A simple and cost-efficient automated floating chamber for continuous measurements of carbon dioxide gas flux on lakes

<p>Freshwaters emit significant amounts of CO₂ on a global scale. However, emissions remain poorly constrained from the diverse range of aquatic systems. The drivers and regulators of CO₂ 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 CO₂ 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 CO₂ 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 CO₂ gas flux with 40&thinsp;min intervals. Combining the direct measurements of gas flux with measurements of air and waterside CO₂ 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

Time-restricted flight ability influences dispersal and colonization rates in a group of freshwater beetles

Variation in the ability to fly or not is a key mechanism for differences in local species occurrences. It is increasingly acknowledged that physiological or behavioral mechanisms rather than morphological differences may drive flight abilities. However, our knowledge on the seasonal variability and stressors creating nonmorphological differences in flight abilities and how it scales to local and regional occurrences is very limited particularly for small, short‐lived species such as insects. Here, we examine how flight ability might vary across seasons and between two closely related genera of freshwater beetles with similar geographical ranges, life histories, and dispersal‐related morphology. By combining flight experiments of >1,100 specimens with colonization rates in a metacommunity of 54 ponds in northern and eastern Europe, we have analyzed the relationship between flight ability and spatio‐environmental distribution of the study genera. We find profound differences in flight ability between the two study genera across seasons. High flight ability for Acilius (97% of the tested individuals flew during the experiments) and low for Graphoderus (14%) corresponded to the different colonization rates of newly created ponds. Within a 5‐year period, 81 and 31% of the study ponds were colonized by Acilius and Graphoderus, respectively. While Acilius dispersed throughout the season, flight activity in Graphoderus was restricted to stressed situations immediately after the emergence of adults. Regional colonization ability of Acilius was independent of spatial connectivity and mass effect from propagule sources. In contrast, Graphoderus species were closely related to high connectivity between ponds in the landscape. Our data suggest that different dispersal potential can account for different local occurrences of Acilius and Graphoderus. In general, our findings provide some of the first insights into the understanding of seasonal restrictions in flight patterns of aquatic beetles and their consequences for species distributions

More is less : net gain in species richness, but biotic homogenization over 140 years

The research was funded by the ‘15. June Foundation’. MD was funded by a Leverhulme Fellowship.While biodiversity loss continues globally, assessments of regional and local change over time have been equivocal. Here, we assess changes in plant species richness and beta diversity over 140 years at the level of regions within a country. Using 19th-century flora censuses for 14 Danish regions as a baseline, we overcome previous criticisms concerning short time series and neglect of completely altered habitats. We find that species composition has changed dramatically and directionally across all regions. Substantial species losses were more than offset by large gains, resulting in a net increase in species richness in all regions. The occupancy of initially widespread species increased, while initially rare species lost terrain. These changes were accompanied by strong biotic homogenization; i.e. regions are more similar now than they were 140 years ago. Species declining in Denmark were found to be in similar decline all over Northern Europe.PostprintPeer reviewe

Pelagic metabolism in eutrophic coastal waters during a late summer period

ABSTRACT: We measured phytoplankton and bacterial biomass and production at weekly intervals during summer in water samples from 2 sites in the shallow, eutrophic Roskilde Fjord, Denmark. In addition we measured Oz-uptake on unfiltered water and size-fractions &lt; 100 pm and &lt; 1 pm. Phytoplankton gross production in the water column was 6.2 g O2 m 2 d &apos; at Stn 1 and was balanced by pelagic community respiration (Rc, 3.8 g 0; m 2 d &apos; ) and sediment respiration (2.5 g 0; m&quot;&apos; d-l). Phytoplankton gross respiration (3.0 g O2 m 2 d l ) was temporarily exceeded by pelagic community respiration (3.1 g O2 m 2 d &apos; ) plus sediment respiration (2.0 g 0; m 2 d l ) at Stn 2 where there is additional production by Littoral plant communities. Phytoplankters and bacteria together accounted for 72 to 85 % of Re and zooplankters for the remainder Phytoplankters respired a large proportion (ca 30%) of their gross production and were apparently mainly grazed by benthic suspension feeders. Phytoplankters dominated pelagic respiration (50% of Re) at Stn 1, which has most phytoplankton, while bacteria dominated (44 % of Re) at Stn 2. We ascribe the relatively larger respiratory activity of bacteria at Stn 2 to additional supply of organic matter from littoral plant communities and frequent sediment resuspension. The importance of bacteria in the pelagic food web was supported by other findings. Bacterial biomass approached phytoplankton carbon biomass at Stn 2 and bacterial net production in the water column was 10 % (Stn 1) and 35 &quot; % (Stn 2) of phytoplankton gross production. Bacterial net production and respiration were linearly related in the bacterial size-fraction (&lt; 1 pm) with a bacterial growth yield of 47 %. We argue that the conversion factors applied to calculate bacterial net production and the mean growth yield attained are reasonable values considering the other measurements of pelagic carbon pools and processes