PRODUCTION OF TWO PLANKTONIC CARNIVORES (CHAETOGNATH 

 AND CTENOPHORE) IN SOUTH FLORIDA INSHORE WATERS' 



M. R. Reeve and L. D. Bakers 



ABSTRACT 



Seasonal changes in biomass and production of two planktonic carnivores, Sagitfa hispida Conant and 

 Mnemiopsis mccradyi Mayer, were followed in a subtropical inshore marine environment. Production 

 was estimated as the product of mean daily biomass (calculated from the sampled biomass and 

 computed mortality rates) and daily growth rate. The latter was determined from laboratory culture 

 experiments at three temperatures. Seasonal fluctuations of ctenophore biomass and production were 

 much greater than those of chaetognaths. Mean daily production in milligram carbon per square meter 

 was 2.00 and 4.80 for Sagitta in Card Sound and Biscay ne Bay respectively, and 1.01 for Mnem iopsis in 

 Biscayne Bay. The ctenophore was absent from Card Sound, possibly because the zooplankton standing 

 crop was an order of magnitude lower than in Biscayne Bay (excluding ctenophores). Average produc- 

 tion/biomass ratios were 0.31 for Sagitta and 0.12 for Mnemiopsis. 



Most production data for zooplankton are re- 

 stricted to the herbivorous copepods in temperate 

 and cold w^aters (see review of MuUin 1969; Mullin 

 and Brooks 1970; Riley 1972). Estimates for car- 

 nivores are very few and include Sagitta elegans 

 (McLaren 1969; Zo 1969; Sameoto 1971) and 

 Pleurobrachia bachei (Hirota 1974). 



As pointed out by Mullin (1969) there is no 

 simple technique for the measurement of produc- 

 tion of natural populations of zooplankton com- 

 parable to the relatively routine '*C uptake method 

 for the determination of primary production by 

 phytoplankton. Unlike the phytoplankton, which 

 share a common characteristic of a single trophic 

 level, zooplankton extend over at least two trophic 

 levels, and an individual species may vary its 

 trophic status on the basis of food availability or 

 life history stage. In addition, zooplankton range 

 in size from 20ju.m or less to 20 cm or more and have 

 widely differing growth and reproduction rates. 

 Attempts to measure total zooplankton production 

 have been made, especially where a single species 

 dominates the population over a period (e.g., 

 Gushing and Vucetic 1963) or where a single group 

 (such as copepods) dominates and is treated as a 

 unit (e.g., Riley 1972), and most recently by relat- 

 ing respiration to temperature and body weight 



' Contribution from the Rosenstiel School of Marine and At- 

 mospheric Science, University of Miami, Miami, FL 33149. 



' Division of Biology and Living Resources, Rosenstiel School 

 of Marine and Atmospheric Science, University of Miami, Miami, 

 FL 33149. . y , , 



and applying these data to the plankton biomass 

 of the Kuroshio (Ikeda and Motoda in press). 



The data reported below are based on the in- 

 dividual species approach, using experimentally 

 determined growth rates to compute production 

 from environmental biomass estimates for two 

 planktonic carnivores, widely separated 

 phylogenetically but dependent upon the same 

 source of food. 



STUDY SITE AND SAMPLING 

 METHODS 



The study area consisted of Biscayne Bay and 

 Card Sound which form part of an extensive sys- 

 tem of shallow, warm, semiestuarine, and 

 semienclosed interconnected water bodies typical 

 of the coastal region of a large part of Florida. 

 Zooplankton sampling programs were conducted 

 at 4 stations on 28 dates throughout 1971 in Card 

 Sound and at 11 stations on 26 dates from October 

 1970 to February 1972 in central Biscayne Bay. 

 Detailed reports of these programs were given by 

 Reeve and Cosper (1973) and Baker (1973), respec- 

 tively. 



In both locations, surface tows were made with a 

 metered, Vz-m mouth diameter net of 200-/im 

 mesh. In addition, a similar net of 64-jU,m mesh was 

 used in Card Sound. In Biscayne Bay, a 1-m, 705- 

 ju,m mesh net with a 14-liter flexible, vinyl cod end 

 was employed to collect ctenophores. It was not 

 used routinely in Card Sound because ctenophores 



Manuscript accepted June 1974. 



FISHERY BULLETIN: VOL. 73, NO. 2, 1975. 



238 



