Factors influencing zooplankton body-size along a latitudinal gradient in South America

LACEROT, G., PAGGI, J.C., JOSÉ DE PAGGI, S.,BRANCO, C., COIMBRA, L., LURLING,M., KOSTEN, S., MAZZEO, N.JEPPESEN, E. SCHEFER, M.

Punta del Este, Uruguay

Simposio; Shallow Lakes Conference; 2008

Universidad de la República, Montevideo, Uruguay

Factors influencing zooplankton body-size along a latitudinal gradient in South America. Lacerot G (1,4)., Paggi, J. C. (2), José de Paggi, S. (2), Branco, C.C. (3), Coimbra, L. (3), Lürling, M. (4), Kosten, S. (4), Mazzeo, N. (1), Jeppesen, E. (5) & M. Scheffer (4) (1)Ecology Department, Faculty of Sciences.Universidad de la República Montevideo-Uruguay, (2) INALI Santo Tomé, Santa Fé-Argentina, (3) NERI-UNIRIO RJ-Brazil, (4) Aquatic Ecology and Water Quality Management Group. Wageningen University-Netherlands, (5) National Environmental Research Institute, Aarhus University, Silkeborg, Denmark It has been observed that zooplankton species in tropical lakes and ponds are generally much smaller than in temperate ones. In particular, large herbivorous zooplankton like Daphnia seem less diverse or absent at lower latitudes. Possible explanatory processes for this pattern include: temperature-mediated increase of physiological rates, lower food quality and stronger predation pressure. However, many of these studies are based on literature reviews with few available explanatory variables or include a small latitudinal range. To asses this issue we analyzed zooplankton body size distribution from 82 shallow lakes along a latitudinal gradient in South America (5-55oS). In each lake we registered several variables including temperature, total phosphorus in the water, and suspended solids and their organic, phosphorus and nitrogen content. We also sampled the fish community and calculated the fraction composed of small omnivorous fish (<10 cm). For the phytoplankton community we measured chlorophyll-a concentration and estimated the biovolume. The relation between zooplankton size and these variables was studied through regression analysis. We found that zooplankton mean body size decreased from temperate to tropical lakes, with minimum body sizes found at the middle (19-30oS) rather than at the lowest latitudes (R2=0.28, p<0.001). However, not all groups exhibited the same pattern. Rotifers and Cyclopoid copepods did not show variation in body size with latitude, while Calanoid copepods exhibited a weak relation (R2=0.12, p<0.001). Cladocera showed the strongest decrease in size from temperate to tropical lakes (R2=0.31, p<0.001). We also found that macrophyte-associated species of Cladocera did not vary their size while pelagic species showed the strongest relation among all groups tested (R2=0.39, p<0.001). Multiple stepwise regression analysis showed that pelagic Cladocera body size was negatively correlated to temperature and the fraction of small omnivorous fish , but positively related to chlorophyll-a (R2=0.38, p<0.001), where temperature and the fraction of small fish were the most important predictors. Our study shows that a combination of abiotic and biotic factors is important in determining zooplankton body size at different climates.