Impacts of added dissolved organic carbon on boreal freshwater pelagic metabolism and food webs in mesocosm experiments

Abstract

We studied the response of pelagic community metabolism, bacterial community structure and the origin of crustacean zooplankton carbon following additions of two different sources of dissolved organic carbon (DOC), natural humic water and cane sugar, in two mesocosm experiments lasting 15-18 days. Experimental water with the natural plankton community originated from two boreal lakes in southern Finland having similar total organic carbon (ca. 11 mg L -1 ) and total phosphorus (ca. 13 μg L -1 ) content, but differing in total nitrogen content (ca. 1450 and 440 mg L -1 ). The experiments demonstrated limitation of bacterial activity mainly by biodegradable DOC in the high nitrogen lake water, and by nitrogen in the low nitrogen lake water, which was also reflected in total pelagic energy metabolism and zooplankton productivity. DOC additions caused changes in the bacterial community structure, shown by length heterogeneity analysis of the PCR amplified 16S rRNA gene (LH-PCR). Cane sugar, differing in stable carbon isotope value (δ 13 C ca.-11‰) from that of humic matter of boreal terrestrial vegetation (δ 13 C ca.-28‰), proved to be a good tracer of DOC via bacteria to metazooplankton, especially in the nitrogen-limited conditions. Utilization of detrital particulate terrestrial organic matter by zooplankton was also evident, but it appeared to be a poor quality food. Our results show that an increased loading of allochthonous DOC to boreal lakes as predicted under future climate scenarios will not necessarily stimulate bacterial production and hence carbon transfer up the food web. Whether this happens will be strongly dependent on the stoichiometry (C: N:P ratios) of the available resources.