Award: OCE-0962201

We determined whether growth and reproduction of copepods, small crustaceans that support many marine fisheries, could be affected by the iron content of their food. Most fisheries models presume that growth of copepods is limited by the overall availability of their algal food, and the energy stored therein. However, in the 30-40% of the ocean in which algal growth is limited by the availability of iron, the iron content of copepod food is typically less than observed in zooplankton (Fig. 1). This mismatch raises the possibility that the growth and reproduction of copepods could be limited not simply by the amount of their food, but also by its iron content. If true, this would make food webs in regions of the ocean where algae are iron limited less efficient at converting algal biomass into fish biomass. We showed in laboratory experiments that the common estuarine copepod, Acartia tonsa, produces fewer eggs (Fig. 2) and exhibits higher larval mortality (Fig. 3) when feeding on algal food depleted in iron. For a range of algal food species, the rate of egg production by A. tonsa varied directly with the rate of iron assimilation and no other measured variable, suggesting that the assimilation of iron was limiting to reproduction in these copepods. When fed diatoms that varied in iron content, A. tonsa exhibited high egg production when iron content of food was above a critical threshold iron content, and a linear relationship with iron content of food below this threshold, again indicating that iron in the diet was the factor limiting egg production. We further showed that A. tonsa is particularly prone to iron limitation because it is not able to compensate for low iron in food by changing ingestion, assimilation or retention of iron or carbon. Both the experimental results and calculations based on physiological rates suggest that copepods must have food with iron content greater than that in their own tissues to reproduce maximally. While A. tonsa is a useful model organism, it is unlikely to experience limitation of reproduction by dietary iron in its native estuarine habitat. We therefore repeated some of these experiments with a common copepod species, Calanus pacificus, which is sometimes exposed to iron-limited algae in its native habitat off the California coast. As with A. tonsa, this species exhibited lower egg production when fed iron-depleted diatom food than when fed iron-replete food (Fig. 2). This effect was reversible; copepods previously exposed to iron-depleted food resumed producing eggs at normal rates when fed iron-replete food. As with A. tonsa, we observed no increase in ingestion rates and fecal pellet production when these organisms were fed food depleted in iron. We also tried to gather further evidence for effects of trace metal limitation of zooplankton production in nature by sampling the Costa Rica Dome, a regions in which phytoplankton are putatively limited by iron and zinc during some seasons of the year. We found that iron contents of food particles (algae and protozoans) were on average lower than measured in many zooplankton size fractions, suggesting that iron limitation of crustacean zooplankton was possible (Fig 4). Indeed, RNA:DNA ratios of zooplankton were low compared to those measured in other regions of the ocean, suggesting low growth rate (Fig. 5). However, a strong correlation between iron, titanium and aluminum in the small zooplankton also suggested that these organisms are ingesting at least some terrestrial iron, possibly by ingesting colloids containing iron oxides. Zinc concentrations in food were low compared to literature values, but generally not low enough to imply limitation of zooplankton by dietary zinc (Fig. 4). However, zinc concentrations increased noticeably with size of zooplankton, suggesting that these larger organisms may be more subject to zinc limitation, especially if they are herbivorous (Fig 6). We have also recently sampled the Cal...