MEPS 611:45-58 (2019)  -  DOI: https://doi.org/10.3354/meps12852

ABSTRACT: Population dynamics of bloom-forming dinoflagellates are regulated both by environmental factors and trophic interactions such as competition and grazing. We investigated trophic interactions of the mixotrophic red tide dinoflagellate Lingulodinium polyedrum in coastal waters off southern California, USA. We conducted 2 laboratory experiments using a natural plankton community in which we manipulated the presence of potential competitors/prey (<20 µm) and the presence of zooplankton consumers (<110 µm) through size fractionation. We further tested the grazing impact of the heterotrophic dinoflagellate Noctiluca scintillans on L. polyedrum in both size fractions containing natural plankton and in a final experiment using a gradient of different L. polyedrum concentrations in culture. Overall, our experiments demonstrated that nanoplankton presence negatively affected L. polyedrum abundances, indicating that the mixotrophic dinoflagellate could not benefit from the presence of potential prey. L. polyedrum was strongly controlled by N. scintillans in the natural plankton community, while natural microzooplankton, mainly consisting of tintinnids and copepod nauplii, were positively correlated with L. polyedrum abundances. This could indicate an indirect positive effect of natural microzooplankton on L. polyedrum, presumably through feeding on other phytoplankton, resulting in the dinoflagellate’s competitive release. The final experiment investigating the density-dependence of N. scintillans grazing on L. polyedrum demonstrated that N. scintillans is able to increase its population size and control the growth of L. polyedrum after an initial lag phase, and up to a certain cell concentration, and may thus play an important role in bloom regulation and possibly even termination.

KEY WORDS: Lingulodinium polyedrum · Harmful dinoflagellates · Bloom dynamics · Grazing control · Noctiluca scintillans · Southern California