Trophic coupling between heterotrophic flagellates and bacteria in hypertrophic shallow lakes

FERMANI, P.; DIOVISALVI, N.; TORREMORELL, A.; ZAGARESE, H.; UNREIN, F.

Simposio; 12th Symposium on Aquatic Microbial Ecology; 2011

Gasol (1994) proposed a model to describe the maximum attainable abundance (MAA) of heterotrophic flagellates (HF) as a function of the heterotrophic bacteria (HB) abundance. He also defined the degree of uncoupling (D) as the difference between the MAA and the realized HF abundance. Several authors had suggested that top-down control of HF should prevail as the trophic state of lakes increases, consequently, D should also increase with trophic level. Here, we investigate the degree of HF-HB coupling / uncloupling in a hypertrophic shallow (Laguna Chascomús, Argentina) and compare the results with Gasol’s model and additional records from other hypertrophic lakes. The HB abundance in Chascomús (up to 1.1 x 108 cells mL-1) often exceeded the highest values included in Gasol's model. In addition, autotrophic picoplankton abundances were also high (max. = 2.7 x 107 cells mL-1). A shift in the dominant zooplankton was observed during two consecutive summers: small cladocerans dominated during 2007-2008 and rotifers during 2008-2009. Plotting our data of HF-HB in the model, two distinct clusters were identified, corresponding to both periods. Higher D values were estimated towards the end of the study. Changes in picoplanktonic preys were unrelated to the contrasting degrees of HF-HB coupling, whereas changes in the abundance and composition of zooplankters seemed to determine a differential top-down control of HF observed in both years. The high rotifer abundance recorded in 2008-2009 (up to 5000 ind. L-1) exerted a stronger top-down control than small cladocerans. Moreover, the abundance of rotifers correlated positively with D. As a rule, the hypertrophic lakes included in this revision show that most datapoints occur above the mean realized abundance (except lakes dominated by Daphnia or characterized by extremely high rotifer abundances). Thus, the available evidence does not provide support for the prediction of increasing of D towards higher trophic levels.