Grazing by ciliates and heterotrophic nanoflagellates on picocyanobacteria in Lago Maggiore, Italy

During the oligotrophication of Lago Maggiore, picocyanobacteria (Pcy) increased in abundance and production. In their bimodal, seasonal cycle, the spring peak was due almost exclusively to single cells of Pcy, whereas in late-summer/autumn the variety of morpho-types increased and larger Aphanothece-like rods appeared. Rates of Pcy cell removal by heterotrophic nanoflagellates and ciliates were measured by using fluorescently labelled Pcy (FLC) in four experiments performed during the Pcy population shift from small cocci to larger rods. The ciliate community appeared to be composed mainly of oligotrichs in the first two experiments, and subsequently of scuticociliates; heterotrophic nanoflagellates decreased in number from May to September, and there was a size shift which might reflect species composition change. Peritrichs emerged as the most efficient Pcy grazers. For the other ciliate groups we observed higher individual ingestion rates during the spring experiments than during those performed in late summer/autumn. The heterotrophic nanoflagellates ingested from 0.5 to3 Pcy h−1 while ciliates ingested from 18 to 80 Pcy h−1. The grazing impact of the heterotrophic nanoflagellate community ranged from 1.9 × 103 to 8 × 103 Pcy ml−1 h−1, whereas the ciliate community ingestion rate was one order of magnitude lower (0.2 × 103–0.4 × 103 Pcy ml−1 h−1). A significant inverse correlation between Pcy size and the clearance rate of heterotrophic nanoflagellates and ciliates was found. Our results indicate that protozoa are less efficient in cell uptake when the Pcy are composed of larger cells. In Lago Maggiore, the carbon flux from Pcy to protozoa decreased from 29.8 to 10.2 μg C l−1 day−1 (May and September respectively). A tentative balance on an annual basis suggests that around 80% of the carbon produced by Pcy is taken up by protozoa and channelled to metazooplankton.