Major effects of alkalinity on the relationship between metabolism and dissolved inorganic carbon dynamics in lakes

Several findings suggest that CO2 emissions in lakes are not always directly linked to changes in metabolism but can be associated with interactions with the dissolved inorganic carbon equilibrium. Alkalinity has been described as a determining factor in regulating the relative contributions of biological and inorganic processes to carbon dynamics in lakes. Here we analyzed the relationship between metabolic changes in dissolved oxygen (DO) and dissolved inorganic carbon (DIC) at different timescales in eight lakes covering a wide range in alkalinity. We used high-frequency data from automatic monitoring stations to explore the sensitivity of DIC to metabolic changes inferred from oxygen. To overcome the problem of noisy data, commonly found in high-frequency measurements datasets, we used Singular Spectrum Analysis to enhance the diel signal-to-noise ratio. Our results suggest that in most of the studied lakes, a large part of the measured variability in DO and DIC reflects non-metabolic processes. Furthermore, at low alkalinity, DIC dynamics appear to be mostly driven by aquatic metabolism, but this relationship weakens with increasing alkalinity. The observed deviations from the metabolic 1:1 stoichiometry between DO and DIC were strongly correlated with the deviations expected to occur from calcite precipitation, with a stronger correlation when accounting also for the benthic contribution of calcite precipitation. This highlights the role of calcite precipitation as an important driver of CO2 supersaturation in lakes with alkalinity above 1 meq L−1, which represents 57% of the global area of lakes and reservoirs around the world

The relevance of pelagic calcification in the global carbon budget of lakes and reservoirs

Calcite precipitation acts as a carbon sink in the sediments and a short-term source of carbon dioxide (CO2) to the atmosphere, as widely acknowledged in marine studies. However, pelagic calcite precipitation has received limited attention in lakes. Here we use the relationship between lake water alkalinity and reported calcification rates to provide the first global estimate of pelagic calcification in lakes. Global gross calcification rates amount to 0.03 Pg C yr-1 (0.01 - 0.07) comparable to rates of or­ganic carbon burial, whereas its related CO2 release is largely buffered by the carbonate equilibria. Calcification occurs at water alkalinity above 1 meq/L corresponding to 57 % of global lake and reservoir surface area. Pelagic calcification therefore is a prevalent process in lakes and reservoirs at the global scale, with a potentially relevant role as a sedimentary inorganic carbon sink, comparable in magnitude to the total calcite accumulation rates in ocean sediments

The zooplankton community in a small, hypertrophic mediterranean reservoir (Foix reservoir, NE Spain)

A small hypertrophic reservoir (Foix Reservoir, Spain) was sampled from March 1997 to January 2000 to document the composition and spatial and temporal variability of the zooplankton assemblage. Twenty five species of Rotifera, 10 of Cladocera, and one species of copepod (Acanthocyclops robustus (Sars)) were collected. The most abundant rotifer species were Brachionus angularis (Gosse) and Polyarthra spp. Daphnia galeata Sars, Ceriodaphnia quadrangula (Müller), and Chydorus sphaericus (Müller) were the main cladocerans. For each species, brief information is given on ecology, distribution, and occurrence in Spain and in the Foix Reservoir. The zooplankton community of the Foix Reservoir was characterized by the abundance of heleoplanktonic species, which accounted for 50 % of the taxa. Remarkable changes over time in species' composition and occurrence were found for Rotifera and Cladocera during the study period. A. robustus showed high density and biomass values throughout the sampling period. In the summer of 1999, copepods avoided deep layers, whereas some Rotifera (Polyarthra spp,, Filinia spp,, Brachionus angularis) and Chydorus sphaericus were found near the anoxic bottom. A rare rotifer in the Iberian Peninsula, Itura aurita aurita (Ehrenberg), was also found in lotic sections of the reservoir.Un embalse pequeño e hipereutrófico (Embalse de Foix, España) fue muestreado desde Marzo de 1997 a Enero de 2000, con el fin de documentar la composición y variabilidad espacial y temporal de la comunidad zooplantónica. Veinticinco especies de rotíferos, 10 de cladóceros y una especie de copépodo (Acanthocyclops robustus (Sars)) fueron recogidos. Las especies de rotíferos más abundantes fueron Brachionus angularis (Gosse) y Polyarthra spp. Daphnia galeata Sars, Ceriodaphnia quadrangula (Müller) y Chydorus sphaericus (Müller) fueron los principales cladóceros. Para cada especie se da breve información sobre su ecología, distribución y ocurrencia en España y en el Embalse de Foix. La comunidad zooplantónica del embalse se caracterizó por una fuerte presencia de especies heleoplantónicas, que supusieron un 50 % de los taxones. Se constataron cambios remarcables en la composición específica y densidades de rotíferos y cladóceros a lo largo del periodo estudiado. Por el contrario, A. robustus mostró densidades y biomasas altas durante todo el periodo. En el verano de 1999 los copépodos evitaron las capas profundas, mientras que algunos rotíferos (Polyarthra spp, Filinia spp, Brachionus angularis) y Chydorus sphaericus fueron encontrados cerca del fondo anóxico. Un rotífero raro en la Península Ibérica, Itura aurita aurita (Erhenberg), fue encontrado en la sección lótica del embalse

