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Abstract: Evolutionary hypotheses for diel vertical migrations (DVM) of aquatic animals include foraging opportunity, predator avoidance, and bioenergetics efficiency. Here we test which hypothesis predicts DVM in the small planktivorous [...]

The use of Shiny in research publications is investigated over the six and a half years since the appearance of this popular web application framework for R, which has been utilised in many varied research areas. While it is demonstrated that the complexity of Shiny applications is limited by the background architecture, and real security concerns exist for novice app developers, the collaborative benefits are worth attention from the wider research community. Shiny simplifies the use of complex methodologies for people of different specialities, at the level of proficiency appropriate for the end user. This enables a diverse community of users to interact efficiently, and utilise cutting edge methodologies. The literature reviewed demonstrates that complex methodologies can be put into practice without insisting on investment in professional training, for a comprehensive understanding from all participants. It appears that Shiny opens up concurrent benefits in communication between those who analyse data and other disciplines, that would enrich much of the peer-reviewed research. [ABSTRACT FROM AUTHOR]

Feldberger Haussee (NE Germany) was polluted for almost a century. During the late 1970s, the nutrient input reached a maximum of approximately 1.9/11.5 g TP/TN m −2  yr −1 . As a result, the lake became a hypertrophic ecosystem and had largely lost its recreational value. In 1980, the sewage discharge was stopped, decreasing the external loading by approximately 90%. Because of vast amounts of phosphorus stored in the sediment, the lake remained highly eutrophic until 1985 with a TP concentration of ca. 1 mg L −1 . To accelerate recovery, biomanipulation was applied from 1985 to 2002 but was successful to only a minor extent. Eventually, due to sediment sequestration and discharge to downstream lakes the TP spring maximum (2006–2010) dropped to 0.112–0.078 mg L −1 . However, given the trend, it was obvious that it would take another 10–15 years for the concentration to approach the desired mesotrophic level. Thus, it was suggested to inactivate the surplus phosphorus by treating the lake with poly-aluminium chloride (PAC) as precipitant. To ensure good water quality, the objective was to decrease concentrations below 0.035 mg TP L −1 , while optimising the amount of PAC applied (as much as needed, but as little as possible). As a prerequisite, the status of the lake was carefully studied; external phosphorus loading and the amount of mobile phosphorus stored in the sediment being of specific interest. Laboratory experiments, modelling studies and field observations eventually resulted in an estimated dosage of 27 g Al m −2 (molar Al/P 12). Following the treatment in April 2011, prime water quality parameters showed two opposing trends: (1) TP concentration immediately dropped below the restoration target (≤0.025/0.035 mg L −1 ), primary production and phytoplankton biomass declined substantially. (2) However, water clarity did not improve for another four years. The likely reason for the delay was the structure of the phytoplankton community. It was dominated by cyanobacteria with the potential to trigger intensive calcite precipitation and thus impair transparency. In spring 2015, the cyanobacteria suddenly disappeared and transparency increased significantly. We speculate that these changes of the planktonic community shifted the ratio of assimilation and respiration in favour of the latter. This allowed an increase of free CO 2 * (dissolved CO 2 & dissociated carbonic acid) altering the carbonate buffering system and thus halting the formation of calcite crystals. Also, the phytoplankton, now represented by small readily ingestible taxa, promoted a flourishing Daphnia population inflicting heavy grazing losses, resulting in clear water stages (2015 June, 4.20 m; 2016, August 3.70 m; 2017 August, 4.5 m). We conclude that the drastic decrease of phosphorus availability in concert with structural and functional changes of the plankton community eventually improved the water quality of Haussee significantly. The lake is now in a mesotrophic status; well in accordance with the aim of the restoration project. [ABSTRACT FROM AUTHOR]

