Shallow Subtropical Lake Research Articles

Temporal and Spatial Variation of Zooplankton Community Structure and Its Relationship with Environmental Factors in Dianshan Lake, Shanghai

Dianshan Lake, a subtropical shallow lake, is the largest freshwater body located in Shanghai. To reveal the temporal and spatial variation of zooplankton community structure and its relationships with environmental factors, monthly data of zooplankton and phytoplankton content and associated physicochemical parameters for 2017 were analyzed using multivariate regression trees (MRT) and principal coordinates analysis (PCoA). The results indicated that there were significant seasonal differences in the community structure of zooplankton (P<0.05). However, spatial variations were significant only in spring and summer (P<0.05). The results indicated that water temperature (WT), chlorophyll-a (Chla), ammonia nitrogen, and cyanobacteria were the key driving factors in the observed spatial and temporal variations in the zooplankton community structure. The MRT analysis illustrated that zooplankton community structure varied strongly across four groups, including spring (13.07℃ ≤ WT<19.57℃), summer (WT ≥ 19.57℃, Chla ≥ 9.03 μg·L-1), autumn (WT ≥ 19.57℃, Chla<9.03 μg·L-1), and winter (WT<13.07℃). In addition, three distinct regions were identified by the cluster analysis. The MRT analysis illustrated that the zooplankton community structure was distinct between samples with relatively lower (<1.11 mg·L-1) and higher concentrations (≥ 1.11 mg·L-1) of ammonia nitrogen in spring. Furthermore, cyanobacteria were identified as a major stressor on zooplankton in summer. These observations further show that that zooplankton community structure in area I (with a cyanobacterial biomass of ≥ 2.58 mg·L-1) was significantly different from other regions (with a cyanobacterial biomass of <2.58 mg·L-1).

Role of plant architecture on littoral macroinvertebrates in temperate and subtropical shallow lakes: a comparative manipulative field experiment

espanolLa complejidad del habitat puede modular interacciones entre depredadores y presas al ofrecer refugio fisico y nuevos micro-habitats para competidores potenciales o presas alternativas. En los lagos someros, las plantas sumergidas y flotantes afectan las interacciones depredador-presa, con diferencias relacionadas a diferencias en la comunidad de peces influenciadas por el clima. Mediante la introduccion de plantas flotantes libres y plantas sumergidas artificiales en un total de 14 lagos someros en dos regimenes climaticos contrastantes (templado y subtropical), analizamos la hipotesis de que la estructura de la comunidad de macroinvertebrados litorales esta principalmente determinada por la presion de depredacion de los peces. Por lo tanto, esperamos encontrar mayores densidades en la zona templada, donde la depredacion por parte de los peces litorales es comparativamente mas debil que en la zona subtropical. Tambien analizamos la hipotesis de que las diferencias en compleji-dad estructural (mayor en las flotantes), asi como en volumen ocupado por plantas (mayor en las sumergidas) promueve diferentes patrones de asociacion en los macroinvertebrados. La riqueza taxonomica, la diversidad y la densidad de los macroinvertebrados litorales fueron varias veces mayores en los lagos templados. Las densidades de macroinvertebrados (por unidad de masa de las plantas) fueron en general mayores en las plantas flotantes libres. La estructura de los macroinvertebrados litorales no mostro una relacion clara con el gradiente de estado trofico estudiado. Los peces, cuya biomasa, densidad y uso del habitat difirieron de forma consistente entre ambas zonas, determinaron la estructura de los macroinvertebrados probablemente via depredacion y en la zona subtropical poten-cialmente tambien por competencia. Las plantas flotantes libres parecieron actuar como un refugio mas efectivo o habitat preferido particularmente en la zona subtropical, pero con el aumento de la turbidez del agua el patron se hizo menos evidente. En los lagos templados, el uso del habitat estuvo mas vinculado a la biomasa de perifiton, sugiriendo la ocurrencia de contro-les ascendentes. Una mayor presion de consumo por los peces (esperada con el calentamiento climatico y un aumento de la eutrofizacion), puede reducir la riqueza y abundancia de los macroinvertebrados litorales con impactos potencialmente fuertes sobre el funcionamiento de los lagos someros. Una expansion de las plantas flotantes libres promovida por el calentamiento climatico, podria, si fuera moderada, debilitar estos efectos. Sin embargo, una comunidad diversa de macrofitas seguramente promoveria la coexistencia entre macroinvertebrados y peces EnglishWe dedicate this paper to the late Maria Rosa Miracle, who was a key researcher in the shallow lakes community, always enthu-siastically dedicated to science, and always ready to discuss and guide younger scientists and fellow colleagues on the dynamics of zooplankton as well as on shallow lakes functioning at large. She will be sadly missed.Plant habitat complexity can buffer against predator-prey interactions by offering physical refuges, and also novel microhabi-tats for alternative prey. In shallow lakes, submerged and free-floating plants affect predator-prey interactions with expected differences related to climate-driven differences in fish community structure. We tested the overall hypothesis that fish preda-tion shapes the structure of plant-associated macroinvertebrate communities. By introducing artificial free-floating and submerged plants in a total of 14 shallow lakes under contrasting climate regimes (temperate and subtropical), we predicted that higher densities of macroinvertebrates would occur in the temperate zone where littoral fish predation is comparatively weaker than in the subtropics. We also tested the hypothesis that different structural complexities and % PVI would lead to different association patterns. Taxonomic richness, diversity, and densities of plant-attached macroinvertebrates were several fold higher in the temperate lakes. Macroinvertebrate densities per unit of plant weight were generally higher on the more complex free-floating than on the more abundant (as % PVI) submerged plants. The structure of littoral macroinvertebrates showed no clear relationship with the trophic state gradient. Fish communities, whose structure (biomass, density and habitat use) differed consistently between the temperate and subtropical locations, seemingly shaped the macroinvertebrate communities, most likely via predation and in the subtropics potentially also by competition. Free-floating plants appeared as a better anti-preda-tion refuge or a preferred habitat for macroinvertebrates, particularly in the subtropics, but with increasing water turbidity this pattern became less distinct. In contrast, in the temperate lakes, the use of artificial plants by macroinvertebrates was rather related to the biomass of periphyton than to their refuge capacity, thus suggesting stronger bottom-up effects. A stronger fish predation pressure, which is to be expected with climate warming and eutrophication, may reduce the richness and abundance of plant-associated macroinvertebrates, with potentially strong impacts on shallow lake functioning. A warming-driven expan-sion of free-floating plants could, if moderate, weaken such effects, but a more diverse macrophyte community would facilitate co-existence of macroinvertebrate and fish.

