Abundance and Biogeography of Picoprasinophyte Ecotypes and Other Phytoplankton in the Eastern North Pacific Ocean.

Eukaryotic algae within the picoplankton size class (≤2 µm in diameter) are important marine primary producers, but their spatial and ecological distributions are not well characterized. Here, we studied three picoeukaryotic prasinophyte genera and their cyanobacterial counterparts, Prochlorococcus and Synechococcus, during two cruises along a North Pacific transect characterized by different ecological regimes. Picoeukaryotes and Synechococcus reached maximum abundances of 1.44 × 10(5) and 3.37 × 10(5) cells · ml(-1), respectively, in mesotrophic waters, while Prochlorococcus reached 1.95 × 10(5) cells · ml(-1) in the oligotrophic ocean. Of the picoeukaryotes, Bathycoccus was present at all stations in both cruises, reaching 21,368 ± 327 18S rRNA gene copies · ml(-1). Micromonas and Ostreococcus clade OI were detected only in mesotrophic and coastal waters and Ostreococcus clade OII only in the oligotrophic ocean. To resolve proposed Bathycoccus ecotypes, we established genetic distances for 1,104 marker genes using targeted metagenomes and the Bathycoccus prasinos genome. The analysis was anchored in comparative genome analysis of three Ostreococcus species for which physiological and environmental data are available to facilitate data interpretation. We established that two Bathycoccus ecotypes exist, named here BI (represented by coastal isolate Bathycoccus prasinos) and BII. These share 82% ± 6% nucleotide identity across homologs, while the Ostreococcus spp. share 75% ± 8%. We developed and applied an analysis of ecomarkers to metatranscriptomes sequenced here and published -omics data from the same region. The results indicated that the Bathycoccus ecotypes cooccur more often than Ostreococcus clades OI and OII do. Exploratory analyses of relative transcript abundances suggest that Bathycoccus NRT2.1 and AMT2.2 are high-affinity NO3 (-) and low-affinity NH4 (+) transporters, respectively, with close homologs in multiple picoprasinophytes. Additionally, in the open ocean, where dissolved iron concentrations were low (0.08 nM), there appeared to be a shift to the use of nickel superoxide dismutases (SODs) from Mn/Fe/Cu SODs closer inshore. Our study documents the distribution of picophytoplankton along a North Pacific ecological gradient and offers new concepts and techniques for investigating their biogeography.

Vertical distributions of dolphinfish (Coryphaena hippurus) in the Eastern Pacific Ocean suggest variability in potential associations with floating objects.

Floating objects play a pivotal role in pelagic ecosystems by serving as shelters, meeting points, cleaning stations, nurseries, and feeding grounds. The abundance of these objects is increasing globally in the form of flotsam, plastics, discarded or lost fishing gear, and fish aggregating devices (FADs) deployed by commercial fisheries. However, it is difficult to measure how often and in what ways fish interact with floating objects in pelagic environments. Dolphinfish (Coryphaena hippurus) is prevalent among the fish species that associate with floating objects, but the extent to which dolphinfish utilize them is unclear. This study applies existing knowledge of FAD-associated dolphinfish diving behavior to identify periods of potential association with floating objects in a remote telemetry dataset of 23 fish with a total of 678 days at liberty spanning two distinct regions within the Eastern Pacific Ocean. Fish inhabiting waters off the western coast of Baja California Sur, Mexico spent significantly more time exhibiting behavior indicative of association with floating objects than those off the coast of Oaxaca, Mexico. When not exhibiting this behavior, dolphinfish in both regions occupied similar vertical habitats, with western Baja fish utilizing more of the water column than Oaxaca fish. Observed regional differences in behavior were coincident with regional differences in size (Oaxaca fish fork lengths ranged from 103 to 118 cm (mean = 110 cm), while Baja fish ranged from 85 to 106 cm (mean = 93 cm)). Although larger fish in the Baja region displayed behavior consistent with smaller Baja fish, future studies should investigate whether the observed regional differences are due to (i) size, (ii) sex, (iii) oceanography, or (iv) availability of floating objects. Dolphinfish are an important mid-trophic level predator and potentially sustainable fishery resource. Understanding their behavior and use of floating objects is of both ecological and economic importance-particularly in the context of expanding international FAD-based fisheries. Our study suggests dolphinfish spend a large amount of their time exhibiting potential floating object associated behavior, and this could influence their population structure and growth.

Using stable isotope analysis to understand the migration and trophic ecology of northeastern Pacific white sharks (Carcharodon carcharias).

The white shark (Carcharodon carcharias) is a wide-ranging apex predator in the northeastern Pacific (NEP). Electronic tagging has demonstrated that white sharks exhibit a regular migratory pattern, occurring at coastal sites during the late summer, autumn and early winter and moving offshore to oceanic habitats during the remainder of the year, although the purpose of these migrations remains unclear. The purpose of this study was to use stable isotope analysis (SIA) to provide insight into the trophic ecology and migratory behaviors of white sharks in the NEP. Between 2006 and 2009, 53 white sharks were biopsied in central California to obtain dermal and muscle tissues, which were analyzed for stable isotope values of carbon (δ(13)C) and nitrogen (δ(15)N). We developed a mixing model that directly incorporates movement data and tissue incorporation (turnover) rates to better estimate the relative importance of different focal areas to white shark diet and elucidate their migratory behavior. Mixing model results for muscle showed a relatively equal dietary contribution from coastal and offshore regions, indicating that white sharks forage in both areas. However, model results indicated that sharks foraged at a higher relative rate in coastal habitats. There was a negative relationship between shark length and muscle δ(13)C and δ(15)N values, which may indicate ontogenetic changes in habitat use related to onset of maturity. The isotopic composition of dermal tissue was consistent with a more rapid incorporation rate than muscle and may represent more recent foraging. Low offshore consumption rates suggest that it is unlikely that foraging is the primary purpose of the offshore migrations. These results demonstrate how SIA can provide insight into the trophic ecology and migratory behavior of marine predators, especially when coupled with electronic tagging data.