Search for: [Abstract = "This is also true of species which do not change their food uptake mechanizmnor feeding behaviour from one to another environment . In this case a sufficientreason for changing the trophic status of a species might be exclusively a modificationof the food conditions. A typical filter feeder grazes mostly on small nannoplanctonicalgae ranging in size from a few to about 20 microns in diameter whenin oligotrophic waters, whereas in eutrophic lakes it becomes a typical detrito andbacteriovore, because, as a result of the very same filtration process, notalgae but bacteria and detritus will be collected in its filtering chamber \( Gliwicz 1969a, 1969b\).It seems that correct classification of limnetic animals into trophic types, oreven trophic levels, is possible only in the case of oligotrophic lakes, where theclassic structure of a food chain is usually maintained\: phytoplankton — herbivorouszooplankton — carnivorous zooplankton — fish. This is clearly demonstratedby the succes in building very accurate models of the Alpine Unterer FinstertalerSee \(Pechlaner et al. 1972\) and the Arctic Char Lake \(Rigler 1972\). Thetrophic statuses of zooplankton species there are precisely defined thanks to thecharacter of plankton primary production and the lack of an intensive inflow ofallochtonic organic matter.In such oligotrophic waters, because of the low nutrient concentrations thenannoplanctonic algae are the main or exclusive primary producers. This probablybecause their higher surface\/volume ratio makes them more effective nutrientutilizers. These small algae \(flagellates, greens, small diatoms\) fall perfectly intothe size range of food particles available to the typical filter feeder. Furthermore,the small income of allochtonic organic material holds bacteria concentration onlow level, While the low plankton primary production causes insignific ant concentrationof detritus particles. Although both of these are mostly also within thesize range of available food particles, the main food collected in the filteringchamber is the fresh living algae \(Fig. 2\). So here we have to do with true herbivores— well distinguis hed primary consumers’ trophic level.The situation is different in more productive lakes — waters, richer innutrients and usually supplied with a greater flow of allochtonous organic matter.The phytoplankton standing crop here is strongly dominated by net algae \(for anattempt of explanation of this phenomenon see Gliwicz 1973\), frequently byhuge blue\-green colonies and peridinian cells, several hundred microns in length,much too large to be available for filter feeders. The organic matter produced bysuch a phytoplankton community becomes eventually available as decomposedmaterial. The large amount of organic substratum, both of net phytoplankton andallochtonous origin, increases the concentration of bacterial cells and particles of detritus \(tripton\) in the environment. Thus the number of live algae within thesize range of food particles available for a filter feeder decreases, whereas theamount of detritus and bacteria increases. In other words, the typical filter feederunable to actively select food particles grazes on totally different food in oligotrophicand eutrophic conditions \(Fig. 2\).In this case, not only does the trophic status of the filtrators changes, butalso that of predators, generally represented in plankton by cyclopoid copepods."]

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