Can zooplankton grazing affect the functional features of phytoplankton in subtropical shallow lakes? - Experiment in situ in the south of Brazil

Herbivory in aquatic environments reflects the organization of trophic webs as a structuring factor in the communities of prima-ry producers. We experimentally evaluated the influence of grazing by zooplankton and by different predator groups on the abundance and biomass of phytoplankton and morphologically-based functional groups (MBFGs), in a subtropical floodplain shallow lake (Brazil). We tested if different grazing levels modify the structure of the phytoplankton, considering that higher zooplankton abundance reduces the abundance and biomass of phytoplankton and the MBFGs. We used five treatments (5 L) with different zooplankton abundance and five replicas to represent an increasing grazing gradient. Sampling of phytoplankton and zooplankton was carried out after 24 hours, in each treatment. The zooplankton abundance presented significant differences among treatments (F = 11.79; p < 0.01), showing the evident grazing gradient in the experiment. Rotifers dominated all the treatments, followed by copepods, although their abundance decreased at the end of the experiment. In contrast, cladoceran abundance increased. An inverse and significant relationship was observed between zooplankton and rotifer abundances and MBFG III abundance (β = -0.85, p < 0.01; r2 = 0.32; β = -0.54; p < 0.01; r2 = 0.29, respectively), showing an effective preda-tor-prey interaction. Many positive and significant relationships were also observed among the grazers and the prey. The increase of the abundance and biovolume of phytoplankton was related to the bottom-up mechanisms, the rapid population growth of prey, and the short time of the experiment. The change in the MBFGs’ contribution was related to the increase in the zooplankton and group abundance, grazer competition and exploitative competition among the MBFGs. The experiment contributes to the knowledge on an important part of the trophic dynamics among planktonic communities in subtropical floodplain shallow lakes

Utility of a macroinvertebrate-based multimetric index in subtropical shallow lakes

Abstract Benthic macroinvertebrates have a long history of use in environmental monitoring. However, macroinvertebrate-based multimetric approach has not been well developed to assessment aquatic ecosystems in China, particularly in lake ecosystems. Eutrophication has been recognized as one of the main pressures affecting shallow lakes in China and elsewhere and it is thus desirable to quantify its effects on ecological integrity of lake ecosystems. Here, 41 subtropical shallow lakes in the Eastern Plain Lake Ecoregion of China were sampled and defined as three groups along eutrophication gradients. A macroinvertebrate-based multimetric index, consisting of three taxonomic metrics (No. of Gastropoda taxa, biological pollution index, and Margalef index) and two functional metrics (functional evenness and relative abundance of deposit feeder) was developed based on the datasets of the 41 lakes, and the results of multiple linear regression showed that the MMI was significantly related to the concentration of TP and macrophyte cover. This finding indicated that the MMI could characterize the influence of eutrophication on lake ecosystems. Our results also indicated that functional metrics could complement the information provided by taxonomic metrics, and we recommend to properly use functional metrics in lake biomonitoring. The macroinvertebrate-based MMI proposed here is the first attempt using biological quality element to assess the ecological status of shallow lakes in the view of eutrophication at the regional scale and could provide complementary information to traditional water quality assessment for the development of management strategies.

