Neutral Theory and Plankton Biodiversity.

The biodiversity of the plankton has been interpreted largely through the monocle of competition. The spatial distancing of phytoplankton in nature is so large that cell boundary layers rarely overlap, undermining opportunities for resource-based competitive exclusion. Neutral theory accounts for biodiversity patterns based purely on random birth, death, immigration, and speciation events and has commonly served as a null hypothesis in terrestrial ecology but has received comparatively little attention in aquatic ecology. This review summarizes basic elements of neutral theory and explores its stand-alone utility for understanding phytoplankton diversity. A theoretical framework is described entailing a very nonneutral trophic exclusion principle melded with the concept of ecologically defined neutral niches. This perspective permits all phytoplankton size classes to coexist at any limiting resource level, predicts greater diversity than anticipated from readily identifiable environmental niches but less diversity than expected from pure neutral theory, and functions effectively in populations of distantly spaced individuals.

Quantifying the Ocean's Biological Pump and Its Carbon Cycle Impacts on Global Scales.

The biological pump transports organic matter, created by phytoplankton productivity in the well-lit surface ocean, to the ocean's dark interior, where it is consumed by animals and heterotrophic microbes and remineralized back to inorganic forms. This downward transport of organic matter sequesters carbon dioxide from exchange with the atmosphere on timescales of months to millennia, depending on where in the water column the respiration occurs. There are three primary export pathways that link the upper ocean to the interior: the gravitational, migrant, and mixing pumps. These pathways are regulated by vastly different mechanisms, making it challenging to quantify the impacts of the biological pump on the global carbon cycle. In this review, we assess progress toward creating a global accounting of carbon export and sequestration via the biological pump and suggest a path toward achieving this goal.

Phytoplankton community structuring in the absence of resource-based competitive exclusion.

Under most natural marine conditions, phytoplankton cells suspended in the water column are too distantly spaced for direct competition for resources (i.e., overlapping cell boundary layers) to be a routine occurrence. Accordingly, resource-based competitive exclusion should be rare. In contrast, contemporary ecosystem models typically predict an exclusion of larger phytoplankton size classes under low-nutrient conditions, an outcome interpreted as reflecting the competitive advantage of small cells having much higher nutrient 'affinities' than larger cells. Here, we develop mechanistically-focused expressions for steady-state, nutrient-limited phytoplankton growth that are consistent with the discrete, distantly-spaced cells of natural populations. These expressions, when encompassed in a phytoplankton-zooplankton model, yield sustained diversity across all size classes over the full range in nutrient concentrations observed in the ocean. In other words, our model does not exhibit resource-based competitive exclusion between size classes previously associated with size-dependent differences in nutrient 'affinities'.