Vertical distribution of zooplankton in a seepage lake (Lacombe Lake, Argentina) during an annual cycle

ARDOHAIN, D.M.; GABELLONE, N.A.; CLAPS, M.C.

Punta del Este, Uruguay

Congreso; VI Shallow Lakes Congress: Structure and Function of World Shallow Lakes; 2008

The shallow Lacombe Lake is located in the Pampean plain (35º 50` S - 57º 53` W). Its area is approximately 130 ha with a maximum length of 1750 m and a maximum width of 1500 m. The shoreline length is 5.6 km and the maximum depth during the sampling period was 2.5 m. The lake is colonized by emergent (Scirpus californicus Steud) and submerged macrophytes (Potamogeton pectinatus L and Myriophyllum quitense HBK). Extensive cattle breeding and agriculture are developed in the surroundings. Sport fishing takes place all year round for the planktivorous Odonthestes bonariensis Cuvier et Valenciennes and predatory Hoplias malabaricus Bloch. The zooplankton samples were obtained at three stations in a vertical profile divided into five layers at each site, A volume of 100 l was pumped from each layer into a 35 μm mesh net. The samples were taken at three or four-week intervals during the period July 2001 - June 2002. Simultaneously, temperature, pH, dissolved oxygen, conductivity and turbidity were measured with a Horiba Multimeter U-10. 79 species were identified: 14 protists, 48 rotifers and 17 crustaceans. Rotifers predominated mainly in summer and autumn. The number of rotifer species usually exceeded those of cladocerans and copepods. Significant differences during the annual cycle were found (Kruskall±Wallis test) for turbidity (p < 0,001), total phosphorus (p < 0,001), temperature (p < 0,001) and conductivity (p < 0,001). The temporal differences of density and biomass of the main zooplankton groups (rotifers, cladocerans and copepods) were significant (P = < 0,001).  The analysis of  principal component of physical, chemical and biotic variables showed the existence  of three phases during the annual cycle. The sequential phases identified correspond to the following months: July, August, September and October (turbid phase); November, December, January and February (Clear phase with macrophytes) and March, April, May and June (clear phase without macrophytes). The highest zooplankton densities were recorded during the clear phase with submerged macrophytes related to their important role in the community structure, acting as refuge, providing food (epiphyton) and changing i predator prey interaction. During the turbid phase benthic cladocerans predominated related to shallowness of the water column and their ingestion capacity of bacteria, and detritus. The euplanktonic cladocerans founded enhanced conditions during the clear phase with macrophytes. The adults of calanoids were registered in low number all the year due to the intense and continuous predation pressure of O. bonariensis. Considering that the planktivorous fish’s performed an uniform predation pressure, the main factors involved in changes of the zooplankton structure can be categorized in physical non aleatory such seasonality and internal biological such the beds of submerged macrophytes. The response of the zooplankton groups to these factors was dissimilar related to the history life of the involucrate species.