Incubation duration effects on copepod naupliar grazing estimates

- Long incubations promote increasingly artificial prey and grazing rate estimates.
- Ingestion rate estimates decrease by up to 75% between 6 and 24 h incubations.
- Short incubations minimize deviation from natural prey and minimize cascade effects.

Copepod naupliar grazing estimates often appear to represent an insignificant fraction of prey community mortality, despite high naupliar abundances and weight-specific ingestion rates. To address this seeming paradox, the impact of incubation time on grazing by nauplii of the subtropical copepod Parvocalanus crassirostris on natural prey assemblages was evaluated. Measurements of prey removal during feeding experiments were taken every 6-h over a 24-h period during two experiments (E1, E2), where the initial 2-35 μm natural prey biomass differed by 3-fold, i.e., 66 μg C L− 1 (E1) vs. 198 μg C L− 1 (E2). Results showed that total prey ingestion rate estimates decreased over the course of incubation by up to 75% after 24-h, with highest ingestion rates obtained during the initial 6-h. In no predator controls, total prey biomass also decreased significantly during the 24-h incubation particularly in the smallest prey size groups despite the absence of nauplii, and positive non-significant trends were observed in prey > 10 μm in the experiment with higher initial prey abundances. Predator density effects in this experiment provided additional evidence for trophic cascades. Results of this work suggest that in communities with rapidly changing natural prey assemblages and predators with short development times, short incubations minimize bottle effects and reduce the risk of measuring grazing rates on prey communities that no longer resemble the in situ populations.