Effects of dry-wet cycles on nitrous oxide emissions in freshwater sediments: a synthesis

Background Sediments frequently exposed to dry-wet cycles are potential biogeochemical hotspots for greenhouse gas (GHG) emissions during dry, wet and transitional phases. While the effects of drying and rewetting on carbon fluxes have been studied extensively in terrestrial and aquatic systems, less is known about the effects of dry-wet cycles on N2O emissions from aquatic systems. As a notable part of lotic systems are temporary, and small lentic systems can substantially contribute to GHG emissions, dry-wet cycles in these ecosystems can play a major role on N2O emissions. Methodology This study compiles literature focusing on the effects of drying, rewetting, flooding, and water level fluctuations on N2O emissions and related biogeochemical processes in sediments of lentic and lotic ecosystems. Results N2O pulses were observed following sediment drying and rewetting events. Moreover, exposed sediments during dry phases can be active spots for N2O emissions. The general mechanisms behind N2O emissions during dry-wet cycles are comparable to those of soils and are mainly related to physical mechanisms and enhanced microbial processing in lotic and lentic systems. Physical processes driving N2O emissions are mainly regulated by water fluctuations in the sediment. The period of enhanced microbial activity is driven by increased nutrient availability. Higher processing rates and N2O fluxes have been mainly observed when nitrification and denitrification are coupled, under conditions largely determined by O2 availability. Conclusions The studies evidence the driving role of dry-wet cycles leading to temporarily high N2O emissions in sediments from a wide array of aquatic habitats. Peak fluxes appear to be of short duration, however, their relevance for global emission estimates as well as N2O emissions from dry inland waters has not been quantified. Future research should address the temporal development during drying-rewetting phases in more detail, capturing rapid flux changes at early stages, and further explore the functional impacts of the frequency and intensity of dry-wet cycles.

Selenium Interactions with Algae: Chemical Processes at Biological Uptake Sites, Bioaccumulation, and Intracellular Metabolism

Selenium (Se) uptake by primary producers is the most variable and important step in determining Se concentrations at higher trophic levels in aquatic food webs. We gathered data available about the Se bioaccumulation at the base of aquatic food webs and analyzed its relationship with Se concentrations in water. This important dataset was separated into lotic and lentic systems to provide a reliable model to estimate Se in primary producers from aqueous exposure. We observed that lentic systems had higher organic selenium and selenite concentrations than in lotic systems and selenate concentrations were higher in lotic environments. Selenium uptake by algae is mostly driven by Se concentrations, speciation and competition with other anions, and is as well influenced by pH. Based on Se species uptake by algae in the laboratory, we proposed an accurate mechanistic model of competition between sulfate and inorganic Se species at algal uptake sites. Intracellular Se transformations and incorporation into selenoproteins as well as the mechanisms through which Se can induce toxicity in algae has also been reviewed. We provided a new tool for risk assessment strategies to better predict accumulation in primary consumers and consequently to higher trophic levels, and we identified some research needs that could fill knowledge gaps.

Plankton characterization of alpine ponds: a case of study for the assessment of water quality in Serra da Estrela (Portugal)

Serra da Estrela is Portugal's mainland highest mountain, with distinctive geological, climatic and geomorphological characteristics which play a key role on surface and groundwater resources. These mountains have the largest set of glacial ponds of Portugal and are scarcely studied. Besides, there are concerns of possible impacts of the use of salts in road de-icing on ponds' biotic communities. This research intends to characterize the hydrology of five Mediterranean alpine ponds and assess their planktonic communities. The biotic index proposed by the Water Framework Directive for lentic systems was used to check its applicability to these systems. Sampling was conducted monthly from June to November 2015 alongside with hydrological characterization. Water parameters and nutrients' content were quantified. Phytoplankton and macrozooplankton were identified. Ponds' water level is controlled by precipitation (input) and evaporation, infiltration and streamflow (output). Ponds were in good ecological quality. However, the current ecological assessment tools proposed by the WFD for lentic systems do not fit alpine ponds, showing a gap in the current methods. Salts used for road de-icing did not impact the communities and the ecological state of ponds. Phytoplankton was dominated by Cyanobacteria, Dinophyceae and Bacillariophyceae; Cyclopoida was the most abundant zooplanktonic group. For macrozooplankton, RDA shows association between cladocerans and phosphorus content. Phosphorus changes promoted shifts in zooplankton, what may be of major concern from a conservation point of view, once alpine ponds and its species are still mostly unknown. Efforts should be done to develop tools to classify aquatic ecosystems from harsh environments.

