Convective mixing and high littoral primary production can establish systematic errors in lake diel oxygen curves in shallow, eutrophic lakes

The diel (24-h) oxygen (O2) curves approach has become a popular method for analyzing gross primary production (GPP) and ecosystem respiration (ER) rates in aquatic systems. Despite the simplicity of this approach, there remain aspects of the calculation and interpretation of diel O2 curves which may skew results, with potentially large implications for estimates of metabolic rates. One common problem in lakes is the occurrence of unexpected changes in O2concentrations (for instance, increasing overnight O2 concentrations). Such changes have typically been ascribed to the random mixing of pockets of O2. It has thus been suggested that negative GPP or positive ER values should be included in calculations, on the assumption that under- and overestimates should occur with equal frequency, and thus cancel each other out. Our data from a shallow, eutrophic lake provided a high share of negative GPP values. We argue that these may have been the result of elevated littoral productivity coupled with convective currents produced by consistent differences in the heating or cooling of littoral and offshore waters. Such phenomena might be common in small, sheltered lakes where the role of mixing by wind is diminished. We conclude that a failure to account for consistent metabolic gradients and periodic convective mixing may lead to a chronic underestimation of metabolic rates in lakes when using the diel O2 curves method

Primary Production in Shallow Freshwater Systems amid a Rapidly Changing World

Kleine, flache Gewässer gelten als sogenannte „hotspots“ der Primärproduktion und Kohlenstoffbindung. Diese Doktorarbeit zielt darauf ab, die Primärproduktion verschiedener kleiner Gewässer zu quantifizieren sowie die Mechanismen, die den Kohlenstoffkreislauf dieser Systeme beeinflussen, zu analysieren. Der Fokus liegt dabei auf dem Einfluss globaler Veränderungen, die diese Mechanismen verändern können Im ersten Abschnitt wurde die Primärproduktion (PP) in kleinen, temporären Söllen untersucht, die sehr anfällig für Störungen sind. Ich konnte zeigen, dass die PP der Sölle im Sommer außergewöhnlich hoch ist, was hauptsächlich auf eine hohe Makrophytenproduktion zurückzuführen ist Im zweiten Teil analysiere ich die Ergebnisse eines Experiments zum Einfluss erhöhter Temperaturen auf die benthische PP kleiner Gewässer im Frühjahr. Acht Mesokosmen wurden bei normalen und um 4°C erhöhten Wassertemperaturen gemäßigter Breiten betrieben. In der ersten Hälfte des Experiments konnte ich eine erhöhte benthische PP in den erwärmten Mesokosmen feststellen, die auf direkte Temperatureffekte und indirekte Auswirkungen einer höheren Nährstoffverfügbarkeit zurückzuführen war. Anfang Juni stieg jedoch der Einfluss der Makroinvertebraten auf das Periphyton in den erwärmten Mesokosmen, so dass keine Unterschiede in der PP mehr auftraten. Schließlich, untersuche ich die Resilienz eines Sees gegenüber einem plötzlichen Eintrag gelösten organischen Kohlenstoffs (DOC) aus dem terrestrischen Umland, der zu einer starken Braunfärbung des Wassers führte. Der Fokus liegt dabei auf Veränderungen der Wasserqualität und der aquatischen PP des Sees, nachdem sich die DOC-Konzentration verfünffacht hatte. Drei Jahren nach Erreichen der maximalen DOC- und Gesamt-Phosphor im See sanken diese signifikant, lagen jedoch noch immer 1,5- bzw. 2-fach oberhalb der Ausgangskonzentrationen vor dem DOC-Eintrag. Die benthische PP zeigte eine teilweise Erholung, erreichte jedoch ebenfalls nicht die Ausgangswerte

Cross continental increase in methane ebullition under climate change

Contains fulltext : 181027.pdf (publisher's version ) (Open Access)8 p

Warming enhances sedimentation and decomposition of organic carbon in shallow macrophyte-dominated systems with zero net effect on carbon burial

Temperatures have been rising throughout recent decades and are predicted to rise further in the coming century. Global warming affects carbon cycling in freshwater ecosystems, which both emit and bury substantial amounts of carbon on a global scale. Currently, most studies focus on the effect of warming on overall carbon emissions from freshwater ecosystems, while net effects on carbon budgets may strongly depend on burial in sediments. Here, we tested whether year‐round warming increases the production, sedimentation, or decomposition of particulate organic carbon and eventually alters the carbon burial in a typical shallow freshwater system. We performed an indoor experiment in eight mesocosms dominated by the common submerged aquatic plant Myriophyllum spicatum testing two temperature treatments: a temperate seasonal temperature control and a warmed (+4°C) treatment (n = 4). During a full experimental year, the carbon stock in plant biomass, dissolved organic carbon in the water column, sedimented organic matter, and decomposition of plant detritus were measured. Our results showed that year‐round warming nearly doubled the final carbon stock in plant biomass from 6.9 ± 1.1 g C in the control treatment to 12.8 ± 0.6 g C (mean ± SE), mainly due to a prolonged growing season in autumn. DOC concentrations did not differ between the treatments, but organic carbon sedimentation increased by 60% from 96 ± 9.6 to 152 ± 16 g C m−2 yaer−1 (mean ± SE) from control to warm treatments. Enhanced decomposition of plant detritus in the warm treatment, however, compensated for the increased sedimentation. As a result, net carbon burial was 40 ± 5.7 g C m−2 year−1 in both temperature treatments when fluxes were combined into a carbon budget model. These results indicate that warming can increase the turnover of organic carbon in shallow macrophyte‐dominated systems, while not necessarily affecting net carbon burial on a system scale

