Structure of Tubificid and Lumbriculid Worm Communities, and Three Indices of Trophy Based upon These Communities, as Descriptors of Eutrophication Level of Lake Geneva (Switzerland)

Abstract

In 1977, the nearshore zone of Lake Geneva (Switzerland) between isobaths 10 and 40 m was divided into 17 areas. In each of these, 19 to 26 sediment cores were collected, the tubificid and lumbriculid worms quantitatively sampled and then pooled together. The subsequent biological analyses based on 17 pooled samples were undertaken to describe the eutrophication level of this zone. Four basic worm communities, each characterized by a different species, were singled out: (1) oligotrophic Stylodrilus heringianus; (2) mesotrophic Potamothrix heuscheri; (3) eutrophic Potamothrix hammoniensis; and (4) eutrophic Peloscolex ferox. Communities (3) and (4) were related to heavy inputs of allochthonous organic matter which also increases the worm density in these areas. Species were divided into three groups according to their value as trophic level indicators established in a previous study. Three indices of trophy based upon these results were calculated for each community: (1) an index of oligotrophy characterized by the relative abundance in the community of Stylodrilus lemani, S. heringianus, Peloscolex velutinus, and Potamothrix vejdovskyi; (2) an index of mesotrophy based mainly on Potamothrix heuscheri, Aulodrilus pluriseta, and Limnodrilus hoffmeisteri; (3) an index of eutrophy based mainly on Potamothrix hammoniensis, Tubifex tubifex, and Peloscolex ferox. According to these indices, most communities were mesotrophic (65%), 29% were eutrophic, and 6% oligotrophic. Total nitrogen and organic carbon in the sediment were negatively correlated with the index of oligotrophy, positively with the index of mesotrophy. The index of eutrophy was positively correlated with total phosphorus and heavy metal content in the sediment.