Utilization of dissolved organic matter (DOM), from living macrophytes, by pulmonate snails: Implications to the ''food web'' and ''module'' concepts

The rates at which freshwater macrophytes (axenic and nonaxenic Lemna minor L. and nonaxenic Ceratophyllum demersum L.) accumulated inorganic carbon (IC) into plant organic carbon (PLOC) and then released it as dissolved organic carbon (DCC) and fine particulate organic carbon (FPOC) were measured by incubating the plants in media containing C-14-labelled inorganic carbon (IC = CO2 + HCO3-). Both the rate of fixation of C-14 and the rate of loss of PLO(14)C were higher in the case of the faster growing Lemna than for Ceratophyllum. The fraction of the C-14 fixed by nonaxenic Lemna and Ceratophyllum and then released as (DOC)-C-14 increased significantly with time to give values of 4.2% and 4.6%, respectively, after 9 days' incubation. There were no significant differences between the rates of release of (DOC)-C-14 by axenic and nonaxenic Lemna but nonaxenic Lemna released significantly more of the fixed C-14 as (FPOC)-C-14 (1%) than axenic Lemna (0.1%). Nonaxenic Ceratophyllum released significantly more (FPOC)-C-14 (8.2%) than Lemna (0.9%) after 9 days of conditioning. Some of the C-14 in the (DOC)-C-14 accumulated as carboxylic acids (C-6, C-7 C-16, C-18) in the surface film. Living adult snails incubated for 38 hr in (DOC)-C-14, from nonaxenic Lemna, accumulated significantly more of the C-14 in their body tissues, haemolymph and shell (9.5%, 6.3%, 0.4% of the total carbon, respectively; concentration factor 10-15) than control snails. Snails incubated in media with (DOC)-C-14 from axenic Lemna accumulated proportionately less of the C-14 label and produced more respiratory CO2 than snails incubated in media from nonaxenic Lemna. The results are discussed with particular reference to the food web and modular concepts. (C) 1997 Elsevier Science Inc.