The riverine bioreactor: an integrative perspective on biological decomposition of organic matter across riverine habitats

Riverine ecosystems can be conceptualized as ‘bioreactors’ (the riverine bioreactor) which retain and decompose a wide range of organic substrates. The metabolic performance of the riverine bioreactor is linked to their community structure, the efficiency of energy transfer along food chains, and complex interactions among biotic and abiotic environmental factors. However, our understanding of the mechanistic functioning and capacity of the riverine bioreactor remains limited. We review the state of knowledge and outline major gaps in the understanding of biotic drivers of organic matter decomposition processes that occur in riverine ecosystems, across habitats, temporal dimensions, and latitudes influenced by climate change. We propose a novel, integrative analytical perspective to assess and predict decomposition processes in riverine ecosystems. We then use this model to analyse data to demonstrate that the size-spectra of a community can be used to predict decomposition rates by analysing an illustrative dataset. This modelling methodology allows comparison of the riverine bioreactor's performance across habitats and at a global scale. Our integrative analytical approach can be applied to advance understanding of the functioning and efficiency of the riverine bioreactor as hotspots of metabolic activity. Application of insights gained from such analyses could inform the development of strategies that promote the functioning of the riverine bioreactor across global ecosystems

Natural events of anoxia and low respiration index in oligotrophic lakes of the Atlantic Tropical Forest

Hypoxia is a well-recognized condition reducing biodiversity and increasing greenhouse gas emissions in aquatic ecosystems, especially under warmer temperatures of tropical waters. Anoxia is a natural event commonly intensified by human-induced organic inputs in inland waters. Here, we assessed the partial pressure of O2 (pO2) and CO2 (pCO2), and the ratio between them (represented by the respiration index, RI) in two oligotrophic lakes of the Atlantic Tropical Forest, encompassing dry and rainy seasons over 19 months. We formulated the hypothesis that thermal stratification events could be coupled to natural hypoxia in deep waters of both lakes. Our results indicated a persistence of CO2 emissions from these tropical lakes to the atmosphere, on average ± standard error (SE) of 17.4 mg Cm-2h-1 probably subsided by terrestrial C inputs from the forest. Additionally, the thermal stratification during the end of the dry season and the rainy summer was coupled to anoxic events and very low RI in deep waters, and to significantly higher pO2 and RI at the surface (about 20 000 μatm and 1.0, respectively). In contrast, the water mixing during dry seasons at the beginning of the winter was related to a strong destratification in pO2, pCO2 and RI in surface and deep waters, without reaching any anoxic conditions throughout the water column. These findings confirm our hypothesis, suggesting that lakes of the Atlantic Tropical Forest could be dynamic, but especially sensitive to organic inputs. Natural anoxic events indicate that tropical oligotrophic lakes might be highly influenced by human land uses, which increase organic discharges into the watershed. © 2012 Author(s). CC Attribution 3.0 License

Planktonic production and respiration in a subtropical lake dominated by Cyanobacteria

Planktonic primary production and respiration rates were estimated in a subtropical coastal lake dominated by Cyanobacteria in order to investigate the temporal and vertical variation in this lake and to evaluate its relationships with limnological variables and phytoplankton. Light and dark bottles were incubated at four different depths in the central part of the lake and were performed bimonthly from June/2009 to December/2010. No significant difference was evident among depths in relation to phytoplankton, limnological variables and metabolic rates. However, the highest production rates were recorded at the surface, and decreased towards the bottom, coupled with phytoplanktonic photosynthetic capacity. Wind induced mixing in Peri Lake played an important role in nutrient and phytoplankton redistribution, characterizing this lake as polymictic. According to density and biovolume, the phytoplankton community was dominated by filamentous Cyanobacteria, especially Cylindrospermopsis raciborskii (Woloszynska) Seenayya and Subba-Raju. This study has shown that both water temperature and nutrient availability drive phytoplankton growth and consequently the temporal variation in metabolic rates, where respiration is higher than primary production

Inter- and intra-annual variations of pCO<inf>2</inf> and pO<inf>2</inf> in a freshwater subtropical coastal lake

© International Society of Limnology 2015. Inland waters emit significant amounts of carbon dioxide to the atmosphere, but tropical and subtropical lakes are underrepresented in current assessments. Here we present results of a 6-year study of the dynamics of surface partial pressures of carbon dioxide and oxygen (pCO2 and pO2) in a subtropical lake, Lake Peri, Brazil, to determine how temperature, rainfall, and wind moderate surface concentrations. Both pCO2 and pO2 tended to increase during the transitions between seasons when rainfall increased, with pCO2 averaging 2.5-3-fold higher than atmospheric values. Occasionally during autumn/winter, pCO2 similarly increased and pO2 decreased. We infer that the increases in both gases during the transition periods resulted from increasing inputs of allochthonous material into the lake. Those in winter resulted from near-bottom intrusions that intermittently reach the depth of measurement. In autumn/winter, pCO2 was 3-fold higher (average 1700 μatm) compared to spring/summer (550 μatm), whereas changes in pO2 did not have a clear seasonal pattern. Overall median net CO2 evasion was 11 mg C m-2 d-1. Variability in the extent of rainfall and the associated high intra- and inter-annual variability in CO2 and CO2 emissions are in part controlled by atmospheric processes related to the South American Monsoon System and to El Niño Southern Oscillation cycles