puree of microcrustacea , chironimids , and decay- 

 ing algal masses. A few days later the dead fish 

 floated to the surface and were washed ashore, 

 forming a veritable "windrow" along several miles 

 of lake front. Such events often occur, unfortu- 

 nately when water temperatures are high (22 to 30 

 degrees C.) and when the oxygen-content is ac- 

 cordingly low. Rarely, blooms of algae, diatoms, 

 and planktonic blue-greens decay under winter ice. 

 This leads indirectly to fish deaths. Such events 

 not infrequently follow fertilization experiments 

 (as previously mentioned) due to oxidation of the 

 surplus organic matter, as well as to the break- 

 down of the dense population of algae which has 

 accrued as a result of the fertilization practice. A 

 thick blanket of snow over winter ice decreases 

 illumination so that a growth of winter-time plank- 

 ton 'crashes' and decays. 



Whereas blue-green algae are accountable 

 for toxic effects, other disturbances are caused 

 mostly by those organisms in which yellow and 

 brown pigments predominate and in which oil and 

 leucosin accumulate as food reserve. These are 

 representatives of the Chrysophyta and the Pyrrho- 

 phyta, especially the former. Two classes of the 

 Chrysophyta , the Chrysophyceae and the Bacillar- 

 iophyceae contain several species which are in- 

 famous pests in aquatic habitats. Dinobryon spp. 

 and Synura uvella become abundant enough to pro- 

 duce obnoxious odors and fishy tastes. These 

 genera are often most abundant early in spring 

 when silicon- and calcium-content are low, these 

 elements having been exhausted by winter pulses 

 of diatoms . 



In suitable habitats (eutrophic lakes, pH 

 7.2-8.5) diatoms often develop dense blooms or 

 form thick encrustments on submerged surfaces 

 (Asterionella , Melosira , Fragilaria , Tabellaria ) . 

 The 'fishy' odor of water is caused not by fish but 

 by diatoms which either alive or dead, but espe- 

 cially after a peak development, release large 

 quantities of oil into the water. 



Chrysophyceae and the diatoms are physio- 

 logically suited to development in cold water and 

 in weak illumination. As classes they often de- 

 velop peak populations in the winter, or in early 

 spring, following a fall dominance of blue-green 

 species. For example, diatoms composed 100 per 

 cent of the plankton during winter and spring 

 months in Cleveland harbor of Lake Erie . Accord- 

 ingly disagreeable tastes in domestic water sup- 

 plies may appear during winter as well as in sum- 

 mer . 



The Pyrrhophyta that cause trouble are mostly 

 marine and are not of direct concern here . As men- 

 tioned previously, Noctiluca , Gymnodinium spp. 

 and Gonyaulax are responsible for fish deaths (red 

 tide) along shores. The former is reported to be 

 especially troublesome in the eastern Pacific (East 

 Tndies) . In fresh-waters only Ceratium hirundi- 



nella appears to form blooms sufficiently dense to 

 cause objectionable tastes in drinking water. This 

 species, although it does not form scums, may 

 color water a distinct gray-brown with as many as 

 13,000,000 organisms per liter. To my knowledge, 

 Ceratium hirundinella has never brought about fish 

 deaths . 



Another incidental way in which diatoms 

 cause disturbances is by clogging sand filters in 

 city water systems which use impounded water. 

 This calls for additional work and expense for san- 

 itary engineers. Diatoms are successful in this 

 because of their persistent silicious walls and be- 

 cause of their large numbers which produce a 

 dense film over a filter bed . 



Some chlorophycean algae cause minor dis- 

 turbances . Unlike the blue-green algae and the 

 diatoms , they have a slower reproductive rate and 

 seldom form superabundant growths. The product 

 of photosynthesis is starch and not ill-smelling 

 fats and oils. They have no pseudovacuoles and 

 do not form surface scums . Even the Volvocales 

 which frequently form conspicuous blooms cause 

 no particular disturbance. They are excellent oxy- 

 genators and remain evenly distributed throughout 

 the medium. 



Such coarse filamentous forms as Rhizoclon- 

 ium and Hydrodictyon , however, develop thick mat 

 mats which do cause some minor troubles. Boating, 

 fishing and vacation sites are spoiled by these and 

 other species. The weight of filamentous blankets 

 causes larger aquatic plants to sink and to undergo 

 decay with attendant objectionable effects. 



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