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Processes regulating the rate of oxygen depletion determine whether hypoxia occurs and the extent to which greenhouse gases accumulate in seasonally ice-covered lakes. Here, we investigate the oxygen budget of four arctic lakes using high-frequency data during two winters in three shallow lakes (9–13 m maximal depth) and four winters in 24 m deep main basin of Toolik Lake. Incubation experiments measured sediment metabolism. Volume-averaged oxygen depletion measured in situ was independent of water temperature and duration of the ice-covered period. Average rates were between 0.2 and 0.39 g O2 m−2 d−1 in the shallow lakes and between 0.03 and 0.14 g O2 m−2 d−1 in Toolik Lake, with higher rates in smaller lakes with their larger sediment area to volume ratio. Rates decreased to ~ 20%–50% of initial values in late winter in the shallow lakes but less or not at all in Toolik. The lack of a decline in Toolik Lake points to continued oxygen transport to the sediment–water interface where oxygen consumption occurs. In all lakes, lower in situ oxygen depletion than in incubation measurements points toward increasing anoxia in the lower water column depressing loss rates. In Toolik, oxygen loss during early winter was less in years with minimal snow cover. Penetrative convection occurred, which could mix downwards oxygen produced by photosynthesis or excluded during ice formation. Estimates of these terms exceeded photosynthesis measured in sediment incubations. Modeling under ice-oxygen dynamics requires consideration of optical properties and biological and transport processes that modify oxygen concentrations and distributions.

Estuaries are among the most productive of aquatic ecosystems. Yet the collective understanding of patterns and drivers of primary production in estuaries is incomplete, in part due to complex hydrodynamics and multiple controlling factors that vary at a range of temporal and spatial scales. A whole-ecosystem experiment was conducted in a deep, pelagically dominated terminal channel of the Sacramento-San Joaquin Delta (California, USA) that seasonally appears to become nitrogen limited, to test whether adding calcium nitrate would stimulate primary productivity or increase phytoplankton density. Production did not respond consistently to fertilization, in part because nitrate and phytoplankton were dispersed away from the manipulated area within 1–3 days. Temporal and spatial patterns of gross primary production were more strongly related to stratification and light availability (i.e., turbidity) than nitrogen, highlighting the role of hydrodynamics in regulating system production. Similarly, chlorophyll was positively related not only to stratification but also to nitrogen—with a positive interaction—suggesting stratification may trigger nutrient limitation. The average rate of primary production (4.3 g O2 m−2 d−1), metabolic N demand (0.023 mg N L−1 d−1), and ambient dissolved inorganic nitrogen concentration (0.03 mg N L−1) indicate that nitrogen can become limiting in time and space, especially during episodic stratification events when phytoplankton are isolated within the photic zone, or farther upstream where water clarity increases, dispersive flux decreases, and stratification is stronger and more frequent. The role of hydrodynamics in organizing habitat connectivity and regulating physical and chemical processes at multiple temporal and spatial scales is critical for determining resource availability and evaluating biogeochemical processes in estuaries.

Ecosystem metabolism is a key ecological attribute and easy to describe, but quantifying metabolism in estuaries is challenging. Properly scaling measurements through time and space requires consideration of hydrodynamics and mixing water from heterogeneous sources, making any estimation uncertain. Here, we compared three methods for modeling ecosystem metabolism in a portion of the Sacramento-San Joaquin Delta. Metabolism estimates based on laboratory incubations, continuous in situ buoys, and an oxygen isotope approach all indicated the system was net heterotrophic, and calculated rates were comparable in magnitude when averaged over the 2-month study. Daily metabolic rates based on in situ buoys were the most variable, likely due to horizontal and vertical advection and poor portrayal of the dissolved oxygen budget. After temporally averaging in situ buoy estimates or smoothing the dissolved oxygen time series for tidal effects, rates were more comparable to the other methods, which may be necessary to account for tidal advection and unbalanced contributions from subhabitats within the metabolic footprint. Incubation-based rates represent the finest temporal and spatial scale and only account for pelagic processes, which may explain why incubation-based rates were lower than the other two methods. The oxygen isotope method provided temporally and spatially integrated rates that were bracketed by the other two methods and may be a valuable tool in systems matching the model requirements. Because uncertainty arises in each method from a number of assumptions and scaling calculations, the resolution of metabolic rates in estuaries is likely coarser and more variable than in other aquatic ecosystems.

