Daily oscillations in photosynthetically active radiation strongly influence the timing of metabolic processes in picocyanobacteria, but it is less clear how the light-dark cycle affects the activities of their consumers. We investigated the relationship between marine picocyanobacteria and nanoplanktonic consumers throughout the diel cycle to determine whether heterotrophic and mixotrophic protists (algae with phagotrophic ability) display significant periodicity in grazing pressure. Carbon biomass of Prochlorococcus and Synechococcus was estimated continuously from abundances and cell size measurements made by flow cytometry. Picocyanobacterial dynamics were then compared to nanoplankton abundances and ingestion of fluorescently labeled bacteria measured every 4 h during a 4 d survey in the North Pacific Subtropical Gyre. Grazing of the labeled bacteria by heterotrophic nanoplankton was significantly greater at night than during the day. The grazing activity of mixotrophic nanoplankton showed no diel periodicity, suggesting that they may feed continuously, albeit at lower rates than heterotrophic nanoplankton, to alleviate nutrient limitation in this oligotrophic environment. Diel changes in Prochlorococcus biomass indicated that they could support substantial growth of nanoplankton if those grazers are the main source of picocyanobacterial mortality, and that grazers may contribute to temporally stable abundances of picocyanobacteria.

The abundance of non-testate ('naked'), free-living amoebae was examined in surface waters of the Black Sea during 5 cruises in 1992 and 1993. Amoebae (520 pm in size) were observed in samples collected during 2 of the 5 cruises at abundances up to approximately 380 ml-l The abundance and biomass of amoebae were low relative to other nanoplanktonic protozoa when averaged over all stations, but amoebae occasionally constituted a major component of the heterotrophic nanoplanktonic assemblage (up to 16% of abundance and 93% of biomass of the protozoa 2 to 20 pm in size). Abundances of amoebae correlated weakly with the total number of heterotrophic nanoplankton, and with ash free dry weight, but not with several other physical, chemical and biological parameters. Based on the highest abundances of naked amoebae observed in this study we suggest that these protozoa can occasionally be important in energy and elemental cycling in Black Sea plankton.

Marine ecosystem models often consider temporal dynamics on the order of months to years, and spatial dynamics over regional and global scales as a means to understand the ecology, evolution, and biogeochemical impacts of marine life. Large-scale dynamics are themselves driven over diel scales as a result of light-driven forcing, feedback, and interactions. Motivated by high-frequency measurements taken by Lagrangian sampling in the North Pacific Subtropical Gyre, we develop a hierarchical set of multitrophic community ecology models to investigate and understand daily ecological dynamics in the near-surface ocean including impacts of light-driven growth, infection, grazing, and phytoplankton size structure. Using these models, we investigate the relative impacts of viral-induced and grazing mortality for Prochlorococcus; and more broadly compare in silico dynamics with in situ observations. Via model-data fitting, we show that a multi-trophic model with size structure can jointly explain diel changes in cyanobacterial abundances, cyanobacterial size structure, viral abundance, viral infection rates, and grazer abundances. In doing so, we find that a significant component (between 5% to 55%) of estimated Prochlorococcus mortality is not attributed to either viral lysis (by T4- or T7-like cyanophage) or grazing by heterotrophic nanoflagellates. Instead, model-data integration suggests a heightened ecological relevance of other mortality mechanisms -- including grazing by other predators, particle aggregation, and stress-induced loss mechanisms. Altogether, linking mechanistic multitrophic models with high-resolution measurements provides a route for understanding of diel origins of large-scale marine microbial community and ecosystem dynamics.