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. PLANKTON 1.0 INTRODUCTION ~ Plankton are defined here as organisms sus- pended in a body of water and because of their physical characteristics or size, are incapable of sustained mobility in directions counter to the water currents. Most of the plankton are micro- scopic and of essentially neutral buoyancy. All of them drift wi th the currents. Plankton consists of both plants (phytoplank- ton) and animals (zooplankton), and complex interrelationships exist among the various com- ponents of these groups. Chlorophyll-bearing plants such as algae usually constitute the greatest portion of the biomass of the plankton. Phytoplankton use the energy of sunlight to metabolize inorganic nutrients and convert them to complex organic materials. Zooplankton and other herbivores graze upon the phytoplankton and, in turn, are preyed upon by other organ- isms, thus passing the stored energy along to larger and usually more complex organisms. In this manner nutrients become available to large organisms such as macroinvertebrates and fish. Organic materials excreted by plankton, and products of plankton decomposition, provide nutrients for heterotrophic microorganisms (many of which are also members of the plank- ton assemblage). The heterotrophs break down organic matter and release inorganic nutrients which become available again for use by the "primary producers." In waters severely pol- luted by organic matter, such as sewage, hetero- trophs may be extremely abundant, sometimes having a mass exceeding that of the algae. As a result of heterotrophic metabolism, high con- centrations of inorganic nutrients become avail- able and massive algal blooms may develop. Plankton may form the base of the food pyramid and drift with the pollutants; therefore, data concerning them may be particularly signif- icant to the pollution biologist. Plankton blooms often cause extreme fluctuations of the dis- solved oxygen content of the water, may be one of the causes of tastes and odors in the water and, if present in large numbers, are aesthetically objectionable. In some cases, plankton may be of limited value as indicator organisms because the plankton move with the water currents; thus, the origin of the plankton may be obscure and the duration of exposure to pollutants may be unknown. The quantity of phytoplankton occurring at a particular station depends upon many factors including sampling depth, time of day, season of year, nutrient content of water, and the pres- ence of toxic materials. 2.0 SAMPLE COLLECTION AND PRES- ERVATION 2.1 General Considerations Before plankton samples are collected, a study design must be formulated. The objectives must be clearly defined, and the scope of the study must remain within the limitations of available manpower, time, and money. Historical, biolo- gical, chemical, and physical (especially hydro- logical) data should be examined when planning a study. Examination of biological and chemical data often reveals areas that warrant intensive sampling and other areas where periodic or seasonal sampling will suffice. Physical data are extremely useful in the design of plankton studies; of particular impor- tance are data concerning volume of flow, cur- rents, prevailing wind direction, temperature, turbidity (light penetration), depths of reservoir penstock releases, and estuarine salinity "wedges. " After historical data have been examined, the study site should be visited for reconnaissance and preliminary sampling. Based on the results of this reconnaissance and on the preliminary plankton data, the survey plan can be modified to better fulfill study objectives and to facilitate efficient sampling. 2.1.1 Influential factors In planning and conducting a plankton survey, a number of factors influence decisions and often alter collection routines. Since water cur- rents determine the directions of plankton movements, knowing the directions, intensity, and complexity of currents in the sampling area is important. Some factors that influence cur-