Food limitation and growth in temperate epipelagic appendicularians (Tunicata)

Abstract

Through an extensive review of reports on appendicularian ecology, we have developed a set of equations on the feeding, metabolic and growth rates of temperate epipelagic appendicularian species. We have used these equations to study the conditions in which these species are likely to ex- perience food-limited growth. As Oikopleura dioica is the most-studied appendicularian species, we have used equations developed for this one species to construct a metabolic budgetary model. Our re- sults suggest that large O. dioica are less likely to experience food-limited growth than other meso- zooplankton, and that growth during early development could be more limited by food concentration in the environment. The degree of food limitation strongly depends on the assimilation efficiency of non-autotrophic material, which comprises a great proportion of the appendicularian diet. However, examination of direct measurements of growth rate over a wide range of food concentrations showed no significant relationship between food concentration and weight-specific growth rates. Temperature alone explained more than 60% of the variance in appendicularian weight-specific growth rates. Since there were no clear differences in the growth rates of different appendicularian species, we have com- bined the available data to develop a temperature-dependent equation to predict their weight-specific growth rates. We have also modified the growth rate equation to take into consideration the expendi- ture in house secretion. Combination of this growth rate equation with biomass estimations allows us to evaluate their contribution to secondary production. In contrast with the traditional view of appen- dicularians as important members of oligotrophic ecosystems, our results show that in the study area their contribution to secondary production was higher during more productive conditions.