Development and Evaluation of Tracer Particles for Use in Microzooplankton Herbivory Studies

Two methods of preparing algae for use as tracer particles in single species measurements of microzooplankton herbivory were evaluated. Algae were either heat-killed and labelled with 5-(4,6-dichlorotriazin-2-yl amino fluorescein) (DTAF) (Rublee & Gallegos 1989; Mar, Ecol. Prog. Ser. 51: 221-227) or stained with hydroethidine (HYD). Both DTAF and HYD-stained algae were readily visible within the digestive vacuoles of most microzooplankton collected in estuarine and coastal waters of Massachusetts (USA) and preserved with Lugol\u27s iodine. However, DTAF was ineffective at staining several chromophytic algae and the heat-kill process reduced cell volume by ≥ 50% in several of the algae which were effectively stained. HYD effectively stained all algae tested except chlorophytes. Staining with HYD had negligible effects on cell size or morphology but reduced photosynthesis in Isochrysis galbana by about 95%. Ingestion rates of field populations of ciliates differed by up to 25-fold for 2 algal species (I. galbana and Pyramimonas sp.). For a ciliate (Strombidinopsis sp.) which ingested Pyramimonas sp. at a high rate relative to I. galbana, ingestion rates for live HYD-stained Pyramimonas sp. were about twice as high as for the same alga heat-killed and stained with DTAF. In cultures of Strobilidium sp., ingestion of HYD-stained and untreated I. galbana radiolabelled with 14C were similar at concentrations up to 1.6 x 104 cells ml-1. HYD dissolved in seawater had negligible effects on ciliate grazing rate at concentrations up to 0.29-μg ml-1. HYD-stained algae are a new tool for quantifying microzooplankton herbivory which should be particularly useful in examining feeding preferences among field populations