22 Phytoplankton Ecology

The word phytoplankton comes from two Greek words: phytos for “plant” and planktos for “wandering.” Phytoplankton are photosynthetic microorganisms that are adapted to live and wander in the open surface waters of lakes, rivers, and oceans. The definition includes both prokaryotes (cyanobacteria) and a diverse array of eukaryotes. Cyanobacteria, greens, diatoms, cryptomonads, dinoflagellates, haptophytes, and chrysophyceans are especially rich in planktonic species. Phytoplankton play many important ecological roles in aquatic ecosystems and affect human affairs in many ways. Planktonic algae are the primary producers of aquatic ecosystems and form the base that supports the zooplankton and fish of aquatic food webs. For example, cold, deep nutrient-rich waters rise to the ocean surface along the west coast of South America, where the nutrients support abundant phytoplankton, zooplankton, and a rich fishery. When a climatic change such as El Niño occurs, the nutrient-rich waters are shut off, causing a cascade of events ending in a collapse of the fishery and severe economic repercussions. Phytoplankton also play a major role in the global cycle of carbon dioxide. The phytoplankton in the oceans account for a little less than half (48 Pg C yr^–1) of the global net primary production of 105 Pg C yr^–1 (refer to Glossary of Abbreviations on the following page) (Geider et al. 2001). Photosynthesis of marine phytoplankton contributes to the removal of carbon dioxide released by the burning of fossil fuels (about 5.5 ± 0.5 Pg C yr^–1) and agricultural activities such as deforestation and land drainage (around 1.6 ± 1.0 Pg C yr^–1) (Behrenfeld et al. 2002). These human activities are also responsible for the increasing flow of nutrients into lakes, rivers, and marine coastal waters, where they may trigger harmful algal blooms. These algal blooms may produce toxins, as can cyanobacteria in freshwaters and dinoflagellates in marine coastal waters, or the phytoplankton may die and decompose, depriving the waters of oxygen and producing dead zones where marine life is killed by suffocation. The second largest dead zone in the world occurs annually where the Mississippi River enters the Gulf of Mexico. This area is now larger than the state of New Jersey. More than 145 dead zones have been reported globally (43 in coastal U.S. waters), and the number is growing. Phytoplankton ecology is thus relevant to global ecology and human lives.

Phytoplankton ecology has been characterized by the development of a large body of theory and mathematical modeling. Theory and modeling have centered about two topics—competition theory and trophic dynamics. The original source of competition theory lies in the work of Gause (1934) on various microorganisms in test tubes and flasks. These experiments led to the development of the competitive exclusion principle and later to niche theory. The basic idea is that biological interactions—not physical and chemical external factors—are paramount in community dynamics. Species are assumed to exist close to their maximum density in the environment and to compete for scarce resources. If two species occupy the same niche, one must inevitably drive the other out through competitive displacement. These concepts entered plankton ecology when Hutchinson (1961) wrote his famous paper on the paradox of the plankton. If we assume that species are close to their maximum density in aquatic systems and competitive exclusion is a general rule, how can 50 to 100 species of phytoplankton possibly coexist in only a few milliliters of lake or ocean water? Why are phytoplankton communities so diverse? The controversy concerning this paradox has fueled experimental and theoretical research to the present and has even shaped aquatic management practices.

The second focus of theory and modeling is trophic dynamics. The original concept of trophic dynamics involved bottom-up control. Bottom-up control maintains that phytoplankton populations are fundamentally controlled by nutrients rather than herbivory. Food chains were seen in simple terms. More nutrients means more phytoplankton biomass, and more phytoplankton means more zooplankton to feed fish populations. The concept of top-down control of phytoplankton communities originated with Porter (1977) and arose out of observations of clear-water phases in aquatic systems. Top-down theories assume that phytoplankton populations are controlled by herbivory, which directs species compositions and seasonal patterns of biomass. Research on trophic dynamics led to the discovery and widespread acceptance of the importance of the microbial loop, a complex microbial food web consisting of secondary producers such as bacteria and archaea, small phytoplankton, and heterotrophic flagellates and ciliates that graze upon them. All these concepts have shaped management practices. We discuss both competition theory and trophic dynamics in later sections. We consider phytoplankton population dynamics under two general categories: (1) growth processes, including photosynthesis and nutrient uptake, and (2) loss processes, including perennation (the formation of resting stages), mortality, parasitism, sedimentation, competition, and grazing. But first, we consider briefly the importance of size and scale in phytoplankton ecology and the physical and chemical environment of lakes and oceans, the medium in which phytoplankton ecology occurs.