Microzooplankton grazing experiments in the subtropical Indian River Lagoon, Florida challenge assumptions of the dilution technique

• 24 microzooplankton grazing experiments were conducted in a subtropical estuary.
• Positive slopes were observed in the majority of dilution experiments.
• Evidence suggested that trophic cascades caused an assumption violation.
• More studies of trophic dynamics in estuarine plankton communities are critical.

The most widely accepted method for measuring microzooplankton grazing rates, the dilution technique, was developed in oligotrophic, open-ocean environments. Applications of the technique in productive waters have resulted in varied responses that often complicate the interpretation of results in light of potential violations of assumptions. Grazing experiments using the dilution technique were carried out for 12 months at two sites in the Indian River Lagoon (IRL), Florida, an underrepresented ecosystem type with regard to global knowledge of microzooplankton grazing. Positive slopes were observed in the majority of experiments implying that grazing activity actually enhanced phytoplankton growth. In two such experiments, plankton community analysis supported the hypothesis that grazer abundances were not constant over the incubation period and net grazer growth rates were related to dilution level, i.e., an important assumption was violated. Trophic cascades (i.e., grazers eating grazers) are the most likely explanation for these observations. The frequency of positive slopes found in this study highlights the need for more detailed studies of trophic interactions within plankton communities of the IRL, and estuarine systems in general. The IRL has experienced persistent harmful algal blooms, the causes of which are currently under investigation; however, the impact of microbial grazers on harmful algal species cannot be quantified until appropriate methods for resolving complex trophic interactions are identified.