Calcite precipitation is a common phenomenon in calcium-rich hardwater lakes during spring and summer, but the number and spatial distribution of lakes with calcite precipitation is unknown. This paper presents a remote sensing based method to observe calcite precipitation over large areas, which are an important prerequisite for a systematic monitoring and evaluation of restoration measurements. We use globally archived satellite remote sensing data for a retrospective systematic assessment of past multi-temporal calcite precipitation events. The database of this study consists of 205 data sets that comprise freely available Landsat and Sentinel 2 data acquired between 1998 and 2015 covering the Northeast German Plain. Calcite precipitation is automatically identified using the green spectra and the metric BGR area, the triangular area between the blue, green and red reflectance value. The validation is based on field measurements of CaCO3 concentrations at three selected lakes, Feldberger Haussee, Breiter Luzin and Schmaler Luzin. The classification accuracy (0.88) is highest for calcite concentrations ≥0.7 mg/L. False negative results are caused by the choice of a conservative classification threshold. False positive results can be explained by already increased calcite concentrations. We successfully transferred the developed method to 21 other hardwater lakes in Northeast Germany. The average duration of lakes with regular calcite precipitation is 37 days. The frequency of calcite precipitation reaches from single time detections up to detections nearly every year. False negative classification results and gaps in Landsat time series reduce the accuracy of frequency and duration monitoring, but in future the image density will increase by acquisitions of Sentinel-2a (and 2b). Our study tested successfully the transfer of the classification approach to Sentinel-2 images. Our study shows that 15 of the 24 lakes have at least one phase of calcite precipitation and all events occur between May and September. At the lakes Schmaler Luzin and Feldberger Haussee, we illustrated the influence of ecological restoration measures aiming at nutrient reduction in the lake water on calcite precipitation. Our study emphasizes the high variance of calcite precipitation in hardwater lakes: each lake has to be monitored individually, which is feasible using Landsat and Sentinel-2 time series. [ABSTRACT FROM AUTHOR]

Polyaluminium chloride ( PAC) addition has been proposed as a technique to precipitate dissolved phosphorus (P) and reduce the internal loading of P in eutrophic hardwater lakes. However, the direct and indirect detrimental effects of PAC on aquatic communities and ecosystem processes are virtually unknown., We determined how PAC may affect a leaf litter-fungus-shredder decomposition system in the littoral habitat of eutrophic hardwater lakes.We hypothesised that PAC will exert little direct toxicity on aquatic organisms at high pH but trigger indirect bottom-up effects by affecting fungal biomass, leaf litter food quality (Al, N and P contents), trophic transfer and the behaviour and performance of a leaf-shredding consumer., We tested these hypotheses in a microcosm experiment involving a leaf-shredding amphipod, Gammarus pulex, offered conditioned aspen leaves as food. We measured concentrations of Al, N, P and organic C in water, leaves and gammarids; fungal biomass associated with leaf litter; gammarid survival, body mass, feeding rate ( FR) and N assimilation efficiency; and fine-particulate organic matter ( FPOM) generation in microcosms receiving 0, 2 and 20 g Al flakes per m2., Survival rates of G. pulex were similar in all microcosms, suggesting that even the high dose of Al flakes was below lethal levels. However, exposure to Al flakes decreased fungal biomass in leaves, the FR of gammarids, FPOM production and N assimilation efficiency. The C/N and C/P ratios of both leaves and gammarids also decreased in microcosms receiving Al flakes, supporting the hypothesis that stoichiometric imbalances between food and consumers were altered by exposure to Al flakes., These results point to the importance of indirect effects of PAC on lake communities and ecosystems beyond direct toxicity on invertebrates. Although not all those effects are necessarily undesirable, their numbers, interdependencies and potential to act in concert suggest they need to be considered in ecotoxicological assessments. [ABSTRACT FROM AUTHOR]

Background: Mortality is a main driver in zooplankton population biology but it is poorly constrained in models that describe zooplankton population dynamics, food web interactions and nutrient dynamics. Mortality due to non-predation factors is often ignored even though anecdotal evidence of non-predation mass mortality of zooplankton has been reported repeatedly. One way to estimate non-predation mortality rate is to measure the removal rate of carcasses, for which sinking is the primary removal mechanism especially in quiescent shallow water bodies. Objectives and Results: We used sediment traps to quantify in situ carcass sinking velocity and non-predation mortality rate on eight consecutive days in 2013 for the cladoceran Bosmina longirostris in the oligo-mesotrophic Lake Stechlin; the outcomes were compared against estimates derived from in vitro carcass sinking velocity measurements and an empirical model correcting in vitro sinking velocity for turbulence resuspension and microbial decomposition of carcasses. Our results show that the latter two approaches produced unrealistically high mortality rates of 0.58-1.04 d-1, whereas the sediment trap approach, when used properly, yielded a mortality rate estimate of 0.015 d-1, which is more consistent with concurrent population abundance data and comparable to physiological death rate from the literature. Ecological implications: Zooplankton carcasses may be exposed to water column microbes for days before entering the benthos; therefore, non-predation mortality affects not only zooplankton population dynamics but also microbial and benthic food webs. This would be particularly important for carbon and nitrogen cycles in systems where recurring mid-summer decline of zooplankton population due to non-predation mortality is observed. [ABSTRACT FROM AUTHOR]

