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

Abstract

Understanding longitudinal trends in the processing of carbon in rivers represents a much conceptualised, but infrequently tested, issue in aquatic ecology. In this study, we conducted concurrent longitudinal examinations of three very different rivers in eastern Australia to determine whether general principles in river functioning exist across broad geographic and hydrologic scales. Specifically, we examined trends in ambient basic water chemistry, nutrient concentrations, dissolved organic carbon (DOC), extracellular enzymes and food web structure and functioning and conducted bioassays to examine the degree to which DOC and nutrients limit heterotrophic bacterial respiration. These parameters revealed striking similarities across all sites. For metazoan communities, stable isotope analysis showed that algal carbon was the dominant basal resource utilised by consumers in all three rivers, suggesting that in-stream primary producers strongly underpin trophic pathways regardless of the position within a catchment or catchment condition. Analyses of extracellular enzymes revealed that microbial communities are actively utilising DOC at all sites. In fact, heterotrophic microbial respiration was strongly limited by DOC at all sites, with nutrient additions resulting in only relatively minor increases in respiration. Ultimately, this study demonstrates that DOC and algal carbon are critically important drivers of ecosystem processes in Australian riverine ecosystems. Furthermore, across all of our sites and rivers, ambient nutrient concentrations did not influence carbon processing. The consistent longitudinal trends in river function identified in this study provide useful insights for catchment managers and modellers with respect to identifiying key principles that underpin ecosystem functioning in Australian rivers.

Sources and Dynamics of Dissolved Inorganic Carbon, Nitrogen, and Phosphorus in a Large Agricultural River Basin in Arid Northwestern China

“…Hence, future regulatory efforts in reducing nutrient loss from soils should place priorities on mitigating runoff from agricultural fields in storm seasons (i.e., June to August). Second, the values of inorganic N and P nutrients in river water and groundwater observed in the present study are largely comparable to those reported in other large arid river basins, such as the San Joaquin River in California [39], the Murray River in Australia [40,41], and the Tigris River in Turkey [42]. These rivers have been all extensively exploited to support agricultural water use, where a significant fraction of river flows (up to 80%) can be diverted for irrigation.…”

River bacterioplankton community responses to a high inflow event

“…Similarly, lotic bacterial community assemblages can shift seasonally, with patterns in bacterial composition typically driven by algal (auto chthonous) carbon during low precipitation seasons and by allochthonous carbon during wetter seasons (Almeida et al 2005, Pinhassi et al 2006, Hitchcock & Mitrovic 2013). Compared to marine environments, bacterial abundance, productivity and respiration rates can be significantly higher in lotic systems (Del Giorgio & Cole 1998, Del Giorgio & Williams 2005 due to higher nutrient inputs from terrestrial inputs (Cole & Caraco 2001, Pollard & Ducklow 2011, and as a consequence riverine ecosystems are often net heterotrophic (Hadwen et al 2010). Our knowledge of the ecology of lotic microbes is much less developed than our understanding of marine and lake microbial communities.…”

“…These studies have indicated that heterotrophic bacterial growth in rivers and estuaries is often limited or co-limited by available DOC, N and P during periods of low river discharge (Jansson et al 2006, Hadwen et al 2010, Hitchcock et al 2010, Hitchcock & Mitrovic 2013), but very little is known about how these events influence the composition and diversity of bacterial assemblages, or the implications for lotic chemical cycling and trophic dynamics. This knowledge is of growing importance because there is a global pattern of high flow events being returned to many major coastal rivers and estuaries as a result of flow management (Dudgeon 2010).…”

Aquatic Fungi

“…Fungal biomass increases with increasing concentration of nitrogen and phosphorus in the water column (Sridhar & Bärlocher, 1997) and decreases with lower dissolved oxygen concentrations (Medeiros et al, 2009). Thus the productivity of fungi and their importance as organic matter producers vary with climate and the availability of nutrients and organic substrates (Ferreira & Chauvet, 2010), and in some instances fungal production will not be a significant resource for the aquatic community (Bunn & Boon, 1993;Hadwen et al, 2010). Additionally, productivity will also be limited by ecological interactions such as competition and mycotrophy (Newell & Bärlocher, 1993;Kagami et al, 2004;Lepere et al, 2007), and physical changes such as burial (Janssen & Walker, 1999;Cornut et al, 2010).…”