6 Continental J. Fisheries and Aquatic Science 2: 6 - 12, 2008 © Wilolud Online Journals, 2008. A MESOCOSM ANALYTICAL STUDY ON THE IMPACT OF FRESHWATER MUSSEL ( LAMELLIDENS MARGINALIS LAMARCK) MEDIATED BIOTURBATION AND BIODEPOSITION ON SOME ECOLOGICAL FACTORS OF A FRESHWATER LAKE P. Jayakumar, N. Jothivel, A. Thimmappa and V.I. Paul Department of Zoology, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India. ABSTRACT The biotic potential of the benthic filter feeding freshwater bivalve mollusc Lamellidens marginalis (Lamarck) influencing the nutrient dynamics of the bottom sediments of the lake by means of biodeposition and bioturbation activities were analysed using a lake mesocosm experiment. Five control as well as experimental mesocosms was maintained up to 60 days (d). The factors studied included the percentage of water content of the sediment, percentage of total nitrogen, percentage of organic matter along with the total phosphorus and humic acid content. While total phosphorus and humic acid content of the experimental mesocosoms showed gradual and significant increases from 30d of the experiment to reach the maximum levels after 60d, the percentage of organic matter registered significant increases right from 15d onwards and reached the maximum values after 60d. On the other hand, while the percentage of water content of the sediments of the experimental mesocosoms increased only up to 30d experiment, percentage of nitrogen was increased during the first half and at the fag end of the experiment. All the investigated ecological factors were found to be significantly influenced by the presence of L. marginalis in the experimental mesocosms. The study indicated that the mussel influence the nutrient dynamics of the inhabitant ecosystem through the processes of excretion, biodeposition of pseudofaeces and faeces, along with the bioturbation of the sediments brought about by their ploughing movements. KEYWORDS: freshwater mussel, Lamellidens marginalis, bioturbation, biodeposition, mesocosms. INTRODUCTION The freshwater mussel (Lamellidens marginalis Lamarck) is a benthic filter feeding organism and is continuously exposed to the water, suspended particles in the water column and bottom sediments. The biotic potential of L. marginalis even though largely remains un-attended, plays very important roles in the ecosystem functions. According to Vaughn and Hakenkemp (2001), freshwater bivalves have the potential to strongly influence the ecosystem processes in freshwater systems. This also holds true with L. marginalis because of their characteristic filter feeding and ploughing movements through the bottom sediments. While filter feeding is an important means of removing particles including plankton suspended in the water column (Widmeyer and Bendell-Young, 2007) and biodepositing it to the bottom sediments as faeces and mucous bound pseudofaeces, the ploughing movements and burrowing activity brings in active bioturbation of the medium leading to sediment mixing, improved oxygen penetration and affects other ecological functions. Due to all these activities mussels can repackage nutrients and act as a nutrient source for other benthic organisms (Christian et al., 2008). Unionid mussels are historically important bioturbating macrobenthic organisms and as they can move and disturb large amounts sediments, they may be designated as “biological bull dozers”. They reportedly burrow themselves and mix the bottom sediments (Vaughn and Hakenkemp, 2001). The digging and burrowing activities of L. marginalis leading to the bioturbation of the bottom sediments is a form of ecosystem engineering. However, according to Vaughn and Hakenkemp (2001), uncertainty over the extent and importance of sediment-related ecological processes performed by bivalves represent the most significant gap in our understanding of the role of burrowing bivalves in freshwater ecosystems. The influence of bioturbators in altering the conditions at the sediment water interface is reported to be due to the biogenic mixing of sediments (Christian et al., 2004; Solan et al., 2004; De Haas et al., 2005). In this context, attempts have been made in this work to understand the ecological importance of bioturbation and biodeposition by the freshwater mussel L. marginalis through a lake mesocosm experiment.