Nitrogen removal by wetlands in a cold climate

Nitrogen (N), a fundamental component of living organisms, has become one of the main global concerns for human society due to the myriad of negative effects of excessive N on ecosystems. Anthropogenic activities such as agriculture, industrial production, urbanisation and mining are major sources of N to freshwaters. Semi-natural and constructed wetlands planted with macrophytes are now widely used in many parts of the world to remove N from water. However, the potential of constructed wetlands for N removal under cold climatic conditions is still not well studied. We also have limited understanding of how macrophyte species and growth form richness as well as functional trait diversity affect N-cycling in constructed wetlands by influencing plant N accumulation, plant associated denitrification and abundance of denitrifying bacterial communities. In mesocosm experiments and in situ studies, I investigated how species and growth form (emerging and submerged macrophytes and bryophytes) richness as well as plant functional trait diversity of macrophytes affect N-cycling in wetlands. Moreover, I tested the applicability of constructed floating wetlands for improved N removal at the local scale in a cold climate. My results highlight that macrophytes are important for both main N removal pathways, viz. uptake and denitrification. Moreover, bacterial denitrification gene abundance on roots and shoots of macrophytes were an important predictor of the denitrification potential of macrophytes. Species and growth form richness of macrophytes enhanced N removal in wetlands. Moreover, I identified complementarity and a selection effect as important diversity related mechanisms, explaining the total N removal from water including plant N accumulation. Functional traits of macrophytes affected N-cycling in wetlands through direct and indirect pathways. The application of constructed floating wetlands at the local scale is feasible in a cold climate with denitrification as the main N removal pathway in these wetland type. Further in situ studies with high numbers of species and growth forms are needed to generalize my findings. Future studies should also consider plant secondary metabolites to better understand the function of the macrophyte-denitrifier interplay

Inverting nutrient fluxes across the land-water interface - Exploring the potential of zebra mussel (Dreissena polymorpha) farming

We studied the potential of zebra mussel farming for nutrient retention in a eutrophic lake. Duplicate experimental long-line cultivation units were deployed and mussel growth and nutrient retention were quantified after 28 months. Mussels grew well at shallow water depth (<3 m) and our 625 m(2) (lake area) experimental units produced 507 and 730 kg dry biomass, respectively, of which 94% were shells. These yields corresponded to an average retention of 92.7 +/- 23.1 kg C, 6.1 +/- 0.68 kg N, and 0.43 +/- 0.04 kg P retention, or 742 kg C, 49 kg N, and 3.5 kg P for a full-size (0.5 ha) mussel farm. We estimate that concentrating the long-lines to a depth of 2.5 m would probably have doubled these yields, based on the differences in mussel growth among depths. We further estimate that a full-size cultivation unit (0.5 ha) thus could compensate for the annual total-P run-off from 23 ha, or the biologically available P from approximately 49 ha of agricultural soils. As traditional measures have proven insufficient, decision-makers need to facilitate novel approaches to mitigate the negative effects of cultural eutrophication. We envision that zebra mussel farming, within their invaded range, provides a promising approach to invert nutrient losses in lakes and coastal lagoons

Disentangling the roles of plant functional diversity and plaint traits in regulating plant nitrogen accumulation and denitrification in freshwaters

1. There is a growing recognition that functional measures of diversity, based on quantification of functionally important species traits, are useful for explaining variation in ecosystem processes. However, the mechanisms linking functional diversity to different processes remain poorly understood, hindering development of a predictive framework for ecosystem functioning based on species traits.2. The current understanding of how the functional traits of aquatic plants (macrophytes) affect nitrogen (N) cycling by regulating microbial communities and their activity in freshwater habitats is particularly limited. Denitrifying bacteria are typically associated with the roots of both aquatic and terrestrial plants and denitrification is the main cause of loss of N from ecosystems. Disentangling the interplay between plants and microbial denitrifiers is key to understanding variation in rates of denitrification from local to landscape scales.3. In a mesocosm experiment, we varied the species richness (monocultures or two-species mixtures) and composition of macrophytes. We quantified effects of both macrophyte functional diversity, quantified as functional trait dissimilarity, and functional trait composition, quantified as community weighted mean trait values, on N removal in wetlands. We used structural equation modelling to disentangle the direct and indirect influences of traits on N accumulation in plant biomass, denitrification activity and abundance of key bacterial denitrification genes (nirS and nirK).4. Both functional diversity and functional trait composition regulated N removal, explaining 70%-94% variation in the underlying ecosystem processes. Increased macrophyte functional diversity increased plant N accumulation, and indirectly enhanced denitrification by increasing denitrification gene abundance. Among traits, greater plant relative growth rates, specific leaf area and above-ground biomass increased plant N accumulation. Denitrification activity increased with increasing below-ground biomass but decreased with increasing root diameter.5. These findings improve our understanding of N removal in freshwater wetlands dominated by macrophytes, and have broad ecological implications for wetland management targeting enhanced ecosystem services. Our results highlight the potential for optimizing denitrification and plant N accumulation in wetlands and thereby improving water purification by increasing macrophyte functional diversity and ensuring the presence of key traits in macrophyte assemblages

