PLANKTON AND NUISANCE CONDITIONS IN SURFACE WATER 59 



There were only 68 high occurrences of blue-green algae, generally the 

 most troublesome group, while diatoms, rarely obnoxious, were high 205 

 times. Obviously the table indicates that any of the groups of algae may 

 pass bloom proportions if conditions are right. No chronology is shown 

 here, but these peak populations occurred at any time; two very dense 

 swarms of Volvocales (Chlamydomonas and Diplostauron) occurred under 

 ice cover in Kegonsa. Diatom abundance was noted through the year and 

 in these lakes was due most frequently to small naviculoid forms and to 

 bandbox forms, such as Cyclotella: small, and not productive of color in 

 surface waters. In rivers, forms like Synedra occur in huge swarms and 

 overnight may make their presence felt in the filter runs. In Lake Michi- 

 gan Tabellaria and Asterionella have been conspicuous. Diatoms, however, 

 are rarely troublesome except to water supplies. 



Dinoflagellates, while frequent in these Wisconsin lakes, were not a 

 nuisance. In oceanic waters their blooming is well known, and their oily 

 inclusions could be very troublesome in water supplies. Cryptophyceae 

 have not been reported as troublesome, but because they sometimes occur 

 in huge numbers, may contribute to tastes and odors. Centrifuging a virtu- 

 ally pure population of Cryptomonas produced almost no whole cells; but 

 the olive-green slime adherent to the centrifuge walls was highly offensive 

 in odor, and these organisms readily disintegrate on sand filters. They 

 tend to be especially abundant in rivers. Chrysophyceae seem to occur in 

 winter and spring or late fall. In this survey they were not troublesome, 

 but are notorious to water works men. They are especially characteristic 

 of non-calcareous waters; many genera, abundant along the Atlantic sea- 

 board or in Lake Michigan, are totally lacking in the Ohio Valley. Volvo- 

 cales probably rank next to blue-green algae as obvious trouble makers, 

 and together with other green flagellates, such as Euglena, are often con- 

 spicuous in waters whose organic content is above normal. But they do 

 not occur in such numbers as to form windrows on the beaches. 



Other bloom-forming organisms such as Azolla, Lemna or small arthro- 

 pod water fleas are either rarely troublesome or do not belong to the 

 plankton. 



The causes of blooms have long been debated. Highly special or local 

 causes may occur at times, but there seems little reasonable doubt at 

 present that one or more optimal conditions — light, temperature, pH, food 

 — produce them, and perhaps the most compelling of these is the nutrient 

 content of the environment. Many laboratory studies have indicated that 

 nitrogen and phosphorus relationships are perhaps the most critical. Trace 

 elements should be sufficiently abundant in natural waters from large drain- 

 age areas to supply the infinitesimal amounts presumably needed. But 

 nitrogen and phosphorus demands are comparatively large. Drainage areas 

 may supply these demands from four sources: Original, virgin land; agri- 

 cultural (fertilized) land; sewage; trade wastes. 



Table II shows inorganic nitrogen relationships in two small Ohio 

 creeks draining small areas; these determinations were made when light 