Abstract: Knowledge about the contribution of food web structure and nutrient concentration in lakes to phosphorus (P) sedimentation and remobilisation at the sediment surface is still poor. Using four large enclosures placed in a eutrophic, thermally stratifying lake, we studied the effects of the structure of the planktonic food web (with and without planktivorous fish, ±F treatments) and nutrient concentration (with and without fertilisation, ±N treatments) on P sedimentation. We investigated the total P content and P binding forms in settling material (TPSM) and of the uppermost 1cm sediment layer (TPSed) during three consecutive stratification periods (2005–2007). Additionally, epilimnetic P (SRPEpi, TPEpi), chlorophyll a and biomass of total crustacean and Daphnia were measured. On a seasonal scale, Daphnia biomass tended to negatively influence chlorophyll a, sedimentation rate of total particulate matter and of P, but the latter two criteria did not differ significantly between treatments due to large fluctuations within each enclosure. The contents of TPSM and loosely adsorbed P in settling material decreased in the following order: −F/+N>+F/+N>−F/−N>+F/−N, indicating greater effects of nutrient addition than of food web structure. In sediments, organically bound P was 9–23% higher in −F variants compared to the corresponding +F treatments, thus indicating an effect of food web structure. Furthermore, positive correlations between SRPEpi, TPEpi, TPSM, TPSed, sediment reductant-soluble P and calcite bound P revealed an effect of the epilimnetic P concentration on P sedimentation and specific P binding forms. Compared to the composition of different P binding forms in the settling material, a considerable decrease of loosely adsorbed P (12–26%) and reductant-soluble P (14–21%), as well as an increase of organic P (14–26%) were observed in the uppermost 1cm-layer of the sediments in all treatments. We conclude that both nutrient enrichment (+N) and food web structure (−F) enhance the P sedimentation and P content at the sediment surface. However, in addition to food web effects on organic P content in settling matter and sediments, factors like iron concentration and calcite precipitation might be of importance for P sedimentation and storage in sediments in complex systems such as lakes. [Copyright &y& Elsevier]

Abstract: Chlorophyll a (chla) concentration was evaluated as a predictor of phytoplankton biomass across a broad trophic gradient of lakes (oligotrophic – highly eutrophic). First, a literature survey was conducted to collect information on the proportion of chla in phytoplankton biomass. As a result of this study (n=21) a mean value of 0.505%±0.197 S.D. chla per unit wet weight of phytoplankton was calculated. Second, analyses were performed on 756 paired measurements from an unpublished database where the specific chla content of phytoplankton biomass was regressed against phytoplankton standing stocks and chla concentration. Within an interval of 0.1–50gm−3 of phytoplankton wet weight, a substantial decrease in chla proportion from approximately 2.5% to 0.18% was found. Likewise, the proportion in phytoplankton wet weight decreased from 0.7% to 0.15% across a chla concentration interval of 0.001–0.150gm−3. These results had a significant impact both on chla-based biomass calculations and the subsequent comparison with phytoplankton biomasses resulting from microscopic counts. Assuming the microscopic method was a measure of the “true” phytoplankton standing stocks, then the precision by which phytoplankton biomass might be predicted based on chla measurements is clearly better when using variable proportions as compared to a constant conversion factor. The same holds for temporal coherence between annual records of phytoplankton biomass. The temporal fit was apparently better when relating the results of microscopic counts and biomass estimation based on variable proportions of chla in phytoplankton biomass. Nevertheless, this effect diminished as the tropic status of the lakes increased. Because of their variable specific chla content, separate taxonomic groups of phytoplankton differently affected the proportion of chla in total phytoplankton wet weight. Chlorophyceae, Cryptophyceae and cyanobacteria had a high impact, while Bacillariophyceae, Dinophyceae and Chrysophyceae were of lesser importance. [Copyright &y& Elsevier]

