Communities of microscopic plant life, or phytoplankton, dominate the Earth's aquatic ecosystems. This important new book by Colin Reynolds covers the adaptations, physiology and population dynamics of phytoplankton communities in lakes and rivers and oceans. It provides basic information on composition, morphology and physiology of the main phyletic groups represented in marine and freshwater systems and in addition reviews recent advances in community ecology, developing an appreciation of assembly processes, co-existence and competition, disturbance and diversity. Although focussed on one group of organisms, the book develops many concepts relevant to ecology in the broadest sense, and as such will appeal to graduate students and researchers in ecology, limnology and oceanography.

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The Ecology of Phytoplankton

Changes in iron supply to oceanic plankton are thought to have a significant effect on concentrations of atmospheric carbon dioxide by altering rates of carbon sequestration, a theory known as the 'iron hypothesis' For this reason, it is important to understand the response of pelagic biota to increased iron supply Here we report the results of a mesoscale iron fertilization experiment in the polar Southern Ocean, where the potential to sequester iron-elevated algal carbon is probably greatest Increased iron supply led to elevated phytoplankton biomass and rates of photosynthesis in surface waters, causing a large drawdown of carbon dioxide and macronutrients, and elevated dimethyl sulphide levels after 13 days This drawdown was mostly due to the proliferation of diatom stocks But downward export of biogenic carbon was not increased Moreover, satellite observations of this massive bloom 30 days later, suggest that a sufficient proportion of the added iron was retained in surface waters Our findings demonstrate that iron supply controls phytoplankton growth and community composition during summer in these polar Southern Ocean waters, but the fate of algal carbon remains unknown and depends on the interplay between the processes controlling export, remineralisation and timescales of water mass subduction

A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization

A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron

The seeding of an expanse of surface waters in the equatorial Pacific Ocean with low concentrations of dissolved iron triggered a massive phytoplankton bloom which consumed large quantities of carbon dioxide and nitrate that these microscopic plants cannot fully utilize under natural conditions. These and other observations provide unequivocal support for the hypothesis that phytoplankton growth in this oceanic region is limited by iron bioavailability.

A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean

The idea that iron might limit phytoplankton growth in large regions of the ocean has been tested by enriching an area of 64 km2 in the open equatorial Pacific Ocean with iron This resulted in a doubling of plant biomass, a threefold increase in chlorophyll and a fourfold increase in plant production Similar increases were found in a chlorophyll-rich plume down-stream of the Galapagos Islands, which was naturally enriched in iron These findings indicate that iron limitation can control rates of phytoplankton productivity and biomass in the ocean

Testing the iron hypothesis in ecosystems of the equatorial Pacific Ocean

Biological nitrogen fixation is an important oceanic nitrogen source, potentially stabilizing marine fertility in an increasingly stratified and nutrient‐depleted ocean. Iron limitation of low latitude primary producers has been previously demonstrated to affect simulated regional ecosystem responses to climate warming or nitrogen cycle perturbation. Here we use three biogeochemical models that vary in their representation of the iron cycle to estimate change in the marine nitrogen cycle under a high CO2 emissions future scenario (RCP8.5). The first model neglects explicit iron effects on biology (NoFe), the second utilizes prescribed, seasonally cyclic iron concentrations and associated limitation factors (FeMask), and the third contains a fully dynamic iron cycle (FeDyn). Models were calibrated using observed fields to produce near‐equivalent nutrient and oxygen fits, with productivity ranging from 49 to 75 Pg C yr−1. Global marine nitrogen fixation increases by 71.1% with respect to the preindustrial value by the year 2100 in NoFe, while it remains stable (0.7% decrease in FeMask and 0.3% increase in FeDyn) in explicit iron models. The mitigation of global nitrogen fixation trend in the models that include a representation of iron originates in the Eastern boundary upwelling zones, where the bottom‐up control of iron limitation reduces export production with warming, which shrinks the oxygen deficient volume, and reduces denitrification. Warming‐induced trends in the oxygen deficient volume in the upwelling zones have a cascading effect on the global nitrogen cycle, just as they have previously been shown to affect tropical net primary production.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1029/2020GB006851

W. Yao

Papers

Testing the iron hypothesis in ecosystems of the equatorial Pacific Ocean

Simulated Future Trends in Marine Nitrogen Fixation Are Sensitive to Model Iron Implementation