Shallow plant-dominated lakes - extreme environmental variability, carbon cycling and ecological species challenges

Do new lakes behave like natural lakes regarding sediment composition and phosphorus fluxes?

Numerous new lakes have been established during the last few decades. Lakes established on former agricultural soils often have high legacy phosphorus (P)-content, which constitutes a risk for potential internal P-loading after the lake is formed. In this study, we compared the P release and sediment P-pools from 31 new lakes and 31 natural lakes, to assess their similarities and differences. A suite of other sediment characteristics was identified and compared for both natural and new lakes; catchment characteristics of the new lakes also were analyzed. P release from the sediment of new lakes was significantly lower than from natural lakes (13.2 mg P m-2 d-1) compared to new lakes (6.9 mg P m-2 d-1). The P release was found to be low when molar Fe:P ratios were above 10. A significant correlation was found between the content of mobile-P (loosely adsorbed P, iron-bound P, and leachable organic P) and TP in the sediment, irrespective of lake type. The composition of the mobile P-pool also differed, with the new non-excavated lakes showing a higher proportion of RP-BD; both new lake types had significantly (p = 0.021) lower proportions of nrP, compared to natural lakes in the uppermost 10 cm sediment. In addition, variance in P release and mobile-P content of new lakes could be explained in terms of the land use of the catchments. Most sediment characteristics of new lakes established without topsoil excavation reached the average levels of natural Danish lakes with respect to density, organic content and P content within 20-30 years, while excavated lakes showed no such tendencies.

Drivers of spatial and seasonal variations of CO2 and CH4 fluxes at the sediment water interface in a shallow eutrophic lake

Shallow eutrophic lakes contribute disproportional to the emissions of CO₂ and CH₄ from inland waters. The processes that contribute to these fluxes, their environmental controls, and anthropogenic influences, however, are poorly constrained. Here, we studied the spatial variability and seasonal dynamics of CO₂ and CH₄ fluxes across the sediment-water interface, and their relationships to porewater nutrient concentrations in Lake Ulansuhai, a shallow eutrophic lake located in a semi-arid region in Northern China. The mean concentrations of CO₂ and CH₄ in porewater were 877.8 ± 31.0 µmol L^-1 and 689.2 ± 45.0 µmol L^-1, which were more than 50 and 20 times higher than those in the water column, respectively. The sediment was always a source of both gases for the water column. Porewater CO₂ and CH₄ concentrations and diffusive fluxes across the sediment-water interface showed significant temporal and spatial variations with mean diffusive fluxes of 887.3 ±124.7 µmol m^-2 d^-1 and 607.1 ± 68.0 µmol m^-2 d^-1 for CO₂ and CH₄, respectively. The temporal and spatial variations of CO₂ and CH₄ concentrations in porewater were associated with corresponding variations in dissolved organic carbon and dissolved nitrogen species. Temperature and dissolved organic carbon in surface porewater were the most important drivers of temporal variations in diffusive fluxes, whereas dissolved organic carbon and nitrogen were the main drivers of their spatial variations. Diffusive fluxes generally increased with increasing dissolved organic carbon and nitrogen in the porewater from the inflow to the outflow region of the lake. The estimated fluxes of both gases at the sediment-water interface were one order of magnitude lower than the emissions at the water surface, which were measured in a companion study. This indicates that diffusive fluxes across the sediment-water interface were not the main pathway for CO₂ and CH₄ emissions to the atmosphere. To improve the mechanistic understanding and predictability of greenhouse gas emissions from shallow lakes, future studies should aim to close the apparent gap in the CO₂ and CH₄ budget by combining improved flux measurement techniques with process-based modeling.

Wind Exposure Regulates Water Oxygenation in Densely Vegetated Shallow Lakes

The presence of dense macrophyte canopies in shallow lakes locally generates thermal stratification and the buildup of labile organic matter, which in turn stimulate the biological oxygen demand. The occurrence of hypoxic conditions may, however, be buffered by strong wind episodes, which favor water mixing and reoxygenation. The present study aims at explicitly linking the wind action and water oxygenation within dense hydrophytes stands in shallow lakes. For this purpose, seasonal 24 h-cycle campaigns were carried out for dissolved gases and inorganic compounds measurements in vegetated stands of an oligo-mesotrophic shallow lake. Further, seasonal campaigns were carried out in a eutrophic shallow lake, at wind-sheltered and -exposed sites. Overall results showed that dissolved oxygen (DO) daily and seasonal patterns were greatly affected by the degree of wind exposure. The occurrence of frequent wind episodes favored the near-bottom water mixing, and likely facilitated mechanical oxygen supply from the atmosphere or from the pelagic zone, even during the maximum standing crop of plants (i.e., summer and autumn). A simple model linking wind exposure (Keddy Index) and water oxygenation allowed us to produce an output management map, which geographically identified wind-sheltered sites as the most subjected to critical periods of hypoxia.

