Ontogenetic growth rate responses of temperate marine copepods to chlorophyll concentration and light

The relationship between growth rate and food concentration often differs between nauplii and copepodites of the same species, but the nature of these relationships varies between systems and species. The artificial cohort method was used to calculate growth rates of Acartia hudsonica and Eurytemora herdmani nauplii and copepodites in Passamaquoddy Bay, the Bay of Fundy, under a natural range of chlorophyll a concentrations and different light regimes. Of 3 predictive maximum growth rate relationships previously published, the Bělehrádek function provided the most precise maximum growth rates, while the others underestimated growth rates of both nauplii and copepodites. No growth rates were correlated with chlorophyll a concentration in any size fraction, but a wide range in growth rates, particularly for the copepodites, was observed. Growth rates of nauplii and copepodites were lower at reduced than at ambient light intensities, but the reason for this is not clear. In general, naupliar growth appears less likely to be food limited than copepodite growth rates. The lack of observed food limitation for the nauplii in this system could be due to (1) nauplii ingesting more food per unit respiration than copepodites, or (2) nauplii not having sufficient lipid reserves and, under food-limiting conditions, rather than responding to food-limiting conditions by lowering growth rates, simply dying. A lower maximum growth rate and a smaller food resource of nauplii compared to copepodites may also explain why nauplii are less susceptible to food limitation in other systems.