Patterns of primary and heterotrophic productivity in an arid lowland river

Abstract

Three river conceptual models make differing predictions about the major source of primary production in lowland rivers, acknowledging the importance of primary productivity in the ecology and management of lowland rivers. Patterns of primary production in lowland rivers are still an area of considerable uncertainty. The objective of this study was to examine the major sources and transformations of organic matter in an Australian lowland river and compare them to the predictions of existing models. The broad approach adopted was to quantify the contribution from the major ecosystem components and compare these with estimates of system metabolism determined using open water measures of diel oxygen change. Three 4-km river reaches were selected to represent the extent of variation found along the free-flowing lowland sections of the Murray River, one of Australia's largest and most regulated rivers. Annual open water gross primary production (GPP) estimates for the Murray R. during this study ranged from 221 to 376 gC m À2 y À1 and were similar to other large rivers. Examination of the net contribution of organic matter to the channel indicates that primary productivity in the Murray R. is derived from a combination of phytoplankton, riparian vegetation and macrophytes, but that the major source varies both spatially and temporally. The present study confirms that the River Continuum Concept (RCC), the Flood Pulse Concept (FPC) and Riverine Productivity Model (RPM) all have some application to Australian lowland rivers, but that synthesis of the models will be difficult until we can incorporate the extent, causes and consequences of primary production variability. This study also highlights the importance of the microbial loop and macrophytes in the ecology of the Murray R.

Longitudinal trends in river functioning: Patterns of nutrient and carbon processing in three Australian rivers

“…Nevertheless, there was also a downstream trend of increasing chlorophyll-a concentrations from LR1 to LR4. This finding is consistent with predictions of water column productivity in riverine systems, with water column productivity and biomass highest in the lower and slower flowing sections of rivers than in the more upstream reaches (Thorp and Delong, 1994;Gawne et al, 2007). In the Logan River, high enzyme activity levels were observed at all sites for both the enzymes responsible for decomposing carbohydrates and those involved in obtaining phosphate.…”

“…Longitudinal trends in the source, concentration and composition of carbon and nutrient pools, and how they influence river function, represent an important yet often overlooked component of river management and understanding (Gawne et al, 2007). This is despite the fact that longitudinal connectivity and transport of both nutrients and carbon within river systems have received considerable conceptual attention.…”

The effects of dams on longitudinal variation in river food webs

“…Understanding longitudinal variation in the contributions of different energy sources to river food webs is important for both ecological theory and river management (Gawne et al 2007). The River Continuum Concept of Vannote et al (1980) suggests that the dominant food resources for primary consumers vary in a predictable manner from headwater streams to large rivers.…”

Flow variability and longitudinal characteristics of organic carbon in the Lachlan River, Australia

“…Given the hydrological variability, it has been hypothesised that floodplain-derived carbon is an important source of energy for aquatic food webs; however, regulation has limited the contribution of floodplain-derived carbon (Gawne et al 2007;Robertson et al 1999). Recent studies have found that autochthonous carbon plays a more significant role than predicted in riverine primary productivity (Oliver and Merrick 2006).…”

“…In the regionally significant Murrumbidgee River, Vink et al (2005) found that the ecosystem was predominantly heterotrophic during a period of in-channel flow but there was a trend of increasing autotrophic activity downstream. Additionally, Gawne et al (2007) found that both heterotrophic and autotrophic productivity were significant contributors to the productivity of the Murray River but that this varied seasonally. These studies have largely taken place under extended low-flow conditions (Bond, Lake et al 2008); however, recent floods in New South Wales, Australia, provided the opportunity to assess carbon dynamics under both low and high flows.…”