Data from: Warming enhances sedimentation and decomposition of organic carbon in shallow macrophyte-dominated systems with zero net effect on carbon burial

Temperatures have been rising throughout recent decades and are predicted to rise further in the coming century. Global warming affects carbon cycling in freshwater ecosystems, which both emit and bury substantial amounts of carbon on a global scale. Currently, most studies focus on the effect of warming on overall carbon emissions from freshwater ecosystems, while net effects on carbon budgets may strongly depend on burial in sediments. Here, we tested whether year‐round warming increases the production, sedimentation, or decomposition of particulate organic carbon and eventually alters the carbon burial in a typical shallow freshwater system. We performed an indoor experiment in eight mesocosms dominated by the common submerged aquatic plant Myriophyllum spicatum testing two temperature treatments: a temperate seasonal temperature control and a warmed (+4°C) treatment (n = 4). During a full experimental year, the carbon stock in plant biomass, dissolved organic carbon in the water column, sedimented organic matter, and decomposition of plant detritus were measured. Our results showed that year‐round warming nearly doubled the final carbon stock in plant biomass from 6.9 ± 1.1 g C in the control treatment to 12.8 ± 0.6 g C (mean ± SE), mainly due to a prolonged growing season in autumn. DOC concentrations did not differ between the treatments, but organic carbon sedimentation increased by 60% from 96 ± 9.6 to 152 ± 16 g C m−2 year−1 (mean ± SE) from control to warm treatments. Enhanced decomposition of plant detritus in the warm treatment, however, compensated for the increased sedimentation. As a result, net carbon burial was 40 ± 5.7 g C m−2 year−1 in both temperature treatments when fluxes were combined into a carbon budget model. These results indicate that warming can increase the turnover of organic carbon in shallow macrophyte‐dominated systems, while not necessarily affecting net carbon burial on a system scale

Data from: Warming advances top-down control and reduces producer biomass in a freshwater plankton community

Global warming has been shown to affect ecosystems worldwide. Warming may, for instance, disrupt plant herbivore synchrony and bird phenology in terrestrial systems, reduce primary production in oceans, and promote toxic cyanobacterial blooms in freshwater lakes. Responses of communities will not only depend on direct species-specific temperature effects, but also on indirect effects related to bottom-up and top-down processes. Here, we investigated the impact of warming on freshwater phytoplankton community dynamics, and assessed the relative contribution of nutrient availability, fungal parasitism, and grazing therein. For this purpose, we performed an indoor mesocosm experiment following seasonal temperature dynamics of temperate lakes and a warmed (+4°C) scenario from early spring to late summer. We assessed phytoplankton biomass, C:N:P stoichiometry and community composition, dissolved nutrient availabilities, fungal parasite (i.e., chytrid) prevalence, and zooplankton abundance. Warming led to an overall reduction in phytoplankton biomass as well as lower C:P and N:P ratios, while phytoplankton community composition remained largely unaltered. Warming resulted in an earlier termination of the diatom spring bloom, and an epidemic of its fungal parasite ended earlier as well. Furthermore, warming advanced zooplankton phenology, leading to an earlier top-down control on phytoplankton in the period after the spring bloom. Linear model analysis showed that most of the observed variance in phytoplankton biomass was related to seasonal temperature dynamics in combination with zooplankton abundance. Our findings showed that warming advanced grazer phenology and reduced phytoplankton biomass, thereby demonstrating how bottom-up and top-down related processes may shape future phytoplankton dynamics

Warming advances virus population dynamics in a temperate freshwater plankton community

Viruses are important drivers in the cycling of carbon and nutrients in aquatic ecosystems. Since viruses are obligate parasites, their production completely depends on growth and metabolism of hosts and therefore can be affected by climate change. Here, we investigated if warming (+4°C) can change the outcome of viral infections in a natural freshwater virus community over a 5‐month period in a mesocosm experiment. We monitored dynamics of viruses and potential hosts. Results show that warming significantly advanced the early summer peak of the virus community by 24 d, but neither affected viral peak abundances nor time‐integrated number of viruses present. Our results demonstrate that warming advances the timing of viruses in a natural community. Although warming may not necessarily result in a stronger viral control of bacterial and phytoplankton communities, our results suggest it can alter host population dynamics through advanced timing of infections, and thus timing of carbon and nutrient recycling