Large pools and fluxes of carbon, calcium and phosphorus in dense charophyte stands in ponds

Bicarbonate and calcium set bounds on photosynthesis and degradation processes in calcareous freshwaters. Charophytic algae use bicarbonate in photosynthesis, and direct variable proportions to assimilate organic carbon and to precipitate calcium carbonate on their surfaces. To evaluate pools of organic carbon (C_org), carbonate carbon (C_carbonate), and phosphorus (P) in dense charophyte vegetation, we studied apical and basal tissue and carbonate surface precipitates, as well as underlying sediments in ten calcareous ponds. We also quantified the release of calcium, bicarbonate and phosphate from charophyte shoots in dark experiments. We found that the C_org:C_carbonate quotient in charophyte stands averaged 1.19 during spring and summer. The C_org:C_carbonate quotient in the sediments formed by dead charophytes averaged 0.97 in accordance with some respiratory CO₂ release without carbonate dissolution to bicarbonate. The molar quotient of carbon to calcium was close to 2.0 in sediment and pond water. In dark incubations, shoots subjected to calcium carbonate dissolution released bicarbonate and calcium with a molar quotient of 2:1; lowered pH (7.0-8.0) increased the release. Thus, the carbonate surface crust on living charophytes was not inert, as hitherto anticipated. Phosphate dark release occurred from basal shoots only, was unrelated to pH, and may have derived from organic decomposition, rather than from carbonate dissolution. Extensive phosphorus pools were associated with the charophyte stands (200-600 mg m^-2) and had about 2/3 incorporated in alga tissue and 1/3 in carbonate crust. Overall, the biogeochemistry of carbon, calcium and phosphorus are closely linked in calcareous charophyte ponds. Carbonate dissolution from charophyte crusts at night and continuously from sediment might balance extensive carbonate precipitation during daytime photosynthesis. The substantial P-pool in charophyte stands may not derive from P-deprived water, but from P-rich sediment. Charophyte photosynthesis may still contribute to nutrient-poor conditions by forming carbonate-rich sediment of high P-binding capacity.

Biogeochemistry of macrophytes, sediments and porewaters in thermokarst lakes of permafrost peatlands, western Siberia

The chemical composition of thermokarst lake ecosystem components is a crucial indicator of current climate change and permafrost thaw. Despite high importance of macrophytes in shallow permafrost thaw lakes for control of major and trace nutrients in lake water, the trace element (TE) partitioning between macrophytes and lake water and sediments in the permafrost regions remains virtually unknown. Here we sampled dominant macrophytes in thermokarst lakes of discontinuous and continuous permafrost zones in the Western Siberia Lowland (WSL) and measured major and trace elements in plant biomass, lake water, lake sediments and sediment porewater. All six plant species (Hippuris vulgaris L., Glyceria maxima (Hartm.) Holmb., Comarum palustre L., Ranunculus spitzbergensis Hadac, Carex aquatilis Wahlenb s. str., Menyanthes trifoliata L.) sizably accumulated macronutrients (Na, Mg, Ca), micronutrients (B, Mo, Nu, Cu, Zn, Co) and toxicants (As, Cd). Accumulation of other trace elements, including rare earth elements (REE), in macrophytes relative to pore waters and sediments was highly variable among species. Using miltiparametric statistics, we described the behavior of ТЕ across two permafrost zones and identified several group of elements depending on their sources in the lake ecosystems and their affinity to sediments and macrophytes. Under future climate warming and shifting the permafrost border to the north, we anticipate an increasing uptake of heavy metals and lithogenic low mobile elements such as Ti, Al, Cr, As, Cu, Fe, Ni, Ga, Zr, and REEs by macrophytes in the discontinuous permafrost zone and Ba, Zn, Pb and Cd in the continuous permafrost zone. This may eventually diminish transport of metal micronutrients and geochemical tracers from soils to lakes and rivers and further to the Arctic Ocean.

Principal Determinants of Aquatic Macrophyte Communities in Least-Impacted Small Shallow Lakes in France

Small shallow lakes (SSL) support exceptionally high and original biodiversity, providing numerous ecosystem services. Their small size makes them especially sensitive to anthropic activities, which cause a shift to dysfunctional turbid states and induce loss of services and biodiversity. In this study we investigated the relationships between environmental factors and macrophyte communities. Macrophytes play a crucial role in maintaining functional clear states. Better understanding the factors determining the composition and richness of aquatic plant communities in least-impacted conditions may be useful to protect these shallow lakes. We inventoried macrophyte communities and collected chemical, climatic, and morphological data from 89 least-impacted SSL widely distributed in France. SSL were sampled across four climatic ecoregions, various geologies, and elevations. Hierarchical cluster analysis showed a clear separation of four macrophyte assemblages strongly associated with mineralization. Determinant factors identified by distance-based redundancy analysis (db-RDA) analysis were, in order of importance, geology, distance from source (DIS, a proxy for connectivity with river hydrosystems), surface area, climate, and hydroperiod (water permanency). Surprisingly, at a country-wide scale, climate and hydroperiod filter macrophyte composition weakly. Geology and DIS are the major determinants of community composition, whereas surface area determines floristic richness. DIS was identified as a determinant in freshwater lentic ecosystems for the first time.

