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Over the 20th century, surface water temperatures have increased in many lake ecosystems around the world, but long-term trends in the vertical thermal structure of lakes remain unclear, despite the strong control that thermal stratification exerts on the biological response of lakes to climate change. Here we used both neo- and paleoecological approaches to develop a fossil-based inference model for lake mixing depths and thereby refine understanding of lake thermal structure change. We focused on three common planktonic diatom taxa, the distributions of which previous research suggests might be affected by mixing depth. Comparative lake surveys and growth rate experiments revealed that these species respond to lake thermal structure when nitrogen is sufficient, with species optima ranging from shallower to deeper mixing depths. The diatom-based mixing depth model was applied to sedimentary diatom profiles extending back to 1750 AD in two lakes with moderate nitrate concentrations but differing climate settings. Thermal reconstructions were consistent with expected changes, with shallower mixing depths inferred for an alpine lake where treeline has advanced, and deeper mixing depths inferred for a boreal lake where wind strength has increased. The inference model developed here provides a new tool to expand and refine understanding of climate-induced changes in lake ecosystems.

Oxygenic photosynthesis evolved at least 2.4 Ga; all oxygenic organisms use the ribulose bisphosphate carboxylase-oxygenase (Rubisco)—photosynthetic carbon reduction cycle (PCRC) rather than one of the five other known pathways of autotrophic CO 2 assimilation. The high CO 2 and (initially) O 2 -free conditions permitted the use of a Rubisco with a high maximum specific reaction rate. As CO 2 decreased and O 2 increased, Rubisco oxygenase activity increased and 2-phosphoglycolate was produced, with the evolution of pathways recycling this inhibitory product to sugar phosphates. Changed atmospheric composition also selected for Rubiscos with higher CO 2 affinity and CO 2 /O 2 selectivity correlated with decreased CO 2 -saturated catalytic capacity and/or for CO 2 -concentrating mechanisms (CCMs). These changes increase the energy, nitrogen, phosphorus, iron, zinc and manganese cost of producing and operating Rubisco—PCRC, while biosphere oxygenation decreased the availability of nitrogen, phosphorus and iron. The majority of algae today have CCMs; the timing of their origins is unclear. If CCMs evolved in a low-CO 2 episode followed by one or more lengthy high-CO 2 episodes, CCM retention could involve a combination of environmental factors known to favour CCM retention in extant organisms that also occur in a warmer high-CO 2 ocean. More investigations, including studies of genetic adaptation, are needed.

Upper layer thickness (mixing depth) is an essential parameter for estimating the dissolved inorganic carbon and carbon flux at the water surface based on their association with the vertical flux of dissolved inorganic carbon. Previous studies quantified the mixing depth without SAV meadow or penetration depth in the SAV meadow without stratification and wind stress. However, mixing depth related to interaction with submerged aquatic vegetations (SAVs), stratification, and wind stress has yet to be quantified in the previous studies. Our study is the first to quantify the theoretical mixing depth with SAVs according to wind stress, SAV height, and drag coefficient. Theoretical mixing depth was quantified from modeled vertical temperature profile, vertical profile of horizontal velocity, and gradient Richardson number (Rig,veg). We found that mixing depth at a Rig,veg of 100 demonstrated good agreement with numerical results on average, with the mixing depth estimated in this study (hU,this study) showing high applicability to observations at Komuke Lagoon. Moreover, hU,this study increased with the increasing wind stress and decreasing drag coefficient and SAV height. Further, we found that SAV meadows with stratification and wind stress could be divided into four hydrodynamic regimes: non‐vegetated layers, upper vegetated layers, thermoclines, and benthic boundary layers. Our findings help us estimate mixing depth or vertical flux without complicated numerical simulation and understand flow interaction with SAV, wind stress, and stratification. Plain Language Summary Upper layer thickness (mixing depth) and flow fields are important to estimate the carbon flux (e.g., “blue carbon”) and the transportation of dissolved materials (e.g., dissolved oxygen, dissolved inorganic carbon, dissolved inorganic nitrogen, etc) in submerged aquatic vegetation (SAV) meadows. However, it may not be easy to estimate mixing depth without complex numerical simulations. Additionally, we have not understood the interaction of SAV meadows with stratification, currents, or waves. Our study is the first to quantify the mixing depth analytically and to show the hydrodynamic regimes in SAV meadows with stratification. Our finding helps us to estimate carbon flux and the transportation rate of dissolved materials easily without complicated numerical simulation. Key Points Mixing depth with submerged aquatic vegetation (SAV) meadows was estimated using an average gradient Richardson number 100 SAV meadows with stratification and wind stress were divided into four hydrodynamic regimes Wind stress, SAV, and stratification effects were used to accurately estimate the mixing depth