Charophytes collapse beyond a critical warming and brownification threshold in shallow lake systems

Charophytes play a critical role for the functioning of shallow lake ecosystems. Although growth of charophytes can be limited by many factors, such as temperature, nutrients and light availability, our understanding about concomitant effects of climate warming and other large-scale environmental perturbations, e.g. increases in humic matter content ('brownification') is still limited. Here we conducted an outdoor mesocosm experiment during 71 days with a common charophyte species, Chara vulgaris, along an increasing gradient of temperature and brownification. We hypothesized the growth of C. vulgaris to increase with temperature, but to level off along the combined temperature and brownification gradient when reaching a critical threshold for light limitation via brownification. We show that C. vulgaris increases the relative growth rate (RGR), main and total shoot elongation, as well as number of lateral shoots when temperature and brownification increased by +2 degrees C and + 100%, respectively above today's levels. However, the RGR, shoot elongation and number of lateral shoots declined at further increment of temperature and brownification. Macrophyte weight-length ratio decreased with increased temperature and brownification, indicating that C. vulgaris allocate more resources or energy for shoot elongation instead of biomass increase at warmer temperatures and higher brownification. Our study shows that C. vulgaris will initially benefit from warming and brownification but will then decline as a future scenario of increased warming and brownification reaches a certain threshold level, in case of our experiment at +4 degrees C and a 2-fold increase in brownification above today's levels. (C) 2019 Elsevier B.V. All rights reserved

Does an evolutionary change in the water sowbug Asellus aquaticus L. alter its functional role?

The ecology behind evolutionary diversification is a well studied area of research, whereas the effects of evolution on ecosystems get little attention. In line with ecological theory, evolutionary diversification of a species could influence different ecosystem aspects such as food web composition, energy flow, nutrient cycling etc. The main objective of this study was to investigate whether two diverging ecotypes (reed and chara) of Asellus aquaticus differ regarding their role in two aquatic ecosystem processes: decomposition of terrestrial leaves and grazing of periphyton. Their role in ecosystem process as well as treatment effects on fitness, measured as growth and survival, were investigated in a laboratory experiment with various levels of intra-specific competition and inter-specific interactions with the amphipod Gammarus pulex. The isopods were collected from two Swedish lakes: Lake Tåkern and Lake Fardume. These two lakes represent different history of ecotype divergence. The experimental design consisted of 2-L aquaria, each providing elm leaves (Ulmus glabra), oak leaves (Quercus roburleaves) and periphyton as food sources. Ten treatments with five replicates were applied for each lake and the experiment lasted for four weeks. The study showed that there was no significant difference between chara and reed ecotype in their functional role. However, the rate of ecosystem processes per individual decreased in competitive interactions. In high density, decomposition per dry weight consumer was low and total algae biomass was high at the end of four weeks due to intra-specific competition. Moreover, ecosystem processes were lowest in inter-specific competition between Gammarus pulex and each ecotype. Present study also shows that ecotypes from the different lakes, having different history, had different responses to mortality and growth.

Does an evolutionary change in the water sowbug Asellus aquaticus L. alter its functional role?

