Influence of floating macrophytes on populations of planktonic microcrustaceans

Background/Question/Methods

Planktonic microcrustaceans may respond differently to the presence of macrophytes, whose roots can serve as refuge from predators or can difficult the foraging for filter feeding species. The aim of this study is to assess the influence of macrophytes on planktonic microcrustaceans and detect the horizontal daily migration behavior of these organisms. An experiment was conducted using 12 small ponds (13x6x1.50m) in two treatments: six ponds had floating macrophytes - Eichhornia crassipes - in one of its edges and six without macrophytes. Predators as fish and Chaobrus larvae were absent. After an acclimatation period, animals were sampled in the littoral and limnetic zones with a suction pump and a plankton 45μm mesh net, at midday and midnight.

Results/Conclusions

The results between treatments and periods were compared by t-test and variance analysis. Total density of microcrustaceans did not vary significantly between treatments and between hours; Macrothrix paulensis was more abundant in tanks with macrophytes while Argyrodiaptomus furcatus did not occur; Moina minuta and Thermocyclops decipiens were more abundant in the night samplings, indicating a daily migration for these species. An analysis of similarity (ANOSIM) showed that species composition did not vary between treatments (r = 0.1 and p = 0.7). Our results show that macrophytes influence differently the species of microcrustaceans and for the occurrence of horizontal migration, macrophytes may not be the only necessary stimulus. Other factors, as predation, should be also considered as important factor for horizontal migration.