Diatom phytoplankton populations are the usual food for zooplankton and filter feeding fishes and contribute in a direct way to the large fishable populations in coastal zones. Flagellates, on the other hand, are frequently poor foods for most grazers and can lead to undesirable eutrophication effects. Arguments are presented that silicon is often the controlling nutrient in altering a diatom to a flagellate community. The alteration is governed by the relative magnitudes of the natural fluxes of the nutrients nitrogen, phosphorus and silicon to the receiving water body and the recycled fluxes of nitrogen and phosphorus from zooplankton grazing and phytoplankton respiration and decomposition. Examples of such alterations are presented for oceanic, estuarine and inland water bodies.

Grazing by Acartia tonsa in large-volume ( l m3) enclosures caused significant changes in the abundance and species composition of natural phytoplankton. Cell densities and biomass were reduced 20-34 % in two separate experiments. Calculations suggest that grazing was responsible for all of the biomass reduction that occurred. Centric diatoms, especially Skeletonema costatum, were reduced in grazed tanks relative to controls, while microflagellates were greatly increased. Accordingly, size distribution was also affected by zooplankton grazing. Grazed phytoplankton assemblages were composed of a much smaller dominant size class (< 7 pm) than were control assemblages (10-15 pm).

Cyclo~ella na?zn I-Iustedt, a small centric diatom requiring vitamin Blr, was tested as a potential assay organism for that vitamin. Dose-respo~lse curves, measured by optical density and cell count, showed a linear relation between gro\vth and BIZ concel~tration between 0 and a t least 2 kkg/rnl of the vitamin. Sincc the diatom is hardy, grows rapidly under a variety of conditions, and Inay be preserved for later esaminatio~l, it represents a ~~s e f u l assay organism for certain applications such as ocearlographic exploration.

The respiratory coefficient, expressed here as respiration per unit of chlorophyll, of six clones (five species) of diatoms was measured at temperatures of 5°, 10°, 15°, 20°, and 25 °C. The acclimatized cultures used were grown at each of these temperatures that supported sufficient growth. Considering any one of the five temperatures of measurement, the respiratory coefficient varied with temperature of acclimatization, but not consistently in the different clones. Moreover, for any of the given temperatures of acclimatization, there was no trend common to all clones in the relationship between respiratory coefficient and temperature at which respiration was measured. In any respiration experiment, greater differences were observed between two clones of the same species than between species differing in size by as much as two orders of magnitude. The highest respiratory coefficients were observed in the two eurythermal clones, and the increase in respiration with temperature was more regular, with Q10 = ca. 2.0. Respiration of the four stenothermal clones was more variable in relation to temperature, and both respiratory coefficients and Q10 were lower.

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