Trophic status index of lentic systems from the diversity of aquatic plants from continental Ecuador (IMAE)

Aquatic plants are a priority biological group to study due to their high representativeness in ecosystem services and because they also indicate the state of conservation of lake systems. The goal of the study is to develop an Index of Macrophytes from continental Ecuador (IMAE), which can also be applied in the Andean region as a bioindicator, thus assessing the ecological health of the continental lakes and lagoons from 12 to 4000 m, evaluating the structure and floristic composition of aquatic plants and identifying the habitat preferences of the species according to the concentration of nutrients (nitrites, phosphates and ammonium), as determining elements of their presence. 104 species of aquatic plants were found. The tolerance and indicator values have been calculated for each species meanwhile seven levels of coverage were established. Last, four classes of trophic status or water quality were defined to asses lentic systems. The present study will significantly strengthen the environmental control tools for Ecuador.

Diet of European Catfish in a Newly Invaded Region

Biological invasions are considered to be one of the leading causes of biodiversity loss worldwide. Freshwater ecosystems are amongst the most biodiverse, containing about 50% of the world’s known fish species but, ironically, are the most invaded ecosystems. In Portuguese freshwaters, there are currently 20 identified non-native fishes, arriving at an unprecedented rate of a new species every two years. A recent non-native is the European catfish Silurus glanis, a top predator native to Central and Eastern Europe, that arrived at the Lower Tagus (Portugal) circa 2006. This study compares the dietary habits of this top predator in two different habitats of the Lower Tagus in order to evaluate its potential impact. The stomach contents of 96 individuals were analyzed, 61 from the lotic environment and 35 from the lentic systems. A total of 66 stomachs contained prey items, 38 from the lotic and 28 from the lentic. Diet varied considerably between habitats, with shrimp and crayfish being the most abundant prey items in the lentic systems and shrimp and fish the most abundant prey items in the lotic section. Tendency to piscivory was observed in large-sized individuals. Overall, the findings in this study provide the first evidence of the potential impacts of European catfish through predation on the fish communities.

Community composition of zooplankton exported from a shallow polymictic reservoir linked to wind conditions

Zooplankton exported from lentic systems provision lotic systems with easily captured, consumed, and assimilated prey items. Previous studies have demonstrated that the community composition of zooplankton exports (CCZE) vary over time, which introduces temporal differences in lotic resource availability (zooplankton prey) in downstream habitats. In the study presented here, we monitored variation in CCZE from a polymictic reservoir outfall in response to physical–chemical and atmospheric conditions bi-hourly over three different 24-h periods. Community composition of zooplankton export varied over the course of the day, and exports were most closely associated with wind directionality. Future studies of temporal variation in CCZE should incorporate wind conditions, especially in shallow systems where holomixis occurs frequently. Polymictic reservoirs are becoming increasingly common as the global pace of small dam construction quickens, making both the identification of factors influencing CCZE and the impact of zooplankton exports on local biodiversity and ecosystem function increasingly important to understand.

Proceedings of the First International Snakehead Symposium

<em>Abstract.</em>—The nonnative Northern Snakehead <em>Channa argus </em>was first documented in the Potomac River system in 2004. Since then, their range in Virginia has expanded to include other rivers and numerous lakes as a result of dispersal and illegal introductions. Most Northern Snakehead lake populations were discovered after 2012. Through 2017, nearly 4,000 Northern Snakehead were collected via Virginia Department of Game and Inland Fisheries (VDGIF) electrofishing surveys, resulting in a robust dataset. These collections provided an opportunity to investigate food habits of Northern Snakehead in both lotic and lentic systems which may assist with management and a better understanding of potential community effects. Incidence of identifiable prey items (<em>n </em>= 677) was evaluated since 2004, however wet weights (<em>n </em>= 370) were not recorded until 2014. A total of 30 prey types were identified from Northern Snakehead stomachs taken from rivers, whereas 7 prey types were identified from lakes. Banded Killifish, Bluegill, and crayfish were the most abundant prey types (in order) based on frequency of occurrence for Northern Snakehead collected from rivers; whereas Bluegill, frogs, and Yellow Perch were most common in Northern Snakehead collected from lakes. Most important food types (in order) based on % wet weight for Northern Snakehead collected from rivers were Bluegill, Gizzard Shad, and Banded Killifish; whereas Bluegill, Yellow Perch, and frogs contributed the most mass for Northern Snakehead from lakes.