Carbon dioxide concentrating mechanisms (also known as inorganic carbon concentrating mechanisms; both abbreviated as CCMs) presumably evolved under conditions of low CO 2 availability. However, the timing of their origin is unclear since there are no sound estimates from molecular clocks, and even if there were, there are no proxies for the functioning of CCMs. Accordingly, we cannot use previous episodes of high CO 2 (e.g. the Palaeocene–Eocene Thermal Maximum) to indicate how organisms with CCMs responded. Present and predicted environmental change in terms of increased CO 2 and temperature are leading to increased CO 2 and HCO 3 − and decreased CO 3 2− and pH in surface seawater, as well as decreasing the depth of the upper mixed layer and increasing the degree of isolation of this layer with respect to nutrient flux from deeper waters. The outcome of these forcing factors is to increase the availability of inorganic carbon, photosynthetic active radiation (PAR) and ultraviolet B radiation (UVB) to aquatic photolithotrophs and to decrease the supply of the nutrients (combined) nitrogen and phosphorus and of any non-aeolian iron. The influence of these variations on CCM expression has been examined to varying degrees as acclimation by extant organisms. Increased PAR increases CCM expression in terms of CO 2 affinity, whilst increased UVB has a range of effects in the organisms examined; little relevant information is available on increased temperature. Decreased combined nitrogen supply generally increases CO 2 affinity, decreased iron availability increases CO 2 affinity, and decreased phosphorus supply has varying effects on the organisms examined. There are few data sets showing interactions amongst the observed changes, and even less information on genetic (adaptation) changes in response to the forcing factors. In freshwaters, changes in phytoplankton species composition may alter with environmental change with consequences for frequency of species with or without CCMs. The information available permits less predictive power as to the effect of the forcing factors on CCM expression than for their overall effects on growth. CCMs are currently not part of models as to how global environmental change has altered, and is likely to further alter, algal and aquatic plant primary productivity.

Variability of bioturbation on different spatial scales was revealed through a survey at 6 stations in the southwestern Baltic Sea with different sediment types, salinities and macrozoobenthic communities. At each station, 6 sampling locations were investigated with 4 cores each (24 cores per station). The cores were analyzed for vertical chlorophyll (chl) profiles, which were modeled with both a local (tracer distribution decreasing exponentially with depth indicative of diffusive transport, D B) and a non-local (presence of subsurface maximum of the tracer, injection flux J and ingestion rate r) mixing model developed by Soetaert et al. (1996; J Mar Res 54: 1207–1227). Degradation of chl was determined experimentally by an incubation of fresh sediment under anoxic, dark conditions and provided decay constants k D of 0.01 d−1 for mud and 0.02 d−1 for sand. Mixing depths reach 7.1 ± 1.6 cm at stations in the west (except Lübeck Bay, LB), 2 cm deeper than at stations in the east, which reach 5.2 ± 1.7 cm (including LB), mainly depending on the macrozoobenthic community present. Bioturbation intensities indicate high variability between closely located sampling sites as well as across the southern Baltic Sea, and depend on the food supply from the water column. Stations indicate a difference in local mixing (D B) of a factor of 20 and in non-local processes (J) of 6. Non-local transports account for 33 to 50% of the investigated area in the west and for 70 to 100% in the east. The statistical description of the results indicates the necessity of high sampling effort when using chl as a particle tracer.

Phytoplankton productivity standardized to chlorophyll a and photon flux (mg C mg chl. a−1 mol photons−1) of natural communities from northern Bothnian Sea under dynamic (vertically rotating) incubations and different optical conditions was studied during four mesocosm experiments between April 2013 and April 2016. The standardized productivity showed a positive exponential relationship with calculated optical depth (P < 0.001 in all four cases) although a considerably weaker one for one of the series where the community was pre-adapted to the same optical condition as used in the measurements. This series also showed a lower regression slope than the three non-adapted series, which in turn showed identical regression slopes, thus indicating a similar response on the standardized productivity to short-term changes in average ambient photon flux and mixing depth. These results indicate that phytoplankton communities in environments with episodic inflow and mixing of humus-rich water can partly compensate for the reduced photon flux by increased production efficiency.

Since phytoplankton production is usually estimated from static incubations (fixed depths or light levels), a mesocosm study was performed to evaluate the significance of mixing depth, mixing intensity and load of humus of natural phytoplankton assemblages. Vertically rotated (dynamic) incubations usually gave higher results than static incubations in humus-rich water. Mixing intensity was of significant importance in one of 2 years tested, but strong interaction effects with humus complicated the explanation. Differences in primary production between dynamic incubations did not fully reflect the received PAR dose, and increased humus and increased mixing depth increased the photo-assimilation efficiency. Different single-depth incubations did not provide a shortcut method to measure water-column primary production with high accuracy. Results diverged from theoretical estimates based on recent combined photo-biological and physical environmental models. The large variability in responses to mixing is supposed to reflect species-specific adaptations and pre-history regarding quantity (photons) and quality (spectral distribution) of the optical environment in an assemblage of different species. The proportional abundance of each species with its specific characters will therefore strongly influence bulk primary production. Due to such variable responses, clear guidelines for a “best practice” in primary production measurements cannot be given, based on the present results.

