Integrated Aquatic CO2 Emission into Boreal Landscape Perspective
As the largest biome on Earth, the boreal ecosystem plays a crucial role on the global C cycling. Freshwater systems as a major component of boreal landscape are highly saturated in carbon dioxide with respect to atmospheric equilibrium, and therefore, likely to contribute significantly to atmospheric C emissions. Yet, their role in regional or global C budgets is still unclear. We estimated the variability of total CO2 emissions from surface waters in a large block of boreal territory in Northern Quebec, by combining GIS-based landscape analysis with ecosystem-specific empirical models of CO2 emissions for streams, rivers and lakes. The resulting C emissions are not only regionally significant relative to other better known aspects of the boreal C balance, but also remarkably constant. Aquatic CO2 fluxes per unit landscape varied less than two-folds across this boreal landscape, despite a 10-fold range in aquatic distribution. Our results suggest a complex interplay between the physical and biogeochemical processes that determine the intensity of C emissions in the different types of aquatic systems, and the geomorphologic and climatic features that determine the distribution of the aquatic in the landscape. This rather constant C emission, for which we propose the general term of Net Biome Aquatic Carbon Evasion (NBACE), can be seen as a new property of the boreal biome next to GPP, NPP, NEP or NEE.
Integrating hydrology and ecology into perspectives of lake dissolved organic carbon cycling and chemical character
Dissolved organic carbon (DOC) present in aquatic ecosystems originates from both terrestrial and aquatic sources. A portion of the DOC pool becomes modified through in-lake processes including photodegradation and microbial metabolism. The extent of modification of the bulk DOC pool depends upon the relative magnitude of DOC delivery and in-lake processing. Despite the wide interest in DOC biogeochemistry in aquatic ecosystems, relatively few studies have directly compared the magnitude of DOC delivery and modification in lake ecosystems. We quantified the magnitude of hydrologic DOC export (DOC-X) and DOC biodegradation (DOC-R) in seven lakes with contrasting water residence time. In lakes with long water residence time, DOC-R was larger than DOC-X and chemical characteristics of the DOC pool including ultraviolet light (UV) absorbance and fluorescent properties (excitation-emission spectra, EEMs) indicated that it had undergone extensive modification due to in-lake processing. In contrast, DOC-X was predominant in lakes with short water residence time. UV absorbance and EEMs in these lakes suggested that this material was mostly allochthonous and had undergone relatively little in-lake processing. These results emphasize the importance of hydrology and ecology to lake DOC cycling and highlight the utility of UV absorbance and EEMs to characterize lake carbon biogeochemistry.
Whole watershed spectrometric and fluorometric characterization of dissolved organic matter in two large rivers in Canada with differing impact
Characterizing Dissolved Organic Matter (DOM) Isolated From Specific Allochthonous and Autochthonous Sources in a North-Temperate Stream Ecosystem
Detrital energy in temperate headwater streams is mainly derived from the annual input of leaf litter from the surrounding landscape. Presumably, its decomposition and other sources of autochthonous organic matter will change dissolved organic carbon (DOC) concentrations and dissolved organic matter (DOM) quality. To investigate this, DOM was leached from two allochthonous sources: white birch (Betula papyrifera) and white cedar (Thuja occidentalis); and one autochthonous source, streambed biofilm, for a period of 7 days on 3 separate occasions in fall 2007. As a second treatment, microorganisms from the water column were filtered out. Deciduous leaf litter was responsible for high, short-term increases to DOC concentrations whereas the amounts leached from conifer needles were relatively constant in each month. Using UV spectroscopy, changes to DOM characteristics like aromaticity, spectral slopes, and molecular weight were mainly determined by source and indicated a preferential use of the labile DOM pool by the microorganisms. Excitation-emission matrices (EEMs) collected using fluorescence spectroscopy suggested that cedar litter was an important source of protein-like fluorescence and that the nature of the fluorescing DOM components changed in the presence of microorganisms. This study demonstrates that simultaneous examination of DOC concentrations and DOM quality will allow a better understanding of the carbon dynamics that connect terrestrial with aquatic ecosystems.
