The physiological and ecological responses of marine zooplankton to iron
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Crustacean zooplankton provide the energetic link between phytoplankton and higher trophic levels in the ocean, including economically important fish species. Their activities also influence biogeochemical cycling through production of fast sinking fecal pellets and the regeneration of nutrients. Previous studies have indicated that zooplankton productivity can be limited by energy (carbon) and by a deficiency of the macronutrients nitrogen and phosphorus in their diets. However, the influence of dietary supply of trace elements on zooplankton productivity has been largely unstudied. Broad limitation of phytoplankton productivity by dissolved iron in the ocean results in larger variability in Fe:C ratios in phytoplankton than in their zooplankton grazers. In my dissertation I tested the general hypothesis that low Fe contents in Fe-limited phytoplankton can cause physiological stress in crustacean zooplankton. I addressed this problem using laboratory experiments and field studies in a region with low trace element concentrations. First the egg production rates and naupliar survivorship of a common temperate copepod Acartia tonsa were assessed with feeding three species of algae Thalassiosira oceanica, Rhodomonas salina, and Isochrysis galbana grown under Fe-replete and Fe-depleted conditions. Reduced egg production rates and naupliar survivorship of the copepods fed Fe-depleted algae clearly indicated that secondary production can indeed be limited by Fe content of food; egg production rates of A. tonsa were positively correlated with the rate of Fe assimilated from food. Fatty acids and sterol concentrations in Fe-replete and Fe-depleted algae and in the copepods consuming these algae were further analyzed to determine whether the effect of Fe on copepod reproduction can be explained by alteration of lipid content and composition in algae and copepods in response to Fe limitation. Gas chromatography coupled with mass spectrometry analysis revealed a negative effect of Fe deficiency on the total fatty acid concentration, total polyunsaturated FA concentration and combined concentration of eicosapentaenoic acid and docosahexaenoic acid in algal food. However, the differences between treatments in total and single fatty acid(s) as well as sterols of algae were not proportional to copepod egg production rates, nor were the copepod lipids consistently affected. Radiotracer techniques were used to determine whether zooplankton are able to adjust their assimilation and retention of Fe and C appropriately in face of Fe deficiency in food. Pulse-chase experiments were conducted on A. tonsa fed Fe-replete and Fe-depleted algae radio-labeled with <super>59Fe</super> and <super>14C</super>. No clear indication of stoichiometric regulation on Fe by A. tonsa was seen from the results; instead, this copepod had a tendency of releasing Fe at faster rates when fed food of low Fe:C. As A. tonsa is a coastal and estuarine copepod, which probably never experiences Fe limitation anywhere in its natural range, I further tested whether copepods which possibly live in Fe-deficient water periodically also display an Fe-limited effect by conducting a reproductive study with the copepod Calanus pacificus collected from water off La Jolla, California, which belongs to a seasonally Fe limiting current system (California Current System). Results clearly show that the reproduction of C. pacificus can also be limited by Fe-deficiency in their diets, although the biovolume of dietary algal cells is another factor affecting reproduction in addition to the dietary Fe content. For the field portion of this dissertation study, I participated in a cruise to the Costa Rica Upwelling Dome - a low Fe and presumbably low Zinc area - to determine if size fractionated zooplankton samples displayed different C, N, P, and trace metal content as well as RNA:DNA ratios from the samples collected from areas of different Fe availability. Analyzed data revealed that P, Fe, Zn, and RNA:DNA ratios of zooplankton samples were all significantly lower than the analysis performed for zooplankton from other waters, presumably enriched in trace metal Fe and Zn. Furthermore, the elemental ratios varied with zooplankton size, suggesting a growth tradeoff for dealing with trace nutrient deficiency. Results from this dissertation study indicate that marine mesozooplankton can be affected by Fe deficiency in food. Given the role that zooplankton play in the cycling of Fe and C, these results could have implications for biogeochemical cycles. In addition, as copepods are an important food source of many economically important fish species, the effect of dietary Fe on the growth and reproduction of copepods could eventually help us to better understand variability in fisheries production. Overall, this research has the potential to transform our understanding of spatial and temporal patterns in marine zooplankton productivity and community structure.