Effects of submerged and emergent macrophytes and predation risk on the diel distribution of
cladocerans in subtropical Lake Blanca (Uruguay, South America)
Carlos Iglesias1, Guillermo Goyenola1, Mariana Meerhoff 1,2 & Néstor Mazzeo1
1-Sección Limnología, Facultad de Ciencias, Universidad de la República, Iguá 4225, CP 11400, Montevideo-Uruguay
2-Universidad de Silkeborg. Silkeborg Denmark.
E-mail: caif@fcien.edu.uy
Introduction
In shallow temperate lakes the presence of macrophyte play a key role in fish-zooplancton interactions. The
presence of planktivorous fish may alter the distribution of zooplankton. Timms & Moss (1984) proposed that
pelagic zooplancton move into macrophyte beds during daytime using it as spatial refuge against fish
predation performing diel horizontal migrations (DHM) between open waters and the strucutred nearshore
zone. Jeppesen et. al (1997) showed that the presence and extent of DHM is conditioned not only by the type
and density of macrophytes but by the density and distribution of predators.
Lazzaro (1997) pointed on the difficulty to aplly this framework to tropical and subtropical lakes without a
previous carefull testing of them based on the difference principally of fish community.
The objective of this work is to determine how Cladocerans are using the different compartmens in the lake
and how the distribution of predators (planktivurous fish and Chaoborus larvae) influence their distribution
and the perform of DHM.
Methods
Lake Blanca (34◦54 S; 54◦50 W) is a shallow eutrophic system (total area = 40.5 ha and Z max = 1.5–3.6 m)
with no fishivurous fishes, located in the main tourist area of Uruguay, and used for water supply. Stratified
seasonal sampling were carried out from winter 2003 to autumn 2004, samples were taken at 5 points by
stratta (open waters (OW), submerged macrophytes, Egeria densa & Ceratophylum demmersum (SP), and
emergent macrophytes, mainly Schoenoplectus californicus (EP) at midday and midnight once for each
year’s season. Integrated zooplankton samples were taken using a core sampler. Fish community were
sampled with electrofishing and meanows traps. Differences in day night densities were tested by ANOVA’s
proves. To detect the relation between predator and zooplankton and fihysicochemical and zooplakton
matrixes we conducted Mantel’s tests.
Results and discussion
Zooplankton community comprised 16 taxa of rotifers, 3 of copepods, and 5 of cladocerans. In auttum we
found the highest abundance ca. 2400 org.l-1, the other seasons were quite simmilar with ca. 1000 org.l-1 .
Mesofiltrators dominated in winter while microfiltrators dominated in auttum, spring and summer.
Cladocerans Bosmina longirostris Müller, Moina micrura Kurz, Diaphanosoma birgei Korineck, Alona spp.
and Chydorus spp. were founded allaround the year with higher abundances in winter and auttum. The
relative composition varied between seasons B. longirostris dominated in auttum and winter, Chidorus spp. in
spring and summer, speccially in PS and PE. D. birgei was present the whole year and dominated the OW
(with M. Micrura) in spring and practically disapeared in summer.
We found evidence of DHM for D. birgei and B. longirostris. Both of them showed high abundance in OW
and PE during the day and low or near zero in PS. By night OW and PE abundances decreased significantly
and rised 50-fold in EP. Diaphanosma birgei showed a strong patern compared with B. longirostris wich is
consistent with observations that large cladocerans are more susceptible to undergo DHM because of the
higher predation risk. Fish community comprised only two species, the planktivurous Jenynsia multidentata
and Cnesterodon decemmaculatus. J. multidentata was the dominant and we found it strongly associated with
EP. We also founded the invertebrate predator Chaoborus spp. mainly in OW and PE and by night. The
evidence of a predator induced DHM behaviour is supported by the Mantel’s tests results that showed a non
significant relation between the zooplankton and the physicochemical matrixes, although there were
significant physicochemical differences betweeen compartmens, and significant with the predator matrix
(r=0.153, p<0.05). We conclude that the combined effect of fish and invertebrate predation risks could be a
good explanation for the undergoing DHM evidenced for B. longirostris and D. Birgei.
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