Zooplankton communities

Zooplankton communities

by Ana Julia Flores (IE/UMSA)

Zooplankton were identified and quantified to genus level. If the sample in the bottle was very dense (opaque), subsampling was performed, which occurred for most of the samples. As the flasks contained different sample volumes, we proceeded to gauging at 100, 150, 200, 250 or 750 mL, depending on the density of the sample. That is, the denser the sample, the larger the gauging volume. For a low density sample, the entire sample was evaluated. Once the subsample was separated, it was deposited on a 60-µm porosity sieve and rinsed with distilled water repeatedly until the formaldehyde content (initial fixation at 4%) and organic matter and/or sediment were eliminated. Then, using a Pasteur pipette, 1-mL aliquots of sample were placed in a Sedgewick-Rafter counting chamber (to count organisms in only 1 mL of water). Sample contents were analyzed under a compound optical microscope with 10x objectives and eyepieces. The quantified zooplankton was expressed in org/L. For rotifers we used the identification keys of Rutner-Kolisko (1974) and Koste (1978); for cladocerans the keys of Ray (1991), Paggi (1995), Covich & Thorp (2009); for copepods the keys of Bayly (1992) and Reid (1985). 

We recorded twenty genera of rotifers, 13 of cladocerans and 7 of copepods, as well as juvenile individuals of the 3 orders of copepods (calanoid, cyclopoid and harpacticoid copepods) and nauplii. Small Herbivores (i.e. rotifers and nauplii; 59 ± 44 org/L) represented double densities of Large Herbivores (cladocera and calanoid copepods and copepodites; 29 ± 35 org/L). Carnivores (cyclopoid copepods and rotifers Polyarthra and Hexarthra) only reached ≤ 1 ± 2 org/L. 

Spatial distribution maps of zooplankton

The most abundant trophic group was Small Herbivores, with Keratella and nauplii standing out, then Brachionus (only in some stations), Anuraeopsis c.f. and Synchaeta. Large Herbivores were represented by cyclopoid copepodites and the cladoceran Ceriodaphnia, followed by Daphnia and calanoid copepodites. Only in the northwestern zone did they reach 50% of the Small Herbivores; in the central zone, Katari and northeastern zones only 7% to 10%. Among the Carnivores, adult cyclopoid copepods were the most abundant, mainly Metacyclops, representing only 1% to 2% of the Large Herbivores.

No pattern of abundance by depth was observed. Organisms were most abundant during the wet season. Several mechanisms could be responsible, such as: the higher photosynthetic active solar radiation (PAR) that favors photosynthesis and consequently the secondary production of zooplankton; the increased discharge of allochthonous nutrients through the basin during the rainy season, with greater influence in the littoral zones.