Cascading effect of exotic fish fry on plankton community in a tropical Andean high mountain lake: a mesocosm experiment

<p>Fishless Andean high mountain lakes may be vulnerable to fish invasion because they tend to be small, oligotrophic and contain low zooplankton diversity. During the first decades of the twentieth century, rainbow trout, <em>Oncorhynchus mikiss</em>, was introduced in South America, and stocking of juvenile stages (fry) in lakes continues today. However, their effect on plankton in these lakes has been little studied. We performed a mesocosms experiment to assess effects of trout of different ontogenetic stages on zooplankton and phytoplankton in a tropical-Andean high mountain lake. The presence of trout fry resulted in declines in several large zooplankton taxa, increases in chlorophyll <em>a</em> and dissolved inorganic nitrogen (DIN). Our results showed that small fry consume large copepods (<em>Colombodiaptomus brandorffi</em>) at a faster rate than larger fry, and also consumed medium sized copepods (<em>Tropocyclops prasinus</em>) that are not affected by the larger trout fry. Fish of both sizes consumed <em>Ceriodaphnia quadrangula</em>, a midsize cladoceran. Fish predation had weak effect on the phytoplankton biomass, but we found a correlation between zooplankton biomass and phytoplankton richness, and significantly larger cell of the alga <em>Peridinium</em> in the presence of fish. Our results indicate that trout introduction produces cascading ecological and phenotypic effects on the plankton communities of tropical mountain lakes, similar to those observed in temperate latitudes.</p>

Food selectivity of Ceriodaphnia quadrangula (O. F. Müller, 1785) (Cladocera) and its impact on competition outcome between two freshwater green algae

The present work tested the food selectivity of the cladoceran Ceriodaphnia quadrangula (O. F. Müller, 1785) and its impact on competition outcome between Chlorella vulgaris Beijerinck, 1890 and Chlamydomonas globosa J. W. Snow, 1902. Freshwater green algae, Chlorella, have heavy cell walls and their size usually exceeds the lower limits of limb size of Ceriodaphnia. According to the optimal foraging theory, it is speculated that Ceriodaphnia would graze on the more exposed and relatively larger Chlamydomonas rather than on Chlorella, and this process would lead to small-sized Chlorella becoming a superior competitor in the presence of Ceriodaphnia. This work used Ceriodaphnia, Chlamydomonas globosa and Chlorella vulgaris to test this hypothesis. The grazing experiment showed that Ceriodaphnia preferred Chl. globosa to Ch. vulgaris, regardless of the concentration and relative abundance of these algae. The decrease in relative abundance of high-quality Chlamydomonas in Chlamydomonas-Chlorella assemblages did not diminish the grazing efficiency of Ceriodaphnia on this algal species, but increased the selectivity of small-sized cells of Chlorella. However, when the concentration of Chlamydomonas was extremely high, the grazing of Ceriodaphnia on Chlamydomonas decreased. In competition experiments, it was observed that the presence of Chlamydomonas restrained the growth potential of Chlorella; however, the introduction of Ceriodaphnia into the competing environment weakened this influence and to some extent enhanced the growth ability of Chlorella. The different densities of Ceriodaphnia had an obvious influence on the competition outcome between Chlamydomonas and Chlorella.