77 research outputs found
A simple and cost-efficient automated floating chamber for continuous measurements of carbon dioxide gas flux on lakes
<p>Freshwaters emit significant amounts of CO<sub>2</sub> on a global scale.
However, emissions remain poorly constrained from the diverse range of
aquatic systems. The drivers and regulators of CO<sub>2</sub> gas flux from
standing waters require further investigation to improve knowledge on both
global-scale estimates and system-scale carbon balances. Often,
lake–atmosphere gas fluxes are estimated from empirical models of gas
transfer velocity and air–water concentration gradient. Direct
quantification of the gas flux circumvents the uncertainty associated with
the use of empirical models from contrasting systems. Existing methods to
measure CO<sub>2</sub> gas flux are often expensive (e.g. eddy covariance) or
require a high workload in order to overcome the limitations of single point
measurements using floating chambers. We added a small air pump, a timer and
an exterior tube to ventilate the floating chamber headspace and passively
regulate excess air pressure. By automating evacuation of the chamber
headspace, continuous measurements of lake CO<sub>2</sub> gas flux can be
obtained with minimal effort. We present the chamber modifications and an
example of operation from a small forest lake. The modified floating chamber
performed well in the field and enabled continuous measurements of
CO<sub>2</sub> gas flux with 40 min intervals. Combining the direct
measurements of gas flux with measurements of air and waterside CO<sub>2</sub>
partial pressure also enabled calculation of gas exchange velocity. Building
and using the floating chamber is straightforward. However, because an air
pump is used to restart measurements by thinning the chamber headspace with
atmospheric air, the duration of the air pump pause–pulse cycle is critical
and should be adjusted depending on system characteristics. This may result
in shorter deployment duration, but this restriction can be circumvented by
providing a stronger power source. The simple design makes modifications of
the chamber dimensions and technical additions for particular applications
and systems easy. This should make this approach to measuring gas flux
flexible and appropriate in a wide range of different systems.</p
Catchment tracers reveal discharge, recharge and sources of groundwater-borne pollutants in a novel lake modelling approach
Groundwater-borne contaminants such as nutrients, dissolved organic carbon
(DOC), coloured dissolved organic matter (CDOM) and pesticides can have an
impact the biological quality of lakes. The sources of pollutants can,
however, be difficult to identify due to high heterogeneity in groundwater
flow patterns. This study presents a novel approach for fast hydrological
surveys of small groundwater-fed lakes using multiple groundwater-borne
tracers. Water samples were collected from the lake and temporary groundwater
wells, installed every 50 m within a distance of 5–45 m to
the shore, were analysed for tracer concentrations of CDOM, DOC, total
dissolved nitrogen (TDN, groundwater only), total nitrogen (TN, lake only),
total dissolved phosphorus (TDP, groundwater only), total phosphorus (TP,
lake only), δ18O ∕ δ16O isotope ratios
and fluorescent dissolved organic matter (FDOM) components derived from
parallel factor analysis (PARAFAC). The isolation of groundwater recharge
areas was based on δ18O measurements and areas with a high
groundwater recharge rate were identified using a microbially influenced FDOM
component. Groundwater discharge sites and the fractions of water delivered
from the individual sites were isolated with the Community Assembly via Trait
Selection model (CATS). The CATS model utilized tracer measurements of TDP,
TDN, DOC and CDOM from the groundwater samples and related these to the
tracer measurements of TN, TP, DOC and CDOM in the lake. AÂ direct comparison
between the lake and the inflowing groundwater was possible as degradation
rates of the tracers in the lake were taken into account and related to
a range of water retention times (WRTs) of the lake (0.25–3.5 years in
0.25-year increments). These estimations showed that WRTs above 2Â years
required a higher tracer concentration of inflowing water than found in any
of the groundwater wells around the lake. From the estimations of inflowing
tracer concentration, the CATS model isolated groundwater discharge sites
located mainly in the eastern part of the lake with a single site in the
southern part. Observations from the eastern part of the lake revealed an
impermeable clay layer that promotes discharge during heavy precipitation
events, which would otherwise be difficult to identify using traditional
hydrological methods. In comparison to the lake concentrations, high tracer
concentrations in the southern part showed that only a smaller fraction of
water could originate from this area, thereby confirming the model results.