Abstract: The strength of trophic cascades in the pelagic area of lakes at low productivity has been discussed intensively, but predictions of trophic coupling differ strongly. Many studies suggest that trophic cascades are weak in oligotrophic lakes, but some models discussed that trophic interactions might be strong at low nutrient concentration. Here, we used time series over 9 (phytoplankton and zooplankton) or 6 (fish) years from the oligo-mesotrophic Lake Stechlin (Germany) to explore correlative relationships between biomasses of these trophic levels. The fourth trophic level of piscivorous fish was almost absent in the pelagic area. The biomass of planktivorous coregonid fishes was not at all correlated to total zooplankton biomass, which was dominated by calanoid copepods. However, there was a strong negative correlation of adult coregonid biomass to the proportion of Daphnia in zooplankton, and a strong positive correlation of adult fish to proportion of calanoid copepods. Zooplankton and phytoplankton biomasses were not correlated except for a significantly negative correlation between Daphnia biomass and biomass of Cyanobacteria. Overall, our results suggest that planktivorous fish may modify the zooplankton structure, but not the zooplankton biomass, in lakes of low productivity. However, this top-down effect by fish does not cascade further down to the phytoplankton biomass or community structure, confirming earlier hypotheses that trophic cascades are weak in oligotrophic lakes. [Copyright &y& Elsevier]

Abstract: The Feldberger Haussee, a highly eutrophic stratified hard-water lake located in the eastern part of Germany's Baltic lake district, was selected for a restoration programme combining external nutrient loading reduction and long-term biomanipulation. In 1980 the external phosphorus loading (1.8 g TP m−2 a−1) decreased by 90%, but water quality did not improve significantly within the following 6 years. In 1985 biomanipulation was initiated, with manual removal of cyprinid fish coupled with piscivore introductions as the principal measures. The expected changes in the pelagic community and improvements of water quality occurred after a delay of several years. Despite intensive manual removal of cyprinids and stocking of piscivores, standing stocks of cyprinid fish remained relatively high (130–260 kg ww ha−1) after some years of decline. Compared to the pre-biomanipulation period, mean seasonal (May-September) Daphnia spp. biomass roughly doubled (0.037 g C m−3 vs. 0.084 g C m−3). However, the predominance of small (<1 mm) D. cucullata throughout the whole investigation period indicated that planktivory was still substantial. Paired observations between edible phytoplankton biomass and Daphnia spp. indicated that a significant decline in algal stocks would only occur if herbivorous biomass was above a certain threshold (0.2 g C m−3). Reduced external and internal loading in concert with pelagic calcite precipitation were most likely responsible for the decline in lake phosphorus concentrations, thereby substantially improving the water quality of Feldberger Haussee. Although this is not supported by quantitative evidence, we hypothesize that resource-related water quality improvements were caused by changes in the structure of the pelagic community leading to increased calcite precipitation. In agreement with the results of other investigations, we conclude that because stabilising mechanisms such as macrophyte growth were lacking in Feldberger Haussee, biomanipulation in stratified lakes may not be as successful as has been observed in shallow lakes. However, in hard-water lakes, calcite precipitation may act as another stabilising resource-related mechanism. Phosphorus associated with sedimenting calcite particles is insensitive to redox-conditions and may therefore not be re-mobilised from lake sediments even if hypolimnetic oxygen is depleted. [Copyright &y& Elsevier]

SUMMARY 1. To illustrate advances made in biomanipulation research during the last decade, seven main topics that emerged after the first biomanipulation conference in 1989 are discussed in relation to the papers included in this special issue and the general literature. 2. The substantially higher success rates of biomanipulations in shallow as opposed to stratified lakes can be attributed to several positive feedback mechanisms relating mainly to the recovery of submerged macrophytes. 3. The role of both nutrient loading and in-lake concentrations in predicting the success of biomanipulations is emphasised and supported by empirically defined threshold values. Nutrient recycling by aquatic organisms (such as fish) can contribute to the bottom-up effects on lake food webs, although the degree can vary greatly among lakes. 4. Ontogenetic niche shifts and size-structured interactions particularly of fish populations add to the complexity of lake food webs and make scientifically sound predictions of biomanipulation success more difficult than was previously envisaged. 5. Consideration of appropriate temporal and spatial scales in biomanipulation research is crucial to understanding food web effects induced by changes in fish communities. This topic needs to be further developed. 6. An appropriate balance between piscivorous, planktivorous and benthivorous fishes is required for long-lasting success of biomanipulations. Recommended proportions and absolute densities of piscivorous fish are currently based on data from only a few biomanipulation experiments and need to be corroborated by additional and quantitative assessments of energy flow through lake food webs. 7. Biomanipulation effects in stratified lakes can be sustained in the long term only by continued interventions. Alternate stable states of food web composition probably exist only in shallow lakes, but even here repeated interventions may be needed as long as nutrient inputs remain high. 8.... [ABSTRACT FROM AUTHOR]

