Response of pelagic food webs to warmer, acidified oceans

In this project, we are employing a combination of laboratory experiments and larger scale ecosystem approaches to test hypotheses that CO2 induced acidification, with warming, will result in a shift of autotrophic plankton communities favoring smaller flagellate species rather than large diatoms and that acidification and warming will favor gelatinous plankton resulting in increased transfer of autotrophic production to the microbial loop. We concentrate our effort on a model gelatinous planktonic organism, Oikopleura, which has a pan-global distribution and plays important roles in marine pelagic ecosystems and in global vertical carbon cycles. In particular, the response of appendicularian zooplankton to climate change may have significant ecosystem implications as they can alter biogeochemical cycling compared to classical copepod dominated food webs. However, the response of appendicularians to multiple climate drivers and effects on carbon cycling are still not well understood. Here we investigated in mesocosms how gelatinous zooplankton (appendicularians) affect carbon cycling of marine food webs under conditions predicted by future climate scenarios. Appendicularians performed well in warmer conditions and benefited from low pH levels, which in turn altered the direction of carbon flow. Increased appendicularians removed particles from the water column that might otherwise nourish copepods by increasing carbon transport to depth from continuous discarding of filtration houses and fecal pellets. To follow up on our mesocosm results we also performed more in depth microcosm analyses. In microcosms, the positive impact of OA, was observed to result from increased fecundity. Oppositely, increased pH, observed for example during phytoplankton blooms, reduced fecundity. Oocyte fertility and juvenile development were equivalent under all pH conditions, indicating that the positive effect of lower pH on O. dioica abundance was principally due to increased egg number. This effect was influenced by food quantity and quality, supporting possible improved digestion and assimilation at lowered pH. Higher temperature resulted in more rapid growth, faster maturation and earlier reproduction. Thus, increased temperature and reduced pH had significant positive impacts on O. dioica fitness through increased fecundity and shortened generation time, suggesting that predicted future ocean conditions may favour this gelatinous zooplankton species.

Atmospheric CO2 is projected to double by 2100, resulting in increased temperatures, ocean acidification (OA) and changes in the balance of marine ecosystems. While chemical effects of OA are well understood, the biological effects are less certain. Predi ctions include a shift in plankton communities towards smaller organisms, reduced carbon (C) export rates, and increased roles of gelatinous zooplankton in C cycling. Using a whole ecosystem approach we will test hypotheses that (H1) CO2 induced acidifica tion, with warming, will result in a shift of autotrophic plankton communities favoring smaller flagellate species rather than large diatoms and (H2) acidification and warming will favor gelatinous plankton resulting in increased transfer of autotrophic p roduction to the microbial loop. To address these hypotheses, we propose to conduct experiments using a multi-factorial design (CO2, temperature, presence/absence of gelatinous plankton). We will quantify and characterize autotrophic, heterotrophic, and b acterial plankton communities, growth and development rates of a model gelatinous plankter (Oikopleura dioica) and dominant copepod species, DOM production, fate, and turnover rates, as well as net microbial community respiration rates. By examining in de tail the 'microbial black box', this proposal will generate data with clear implications for international biogeochemical initiatives which seek to provide understanding of global change and consequent effects on human society. Determining how gelatinous plankton alter C flows in a high CO2 world is also important in managing commercial fisheries as yields are controlled by C bioavailability to higher trophic levels and C transfer efficiency through planktonic food webs. Combining multidisciplinary intern ational science and state of the art research facilities and approaches, provides a unique template for transformative research on impacts of OA on biologically mediated elemental flux through our changing oceans.