Warming winters will reduce ice cover and change under-ice conditions in temperate mountain lakes, where snow contributes most of winter cover on lakes. Snow-dominated mountain lakes are abundant and highly susceptible to climate warming, yet we lack an understanding of how climate variation and local attributes influence winter processes. We investigated climatic and intrinsic controls on ice phenology, water temperature, and bottom-water dissolved oxygen (DO) in 15 morphologically diverse lakes in the Sierra Nevada and Klamath Mountains of California, USA, using high-frequency measurements from multiple (2–5) winters. We found that ice phenology was determined by winter climate variables (snowfall and air temperature) that influence ice-off timing, whereas ice-on timing was relatively invariant among years. Lake size and morphology mediated the effect of climate on lake temperature and DO dynamics in early and late winter. Rates of hypolimnetic DO decline were highest in small, shallow lakes, and were unrelated to water temperature. Temperature and oxygen dynamics were more variable in small lakes because heavy snowfall caused ice submergence, mixing, and DO replenishment that affected the entire water column. As the persistence of snow declines in temperate mountain regions, autumn, and spring climatic conditions are expected to gain importance in regulating lake ice phenology. Water temperature and DO will likely increase in most lakes during winter as snowpack declines, but morphological attributes such as lake size will determine the sensitivity of ice phenology and under-ice processes to climate change.

Citizen science projects have a long history in ecological studies. The research usefulness of such projects is dependent on applying simple and standardized methods. Here, we conducted a citizen science project that involved more than 3500 Swedish high school students to examine the temperature difference between surface water and the overlying air (Tw-Ta) as a proxy for sensible heat flux (QH). If QH is directed upward, corresponding to positive Tw-Ta, it can enhance CO2 and CH4 emissions from inland waters, thereby contributing to increased greenhouse gas concentrations in the atmosphere. The students found mostly negative Tw-Ta across small ponds, lakes, streams/rivers and the sea shore (i.e. downward QH), with Tw-Ta becoming increasingly negative with increasing Ta. Further examination of Tw-Ta using high-frequency temperature data from inland waters across the globe confirmed that Tw-Ta is linearly related to Ta. Using the longest available high-frequency temperature time series from Lake Erken, Sweden, we found a rapid increase in the occasions of negative Tw-Ta with increasing annual mean Ta since 1989. From these results, we can expect that ongoing and projected global warming will result in increasingly negative Tw-Ta, thereby reducing CO2 and CH4 transfer velocities from inland waters into the atmosphere.

We compared the efficacy of stable carbon, hydrogen, and nitrogen isotope ratios in identifying the resources used by insect consumers in headwater streams of southern California. We also compared gut contents with consumer stable isotope ratios and mixing model estimates of resource contributions to predator diet. Stable hydrogen isotope ratios (as [[delta].sup.2]H) of algivores were well separated from ratios for detritivores, whereas relationships between stable carbon (as [[delta].sup.13]H) and nitrogen (as [[delta].sup.15]N) ratios of consumers and their expected diets were weaker and more ambiguous. [[delta].sup.2]H values of primary consumers more strongly reflected the proportions of their gut contents consisting of algae than [[delta].sup.13]H values. The proportions of algivorous prey in predator gut contents increased with mixing model estimates of algivore contributions to predator diet using [[delta].sup.2]H but not [[delta].sup.13]H values. Our findings support the use of hydrogen isotope ratios in food web studies of streams in southern California and their potential use in assessing the effects of anthropogenic and natural disturbance on basal resource contributions to food webs that might not otherwise be identified using carbon isotope ratios. Nous avons compare l'efficacite des isotopes stables de carbone, d'hydrogene et d'azote pour cerner les ressources utilisees par des consommateurs d'insectes dans des cours d'eau d'amont du sud de la Californie. Nous avons egalement compare les contenus stomacaux aux rapports d'isotopes stables de consommateurs et a des estimations obtenues de modeles de melange des contributions de ressources au regime alimentaire de predateurs. Les rapports d'isotopes stables d'hydrogene ([[delta].sup.2]H) d'alguivores etaient bien distincts des rapports de detritivores, alors que les relations entre les rapports d'isotopes stables de carbone ([[delta].sup.13]H) et d'azote ([[delta].sup.15]N) de consommateurs et leurs regimes alimentaires attendus etaient plus faibles et plus ambigues. Les valeurs de [[delta].sup.2]H de consommateurs primaires refletaient plus fortement les proportions d'algues de leurs contenus stomacaux que les valeurs de [[delta].sup.13]H. Les proportions de proies alguivores dans les contenus stomacaux de predateurs augmentaient parallelement aux estimations des modeles de melange de la contribution d'alguivores au regime alimentaire de predateurs obtenues en utilisant les valeurs de [[delta].sup.2]H, mais non les valeurs de [[delta].sup.13]H. Nos constatations appuient l'utilisation des rapports d'isotopes d'hydrogene dans les etudes des reseaux trophiques dans des cours d'eau du sud de la Californie et leur utilite potentielle pour evaluer les effets de perturbations humaines et naturelles sur les contributions de ressources de base aux reseaux trophiques que les rapports d'isotopes de carbone pourraient ne pas faire ressortir. [Traduit par la Redaction], Introduction Terrestrial organic matter and stream algae are recognized as the two most important sources of primary production fueling stream food webs (Cummins 1974; Finlay 2001; Doucett et al. 2007; [...]

