No abstract

Recent unenclosed iron-fertilization experiments in the equatorial Pacific Ocean have shown that phytoplankton biomass can be increased substantially by the addition of iron. Analyses of size-fractionated chlorophyll indicate that much of the increase during the most recent fertilization experiment, IronEx II, occurred in the Ͼ10-m size fraction. We used flow cytometry, combined with taxon-specific pigment measurements by high-performance liquid chromatography (HPLC), to analyze the responses of five different groups of phytoplankton: Prochlorococcus, Synechococcus, ultraplankton, nanoplankton, and pennate diatoms. These results are unique in the suite of measurements from the IronEx studies in that they simultaneously examine individual cell properties, which are grazer independent, and population dynamics, which reflect the net result of growth and grazing. Our results show that the overall increase of chlorophyll a (Chl a) in the patch was due in part to increases in chlorophyll content per cell and in part to increases in cell numbers of specific groups. Cellular fluorescence was stimulated by iron addition in all five groups to a qualitatively similar degree and was correlated with taxon-specific changes in cellular pigments. In terms of net cell growth, however, these groups responded very differently. The groups that dominated the community before the addition of iron increased at most twofold in cell number; Prochlorococcus actually decreased. In contrast, the initially rare pennate diatoms increased 15-fold in number by the peak of the iron-induced bloom. Within 1 week, this differential response led to a dramatic change in the phytoplankton community structure, from one dominated by picoplankton to one dominated by large diatoms. It is not known whether this shift would be sustained over extended periods of fertilization, a response that would ultimately change the structure of the food web.Oceanographers have long been puzzled by the simultaneous abundance of macronutrients, such as nitrate and 1 Corresponding

Microbial size spectra, including bacteria through nanophytoplankton, were measured by use of flow cytometry across the western north Atlantic Ocean and during two nutrient enrichment studies: bottle enrichments in the Sargasso Sea and an in situ iron enrichment in the equatorial Pacific (IronEx II). Spectral shapes, or the relative conformity to a function described by a power law, ranged from smooth and log linear during the spring bloom in the Sargasso Sea to being distinctly non-log linear in coastal waters. Overall, the individual spectra within large regions characterized by similar ecological conditions showed remarkable consistency, inviting speculation that powerful organizing mechanisms are at work in these communities. Moreover, the ensemble average of all of the spectra along the transect displays clear power-law behavior. Slopes ranged from Ϫ1.0, in which biomass was equally distributed between all size classes, to Ϫ1.4, in which proportionally more biomass was contained in smaller size classes; there was no clear relationship between nutrient concentrations and spectral slopes over the entire data set. Species succession in nutrient-enriched bottles caused spectra to evolve from relatively smooth power laws to distributions showing preferred sizes (i.e., nonlinear on a log-log plot). The IronEx II spectra, however, remained similar over the course of the experiment. It could be that the elimination of bottle effects in this experiment buffered the system in ways that maintained the size structure of the microbial community over the size range we measured. Our results suggest conditions that lead to log-linear size distributions; these should be verified over a broader range of scales and environments.Size spectra, which display the relative abundance of organisms of different sizes, convey a synoptic image of ecological communities that is taxon independent. As such, they have been attractive to ecological theorists and have been the subject of periodic interest in marine ecology for the past 30 yr. Sheldon et al. (1972Sheldon et al. ( , 1977 recognized the predictive powers of size spectra, suggesting that fish stocks could be predicted if the planktonic size spectrum were known. Moreover, a spectral approach offers potential for enhancing ecosystem models (Gin et al. 1998) AcknowledgementsWe thank the captain and crew of the R/V Oceanus, who facilitated the collection of the transect data through challenging weather conditions. We thank M. Durand and R. Greene for help developing the Mie fit to our calibration data. We also thank R. Olson and L. Moore for helpful comments on earlier drafts of this manuscript and J. Rodríguez and an anonymous reviewer for valuable critical comments.

Evaluating the component features of 'scaling' planktonic size spectra, commonly observed in marine ecosystems, is crucial for understanding the ecological and evolutionary processes from which they emerge. Here, we develop a theoretical framework that describes such spectra in terms of the size distributions of individual species, and test it against actual datasets of microbial size spectra from the Atlantic Ocean. We describe characteristics of size probability distributions of component species that are suf cient to support the observational evidence and infer that, when a power law describes the community size spectrum (thus suggesting critical self-organization of microbial ecosystem structure and function), a related power law links the total number of individuals of a given species to its mean size.