Project: Effects of Fe:C ratios in food on marine copepod productivity and physiology

Description from NSF award abstract:
Crustacean zooplankton provide the energetic link between phytoplankton and higher trophic levels in the ocean, including economically important fish species. They also affect biogeochemical cycling through production of fast sinking fecal pellets and the regeneration of nutrients. Historically, marine food-web models have assumed that zooplankton productivity is energy limited, and that productivity therefore scales directly with abundance of food in the environment. However, comparisons of the iron contents of phytoplankton and zooplankton, stoichiometric modeling and preliminary experiments all suggest that in iron-limited regions of the ocean zooplankton growth and reproduction may be limited by the supply of iron via the diet. This project will address three questions regarding the response of copepods to food containing different ratios of Fe:C. First, how the assimilation and retention of iron varies with the Fe:C ratio in food will be determined. The critical threshold Fe:C ratio in food at which copepod production is limited by iron depends on the assimilation of iron and carbon from food, retention of assimilated iron and carbon within copepod tissues and the elemental composition of copepod tissues. Under controlled conditions in the laboratory both pulse chase radioisotope experiments and stable elemental analysis will determine how these variables change with Fe:C in the diet. Second, demographic consequences of variation in Fe:C ratios in food will be assessed. Third, it will be determined whether iron limitation affects copepods indirectly by influencing other aspects of food quality. By selectively supplementing the diet of the copepods and by comparing responses to different algal species the investigators will isolate the characteristic of algal food that correlates most closely with copepod production. In all aspects of the work, a diverse set of phytoplankton will be used as composition and responses to iron limitation vary substantially among major groups. The copepod employed in these experiments will be a cultured coastal calanoid copepod, Acartia tonsa. The ability to culture this organism will allow the investigators to conduct experiments throughout the year without worrying about genotypic variability. In addition they will use copepods species collected from the California Coast which actually experience iron limitation somewhere in their range.

By building a fuller understanding of the factors controlling secondary production in the ocean, this research will be of use to fisheries scientists and managers. It will also inform policy regarding potential economic impacts of climate change and the ecological and biogeochemical consequences of anthropogenic ocean iron fertilization.