Vertical diffusion rates ( K ( K z ) were determined by measuring for several weeks the vertical spread of an injection of tritiated water into the thermoclines and hypolimnia of Lake 227 and Lake 224 in the Experimental Lakes Area (ELA) of northwestern Ontario. K z values of 5×10 −5 and 8×10 −4 cm 2 · s −1 were determined from the tracer experiments in the thermoclines of L227 and L224; in the hypolimnia, similar K z determinations of 1.7×10 −3 and 1.8×10 −2 cm 2 ·s −1 are 20−30 times greater than the thermocline rates. Vertical diffusion rates of heat were determined over the same time and depth intervals as the tracer experiments. In each lake, heat is diffusing vertically faster than mass in the thermocline and at more equal rates in the hypolimnion. The low K z values and the greater diffusion rate of heat than mass (tritium) indicate that molecular diffusion is important in determining the rate of vertical transport in the thermoclines of these highly stratified lakes. Vertical eddy diffusion rates ( K z ') determined by the tracer experiments show an inverse proportionality to the static stability of the water column (N 2 ), such that K z ′ ∝ (N 2 ) −0 · 8 . However, K z ′ values determined by measuring the hypolimnetic heating rates of eight ELA lakes (including L227 and L224) and three lakes outside ELA indicate that K z ′ ∝ (N 2 ) −0.44 . These observations suggest that in the absence of large shear where K z ′ < 10 −2 cm 2 · s −1 the vertical diffusion rates of mass and heat show different inverse correlations to the static stability of the water column.
Rates of sulfate reduction and denitrification were measured in the sediments of unacidified, experimentally acidified, and atmospherically acidified lakes in North America and Norway. These data, plus profiles of porewater and sediment chemistry, demonstrated that in all of the lakes H was being actively consumed by both sulfate reducers and denitrifiers. Both of these microbial activities were assayed in sediments overlaid by oxygenated water, demonstrating that anoxic hypolimnia are not required for in situ alkalinity production. Neither short term experimental acidification nor long term atmospheric acidification had detectably inhibited the activity of these two types of bacteria. Both processes were active at pH 4.5. In lakes that were receiving significant quantities of both nitric and sulfuric acids, short term H + consumption from denitrification was 1.5–2 times faster than H consumption by sulfate reduction. However on an annual basis, because of loss of reduced sulfur during fall and winter, long term H + consumption by denitrification was estimated to be 4–5 times as large as H + consumption by sulfate reduction.
We investigated δ 13 carbon (C)‐dissolved inorganic carbon (DIC) values in 72 lakes from diverse regions using literature data as well as new measurements for 32 lakes. δ 13 C‐DIC varied broadly among lakes from ~31 to +2.6‰. This variation of surface‐water δ 13 C‐DIC among lakes is greater than the seasonal variation within most lakes. Several statistical models account for a large portion of the interlake variation and indicate that geochemical (e.g., DIC, pH, alkalinity) and morphometric (area) variables are important, whereas biological (e.g., gross primary productivity [GPP], respiration [R], chlorophyll a) variables are generally not significant. A process‐based model including gas exchange with the atmosphere, inorganic carbon speciation, and ecosystem metabolism was also constructed. The model provides a reasonable fit to the data for lakes, in which respiration exceeded GPP (heterotrophic lakes; 75% of lakes sampled). Lakes for which GPP exceeded respiration (autotrophic) were not fit well by the process‐based model. The data and models indicate that metabolism creates substantial variation in δ 13 C‐DIC around the potential δ 13 C‐DIC that is set by geochemical factors of the watershed.
The variability of surface water carbon dioxide concentration, or partial pressure (pCO 2 ), was studied in 11 lakes of greatly varying size (2.4 ha up to 8 million ha) in Northwest Ontario, Canada. Six of these lakes were chosen to be as similar as possible in all respects except surface area (the Northwest Ontario Lake Size Series [NOLSS], which range from 88 to 35,000 ha). Spatial and temporal variability of p CO2 within a single lake was no greater in the larger lakes than in the smaller lakes. Interannual variability was significant and synchronous, which indicates that weather patterns were important and affected the different lakes within the region in a similar manner. However, annual p CO 2 averages were not related to annual differences in planktonic photosynthetic activity, measured by 14 CO 2 fixation. In the six NOLSS lakes, there was not a significant relationship of average pCO 2 with lake size. For all 11 lakes, however, there was a significant negative correlation of p CO 2 with lake size, which was likely due to several characteristics of the very small and very large lakes that covaried with size. The larger lakes were deeper and had longer water residence times and lower DOC, which suggests lower CO 2 production from allochthonous organic carbon inputs. Also, the ratio of epilimnetic sediment area/epilimnetic volume (Ae/Ve) was smaller in the larger lakes, which likely resulted in lower rates of recycling of fixed carbon to CO 2 during summer stratification.
Vertical diffusion rates (K,) were determined by measuring for several weeks the vertical spread of an injection of tritiated water into the thermoclines and hypolimnia
A direct field comparison was conducted to determine the dependency of gas exchange coefficient ( k x ) on the diffusion coefficient ( D x ). The study also sought to confirm the enhanced vertical exchange properties of limnocorrals and similar enclosures. Gas exchange coefficients for 222 Rn and 3 He were determined in a small northern Ontario lake, using a 226 Ra and 3 H spike to gain the necessary precision. The results indicate that the gas exchange coefficient is functionally dependent on the diffusion coefficient raised to the 1.22 + > 12 −35 power ( k x = ƒ( D x 1.22 )), clearly supporting the stagnant film model of gas exchange. Limnocorrals were found to have gas exchange rates up to 1.7 times higher than the whole lake in spite of the observation of more calm surface conditions in the corral than in the open lake.
Two independent methods for measuring total sulfur were used to show that underestimates of sulfur content of lacustrine sediments can occur when sediments are dried before total sulfur analysis. Different types of sediments were oven dried at 60° or 100°C or lyophilized to assess the effect of the drying method on the amount of sulfur lost. Losses ranged from 0 to 86%. Common losses were 6–22% and dependent on the sample and drying method used. Lyophilization caused greater sulfur losses (1.5‐fold) than the two oven‐drying methods. These sulfur losses caused changes in the sulfur isotopic content of the sediments and could underestimate rates of sulfur burial in sediments, organic‐S formation in sediments, and internal alkalinity production in lakes.
Three parameters must be known to use the thin boundary‐layer model (or other bulk transfer models) for CO 2 flux between water and air: the concentration of dissolved CO 2 , CO 2(aq) , the concentration of CO 2 in the air immediately above the water, CO 2(atm) , and the wind velocity, which is used to determine the appropriate transfer coefficient. These parameters change hourly and from day to day in a nonlinear fashion, so the frequency of measurements is an important factor in determining the accuracy of flux estimates for any period. To achieve a high frequency measurement, we developed a self‐contained, solar‐powered, in situ sampling system that continuously measures and records CO 2(aq) , CO 2(atm) , and windspeed. Unique to this technique is an underwater in situ equilibration chamber (ISEC). The ISEC was tested in a shallow wetland pond in which changes in both CO 2(aq) and CO 2(atm) were large. The data obtained showed that large errors may result from extrapolating flux calculations made from short‐term data (e.g. daily) to longer time periods.
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