Heterotrophic bacterial production in the lower Murray River, south-eastern Australia

Abstract

Bacterial production is important in aquatic carbon cycles because it represents a key component whereby dissolved and particulate carbon can be recycled back into food webs. Despite its acknowledged importance, few studies have examined bacterial production in lowland rivers. Since studies have suggested bacterial production is closely related to some carbon pools, we anticipated this to be the case in the Murray River, but that the timing and type of carbon inputs in the Murray River may lead to bacterial dynamics that differ from studies from other sites. Bacterial abundance and production were measured at three contrasting sites of the lowland Murray River, southeastern Australia, over an 18-month period. Bacterial abundance varied across the three sites on the Murray River and was correlated with chlorophyll a concentrations but not with temperature, nutrients, particulate organic carbon and dissolved organic carbon concentrations. Bacterial production also varied across the sites. Lowest production was at the site most immediately downstream of a large reservoir, with production generally ranging from 0.88 to 8.00 µg C L −1 h −1 . Bacterial production in a reach within a large forest ranged from 4.00 to 17.38 µg C L −1 h −1 . Production at the reach furthest downstream ranged from 1.04 to 23.50 µg C L −1 h −1 . Bacterial production in the Murray River was generally greater than in the European River Spree, reaches of the Meuse and Rhine without immediate impacts from major urban centres and the Amazon River, but was similar to the concentration measured in the Mississippi and Hudson Rivers. Bacterial production was closely correlated with chlorophyll a concentration and total phosphorus, but not with temperature, dissolved organic carbon, particulate organic carbon or inorganic nitrogen. Despite the differences in production and respiration measured at different sites across the Murray River, bacterial growth efficiency was very similar at the three sites. Bacterial populations in the Murray River appear to be influenced by reach-specific conditions rather than broad-scale drivers such as temperature, carbon and nutrient concentrations.

Limitation of lowland riverine bacterioplankton by dissolved organic carbon and inorganic nutrients

“…However, as Rees et al (2005) suggest this does not lessen the potential role of other carbon fractions (e.g. coarse particulate organic matter) in driving bacterial production.…”

“…The changes in aquatic ecosystem function when allochthonous sources of DOC enter a lowland river are not well understood (Rees et al, 2005). Metabolism in many aquatic environments is significantly subsidised by terrestrial sources of carbon, yet few studies have examined the fate of allochthonous carbon (Kritzberg et al, 2006).…”

Microinvertebrate dynamics in riverine slackwater and mid‐channel habitats in relation to physico‐chemical parameters and food availability

“…This parameter is widely accepted as a good measure of food availability for microinvertebrates (Bonecker and Lansac-Tôha, 1996;Kobayashi, 1997), since algae are regarded as the predominant and nutritionally important food resource for many microinvertebrate taxa (Santer and Lampert, 1995;Guelda et al, 2005). In addition, chlorophyll-a concentration is often positively correlated with other potential sources of microinvertebrate food such as bacterial production (Rees et al, 2005) and protist abundance (Kimura et al, 2001). Samples were collected using one litre opaque plastic jars, put on ice, and sheltered from light.…”

Patterns of primary and heterotrophic productivity in an arid lowland river

“…Microbial production was estimated from the rate of tritiated thymidine incorporation into bacterial macromolecules (Rees et al, 2005). Briefly, 50 ml of 2-Ci mmol À1 [methyl-3 H] thymidine was added to six replicate time-course series, each reaction containing 4 ml of water, which were incubated at in situ temperatures in the dark.…”