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PHYTOPLANKTON Phytoplankton is a general term for largely photosynthetic single-celled organisms found in the water column. Phytoplankton form the base for many planktonic food webs, and the type and size of the phytoplankton often dictate which zooplankton will be abundant. However, many of the common phytoplankton are too small (2–20 μm) to be caught or noticed in zooplankton collections. Indeed, production by small photosynthetic picoplankton and nanoplankton often exceeds that of the larger species. Of the many different groups of phytoplankton, the two most likely to be retained in zooplankton collections are larger species of diatoms and dinoflagellates. These two ancient groups of uncertain taxonomic affinities have many distinctive peculiarities. Both groups are widespread and abundant from the fresh headwaters of estuaries to the open sea. Both dinoflagellates and diatoms are present all year, but their relative proportions vary. Along the Atlantic Coast and in many estuaries, there is a large spring diatom bloom and a less predictable secondary fall/winter bloom. In summer, dinoflagellates and a mixed assemblage of other flagellates usually dominate. This is a general cycle, and local exceptions are common. Phytoplankton blooms, often dominated by a single species, can produce cell densities in excess of a half-million cells per milliliter. These blooms are dynamic, appearing suddenly and lasting a few days or weeks as a changing suite of phytoplankton species assume dominance. The abundance and types of phytoplankton present often control, at least in part, the dynamics and species composition of the zooplankton grazers and thereby exert a major influence on the overall dynamics of planktonic food webs. Blooms of pigmented species produce yellow, green, red, or brown “tides,” collectively referred to as harmful algal blooms (HABs). Some blooms produce toxins implicated in the deaths of fish and marine mammals and in the poisoning of humans who eat contaminated shellfish or are otherwise exposed to the toxin. Even nontoxic blooms can result in hypoxic or anoxic conditions. Most nearshore planktonic diatoms are photosynthetic, but dinoflagellate nutrition is less readily categorized. Although many dinoflagellates are entirely autotrophic, some are heterotrophic, and others (mixotrophs) switch between autotrophy and heterotrophy, depending on their developmental stage or their nutritional state. Some dinoflagellates feed on other phytoplankton and steal and retain the still functional chloroplasts (kleptoplastidy ). The prevalence of mixotrophy, especially in many species associated with blooms, has received recent attention. This nutritional flexibility may be advantageous in nutrient -poor conditions where heterotrophic feeding could supply both food and nutrients for growth. At times, grazing of heterotrophic and mixotrophic dinoflagellates on photosyn- 60 IDENTIFICATION AND BIOLOGY OF COMMON ZOOPLANKTON thetic diatoms and flagellates may exceed that of mesozooplankton. Some dinoflagellates also feed on dissolved organic material and on bacteria and are an important link in the microbial loop. Heterotrophic dinoflagellates include the predacious Noctiluca, a number of parasites, and the enigmatic Karlodinium and Pfiesteria associated with fish kills in North Carolina and Maryland. The life cycles of phytoplankton often involve both sexual and asexual stages. During blooms, diatoms reproduce asexually by binary fission to produce two diploid cells. The silica test surrounding each cell is made of two pieces (called valves or frustules) that overlap slightly, much like the two pieces of a petri dish. When cells divide, each daughter gets one of the valves and regenerates the inner half. In this way, the average cell size diminishes with time. Sexual reproduction involving gametes occurs at intervals in the life cycle. At the end of blooms or in unfavorable conditions, diatoms produce asexual resting spores that settle to the bottom. Dinoflagellate life cycles are more complex. Asexual reproduction of the haploid vegetative stages is the norm. The sexual phase, if present, is brief. As in diatoms, a resting cyst or spore may form part of the life cycle. Some species are apparently capable of multiple transformations. See the review in Coats (2002) and Litaker et al. (2002) for more details on the complexities of dinoflagellate life cycles. Dinoflagellates and diatoms differ with respect to locomotion. Dinoflagellates typically have two flagella to propel them through the water. In contrast, diatoms lack flagella. Some diatoms employ oil droplets to assist with buoyancy or cytoplasmic projections to slow sinking. Nevertheless, diatoms ultimately rely on mixing or upwelling to keep them within the photic zone. IDENTIFICATION HINTS Diatoms come in two broad groups, centric and pennate. Centric diatoms (the cells, not the colonies) are typically radially symmetrical, whereas pennate diatoms are slender, often rodlike, and bilaterally symmetrical. Whereas most centric diatoms are...


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