Eutrophication and geochemistry drive pelagic calcite precipitation in lakes

Pelagic calcification shapes the carbon budget of lakes and the sensitivity of dissolved inorganic carbon (DIC) responses to lake metabolism. This process, being tightly linked to primary production, needs to be understood within the context of summer eutrophication which is increasing due to human stressors and global change. Most lake carbon budget models do not account for calcification because the conditions necessary for its occurrence are not well constrained. This study aims at identifying ratios between calcification and primary production and the drivers that control these ratios in freshwater. Using in situ incubations in several European freshwater lakes, we identify a strong relationship between calcite saturation and the ratio between calcification and net ecosystem production (NEP) (p-value < 0.001, R2 = 0.95). NEP-induced calcification is a short-term process that is potentiated by the increase in calcite saturation occurring at longer time scales, usually reaching the highest levels in summer. The resulting summer calcification event has effects on the DIC equilibria, causing deviations from the metabolic 1:1 stoichiometry between DIC and dissolved oxygen (DO). The strong dependency of the ratio between NEP and calcification on calcite saturation can be used to develop a suitable parameterization to account for calcification in lake carbon budgets

Effect of small water retention structures on diffusive CO2 and CH4 emissions along a highly impounded river

The impoundment of running waters through the construction of large dams is recognised as one of the most important factors determining the transport, transformation, and outgassing of carbon (C) in fluvial networks. However, the effects of small and very small water retention structures (SWRS) on the magnitude and spatiotemporal patterns of C emissions are still unknown, even though SWRS are the most common type of water retention structure causing river fragmentation worldwide. Here we evaluated and compared diffusive carbon dioxide (CO2) and methane (CH4) emissions from river sections impounded by SWRS and from their adjacent free-flowing sections along a highly impounded river. Emissions from impounded river sections (mean [SE] = 17.7 [2.8] and 0.67 [0.14] mmol m−2 d−1, for CO2 and CH4, respectively) never exceeded those from their adjacent free-flowing river sections (230.6 [49.7] and 2.14 [0.54] mmol m−2 d−1). We attribute this finding to the reduced turbulence in impounded river sections induced by SWRS compared to free-flowing river sections (i.e., physical driver). Likewise, the presence of SWRS favoured an increase of the concentration of CH4 in impounded waters, but this increase was not sufficient to cause a significant influence in the CH4 efflux from the downstream free-flowing river sections. By contrast, this influenced the larger-scale longitudinal patterns of dissolved CH4, which exhibited a clear shifting pattern along the study stretch, modulated by variables associated with the presence of SWRS, such as higher water residence times, higher sedimentation rates, and higher temperatures. Overall, our results show that the presence of SWRS can modify the concentrations of C gases in highly impounded rivers but exerts a minor influence on diffusive C emissions