We conducted grazing experiments to test whether larger-bodied Daphnia pulicaria have a different effect from smaller-bodied Daphnia galeata mendotae on the composition of summer algal assemblages in eutrophic lakes. Three separate cubitainer experiments were run for 5 days in a replicated factorial design utilizing two algal community types and the two Daphnia species. Inorganic phosphorus and nitrogen were added to prevent nutrient limitation of the algae. Both edible and inedible size fractions of chlorophyll a increased in cubitainers without Daphnia spp. Grazer addition usually resulted in a reduction in edible chlorophyll; reductions were greater in D.pulicaria cubitainers. Grazing by Daphnia spp. on presumed inedible chlorophyll was variable. Algal size was not always a good predictor of grazeability. The results of this study indicate that D.pulicaria, because of its greater filtration potential and ability to ingest larger particles, provides a stronger control on inedible-sized algae when compared to equal numerical densities of D.g.mendotae. However, Aphanizomenon increased as a response to heavy grazing pressure by D.pulicaria on other algal species. This suggests that biomanipulation efforts that promote large-bodied Daphnia may not produce desirable results if nutrient inputs remain high. [ABSTRACT FROM PUBLISHER]

This study looks at two facets of dominant phytoplankton classes during phytoplankton succession. A detailed assessment of this issue is of special interest with regard to realized niches from a theoretical point of view but also for lake management as practical application. A realized niche mirrors the functional adaptability of an organism in a lake-specific constellation of environmental parameters. Therefore, the characterization of realized niches could be a key factor for management of problematic waters. Different strategies exist to control eutrophication and the risk of blooms by harmful algae. During the last decades, many restoration measures were initiated to manage eutrophicated inland lakes. In the past, it has become evident several times that restoration strategies do not necessarily lead to a reduction of biomass of undesirable cyanobacteria but can even promote their development. Due to this uncertainty of success and the high costs for remediation strategies, new prediction tools are required – ideally, based on routine monitoring data. Therefore, we developed a new method to extract potential optimal growth conditions (POGC) as indicators of realized niches for different phytoplankton taxa from existing data to improve existing strategies used in lake remediation and restoration. The analysis presented in this work is based on dominance pattern of different phytoplankton groups relative to environmental variables. Interpretation of these dominance patterns as indicators of POGC showed distinct pattern for several phytoplankton classes for all investigated objects. We identified low nitrogen and phosphate concentrations as favorable condition for cyanobacteria in Lake Auensee and Lake Feldberger Haussee. The reservoir Bleilochtalsperre showed a high N/P-concentration and cyanobacteria dominance was generally very low. [ABSTRACT FROM AUTHOR]

Global environmental change has influenced lake surface temperatures, a key driver of ecosystem structure and function. Recent studies have suggested significant warming of water temperatures in individual lakes across many different regions around the world. However, the spatial and temporal coherence associated with the magnitude of these trends remains unclear. Thus, a global data set of water temperature is required to understand and synthesize global, long-term trends in surface water temperatures of inland bodies of water. We assembled a database of summer lake surface temperatures for 291 lakes collected in situ and/or by satellites for the period 1985-2009. In addition, corresponding climatic drivers (air temperatures, solar radiation, and cloud cover) and geomorphometric characteristics (latitude, longitude, elevation, lake surface area, maximum depth, mean depth, and volume) that influence lake surface temperatures were compiled for each lake. This unique dataset offers an invaluable baseline perspective on global-scale lake thermal conditions as environmental change continues.