The ecology behind evolutionary diversification is a well studied area of research, whereas the effects of evolution on ecosystems get little attention. In line with ecological theory, evolutionary diversification of a species could influence different ecosystem aspects such as food web composition, energy flow, nutrient cycling etc. The main objective of this study was to investigate whether two diverging ecotypes (reed and chara) of Asellus aquaticus differ regarding their role in two aquatic ecosystem processes: decomposition of terrestrial leaves and grazing of periphyton. Their role in ecosystem process as well as treatment effects on fitness, measured as growth and survival, were investigated in a laboratory experiment with various levels of intra-specific competition and inter-specific interactions with the amphipod Gammarus pulex. The isopods were collected from two Swedish lakes: Lake Tåkern and Lake Fardume. These two lakes represent different history of ecotype divergence. The experimental design consisted of 2-L aquaria, each providing elm leaves (Ulmus glabra), oak leaves (Quercus roburleaves) and periphyton as food sources. Ten treatments with five replicates were applied for each lake and the experiment lasted for four weeks. The study showed that there was no significant difference between chara and reed ecotype in their functional role. However, the rate of ecosystem processes per individual decreased in competitive interactions. In high density, decomposition per dry weight consumer was low and total algae biomass was high at the end of four weeks due to intra-specific competition. Moreover, ecosystem processes were lowest in inter-specific competition between Gammarus pulex and each ecotype. Present study also shows that ecotypes from the different lakes, having different history, had different responses to mortality and growth.

Stream flow velocity alters submerged macrophyte morphology and cascading interactions among associated invertebrate and periphyton assemblages

Submerged macrophytes play a key role in the functioning of stream ecosystems since they strongly affect the biological and physical environment of the habitat. On the other hand, flow velocity may affect growth and establishment of submerged macrophytes in streams and rivers. However, little attention has been paid to the morphological responses of submerged macrophytes to different stream flows and in the present study we investigate the intraspecific difference in flow adaptation of a common submerged macrophyte, Myriophyllum spicatum L We found no difference in length of main shoot or total length of lateral shoots of M. spicatum plants grown at high and low stream flow. However, shoot and root dry weight biomass, number of lateral shoots, degree of branching and stem diameter of the main shoot increased significantly with increasing water velocity. In contrast, the opposite trend was observed for leaf whorl area and distances between the internodes of the main shoot. The amount of periphytic algae also decreased with increased water velocity, whereas the macroinvertebrate abundances were nine fold higher at high than at low stream flow, suggesting that grazing may, besides higher stream flow, have been a process behind the lower periphyton growth at high flow. Hence, stream flow not only acts as a stress factor leading to morphological changes in submerged macrophytes, but also induces cascading trophic interactions among periphytic algae and invertebrate assemblages, thereby being a major force in shaping the organism communities of streams and rivers. (C) 2014 Elsevier B.V. All rights reserved

Disentangling the roles of plant functional diversity and plaint traits in regulating plant nitrogen accumulation and denitrification in freshwaters

1. There is a growing recognition that functional measures of diversity, based on quantification of functionally important species traits, are useful for explaining variation in ecosystem processes. However, the mechanisms linking functional diversity to different processes remain poorly understood, hindering development of a predictive framework for ecosystem functioning based on species traits. 2. The current understanding of how the functional traits of aquatic plants (macrophytes) affect nitrogen (N) cycling by regulating microbial communities and their activity in freshwater habitats is particularly limited. Denitrifying bacteria are typically associated with the roots of both aquatic and terrestrial plants and denitrification is the main cause of loss of N from ecosystems. Disentangling the interplay between plants and microbial denitrifiers is key to understanding variation in rates of denitrification from local to landscape scales. 3. In a mesocosm experiment, we varied the species richness (monocultures or two-species mixtures) and composition of macrophytes. We quantified effects of both macrophyte functional diversity, quantified as functional trait dissimilarity, and functional trait composition, quantified as community weighted mean trait values, on N removal in wetlands. We used structural equation modelling to disentangle the direct and indirect influences of traits on N accumulation in plant biomass, denitrification activity and abundance of key bacterial denitrification genes (nirS and nirK). 4. Both functional diversity and functional trait composition regulated N removal, explaining 70%–94% variation in the underlying ecosystem processes. Increased macrophyte functional diversity increased plant N accumulation, and indirectly enhanced denitrification by increasing denitrification gene abundance. Among traits, greater plant relative growth rates, specific leaf area and above-ground biomass increased plant N accumulation. Denitrification activity increased with increasing below-ground biomass but decreased with increasing root diameter. 5. These findings improve our understanding of N removal in freshwater wetlands dominated by macrophytes, and have broad ecological implications for wetland management targeting enhanced ecosystem services. Our results highlight the potential for optimizing denitrification and plant N accumulation in wetlands and thereby improving water purification by increasing macrophyte functional diversity and ensuring the presence of key traits in macrophyte assemblages. biodiversity, denitrification, ecosystem functioning, ecosystem service, functional diversity, functional traits, macrophytes, plant uptakepublishedVersio