Lakes are connected to surrounding terrestrial habitats by reciprocal flows of energy and nutrients. We synthesize data from California's mountain lake catchments to investigate how these reciprocal subsidies change along an elevational gradient and with the introduction of a top aquatic predator. At lower elevations, well-developed terrestrial vegetation provides relatively large inputs of organic material to lakes, whereas at higher elevations, the paucity of terrestrial vegetation provides minimal organic input but allows for higher inputs of inorganic nitrogen. There are also pronounced elevational patterns in amphibians and aquatic insects, which represent important vectors for resource flows from lakes back to land. The introduction of trout can reduce this lake-to-land resource transfer, as trout consume amphibians and aquatic insects. We propose a conceptual model in which within-lake processes influence terrestrial consumers at higher elevations, while terrestrial inputs govern within-lake processes at lower elevations. This model contributes to a more general understanding of the connections between aquatic and terrestrial habitats in complex landscapes. Les lacs sont relies aux habitats terrestres qui les entourent par des flux reciproques d'energie et de nutriments. Nous mettons en rapport des donnees tirees de bassins versants de lacs de montagne en Californie pour etudier les variations de ces apports reciproques le long d'un gradient altitudinal et apres l'introduction d'un predateur aquatique de niveau trophique superieur. A basse altitude, une vegetation terrestre bien developpee fournit de relativement grands apports de matiere organique aux lacs, alors qu'a plus haute altitude, la rarete de la vegetation terrestre fournit peu d'apports organiques, mais permet des apports d'azote inorganique plus importants. Des variations altitudinales marquees sont egalement observees chez les amphibiens et les insectes aquatiques, qui representent d'importants vecteurs pour les flux de ressources des lacs vers la terre. L'introduction de truites peut reduire ce transfert de ressources des lacs vers la terre, puisque les truites consomment des amphibiens et des insectes aquatiques. Nous proposons un modele conceptuel dans lequel les processus internes des lacs influencent les consommateurs terrestres a plus haute altitude, alors que les apports terrestres regissent les processus internes des lacs de plus basse altitude. Ce modele participe a une comprehension plus generale des liens entre les habitats aquatiques et terrestres dans des paysages complexes. [Traduit par la Redaction], Introduction The transport of energy and resources across habitat boundaries has important consequences for food-web dynamics and ecosystem function (Polis et al. 1997, 2004; Loreau et al. 2003; Richardson and [...]

We examined bacterioplankton community composition (BCC; using polymerase chain reaction-based phylogenetic fingerprinting) and the composition of dissolved organic matter (DOM; using scanning fluorometry) within lake chains in high-elevation catchments of the Sierra Nevada, California. We compared temporal BCC dynamics in two connected cirque lakes and their inlets and found that lake : inlet community similarity of both lakes declined with increasing residence time because of seasonal snowpack melting. However, for over 3 yr the headwater lake remained distinct from its inlet even during peak snowmelt (residence time