Award: OCE-1220381

Copepods are the most important marine secondary producers world-wide, and they serve as a dominant food for nearly all larval fish and many sea birds and marine mammals. They make up 70-90% of the zooplankton biomass throughout most of the world?s oceans. Despite this, very few studies have addressed the impact of the emerging climate threat of ocean acidification (OA—the increase in dissolved CO2 and resultant lowered ocean pH) on copepods. The existing studies have primarily focused on how OA directly affects copepods. Few significant effects of OA on copepods have been found, and almost no effects have been found at OA levels that are within realistic future predictions for the world?s oceans. However, there is increasing evidence that OA significantly affects biological characteristics and the nutritional quality of their dominant prey, the phytoplankton. For example, some researchers have shown that OA affects the types and concentrations of fatty acids produced by phytoplankton. This is important because the amount and types of fatty acids present in phytoplankton are crucial for the health and development rate of juvenile copepods. Some fatty acids essential to growth and survival of juvenile copepods cannot be synthesized by the mother copepods themselves, and instead must be acquired through the mothers? phytoplankton diets. If OA changes the nutritional quality of their food, then copepod grazing, egg production, and hatching success could also change. This could mean that copepod populations and, ultimately, marine food webs would also change. This award provided the opportunity to advance our understanding of how copepod populations are affected by OA. Our main objective was to determine how OA-driven changes in phytoplankton physiology and biochemistry affect copepod reproductive output and the development of the resultant offspring. We also included a series of experiments that tested whether OA affects copepod reproductive output independent of changes to the phytoplankton on which they grazed. A conceptual diagram showing the various pathways by which OA may affect copepods, and the pathways that formed the foundation of our hypotheses, is shown in Figure 1. Our study found that under future ocean conditions (higher dissolved CO2 concentration and lower pH), the phytoplankton species used in this study changed in ways that could alter copepod reproductive output. For example, the phytoplankton showed higher population growth rates under future ocean conditions and there was a trend toward increased total amounts of lipids per phytoplankton cell. The biological changes we observed in the phytoplankton gave rise to some modifications in copepod behavior, reproductive output, and developmental rates of juvenile copepods. In the two copepods tested in this study—a large lipid-rich species and a small lipid-poor species—we observed differences in their feeding rates when offered phytoplankton grown under OA. For the small copepods, the developmental rate of copepod juveniles was faster when consuming phytoplankton grown under OA. For the larger copepod, the number of eggs produced by individual adult females was lower when feeding on phytoplankton grown under OA. These results show that the reproductive output and development of juvenile copepods can be affected by OA through the food quality of their prey, and that the degree to which this occurs is dependent on the individual species of copepods. However, the differences found among species also indicates that it may be difficult to generalize how OA will affect copepod species more generally, and therefore how OA will affect complex marine ecosystems. We were committed to broadening the scope of this project beyond just the research activities that most directly advance scientific understanding. This award also supported public outreach initiatives which increased awareness of marine issues in the general public through six presentations by the faculty and students at public events and three professional presentations specifically to ecosystem and fishery managers to assist their management efforts. Three undergraduate students from under-represented minority groups, two female graduate students, and two visiting professors from a developing nation were mentored by the PI on independent research projects as part of this award. Last Modified: 03/19/2018 Submitted by: Julie E Keister