A Euclidean cluster analysis of δ18O isotopes identified
recharge sites corresponding to areas adjacent to drainage channels, and
a cluster analysis of the microbially influenced FDOM component C4 further
identified five sites that showed a tendency towards high groundwater
recharge rate. In conclusion, it was found that this methodology can be
applied to smaller lakes within a short time frame, providing useful
information regarding the WRT of the lake and more importantly the
groundwater recharge and discharge sites around the lake. Thus, it is a tool
for specific management of the catchment
Tracing the Spatial Distribution of Whole-Lake Exchange of Groundwater and Lake Water in Low-Hydraulic Gradient Systems Using delta O-18 and Electrical Conductivity and Uncertain End-Member Mixing Analysis
δ18O and electrical conductivity (EC) were used successfully to trace the spatial distribution of whole-lake groundwater-lake exchange for a small (four ha) groundwater-fed lake situated in a low relief and low hydraulic gradient area. The method relies on quick sampling of shallow groundwater, direct analysis of EC in the field, and relatively in-expensive analysis of δ18O in the laboratory. Ternary uncertain end-member mixing analysis (precipitation, groundwater, and lake water) quantified the composition of water discharging to and recharging from the lake. The tracer distribution and mixing analysis were in agreement with the interpreted groundwater flow near the lake. The use of only one tracer (either δ18O or EC) gave the same results for the recharge segments, but the discharge segments changed the origin of the water from being groundwater to precipitation controlled. The two tracers complemented each other, especially with different signals in precipitation and groundwater. The uncertain end-members were assessed based on local (groundwater and lake water) and off-site (precipitation) data. The off-site data were found to be useful if it contained representative information on local-site seasonality (uncertainty, variance). Final end-member concentrations could explain the transience of the hydrology at the site (i.e., flooding of the area adjacent to the lake during periods with high precipitation, and variability of the δ18O signal in precipitation). This methodology potentially represents a new option to study groundwater-lake systems. The tracer information collected over only two days is useful by itself for developing the next steps like the quantification of fluxes based on other standard methods (Darcy approach, seepage meters, or temperature). The tracer information can provide quantitative estimation of inputs and outputs by using the mixing analysis
Distribution and production of plankton communities in the subtropical convergence zone of the Sargasso Sea. I. Phytoplankton and bacterioplankton
External Phosphorus Loading in New Lakes
The water quality of lakes is highly dependent on external phosphorus (P) loading. The vast external loadings from sewage and other wastewater discharge that European lakes have historically received have been dramatically reduced today by improved wastewater treatment. Gaining knowledge of the catchment characteristics that influence external P-loading should enable predictions of the achievable water quality of lakes. In this study, we tested this proposition. Data from 90 new Danish lakes show no apparent relationship between the mean summer P-concentration and the size or land use of the catchments. The external P-loading and resulting annual P-concentration were further investigated on a representative subset of 12 of the new lakes, using six methods. Three of the methods used empirical estimates of P-transport from catchments, based on the national average P-transport, runoff-dependent P-transport, and crop-dependent P-transport, and the other three methods used different empirical models tested on the lakes. External P-loading was reliably predicted by several of the methods. The predictions of the annual P-concentration were highly dependent on the inclusion of annual runoff. However, the predicted P-concentrations were generally overestimated, most pronounced for the nutrient-poor and most recently established lakes. In these lakes, internal P-loading was found to be the most important factor in predicting achievable water quality
Effect of exposure to sunlight and phosphorus-limitation on bacterial degradation of coloured dissolved organic matter (CDOM) in freshwater
- …