sibility is that chaetognaths are more efficient in 

 collecting food at lower densities than are 

 ctenophores. 



It is of interest that the seasonal variations of 

 biomass and production of the ctenophore popula- 

 tions both in Biscayne Bay and off California are 

 extreme, to the extent that in both cases the 

 months of peak production account for about two- 

 thirds of the annual total. In the case of S. hispida 

 this value is about one-fifth. There is probably 

 some correlation between this extreme population 

 instability of M. mccradyi and the suggestion 

 above that its absence from Card Sound is related 

 to its inefficiency in collecting food at low concen- 

 trations compared to S. hispida. The dry weight of 

 other zooplankton from the 200-ju.m mesh net in 

 Card Sound (Reeve and Cosper 1973) never ex- 

 ceeds the minimum value in central Biscayne Bay 

 (Baker 1973). 



It is possible to get a rough estimate of the 

 relationship between production of S. hispida and 

 M. mccradyi and the rest of the zooplankton by 

 utilizing the standing stock data for that period in 

 the two reports referred to immediately above. 

 The mean annual dry weight of zooplankton 

 (excluding ctenophores and corrected for detritus) 

 was 2.02 and 5.28 mg/m' in the 200- and 64-/xm 

 mesh net respectively in Card Sound and 19.8 

 mg/m^ in the 200-ju,m mesh net in central Biscayne 

 Bay. Assuming the ratio between 64- and 200- 

 mesh plankton in Card Sound is applicable to Bis- 

 cayne Bay, and the ash- free dry weight is the same 

 percentage of dry weight as determined for S. 

 hispida, the mean annual ash-free dry weight in 

 Card Sound and central Biscayne Bay was 6.62 and 

 64.9 mg/m^ respectively. Since it appears that 



even the youngest larvae of Mnemiopsis and 

 Sagitta do not utilize food organisms much smaller 

 than those retained by the 64-/im mesh, and since 

 neither carnivore appears to be able to utilize 

 other sources of potential food such as detritus or 

 phytoplankton (Reeve and Walter 1972; Baker and 

 Reeve 1974), the plankton biomass quoted above is 

 the only source of nutrition for these carnivores. If 

 a production /biomass ratio the same as that de- 

 termined for S. hispida is applied to these biomass 

 figures, the net production available to these car- 

 nivores is 2.05- and 20.1-mg ash-free dry 

 weight/m' per day. Since these figures are for 

 surface waters, they may be related to the 

 equivalent values for Sagitta and Mnemiopsis 

 derived earlier. The daily net production of S. his- 

 pida in Card Sound and Biscayne Bay is then 47.7 

 and 11.7% of the production of potential food in 

 those areas. For M. mccradyi in Biscayne Bay it 

 was 9.5 or 19.2% depending on which production 

 peak was included (see above). The total percen- 

 tage for the two species is then 47.7 for Card Sound 

 and 21.2 or 30.9 for central Biscayne Bay. If the 

 ratio of production to food ingested is taken to be 

 50% on the basis that immature animals are re- 

 sponsible for most of the production, and would 

 have higher growth efficiencies than the 30-40% 

 range for adults quoted by Reeve (1972), then the 

 chaetognaths in Card Sound appear to utilize all 

 the rest of the zooplankton above 64 ^m. For Bis- 

 cayne Bay, the chaetognaths and ctenophores 

 together utilize between 40 and 60% of the avail- 

 able food. As explained earlier, these are overes- 

 timated because the growth rates were maximum 

 growth rates, but they do support the contention 

 that there is little potential food reserve in Card 

 Sound for other carnivores, and that Sagitta is 

 more efficient in competing for the available sup- 

 ply. This is in agreement with the fact that in Card 

 Sound, its population was as high as 42% of that in 

 central Biscayne Bay, while for larger decapod 

 larvae, fish larvae, and ctenophores (the other 

 major first-order plankton carnivores) the values 

 were approximately 25, 25 (see Reeve in press), 

 and 0%. 



ACKNOWLEDGMENTS 



We are grateful to Michael M. Mullin and Jed 

 Hirota for reading this manuscript and for the 

 support of National Science Foundation Grant 

 GA-28522X (Biological Oceanography). 



247 



