A variety of different sampling and analysis methods are found in the literature for determining carbon dioxide (CO₂) in freshwaters, methods that rarely have been evaluated or compared. Here we present an evaluation of an acidified headspace method (AHS) in which the dissolved inorganic carbon (DIC) is measured from an acidified sample and the partial pressure (pCO₂) is calculated from DIC using pH and water temperature. We include information on practical sampling, accuracy, and precision of the DIC/pCO₂ determination and a storage test of samples. The pCO₂ determined from the AHS method is compared to that obtained from the more widely used direct headspace method (DHS) in which CO₂ is equilibrated between the water and gas phases at ambient pH. The method was tested under both controlled laboratory conditions as well as wintertime field sampling. The accuracy of the DIC detection was on average 99% based on prepared standard solutions. The pCO₂ determination in lab, using the DHS method as a reference, showed no significant difference, although the discrepancy between the methods was larger in samples with <1000 µatm. The precision of the pCO₂ determination was on average ±4.3%, which was slightly better than the DHS method (±6.7%). In the field, the AHS method determined on average 10% higher pCO₂ than the DHS method, which was explained by the extreme winter conditions (below −20 °C) at sampling that affected the sampling procedure of the DHS method. Although samples were acidified to pH 2, respiration processes were still occurring (at a low rate), and we recommend that analyses are conducted within 3 days from sampling. The AHS method was found to be a robust method to determine DIC and pCO₂ in acidic to pH-neutral freshwater systems. The simple and quick sampling procedure makes the method suitable for time-limited sampling campaigns and sampling in cold climate.

The boreal forest carbon balance is predicted to be particularly sensitive to climate change. Carbon balance estimates of these biomes stem mainly from eddy-covariance measurements of net ecosystem exchange (NEE). However, a full net ecosystem carbon balance (NECB) must include the lateral carbon export (LCE) through discharge. We show that annual LCE at a boreal forest site ranged from 4 to 28%, averaging 11% (standard deviation of 8%), of annual NEE over 13 years. Annual LCE and NEE are strongly anticorrelated; years with weak NEE coincide with high LCE. The decreased NEE in response to increased precipitation is caused by a reduction in the amount of incoming radiation caused by clouds. If our finding is also valid for other sites, it implies that increased precipitation at high latitudes may shift forest NECB in large areas of the boreal biome. Our results call for future analysis of this dual effect of precipitation on NEE and LCE.

It is frequently observed that the local relative abundances of aquatic microbial taxa are correlated with their average relative abundance at the regional scale, which results in the composition of different communities being more similar than expected by chance or invariant. The degree to which communities within a region match the regional average community is variable and likely depends on several different mechanisms that control the process of microbial community assembly. Here, we show that environmental variables were associated with the community specific degree of regional invariance in 9 of 10 datasets of microbial communities in aquatic systems, being the main set of variables explaining differences in regional invariance in 5 of them. This indicates that variation in local environmental conditions across a region reduces the degree of regional invariance amongst communities. Spatial distances between communities were not related to the degrees of regional invariance, but in 7 of the datasets, regional invariance differed among different parts of the regions, particularly for phytoplankton communities. This suggests an influence of spatial or historical processes on the community specific degree of regional invariance. We conclude that both local environmental conditions and spatial/historical processes cause between-site differences in the degree of invariance between local and regional abundances in aquatic microbial metacommunities. We argue that studies of regional invariance can be an important complement to other statistical methods due to its propensity to detect variation in stochastic processes along gradients.

Microbial ecology has focused much on causes of between-site variation in community composition. By analysing five data-sets each of aquatic bacteria and phytoplankton, we demonstrated that microbial communities show a large degree of similarity in community composition and that abundant taxa were widespread, a typical pattern for many metazoan metacommunities. The regional abundance of taxa explained on average 85 and 41% of variation in detection frequency and 58 and 31% of variation in local abundances for bacteria and phytoplankton, respectively. However, regional abundance explained less variation in local abundances with increasing environmental variation between sites within data-sets. These findings indicate that the studies of microbial assemblages need to consider similarities between communities to better understand the processes underlying the assembly of microbial communities. Finally, we propose that the degree of regional invariance can be linked to the evolution of microbes and the variation in ecosystem functions performed by microbial communities.

Temporal dynamics of microbial communities show seasonal trends and synchronous dynamics between communities in different aquatic habitats, but previous studies have mainly focused on larger systems such as oceans, rivers, or lakes. However, a large part of aquatic water bodies consists of small pools, ponds, and streams, which tend to be environmentally heterogeneous over relatively small spatial scales. We studied a bacterial metacommunity of 16 rock pools at a spatial scale of 600 m², which was sampled at approximately monthly intervals over the course of 1 yr. We show that temporal dynamics were not evidently synchronous between rock pools over time and that there was no clear seasonal pattern. The environmental variable that explained the most of the temporal dynamics in rock pools over time was water colour, which is often not the main variable explaining spatial differences in bacterial composition between pools. Our results suggest that temporal dynamics of bacterial communities both among and within small water bodies show markedly different patterns compared to larger previously investigated systems, presumably due to their larger heterogeneity and less synchronous environmental changes. 

Temporal dynamics of microbial communities show seasonal trends and synchronous dynamics between communities in different aquatic habitats, but previous studies have mainly focused on larger systems such as oceans, rivers, or lakes. However, a large part of aquatic water bodies consists of small pools, ponds, and streams, which tend to be environmentally heterogeneous over relatively small spatial scales. We studied a bacterial metacommunity of 16 rock pools at a spatial scale of 600 m(2), which was sampled at approximately monthly intervals over the course of 1 yr. We show that temporal dynamics were not evidently synchronous between rock pools over time and that there was no clear seasonal pattern. The environmental variable that explained the most of the temporal dynamics in rock pools over time was water colour, which is often not the main variable explaining spatial differences in bacterial composition between pools. Our results suggest that temporal dynamics of bacterial communities both among and within small water bodies show markedly different patterns compared to larger previously investigated systems, presumably due to their larger heterogeneity and less synchronous environmental changes.

The browning of freshwater bodies due to increasing concentrations of dissolved organic carbon (DOC) is becoming more prevalent in light of the changing climate and is therefore a relevant topic of study within the scientific community. An obstacle to overcome however, is what to use as a source of carbon for scientific experiments. Reverse osmosis concentrate (ROC) is a natural source filtered from wetlands, though this can take a long time to collect. Other studies have turned to commercial products which are leonardite-based to simulate browning: HuminFeed, which comes as a powder or SuperHume, which is liquid. Here I present the results that test the effects of these browning agents on zooplankton. After a previous study indicated a severe impact on zooplankton in a lake mesocosm experiment, a controlled lab experiment was conducted. The model test organism Daphnia magna and copepods from a local lake were used to observe the effects of HuminFeed, SuperHume and ROC in a standard immobilisation and reproduction test using similar concentrations as the mesocosm experiment. The browning agents did not affect the immobilisation of D. magna or the copepods after 24 or 48 hours, however the reproduction test on D. magna resulted in a significant decrease in the total number of offspring produced in the HuminFeed treatment compared to the control. This study raises the question if the commercial product HuminFeed is an acceptable substance to use to mimic browning in lab or mesocosm experiments