Trophic status affects the distribution of polycyclic aromatic hydrocarbons in the water columns, surface sediments, and plankton of twenty Chinese lakes

The influence of trophic status on the distribution of hydrophobic organic contaminants (HOCs) in different subtropical shallow waters at large spatial scales remains largely unknown. In this study, samples of surface sediments, water, total suspended particles, phytoplankton, and zooplankton were simultaneously collected from 83 sampling sites in 20 subtropical oligotrophic to hyper-eutrophic shallow lakes in China to investigate the influence of trophic status on the spatial distribution and sinking fluxes of 16 polycyclic aromatic hydrocarbons (PAHs). The total concentration of the 16 PAHs (ΣPAH16) in the water columns of these lakes varied from 0.22 to 5.81 μg L−1, and increased with the trophic state index (TSI) and phytoplankton biomass. Phytoplankton were the dominant reservoir for the PAHs in the water column. However, the fraction of ΣPAH16 in phytoplankton decreased with the TSI. The average sinking flux of ΣPAH16 of the individual lakes varied from 2257.1 to 261674.1 mg m−2 d−1, and increased with the TSI of the lakes. The concentration of ΣPAH16 in the surface sediments ranged from 385.77 to 3784.37 ng gdw−1, and increased with the TSI and the ratio of phycocyanin/sediment organic carbon. It suggested that cyanobacterial biomass affected by trophic status dominated the occurrence of the PAHs in the surface sediments of these lakes. Biomass dilution and the biological pump affected the accumulation of the PAHs in phytoplankton, and zooplankton, and had more influence on the PAHs with higher hydrophobicity. Both the bioconcentration factors and bioaccumulation factors of the PAHs decreased with the TSI. No biomagnification was observed for the PAHs from phytoplankton to zooplankton in these lakes in spring. Our study provided novel knowledge for the coupling between eutrophication and HOCs in 20 subtropical shallow lakes with different trophic status.

The El Niño Southern Oscillation (ENSO) is the main source of variation for the gamma diversity of plankton communities in subtropical shallow lakes

We determined the variation in the composition of plankton communities (zooplankton, phytoplankton, and ciliates) in subtropical lakes at different temporal scales, in relation to the seasons (dry and rainy seasons), as well as at finer (among months) and broader (ENSO—among El Nino, La Nina, and normal climate events) scales. Using a 16-year time-series dataset, we tested the hypothesis that seasonal variation would explain most of the gamma diversity of these plankton communities. We also investigated the environmental and temporal factors responsible for the variations in composition and species turnover. The scale related to dry and rainy seasons explained a considerable percentage of the gamma diversity and variation partitioning, showed that compositional changes occurred mainly over broader temporal scales. Environmental factors varying among seasons and ENSO events explained changes in composition, although some communities did not respond to the environment. Our results suggest that niche and stochastic processes operating at temporal scales correlated with ENSO climate events contributed to changes in species composition. Hence, climate anomalies might be important to maintain diversity in areas with reduced or loss of the natural variations in environmental conditions. Our results also suggest that, although communities show similar patterns of variation in composition, they might respond in a different degree to environmental and temporal factors. Thus, while niche-associated (environment) and stochastic (time) processes drove the phytoplankton, stochastic processes were more important for zooplankton, whereas neither were important for ciliates.

Daily pCO2 and CO2 flux variations in a subtropical mesotrophic shallow lake

Carbon dioxide (CO2) emissions from lakes have been proven to be an important component of the continental carbon balance, but most CO2 evasion estimates ignore daily variability of partial pressure of CO2 (pCO2). To discern the variability of pCO2 and its effect on CO2 outgassing estimations, we conducted in-situ biweekly pCO2 measurements during daylight from November 2017 to June 2018 at 7:00, 10:00, 14:00, and 17:00 Central Standard Time of the United States (CST) in a subtropical shallow lake in Louisiana, USA. Daytime pCO2 varied largely from 154 to 1698 μatm with an average of 736 μatm, while daytime CO2 flux ranged from −43 to 284 mmol m2 h−1 averaging at 44 mmol m2 h−1. Significant decreases in pCO2 and CO2 fluxes from mornings to late afternoons were observed throughout the study period. Specifically, in morning hours average pCO2 and CO2 fluxes were 940 μatm and 83 mmol m2 h−1, respectively, while in afternoon hours average pCO2 and CO2 fluxes were only 410 μatm and 6 mmol m2 h−1, respectively. Relationships between pCO2 and ambient factors reveal that solar radiation, lake trophic status and water temperature play a major role in pCO2 and CO2 dynamics in lake water. We argue that uncertainties may exist in the current regional and global CO2 evasion estimates due to this high daily pCO2 variability.

