R. R. Hopcroft á J. C. Ro áM. K. Webber á J. D. S. Witt Zooplankton growth rates: the in¯uence of size and resources in tropical marine copepodites Received: 30 May 1997 / Accepted: 13 May 1998 AbstractGrowth rates were determined for copepodites of the genera: Acartia, Centropages, Corycaeus, Oithona, Paracalanus, Parvocalanusand Temora in nearshore waters of Jamaica from in situ microcosm incubations. At these high localtemperatures (28 °C), total co- pepodite development time was as short as 4 to 5 d. Mean instantaneous growth rates (g) ranged from as high as 1.2 d )1 to as low as 0.1 d )1 . In general, cyclopoid copepods appeared to grow more slowly than calanoids of the same size. Enhancement of resources by nutrient addition caused a 32% increase in growth rates in experiments from a mesotrophic site, but only a 17% increase ata more eutrophic site. Additionally,cope- poditesat both sitesshowed faster development and generally larger size at stage in response to nutrient addition.Growth rates were positively related to chlo- rophyll concentration in the >2 lm size-fraction. A signi®cant relationship ofgrowth rateto body size (r 2  0.45) emerged across a wide rangeof trophic status, but it was confounded with resource availability. It appears that growth in tropical copepod copepodites may be frequently limited by resources in a size-depen- dent manner. Introduction Our understanding of the functioning of aquatic eco- systems will remain incomplete without detailed know- ledge of rates of processes (Longhurst 1984). Although it has been argued that accurate, spatially representative determination of biomass is currently a more important concern (e.g. Huntley and Lopez 1992), measurements of biomass are constantly improving, and promise to becomeroutinefor somegroupsof organisms.The major block to knowledge of rates of energy transfer has now (or will) become ourunderstanding of ratesof growth. The search for algorithms to predict growth rates of secondary producers in aquatic communities has been persistent; attention has focused on invertebrates, pri- marily arthropod groups, in both marine and freshwater ecosystems, resulting in broad empirical relationships. Growth,or development time, has been related to en- vironmentaltemperature (e.g. McLaren and Corkett 1981; Huntley and Lopez 1992), resource concentration (e.g. Vidal 1980a, b; Berggreen et al. 1988) and to body size (e.g. Banse and Mosher 1980), or to some combi- nation of these variables or their surrogate measures (e.g. Plante and Downing 1989). Empirical models that describe growth rates generally include only one variable, and may be at odds with other possibilities in their theoretical foundations. For exam- ple, Be leÂhraà dek'sfunction assumes that food concen- tration is not limiting, and that development is isochronal,i.e. implicitly thatgrowth rateis not a function of body size. In a proposed global relationship, Huntley and Lopez (1992) argued thatemperature is the major determinant of growth rate, that body size is irrelevant (thus by inference that development is isoch- ronal)and thatfood is never limiting in nature. The generality of isochronal development, however, has been challenged (Landry 1983; Hart 1990),and growth rate variations appear to be related to developmental stage (e.g. Hart 1990) and body size (Webber and Ro 1995b); Marine Biology (1998) 132: 67±77 Ó Springer-Verlag 1998 Communicated by R.J. Thompson, St. John's R.R. Hopcroft (&) 1 áJ.C. Ro áJ.D.S. Witt Department of Zoology, University of Guelph, Guelph, Ontario, N1G 2W1, Canada M.K. Webber Zoology Department, University of the West Indies, Kingston 7, Jamaica, West Indies Present address: 1 Monterey Bay Aquarium Research Institute, 7700 Sandholt Rd., Moss Landing, California 95039-0628, USA