Jussi Huotari

Eddy covariance measurements of carbon exchange and latent and sensible heat fluxes over a boreal lake for a full open‐water period

was >0 W/m 2 at night and <0 W/m 2 in daytime. The latent heat flux dominated clearly over H in spring and summer; that is, the Bowen ratio was less than 1. Highermoment turbulence statistics proved to be efficient in detection of frequent nonstationary situations. Applying the statistical criteria for CO2 concentration and vertical wind speed, averaging over a 5-min period and selecting only the wind direction with longest fetch, we could obtain lake-representative CO2 fluxes. Footprint analysis based on a closure model revealed that the source areas were relatively short because of the presence of turbulence generated by the surrounding forest, compared to a larger lake with an extended smooth surface. We observed a net CO2 source of 0.2–0.4 mmol m � 2 s � 1 excluding July, when the flux was closer to zero. The results are consistent with the gradient method, based on more infrequent sampling, and both methods gave the same average flux, 0.2 mmol m � 2 s � 1 , over the whole open-water period.

Effect of catchment characteristics on aquatic carbon export from a boreal catchment and its importance in regional carbon cycling

Inland waters transport and emit into the atmosphere large amounts of carbon (C), which originates from terrestrial ecosystems. The effect of land cover and land-use practises on C export from terrestrial ecosystems to inland waters is not fully understood, especially in heterogeneous landscapes under human influence. We sampled for dissolved C species in five tributaries with well-determined subcatchments (total size 174.5 km(2)), as well as in various points of two of the subcatchments draining to a boreal lake in southern Finland over a full year. Our aim was to find out how land cover and land-use affect C export from the catchments, as well as CH4 and CO2 concentrations of the streams, and if the origin of C in stream water can be determined from proxies for quality of dissolved organic matter (DOM). We further estimated the gas evasion from stream surfaces and the role of aquatic fluxes in regional C cycling. The export rate of C from the terrestrial system through an aquatic conduit was 19.3 g C m(-2) (catchment) yr(-1), which corresponds to 19% of the estimated terrestrial net ecosystem exchange of the catchment. Most of the C load to the recipient lake consisted of dissolved organic carbon (DOC, 6.1 ± 1.0 g C m(-2) yr(-1)); the share of dissolved inorganic carbon (DIC) was much smaller (1.0 ± 0.2 g C m(-2) yr(-1)). CO2 and CH4 emissions from stream and ditch surfaces were 7.0 ± 2.4 g C m(-2) yr(-1) and 0.1 ± 0.04 g C m(-2) yr(-1), respectively, C emissions being thus equal with C load to the lake. The proportion of peatland in the catchment and the drainage density of peatland increased DOC in streams, whereas the proportion of agricultural land in the catchment decreased it. The opposite was true for DIC. Drained peatlands were an important CH4 source for streams.

Concentrations and quality of DOC along the terrestrial–aquatic continuum in a boreal forested catchment

C cycling and dissolved organic C (DOC) inputs to boreal aquatic systems probably will change substantially with climate change. DOC concentrations already are increasing in surface waters. Terrestrial C is a major source of C to boreal freshwater ecosystems, but the interface between these 2 ecosystems, the riparian zone, has not been studied often. To improve our understanding of the importance of terrestrial inputs of DOC to aquatic systems from surrounding forests, we followed the changes of DOC concentration along a continuum of precipitation, throughfall, soil water, ground water, lake, and brook water in a pristine, boreal, forested headwater catchment and developed a lake C balance based on terrestrial and lacustrine C fluxes. We also examined DOC quality changes using the ratio of absorbance at 465 and 665 nm (E4/E6). DOC concentrations increased from 2.4 mg/L in precipitation to 132.3 mg/L in soil water as water passed through the terrestrial ecosystem. DOC concentrations in the riparian zone were correlated with DOC concentrations in the adjacent outflowing brook but not in the headwater lake. E4/E6 ratios indicated that the DOC in precipitation and throughfall was dominated by higher molecular weight compounds and that the DOC in soil and ground water was dominated by lower molecular weight compounds. The input of terrestrial DOC to the aquatic ecosystem was estimated to be 5 to 13 g C m−2 y−1, which is small compared with the C fluxes between atmosphere and vegetation, but can significantly decrease the net ecosystem exchange of an old-growth forest catchment. Terrestrial DOC was a major source of C in the lake, rendering it heterotrophic. The DOC export (3 g DOC m−2 y−1) made up almost 70% of total C export.

Methanotrophic activity in relation to primary and bacterial production in a boreal humic lake

Microbial decomposition of organic matter causes hypoxic and anoxic conditions in the hypolimnion of polyhumic lakes during summer and winter stratification periods. The concentration of eH4, an end product of anaerobic decomposition of organic matter, may as high as 250mmol m-3 in the hypolimnion of these Jakes (HUTTUNEN et al. 2002, HUOTARI & KANKAALA unpubl.). A large proportion of the eH4 produced is oxidized to eo2 by methanotrophs or converted to microbial biomass (Ruoo & HAMILTON 1974, BASTVIKEN et al. 2003), while the rest is degassed into the atrnosphere mainly during the spring and autumn turnover periods (RIERA et al. 1999). Stable carbon isotope (ô13C) composition of zooplankton has given rise to a hypothesis that methanotrophs may be a significant source of energy and carbon in the food webs o f humic lakes (JONES et al. 1999), where the algal primary production is insufficient to support the productivity of zooplankton (ÜJALA & SALONEN 2001). In 2002 we studied e~ oxidation in relation to primary and bacterial production in the water column of a boreal, polyhumic Lake Valkea-Kotinen. The lake has been intensively monitored since 1990 by Finnish Environment Institute and Lanuni Biological Station (see KESKITALO et al. 1998).

High nitrogen removal in a constructed wetland receiving treated wastewater in a cold climate

Constructed wetlands provide cost-efficient nutrient removal, with minimal input of human labor and energy, and their number is globally increasing. However, in northern latitudes, wetlands are rarely utilized, because their nutrient removal efficiency has been questioned due to the cold climate. Here, we studied nutrient retention and nitrogen removal in a boreal constructed wetland (4-ha) receiving treated nitrogen-rich wastewater. On a yearly basis, most of the inorganic nutrients were retained by the wetland. The highest retention efficiency was found during the ice-free period, being 79% for ammonium-nitrogen (NH4+-N), 71% for nitrate-nitrogen (NO3--N), and 88% for phosphate-phosphorus (PO43--P). Wetland also acted as a buffer zone during the disturbed nitrification process of the wastewater treatment plant. Denitrification varied between 106 and 252 mg N m-2 d-1 during the ice-free period. During the ice-cover period, total gaseous nitrogen removal was 147 mg N m-2 d-1, from which 66% was removed as N2, 28.5% as N2O through denitrification, and 5.5% as N2 through anammox. Nearly 2600 kg N y-1 was estimated to be removed through microbial gaseous N-production which equaled 72% of NO3--N and 60% of TN yearly retention in the wetland. The wetland retained nutrients even in winter, when good oxygen conditions prevailed under ice. The results suggest that constructed wetlands are an efficient option for wastewater nitrogen removal and nutrient retention also in cold climates.