Population biomass, feeding, respiration and growth rates, and carbon budget of the scyphomedusa Aurelia aurita in the Inland Sea of Japan

Abstract

We investigated the seasonal occurrence, wet : dry : carbon : nitrogen weight ratios, population biomass, gastric pouch contents, and rates of feeding, growth and respiration of the scyphomedusa Aurelia aurita in the central part of the Inland Sea of Japan. Aurelia aurita medusae began to appear in January/ February as ephyrae, reached annual maximum body size in July/August, and disappeared, presumably due to death, by November. Initial slow growth in early spring was followed by a period of exponential growth (mean growth rate: 0.069 d-1) between April and July. In the Ondo Strait, which is characterized by strong tidal mixing, the A. aurita population (mean carbon biomass: 66.0 mg C m-3) overwhelmingly dominated the zooplankton-community biomass (mean biomass of micro- and mesozooplankton: 23.7 mg C m-3) between May and early August The gastric content analysis revealed that A. aurita ate almost all micro- and mesozooplankters, of which small copepods were most important. On the basis of digestion time for small copepods (60 min) and their abundance in the gastric pouch of field-collected A. aurita, we determined the weight specific feeding rates and clearance rates. The former increases linearly with increasing copepod abundance, but the latter was relatively constant irrespective of the food supply. We also measured the respiration rates of A. aurita and expressed them as functions of body weight and temperature. These physio-ecological parameters enabled us to construct the carbon budget of the A. aurita population typical of early summer in the Ondo Strait. Predicted population-feeding rate (6.07 mg C m-3 d -1) was higher than the population-food requirement for both metabolism and growth (4.55 mg C m-3 d-1), indicating that food supply was sufficient to sustain the observed growth rate. This feeding rate was equivalent to 26% ofmicro-and mesozooplankton biomass, a significant impact on zooplankton. © Oxford University Press 2005; all rights reserved.