As a result of reductions in sulfate deposition and changing climate, dissolved organic carbon (DOC) concentrations have increased in many lakes situated in forests of northeastern North America and northern Europe since the 1990s. Although this increase is well documented, the associated ecological implications remain unclear. In particular, DOC strongly influences the vertical temperature structure of lakes, with increasing DOC often leading to a shallower epilimnion. We investigated the effect of increased DOC concentrations on lake thermal structure using fossil diatom records from six remote Maine lakes. Sedimentary diatom profiles from three pairs of small (<0.5 km 2 ) lakes were compared, with each pair containing one lake with a documented significant increase in DOC and the other experiencing no change in DOC since the early 1990s. Lake thermal structure was inferred from changes in the relative abundance of Discostella stelligera and Aulacoseira species, two diatom taxa that are associated with changes in thermal stratification. The three lakes without increasing DOC showed no change in diatom-inferred mixing depth over the past few decades. Of the lakes with documented increases in DOC, two showed the highest turnover in diatom community structure over time. Profiles from both of these lakes also indicated directional change in diatom-inferred mixing depth over the past 20 years, but the direction of change differed. This study demonstrates that recent increases in DOC have the potential to alter the physical and biological structure of lakes, but that these responses may differ across lakes.

Water scarcity poses a formidable challenge to agricultural productivity in arid regions, and water retention agents offer promising potential in this regard. Therefore, this study proposes developing and preparing polymers with water retention properties using waste green algae as raw material to explore the effectiveness of enhanced water infiltration and reduce evaporation at different use levels (0%, 0.15%, 0.30%, 0.45% and 0.60%) and maximum mixing depths (10 cm, 20 cm, 30 cm, 40 cm and 50 cm) and determine the optimum management. The results demonstrate that the synthesized polymers exhibited a remarkable swelling rate of 143.6 g/g, along with reusability and excellent temperature stability. The polymer application rate was positively correlated with infiltration duration, with an increase from 161 min to 750 min as the application rate rose from 0% to 0.60%. Concurrently, cumulative infiltration increased from 22.6 cm to 31.1 cm, showcasing the benefits of the polymer in enhancing water retention. Intriguingly, cumulative evapotranspiration initially decreased and then increased with increasing polymer application rates. Moreover, increasing the maximum mixing depth from 10 to 50 cm while maintaining the 0.3% application rate increased the cumulative infiltration (from 22.6 cm to 31.1 cm) and infiltration rate (from 0.03 cm/min to 0.08 cm/min) while decreasing the cumulative evaporation (from 44.4 mm to 31.7 mm). Considering the cumulative infiltration, infiltration rate and evapotranspiration characteristics, an optimized polymer application rate of 0.27% at a mixing depth of 0–50 cm was recommended for efficient soil moisture management. This study highlights the potential of green algae-derived biodegradable polymers as a win–win strategy for achieving waste alleviation of water scarcity in drylands, particularly for maize and wheat cultivation in northern China.

Faunal mediated particle and porewater mixing (bioturbation) alters the structure of the surface sediment layer, forming a distinct mixed layer, where the majority of organic matter degradation takes place. Current methods of assessing benthic habitat quality often reference this mixed layer as an indicator of benthic activity. Whilst a great deal of effort has been devoted to linking macro-invertebrate activity to the mixing depth, less attention has been given to defining what the mixing depth represents in terms of ecosystem process and function. Here,in situsediment profile images are analysed using grey scale intensity analysis to distinguish the mixed zone and relate it to the physicochemical environment in order to determine the biological, chemical and physical variables most influential in its formation. Significant differences were found between biogeochemical conditions within the mixed layer relative to the underlying historic sediment layer. These were attributed to a combination of environmental variables (Fe, Mn, Si, chlorophyllaand NO₃⁻) rather than a single dominant driver of change. Although these findings are consistent across multiple locations, the driver(s) that influence the depth of the mixed layer are site- and season-specific. The mixing depth thus provides a reasonable approximation of benthic ecosystem functioning, but when considering ecosystem process the link between the mixing depth and its driving factors (faunal mixing, food input, environmental conditions) is highly context-dependent. Conclusions on benthic community dynamics and ecosystem process, including assessments of habitat quality, cannot therefore be drawn from estimates of the mixing depth alone.