Emission factor estimation of ca. 160 emerging organic microcontaminants by inverse modeling in a Mediterranean river basin (Llobregat, NE Spain)

Starting from measured river concentrations, emission factors of 158 organic compounds out of 199 analyzed belonging to different groups of priority and emerging contaminants [pesticides (25), pharmaceuticals and hormones (81), perfluoroalkyl substances (PFASs) (18), industrial compounds (12), drugs of abuse (8) and personal care products (14)] have been estimated by inverse modeling. The Llobregat river was taken as case study representative of Mediterranean rivers. Industrial compounds and pharmaceuticals are the dominant groups (range of 104mg·1000inhab-1·d-1). Personal care products, pesticides, PFASs and illegal drugs showed a load approximately one order of magnitude smaller. Considered on a single compound basis industrial compounds still dominate (range of ca. 103mg·1000inhab-1·d-1) over other classes. Generally, the results are within the range when compared to previously published estimations for other river basins. River attenuation expressed as the percentage fraction of microcontaminants eliminated was quantified. On average they were around 60-70% of the amount discharged for all classes, except for PFASs, that are poorly eliminated (ca. 20% on average). Uncertainties associated with the calculated emissions have been estimated by Monte-Carlo methods (15,000 runs) and typically show coefficients of variation of ca. 120%. Sensitivities associated with the various variables involved in the calculations (river discharge, river length, concentration, elimination constant, hydraulic travel time and river velocity) have been assessed as well. For the intervals chosen for the different variables, all show sensitivities exceeding unity (1.14 to 3.43), tending to amplify the variation of the emission. River velocity and basin length showed the highest sensitivity value. Even considering the limitations of the approach used, inverse modeling can provide a useful tool for management purposes facilitating the quantification of release rates of chemicals into the aquatic environment.This study has been financially supported by the EU through the FP7 project GLOBAQUA (Grant Agreement No. 603629), by the Spanish Ministry of Economyand Competitiveness [project Consolider-Ingenio 2010 SCARCE CSD2009-00065] and by the Generalitat de Catalunya (Consolidated Research Groups: 2014 SGR 418−Water and Soil Quality Unit and 2014 SGR 291−ICRA). It reflects only the author’s views. The Community is not liable for any use that may be made of the information contained therein. MK acknowledges an AGAUR fellowship from the Generalitat de Catalunya (Spain).Peer reviewe

Primer C-Hydrochange workshop sobre métodos y técnicas de medida de flujos de CO2 y CH4 en lagos y embalses

La distribución de la reservas de carbono (C) entre compartimentos del Sistema Tierra constituye uno de los principales temasde estudio de la biogeoquímica moderna. Lagos y embalses des-empeñan un papel importante en los intercambios globales de Cmediante la regulación del transporte de este elemento desde loscontinentes hasta los océanos. [...

Nutrient fluxes through boundaries in the hypolimnion of Sau reservoir : expected patterns and unanticipated processes

