Food-limited reproduction and growth of three copepod species in the low-salinity zone of the San Francisco Estuary

Abstract

We determined reproductive and growth rates of three common copepods in the low-salinity zone of the San Francisco Estuary during spring–summer of 2006 and 2007. Rates were low, particularly during summer. The egg production rate of Eurytemora affinis in spring averaged ∼3 eggs female−1 day−1 or 0.04 day−1, while that of Pseudodiaptomus forbesi in summer averaged ∼1 egg female−1 day−1 or 0.02 day−1. Specific growth rates of copepodites were moderate for E. affinis in spring (0.23 and 0.15 day−1 for early and late stages, respectively) and low for P. forbesi in summer (0.15 and 0.03 day−1, respectively). Growth and egg production rates of both species were generally lower than rates predicted from temperature for well-fed copepods, suggesting chronic food limitation. Previously published estimates for the small cyclopoid Limnoithona tetraspina were also low. None of the measures of growth of any species was related to phytoplankton biomass, primary production or abundance of the species, nor did they differ between the 2 years despite large differences in hydrology. To understand patterns of abundance will require investigation of differential mortality rates.