The weir pool Serrig is the deepest one along the impounded river Saar. Damming caused massive changes in the river's hydrodynamics. We analyzed the spatio-temporal variability of thermal density stratification in the weir pool and its effect on phytoplankton and nutrient dynamics. In the analysis, continuous measurements from the years 2014 and 2015 were combined with three two-day sampling campaigns in spring 2015. Thermal stratification occurred regularly in the downstream section of the weir pool during spring and summer and showed a diurnal rhythm. Temperature differences >1 K between the 1 and 2 m water layer were observed during 34 out of 217 days (16%) in 2014, with maximum temperature gradients up to 3.71 K. Whereas the influence of thermal stratification on phytoplankton biomass and distribution was low during the algal bloom in early spring, stratification events between May and July promoted temporary algal blooms in the surface layer with chlorophyll a concentrations up to 98 μg Chla l À1 and a maximum difference between the 1 and 2 m water layer of 36 μg Chla l À1 . Some of the stratification events resulted in reduced concentrations of dissolved nutrients in the surface layer as a result of increased uptake by algae, with maximum gradients between the surface and the 8 m water layer of 0.070 mg ortho-PO 4 3--P l À1 , 0.136 mg NH 4 + ÀN l À1 and 0.24 mg NO 3¯À N l À1 . These vertical gradients should be considered in sampling protocols for the assessment of the water quality of temporarily stratified river sections.

The magnitude of lateral dissolved inorganic carbon (DIC) export from terrestrial ecosystems to inland waters strongly influences the estimate of the global terrestrial carbon dioxide (CO2) sink. At present, no reliable number of this export is available, and the few studies estimating the lateral DIC export assume that all lakes on Earth function similarly. However, lakes can function along a continuum from passive carbon transporters (passive open channels) to highly active carbon transformers with efficient in-lake CO2 production and loss. We developed and applied a conceptual model to demonstrate how the assumed function of lakes in carbon cycling can affect calculations of the global lateral DIC export from terrestrial ecosystems to inland waters. Using global data on in-lake CO2 production by mineralization as well as CO2 loss by emission, primary production, and carbonate precipitation in lakes, we estimated that the global lateral DIC export can lie within the range of to Pg C yr−1 depending on the assumed function of lakes. Thus, the considered lake function has a large effect on the calculated lateral DIC export from terrestrial ecosystems to inland waters. We conclude that more robust estimates of CO2 sinks and sources will require the classification of lakes into their predominant function. This functional lake classification concept becomes particularly important for the estimation of future CO2 sinks and sources, since in-lake carbon transformation is predicted to be altered with climate change.

The partial pressure of CO 2 (pCO 2) in lake water, and thus CO 2 emissions from lakes are controlled by hydrologic inorganic carbon inputs into lakes, and in-lake carbon transformation (mainly organic carbon mineralization and CO 2 uptake by primary producers). In boreal lakes, CO 2 uptake by phytoplankton is often considered to be of minor importance. At present, however, it is not known in which and how many boreal lakes phytoplankton CO 2 uptake has a sizeable influence on the lake water pCO 2. Using water physico-chemical and phytoplankton data from 126 widely spread Swedish lakes from 1992 to 2012, we found that pCO 2 was negatively related to phytoplankton carbon in lakes in which the phytoplankton share in TOC (C phyto :TOC ratio) exceeded 5%. Total phosphorus concentration (TP) was the strongest predictor of spatial variation in the C phyto :TOC ratio, where C phyto :TOC ratios > 5% occurred in lakes with TP > 30 µg l −1. These lakes were located in the hemi-boreal zone of central and southern Sweden. We conclude that during summer, phytoplankton CO 2 uptake can reduce the pCO 2 not only in warm eutrophic lakes, but also in relatively nutrient poor hemi-boreal lakes. Keywords Phytoplankton • CO 2 emission • Authochthonous organic carbon • TOC • Global carbon cycle • Lake carbon cycling Aquatic Sciences

Damming alters carbon processing along river continua. Estimating carbon transport along rivers intersected by multiple dams requires an understanding of the effects of cascading impoundments on the riverine metabolism. We analyzed patterns of riverine metabolism and phytoplankton biomass (chlorophyll a ; Chl a ) along a 74.4-km river reach intersected by six low-head navigation dams. Calculating gross primary production (GPP) from continuous measurements of dissolved oxygen concentration, we found a maximum increase in the mean GPP by a factor of 3.5 (absolute difference of 0.45 g C m −3 d −1 ) along the first 26.5 km of the study reach, while Chl a increased over the entire reach by a factor of 2.9 (8.7 µg l −1 ). In the intermittently stratified section of the deepest impoundment the mean GPP between the 1 and 4 m water layer differed by a factor of 1.4 (0.31 g C m −3 d −1 ). Due to the strong increase in GPP, the river featured a wide range of conditions characteristic of low- to medium-production rivers. We suggest that cascading impoundments have the potential to stimulate riverine GPP, and conclude that phytoplankton CO 2 uptake is an important carbon flux in the river Saar, where a considerable amount of organic matter is of autochthonous origin.

Carbon dioxide (CO 2) uptake by phytoplankton can significantly reduce the partial pressure of CO 2 (pCO 2) in lakes and rivers, and thereby CO 2 emissions. Presently, it is not known in which inland waters on Earth a significant pCO 2 reduction by phytoplankton is likely. Since detailed, comparable carbon budgets are currently not available for most inland waters, we modified a proxy to assess the pCO 2 reduction by phytoplankton, originally developed for boreal lakes, for application on a global scale. Using data from 61 rivers and 125 lakes distributed over five continents, we show that a significant pCO 2 reduction by phytoplankton is widespread across the temperate and sub-/tropical region, but absent in the cold regions on Earth. More specifically, we found that a significant pCO 2 reduction by phytoplankton might occur in 24% of the lakes in the temperate region, and 39% of the lakes in the sub-/tropical region. We also showed that such a reduction might occur in 21% of the rivers in the temperate region, and 5% of the rivers in the sub-/tropical region. Our results indicate that CO 2 uptake by phytoplankton is a relevant flux in regional and global carbon budgets. This highlights the need for more accurate approaches to quantify CO 2 uptake by primary producers in inland waters, particularly in the temperate and sub-/tropical region. Keywords Phytoplankton. Global carbon cycle. Inland waters. Total organic carbon. CO 2 dynamics. Chlorophyll a