Size-related patterns in growth rate and production of tropical marine planktonic communities along a trophic gradient

The size-dependence of growth rates and possible ramifications on structure and regulation of planktonic communities were investigated in tropical marine ecosystems. Manipulations of nutrient, predator and prey concentrations were conducted using 21 litre microcosms in oligotrophic, mesotrophic and eutrophic waters surrounding Jamaica. Responses of size-fractionated chlorophyll, copepods and larvaceans were followed over 5 days. In all environments, there was a pronounced increase of chlorophyll in response to addition of nutrients, response being greatest in oligotrophic waters, and in the netplankton ({dollar}>{dollar}20 {dollar}mu{dollar}m) size-fraction. Microcosms not receiving nutrients showed a decline in chlorophyll as a function of cell-size and grazing pressure. Sedimentation was most pronounced in the netplankton, and insignificant in the picoplankton ({dollar}{lcub}<{rcub}2 mu{dollar}m). Grazing effects were less pronounced than nutrient effects. Phytoplankton biomass was more strongly controlled by nutrient supply (bottom-up) than by metazoan zooplankton grazing in areas of lower productivity, but may be strongly controlled by grazing (top-down) in areas of higher productivity.; Growth rates of copepods and larvaceans were also size-dependent. Smaller species and/or earlier developmental stages displayed higher growth rates than larger species and/or later developmental stages. This pattern arises due to interaction of both intrinsic effects of size and progressive growth limitation by resources, with increasing size. Within the copepods, naupliar growth rates appear relatively uninfluenced, copepodites were mildly influenced, and egg production was highly influenced by resource concentration. Diel patterns were observed in molting and copepod egg production. Larvacean instantaneous growth rates averaged {dollar}sim{dollar}2 d{dollar}sp{lcub}-1{rcub},{dollar} more than 3 times those of the average copepod. Applying these growth rates to a monitoring program in outer Kingston Harbour, secondary productivity was estimated as 1679 kJ {dollar}rm msp{lcub}-2{rcub} ysp{lcub}-1{rcub}{dollar} for the copepods and 544 kJ {dollar}rm msp{lcub}-2{rcub} ysp{lcub}-1{rcub}{dollar} for the larvaceans (excluding larvacean house production). Secondary production was 681 kJ {dollar}rm msp{lcub}-2{rcub} ysp{lcub}-1{rcub}{dollar} for the copepods and 323 kJ {dollar}rm msp{lcub}-2{rcub} ysp{lcub}-1{rcub}{dollar} for the larvaceans at mesotrophic Lime Cay, and 304 kJ {dollar}rm msp{lcub}-2{rcub} ysp{lcub}-1{rcub}{dollar} for the copepods in oligotrophic offshore waters. These production estimates indicate that the importance of tropical waters and larvaceans in global estimates of secondary production has been previously under-appreciated.