By contrast to many natural lakes, the summer hypolimnion in advection-dominated systems like canyon-shaped reservoirs is not isolated from direct inputs from the river. This has important implications in the evolution of limnological features of the hypolimnion through the stratified period, especially if the river water directly plunges as a density current into the hypolimnion as a consequence of temperature differences. Taking the Sau Reservoir (Spain) as a prototype for this kind of systems, we present data from 11 years of monitoring to show how the river water entering the reservoir during summer is the main factor determining hypolimnetic nutrient concentrations. The empirical regression approach used all through the paper also stressed the effect of the improvement in water quality experienced by the river during the studied period on the improvement of the water quality stored in the summer hypolimnion of the reservoir. Since the change in river water quality was the consequence of the implementation of remediation measures at the basin scale, we advocate these solutions to manage reservoir eutrophication problems in this type of systems, which, in addition, had other unexpected benefits for the hypolimnetic water quality in Sau Reservoir.A diferencia de muchos lagos naturales, el hipolimnion que se forma en verano en sistemas dominados por la advección, como los embalses que inundan valles profundos y estrechos, no está aislado de las entradas directas desde el río. Esto tiene importantes implicaciones en la evolución de las características limnológicas del hipolimnion durante el periodo de estratificación, especialmente si el agua del río entra al embalse como una corriente de densidad directamente en el hipolimnion debido a diferencias de temperatura. Tomando el Embalse de Sau (España) como prototipo de este tipo de sistemas, mostramos 11 años de datos de un programa de monitoreo para ejemplificar cómo el agua del río que entra en verano al embalse es el factor determinante de la concentración de nutrientes en el hipolimnion. La aproximación de regresión empírica utilizada en el artículo también puso de manifiesto el efecto de la mejora en la calidad del agua que sufrió el río durante el periodo de estudio en la mejora del agua del agua embalsada en el hipolimnion. Ya que esta mejora en el río fue consecuencia de la implementación de medidas de restauración a nivel de la cuenca, promovemos estas soluciones para la gestión de problemas de eutrofización en este tipo de sistemas, que por otra parte mostraron ventajas inesperadas en el caso del hipolimnion del Embalse de Sau

Hydraulic management drives heat budgets and temperature trends in a Mediterranean reservoir

By contrast to the more regular and predictable temperate lakes, heat budgets and temperature dynamics in Mediterranean reservoirs are characterized by a marked interannual variability. In the present paper, the heat content, annual Birgean heat budget (ABHB), and thermal structure of Sau Reservoir were examined during a period of hypolimnetic withdrawal between 1980 and 1985, and during a period of withdrawal at intermediate depths between 1996 and 2003. The two study periods were also characterized by a wide range of stored water volume fluctuations. Results were used to develop and validate an empirical model to predict annual and monthly heat dynamics statistics and mixed layer depth as a function of hydraulic management parameters such as water volume and selective withdrawal depth. During the hypolimnetic withdrawal period elevated ABHB and deep mixed layer depths were recorded in the reservoir, which behaved as a heat trap. By contrast, intermediate depth withdrawal promoted a shallower and more stable thermocline, thus increasing the cold hypolimnetic water volume and decreasing heat content and ABHB. The study reveals that hydraulic management constitutes the main driver of the heat and thermal dynamics in reservoirs with multiple withdrawal outlets. By contrast with the increasing temperature trends recorded in many natural lakes, the hydraulic management in Sau Reservoir induced a progressive reduction in water temperature and heat content in the system, thus partially counteracting the possible deleterious effects of global warming. Our intensive study in a single, highly‐dynamic ecosystem constitutes a new approximation to the study of thermal structure and heat dynamics in water bodies

Carbon dioxide efflux during the flooding phase of temporary ponds

Small water bodies, such as temporary ponds, have a high carbon processing potential. Nevertheless, despite the global occurrence of these systems, the carbon effluxes from such water bodies have been largely overlooked. In this study, we examined the intra- and intersystem variability of carbon dioxide (CO2) effluxes from a set of Mediterranean temporary ponds during the flooding phase, a hot-spot for biogeochemical cycling in temporary systems. The CO2 effluxes showed higher variability among the various sections of each pond (i.e., inundated, emerged-unvegetated and emerged-vegetated) than among the ponds. The emerged-vegetated sections showed the highest CO2 effluxes per unit area and tended to drive the total effluxes at the whole-ecosystem scale. The mean CO2 efflux (121.3 ± 138.1 mmol m 2 d 1) was in the upper range for freshwater ecosystems. The CO2 effluxes were not related to catchment properties but rather to the organic content of the sediments, especially in the emerged sections of the ponds. Our results indicate that temporary ponds, especially their emerged sections, are important sources of CO2 to the atmosphere, highlighting the need to include the dry phases of these and other temporary aquatic systems in regional carbon budgets