A Mechanistic Model for Bubble Rise in Soft Sediments
Wetland and aquatic sediments (marine and freshwater) are significant sources of methane, due to the degradation of organic matter under anoxic conditions. Once produced, methane migrates from the sediments to the overlying water and eventually the atmosphere by diffusion or bubble ebullition. Ebullition is a significant source because it can release methane directly to the water column or atmosphere, bypassing the methane oxidizing zone, that consumes much of the diffusive flux of methane from sediments. Recently it has been shown that bubbles released from thermokarst lakes (lakes formed due to the melting of permafrost) in northern latitudes explain much of the historic glacial/interglacial methane cycle and represent what could be a significant positive feedback mechanism on the Earth's climate system. Therefore a thorough understanding of how bubbles migrate through sediments is necessary. Bubble release is episodic and patchy with periods of release often triggered by decreases in hydrostatic pressure. Here I present a mechanistic model for bubble rise is soft sediments. The model describes the rise of a single isolated bubble through the sediments and therefore describes the initial stages of bubble tube formation. The bubble migrates by propagating a fracture and the rate of rise is controlled by the viscoelastic response of the sediments to stresses induced by the bubble. This model predicts that a bubble will rise under its own weight when the long axis of the fracture reaches a critical length. This length is determined by the strength of the sediment. The model provides insights into the mechanism behind hydrostatic pressure release and predicts rise velocities as a function of measurable sediments properties. This will aid in the prediction of fluxes to the atmospheric from methane producing sediments and increases our understanding of this important link between the aquatic ecosystem and the atmosphere.
Water availability controls on community structure of an ephemeral meltwater stream ecosystem in the McMurdo Dry Valleys
The McMurdo Dry Valleys of Antarctica contain many glacial meltwater streams that flow during the summer into lakes on the valley floors. Many streams have thriving cyanobacterial mats that are freeze-dried in winter and begin photosynthesis when flow arrives. We studied the community structure in a formerly abandoned channel, which was reactivated by a flow diversion in 1994. Cyanobacterial mats became abundant in the reactivated channel within a week and have remained evident even through cold, low flow summers. We recently compared the abundance and species distribution of invertebrates and diatoms in the cyanobacterial mats and in hyporheic zone during cold (low flow) and warm (high flow) summers. During the warm summer, there were sites where the invertebrate abundance was greater in the mats than in the underlying hyporheic sediments. In contrast, during the cold summer the invertebrate biomass was lower in the mats than in the hyporheic sediments. These findings suggest that the optimal micro-habitat for invertebrates in these mats and sediments is partially driven by ephemeral stream hydrology. This limitation on potential invertebrate grazers (which are important nutrient transformers) may account for the accumulation of algal biomass and subsequent nutrient immobilization in the mats over many summers.
The Effects of Photolysis on DOC Fate, POC Production, and the δ13C of DOC and δ13C of POC in Forested Northern Temperate Streams.
Northern temperate and boreal lakes are generally net heterotrophic ecosystems, relying on dead or decaying organic matter produced in the terrestrial catchment for sustenance. Carbon dynamics within these lakes can be affected by a number of processes including photodegradation, microbial decomposition, and direct sedimentation of dissolved organic matter (DOM), in addition to gas exchange with the atmosphere and photosynthesis. Similarly, DOM photodegradation can also affect particulate organic carbon (POC) production within aquatic systems. During the photodegradation of DOM, dissolved oxygen is used as an electron acceptor in the overall oxidation of dissolved organic carbon (DOC) into lower molecular weight carboxylic acids and DIC. Although extensive research has addressed the production of DIC and the degradation of DOC from this process, minimal work has examined the fractionation of δ13C-DOC during this process. Furthermore, the production and the stable carbon isotopic signatures of POC during photolysis have been neglected. This study tracks the photolytic production of POC and degradation of DOC in 10 temperate forested streams and the isotopic effects of photodegradation in addition to the fractionation of DOC during this process. As δ13C applications in aquatic system studies currently do not separately include DOM photodegradation as a process affecting isotopic signatures, δ13C values from carbon cycling studies, and interpretations of δ13C records in lake sediments could be affected in high DOC aquatic systems.
Changes of Photochemical Properties of Dissolved Organic Matter During a Hydrological Year
The fate of dissolved organic matter (DOM) in lakes and streams is significantly affected by photochemical transformation of DOM. A series of laboratory photochemical experiments has been conducted to describe long term changes in photochemical properties of DOM. The stream samples used in this study originated from three different watersheds in Dorset area (Ontario, Canada), the first watershed has predominantly coniferous cove, the second one is dominated by maple and birch, and a large wetland dominates to the third one. The first order kinetic constant rate was used as a suitable characteristic of photochemical properties of DOM. The higher rates were observed in samples from watershed dominated by coniferous forest while the lower rates were determined in deciduous forest. Kinetic rates from all three watersheds showed sinusoidal pattern during the hydrological year. The rates increased steadily during autumn and winter and decreased during spring and summer. The highest values were observed during the spring melt events when the fresh DOM was flushed out from terrestrial sources. The minimum rate constants were in summer when the discharge was lower. The photochemical properties of DOM changes during the hydrological year and correspond to the seasonal cycles of terrestrial organic matter.