Warming shows differential effects on late-season growth and competitive capacity of Elodea canadensis and Potamogeton crispus in shallow lakes

Submerged macrophytes are likely to be affected by climate changes through changes in water temperatures and length of growing season. We conducted a lab experiment to examine the influence of a late-season temperature increase on growth, biomass allocation, and acclimation of 2 submerged macrophyte species, Elodea canadensis and Potamogeton crispus. We also ran competitive interaction experiments between the 2 species with mono- and mixed-species cultures in pots placed in outdoor heated mesocosms (5 years at ambient temperature and a higher temperature following the IPCC A2 scenario downscaled to local conditions but enhanced by 50%). In the lab, macrophytes collected in the 2 types of mesocosms were grown at ambient temperatures (12 °C in September and 8 °C in October) and 4 °C higher. Warming had an overall stronger effect on E. candensis than P. crispus, particularly within the low temperature range studied. Hence, the relative growth rate (RGR) of E. canadensis acclimated to ambient mesocosm conditions increased 6-fold from low (8 °C) to high (16 °C) temperature whereas the RGR of P. crispus increased <2-fold. In the competitive interaction experiment, warming increased the biomass and RGR of E. canadensis in the monoculture. In addition, warming increased shoot elongation of the plant in both the monoculture and mixed culture. P. crispus was generally unaffected by warming when grown in both monoculture and mixed culture, but competition decreased the elongation of shoots pre-adapted to ambient conditions and grown in the warmer mesocosms. The decomposition rate of E. canadensis accelerated with warming but was unaffected in P. crispus. We conclude that E. canadensis is a stronger competitor than P. crispus under warmer late-season conditions; however, it may have a higher demand for oxygen due to the increased decomposition rates at higher temperatures, particularly in the peripheral growing season, with potential profound effects on lake ecosystems. Although acclimatisation was evident, we suggested that temperature changes will affect the growth pattern of the 2 plant species and thereby perhaps induce a switch in macrophyte species dominance

Benthic–planktonic Coupling, Regime Shifts, and Whole-lake Primary Production in Shallow Lakes

Alternative stable states in shallow lakes are typically characterized by submerged macrophyte (clear-water state) or phytoplankton (turbid state) dominance. However, a clear-water state may occur in eutrophic lakes even when macrophytes are absent. To test whether sediment algae could cause a regime shift in the absence of macrophytes, we developed a model of benthic (periphyton) and planktonic (phytoplankton) primary production using parameters derived from a shallow macrophyte-free lake that shifted from a turbid to a clear-water state following fish removal (biomanipulation). The model includes a negative feedback effect of periphyton on phosphorus (P) release from sediments. This in turn induces a positive feedback between phytoplankton production and P release. Scenarios incorporating a gradient of external P loading rates revealed that (1) periphyton and phytoplankton both contributed substantially to whole-lake production over a broad range of external P loading in a clear-water state; (2) during the clear-water state, the loss of benthic production was gradually replaced by phytoplankton production, leaving whole-lake production largely unchanged; (3) the responses of lakes to biomanipulation and increased external P loading were both dependent on lake morphometry; and (4) the capacity of periphyton to buffer the effects of increased external P loading and maintain a clear-water state was highly sensitive to relationships between light availability at the sediment surface and the of P release. Our model suggests a mechanism for the persistence of alternative states in shallow macrophyte-free lakes and demonstrates that regime shifts may trigger profound changes in ecosystem structure and function

Contrasting roles of water chemistry, lake morphology, land-use, climate and spatial processes in driving phytoplankton richness in the Danish landscape

Understanding of the forces driving the structure of biotic communities has long been an important focus for ecology, with implications for applied and conservation science. To elucidate the factors driving phytoplankton genus richness in the Danish landscape, we analyzed data derived from late-summer samplings in 195 Danish lakes and ponds in a spatially-explicit framework. To account for the uneven sampling of lakes in the monitoring data, we performed 1,000 permutations. A random set of 131 lakes was assembled and a single sample was selected randomly for each lake at each draw and all the analyses were performed on permuted data 1,000 times. The local environment was described by lake water chemistry, lake morphology, land-use in lake catchments, and climate. Analysis of the effects of four groups of environmental factors on the richness of the main groups of phytoplankton revealed contrasting patterns. Lake water chemistry was the strongest predictor of phytoplankton richness for all groups, while lake morphology also had a strong influence on Bacillariophyceae, Cyanobacteria, Dinophyceae, and Euglenophyceae richness. Climate and land-use in catchments contributed only little to the explained variation in phytoplankton richness, although both factors had a significant effect on Bacillariophyceae richness. Notably, total nitrogen played a more important role for phytoplankton richness than total phosphorus. Overall, models accounted for ca. 30% of the variation in genus richness for all phytoplankton combined as well as the main groups separately. Local spatial structure (< 30 km) in phytoplankton richness suggested that connectivity among lakes and catchment-scale processes might also influence phytoplankton richness in Danish lakes

Factors controlling the stable isotope composition and C:N ratio of seston and periphyton in shallow lake mesocosms with contrasting nutrient loadings and temperatures

