Organic carbon Mineralisation in a Subtropical River

Abstract

Microorganisms mediate the many biochemical transformations of aquatic ecosystems. Heterotrophic bacteria mineralise most of the total carbon of freshwater and marine ecosystems via the dissolved organic carbon (DOC) fraction. This paper describes the relationship between primary and heterotrophic microbial production in a major subtropical river in South East Queensland, Australia, in dry weather. Stormwater was not the major external source of organic carbon in dry weather. The rate of heterotrophic bacterial production was 4 g C m-2 d-1. The bacteria were decomposing over four times more organic carbon per day than the daily primary production (1.1 g C m-2 d-1). There was not sufficient organic carbon generated through photosynthesis to support the heterotrophic bacterial growth. The river ecosystem was net heterotrophic. However, the concentration of the DOC in the water column remained relatively constant. The bacteria mineralised the DOC pool every few days. This suggests that organic carbon was constantly being supplied to this aquatic ecosystem from a terrestrial source that was readily degradable. The organic carbon pollution was likely due to non-point sources related to human activities in the catchment.