Disentangling the direct and indirect effects of agricultural runoff on freshwater ecosystems subject to global warming: A microcosm study

Aquatic ecosystems are exposed to multiple stressors such as agricultural run-off (ARO) and climate-change related increase of temperature. We aimed to determine how ARO and the frequency of its input can affect shallow lake ecosystems through direct and indirect effects on primary producers and primary consumers, and whether warming can mitigate or reinforce the impact of ARO. We performed a set of microcosm experiments simulating ARO using a cocktail of three organic pesticides (terbuthylazine, tebuconazole, pirimicarb), copper and nitrate. Two experiments were performed to determine the direct effect of ARO on primary producers (submerged macrophytes, periphyton and phytoplankton) and on the grazing snail Lymnaea stagnalis, respectively. Three different ARO concentrations added as single doses or as multiple pulses at two different temperatures (22°C and 26°C) were applied. In a third experiment, primary producers and consumers were exposed together to allow trophic interactions. When functional groups were exposed alone, ARO had a direct positive effect on phytoplankton and a strong negative effect on L. stagnalis. When exposed together, primary producer responses were contrasting, as the negative effect of ARO on grazers led to an indirect positive effect on periphyton. Periphyton in turn exerted a strong control on phytoplankton, leading to an indirect negative effect of ARO on phytoplankton. Macrophytes showed little response to the stressors. Multiple pulse exposure increased the effect of ARO on L. stagnalis and periphyton when compared with the same quantity of ARO added as a single dose. The increase in temperature had only limited effects. Our results highlight the importance of indirect effects of stressors, here mediated by grazers and periphyton, and the frequency of the ARO input in aquatic ecosystems.

Conditions that promote the formation of black bloom in aquatic microcosms and its effects on sediment bacteria related to iron and sulfur cycling

Black bloom occurs frequently in eutrophic waters. We investigated the conditions promoted the formation of black bloom via in-situ measurement in two aquatic microcosms and the effects of black bloom on the bacterial community composition. Although larger changes in dissolved oxygen (DO) were detected in the Hydrilla verticillata-dominated microcosm over the 90-day simulation, black bloom occurred more readily in the phytoplankton-dominated than macrophyte-dominated microcosm under conditions of O₂ depletion and temperature above 30 °C. The sediment bacterial community composition shifted after black bloom; the relative abundance of Thiobacillus and Sideroxydans, which oxidize iron (Fe) and sulfur (S), decreased by 47% and 48%, respectively, in the phytoplankton-dominated microcosm and by 18% and 20% in the macrophyte-dominated microcosm. By contrast, Desulfatiglans increased by 13% and 19%, respectively, after black bloom. Furthermore, inter-taxa correlations remarkably changed according to co-occurrence network analysis. Thirty-six different taxa from the phylum to the genus level were identified as biomarkers of sediments collected before and after the black bloom event. Most of these biomarkers are related to Fe/S cycling in aquatic ecosystems.

Structural basis for C4 photosynthesis without Kranz anatomy in leaves of the submerged freshwater plant Ottelia alismoides

BACKGROUND AND AIMS

Ottelia alismoides (Hydrocharitaceae) is a freshwater macrophyte that, unusually, possesses three different CO2-concentrating mechanisms. Here we describe its leaf anatomy and chloroplast ultrastructure, how these are altered by CO2 concentration and how they may underlie C4 photosynthesis.

METHODS

Light and transmission electron microscopy were used to study the anatomy of mature leaves of O. alismoides grown at high and low CO2 concentrations. Diel acid change and the activity of phosphoenolpyruvate carboxylase were measured to confirm that CAM activity and C4 photosynthesis were present.

KEY RESULTS

When O. alismoides was grown at low CO2, the leaves performed both C4 and CAM photosynthesis whereas at high CO2 leaves used C4 photosynthesis. The leaf comprised an upper and lower layer of epidermal cells separated by a large air space occupying about 22 % of the leaf transverse-section area, and by mesophyll cells connecting the two epidermal layers. Kranz anatomy was absent. At low CO2, chloroplasts in the mesophyll cells were filled with starch even at the start of the photoperiod, while epidermal chloroplasts contained small starch grains. The number of chloroplasts in the epidermis was greater than in the mesophyll cells. At high CO2, the structure was unchanged but the thicknesses of the two epidermal layers, the air space, mesophyll and the transverse-section area of cells and air space were greater.

CONCLUSIONS

Leaves of O. alismoides have epidermal and mesophyll cells that contain chloroplasts and large air spaces but lack Kranz anatomy. The high starch content of mesophyll cells suggests they may benefit from an internal source of CO2, for example via C4 metabolism, and are also sites of starch storage. The air spaces may help in the recycling of decarboxylated or respired CO2. The structural similarity of leaves at low and high CO2 is consistent with the constitutive nature of bicarbonate and C4 photosynthesis. There is sufficient structural diversity within the leaf of O. alismoides to support dual-cell C4 photosynthesis even though Kranz anatomy is absent.