Carbon (C) and nitrogen (N) stable isotope composition (15N:14N, δ15N and 13C:12C, δ13C) have been widely used to elucidate changes in aquatic ecosystem dynamics created by eutrophication and climate warming, often, however, without accounting for seasonal variation. Here, we aim to determine the factors controlling the stable isotope composition and C:N ratio of seston and periphyton in shallow lakes with contrasting nutrient loadings and climate; for this purpose, we followed the monthly stable isotope composition (c. 1 year) of seston (SES) and periphyton (PER) in 24 mesocosms mimicking shallow lakes with two nutrient treatments (enriched and unenriched) and three temperature scenarios (ambient, +3 and +5°C). Nutrient enrichment and warming had a stronger impact on the δ15N and δ13C values of seston than on periphyton, and the temporal isotopic variability in both communities was large. δ15NPER did not differ markedly between nutrient treatments, whereas δ15NSES was lower in the enriched mesocosms, possibly reflecting higher N2-fixation by cyanobacteria. δ15NSES was higher in winter in the heated mesocosms and its dynamics was linked with that of NH4-N, whereas δ15NPER showed a stronger association with NO3-N. δ15NSES demonstrated a positive relationship with mean monthly temperature, indicating less isotope fractionation among autotrophs when production increased. δ13CSES was lowest in the enriched mesocosms during winter, whereas δ13CPER did not differ between nutrient treatments. δ13CSES and δ13CPER were positively related to pH, likely reflecting a pH-induced differential access to dissolved carbon species in the primary producers. The positive δ13C-temperature relationship suggested less fractionation of CO2 and HCO3 − and/or larger use of HCO3 − at higher temperatures. The C:N ratios varied seasonally and the differences between the enriched and unenriched mesocosms were stronger for seston than for periphyton. Particularly, the C:NSES ratios did not indicate deficiencies in N as opposed to the C:NPER ratios, supporting the observed changes in δ15N and suggesting that seston and periphyton have access to different sources of nutrients. We did not observe any clear effect of temperature warming on the C:N ratios. Our study provides evidence of strong seasonality in the isotopic composition and C:N ratios of seston and periphyton across nutrient and temperature levels; also, we identified several factors that are likely to modulate the strength and variability in stable isotopes values and stoichiometry of sestonic and periphytic communities under these scenarios.Fil: Trochine, Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; Argentina. University Aarhus; DinamarcaFil: Guerrieri, Marcelo. University Aarhus; DinamarcaFil: Liboriussen, Lone. University Aarhus; DinamarcaFil: Willems, Priscila. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Norte. Estación Experimental Agropecuaria San Carlos de Bariloche; ArgentinaFil: Lauridsen, Torben L.. University Aarhus; Dinamarca. Sino-Danish Education and Research Centre; ChinaFil: Søndergaard, Martín. University Aarhus; Dinamarca. Sino-Danish Education and Research Centre; ChinaFil: Jeppesen, Erik. University Aarhus; Dinamarca. Sino-Danish Education and Research Centre; Chin

Contrasting patterns of freshwater microbial metabolic potentials and functional gene interactions between an acidic mining lake and a weakly alkaline lake

Acidic coal mining lakes (AMLs) are characterized by low pH, low biodiversity, and high concentrations of sulfate and metals. However, to date, knowledge is inadequate on the involvement of the aquatic microbial functions in biogeochemical processes of an AML. Here, we use a natural weakly alkaline lake (WAL) as a reference to investigate the functional gene structure of microbial communities in the aerobic surface waters of an AML by using a metagenomic approach with GeoChip-based functional gene arrays (GeoChip 5.0). We found significant differences in the compositions of the microbial functional genes between the two lake types. The AML had significantly lower gene diversity than the WAL, and the functional genes detected in the AML were almost a subset of those in the WAL. We also found significant differences in the microbial metabolic potentials between the two lake types. In the AML, the microbial communities had higher functional capacities in phosphorus-utilization, sulfur oxidation, and metal-detoxification and had lower metabolic potentials of carbon and nitrogen fixation, and the degradation of starch, hemicellulose, cellulose, chitin, and lignin than those in the WAL. By performing null model and network analyses, we observed stronger deterministic effects of the functional gene assemblages and greater clustering among the co-occurring functional genes in the AML than those in the WAL. Altogether, we demonstrated contrasting patterns of freshwater microbial metabolic potentials and functional gene interactions between an AML and a WAL, which may advance the understanding of microbial functional response to acidification in freshwater lakes

Winter ecology of shallow lakes: strongest effect of fish on water clarity at high nutrient levels

While the structuring role of fish in lakes is well studied for the summer season in North temperate lakes, little is known about their role in winter when fish activity and light irradiance potentially are lower. This is unfortunate as the progressing climate change may have strong effects on lake winter temperature and possibly on trophic dynamics too. We conducted an enclosure experiment with and without the presence of fish throughout winter in two shallow lakes with contrasting phosphorus concentrations. In hypertrophic Lake Søbygard, absence of fish led to higher biomass of zooplankton, higher grazing potential (zooplankton:phytoplankton ratio) and, accordingly, lower biomass of phytoplankton and chlorophyll a (Chl a), while the concentrations of total nitrogen (TN), total phosphorus (TP), oxygen and pH decreased. The average size of egg-bearing Daphnia and Bosmina and the minimum size of egg-bearing specimens of the two genera rose. In the less eutrophic Lake Stigsholm, zooplankton and their grazing potential were also markedly affected by fish. However, the decrease in Chl a was slight, and phytoplankton biovolume, pH and the oxygen concentration were not affected. TN was higher when fish were absent. Our results indicate that: (i) there is a notable effect of fish on zooplankton community structure and size during winter in both eutrophic and hypertrophic North temperate lakes, (ii) Chl a can be high in winter in such lakes, despite low light irradiance, if fish are abundant, and (iii) the cascading effects on phytoplankton and nutrients in winter may be more pronounced in hypertrophic lakes. Climate warming supposedly leading to reduced winter mortality and dominance of small fish may enhance the risk of turbid state conditions in nutrient-enriched shallow lakes, not only during the summer season, but also during winter.JRC.DDG.H.5-Rural, water and ecosystem resource