We investigated biomass, size-structure, composition, depth distributions and spatial variability of the phytoplankton community in the Costa Rica Dome (CRD) in June-July 2010. Euphotic zone profiles were sampled daily during Lagrangian experiments in and out of the dome region, and the community was analyzed using a combination of digital epifluorescence microscopy, flow cytometry and HPLC pigments. The mean depth-integrated biomass of phytoplankton ranged 2-fold, from 1089 to 1858 mg C m (mean ± SE = 1378 ± 112 mg C m), among 4 water parcels tracked for 4 days. Corresponding mean (±SE) integrated values for total chlorophyll (Chl) and the ratio of autotrophic carbon to Chl were 24.1 ± 1.5 mg Chl m and 57.5 ± 3.4, respectively. Absolute and relative contributions of picophytoplankton (∼60%), (>33%) and (17%) to phytoplankton community biomass were highest in the central dome region, while>20 µm phytoplankton accounted for ≤10%, and diatoms <2%, of biomass in all areas. Nonetheless, autotrophic flagellates, dominated by dinoflagellates, exceeded biomass contributions of at all locations. Order-of-magnitude discrepancies in the relative contributions of diatoms (overestimated) and dinoflagellates (underestimated) based on diagnostic pigments relative to microscopy highlight potential significant biases associated with making community inferences from pigments.

Marine debris is a worldwide ocean pollution problem and evidence suggests that the tiniest pieces of plastic in the ocean, microplastics, may be the most abundant forms of plastic pollution in the ocean; however, the smallest microplastics have yet to be accurately quantified due to methodological limitations. We successfully developed and tested a new method for collecting and counting the smallest microplastic pieces in seawater and ingested inside the guts of salps, a planktonic species at the base of food webs and key to transport of carbon and particles from the sea surface to the deep sea. We determined that the true abundance of these tiniest microplastics far outnumber previously reported counts, and that every salp we examined had ingested plastic. AbstractMicroplastics (< 5 mm) have long been a concern in marine debris research, but quantifying the smallest microplastics (< 333 μm) has been hampered by appropriate collection methods, like net tows. We modified standard epifluorescence microscopy methods to develop a new technique to enumerate < 333 μm microplastics (minimicroplastics) from filtered surface seawater samples and salp stomach contents. This permitted us to distinguish mini-microplastics from phytoplankton and suspended particles. We found seawater mini-microplastic

We investigated biomass and composition of heterotrophic microbes in the Costa Rica Dome during June-July 2010 as part of a broader study of plankton trophic dynamics. Because picophytoplankton (<2 μm) are known to dominate in this unique upwelling region, we hypothesized tight biomass relationships between size-determined predator-prey pairs (i.e. picoplankton-nano-grazers, nanoplankton-micro-grazers) within the microbial community. Integrated biomass of heterotrophic bacteria ranged from 180 to 487 mg C m and was significantly correlated with total autotrophic carbon. Heterotrophic protist (H-protist) biomass ranged more narrowly from 488 to 545 mg C m, and was comprised of 60% dinoflagellates, 30% other flagellates and 11% ciliates. Nano-sized (<20 μm) protists accounted for the majority (57%) of grazer biomass and were positively correlated with picoplankton, partially supporting our hypothesis, but nanoplankton and micro-grazers (>20 μm) were not significantly correlated. The relative constancy of H-protist biomass among locations despite clear changes in integrated autotrophic biomass, Chl , and primary production suggests that mesozooplankton may exert a tight top-down control on micro-grazers. Biomass-specific consumption rates of phytoplankton by protistan grazers suggest an instantaneous growth rate of 0.52 day for H-protists, similar to the growth rate of phytoplankton and consistent with a trophically balanced ecosystem dominated by pico-nanoplankton interactions.