Comparison of Methods to Estimate Lake-Surface-Water Temperature Using Landsat 7 ETM+ and MODIS Imagery: Case Study of a Large Shallow Subtropical Lake in Southern Brazil

Water temperature regulates many processes in lakes; therefore, evaluating it is essential to understand its ecological status and functioning, and to comprehend the impact of climate change. Although few studies assessed the accuracy of individual sensors in estimating lake-surface-water temperature (LSWT), comparative analysis considering different sensors is still needed. This study evaluated the performance of two thermal sensors, MODIS and Landsat 7 ETM+, and used Landsat methods to estimate the SWT of a large subtropical lake. MODIS products MOD11 LST and MOD28 SST were used for comparison. For the Landsat images, the radiative transfer equation (RTE), using NASA’s Atmospheric Correction Parameter Calculator (AtmCorr) parameters, was compared with the single-channel algorithm in different approaches. Our results showed that MOD11 obtained the highest accuracy (RMSE of 1.05 ° C), and is the recommended product for LSWT studies. For Landsat-derived SWT, AtmCorr obtained the highest accuracy (RMSE of 1.07 ° C) and is the recommended method for small lakes. Sensitivity analysis showed that Landsat-derived LSWT using the RTE is very sensitive to atmospheric parameters and emissivity. A discussion of the main error sources was conducted. We recommend that similar tests be applied for Landsat imagery on different lakes, further studies on algorithms to correct the cool-skin effect in inland waters, and tests of different emissivity values to verify if it can compensate for this effect, in an effort to improve the accuracy of these estimates.

Exploring the influence of hydrology on the threshold phosphorus-loading rate in shallow lakes

Abstract The Alternative Stable States Model describes the two contrasting conditions of shallow lakes that occur in response to external phosphorus (P) loading: 1) a clear-water, macrophyte-dominated state during times of lesser external P loading, and 2) a turbid, phytoplankton-dominated state during periods of greater P loading. Determination of the P loading rate that distinguishes the two stable states, i.e. the threshold phosphorus loading rate (TPL), is fundamental to shallow-lake science and management. We used data from nine shallow subtropical lakes in Florida (USA) to generate a parsimonious model that predicts the TPL from the hydraulic detention time (HDT). The model reveals that the TPL is more sensitive to changes in HDT in lakes with short ( 10 years) HDTs. We compared our results with published data from 54 geographically distributed shallow aquatic systems and found that the Florida-based HDT-TPL model has general applicability. Given many lakes worldwide are shallow, our findings have the potential to improve understanding and management of numerous aquatic ecosystems around the globe.

Quantifying the effects of different cold air events on latent and sensible heat fluxes of Lake Taihu

作为冷季主要的天气事件，冷空气过境会改变湖泊上方的气团性质，对湖泊的水热通量产生影响，进而影响湖泊的生物物理和化学过程.以亚热带大型浅水湖泊——太湖为研究对象，基于2012-2017年5个冷季（11月-翌年3月）的太湖中尺度通量网观测数据，量化不同强度冷空气（寒潮、强冷空气和较强冷空气）对太湖水热通量的影响.结果表明：在5个冷季中，寒潮、强冷空气和较强冷空气发生的总次数分别为4、11和33次，累积持续天数分别为14、31和78天.冷空气过境显著增强太湖的水热通量，3种冷空气过境使太湖的感热通量分别增至无冷空气时的10.3、6.0和4.3倍，潜热通量分别增至无冷空气时的4.0、2.1和2.7倍.虽然冷空气影响天数仅占冷季天数的16.4%，但对整个冷季的潜热和感热通量贡献分别为34.9%和51.7%，以较强冷空气贡献最大.冷空气影响时，水-气界面的温度梯度是太湖感热通量的主控因子，而潜热通量的主控因子为风速.与深水湖泊相比，太湖等浅水湖泊对冷空气过境的响应更快，寒潮过境时尤为明显.;As the major synoptic system, cold air events influence the water vapor and heat exchanges between lake and atmosphere by changing the meteorological conditions of air masses over the lake. Then biophysical and biogeochemical cycles in the lake ecosystem would be moderated by cold air passage. Based on dataset of the Taihu Eddy Flux Network observed during the five cool seasons (2012-2017), the effects of different cold air events (cold wave, severe cold air events and strong cold air events) on latent and sensible heat fluxes were quantified on the large subtropical shallow Lake Taihu. The results showed that cold wave, severe cold air events and strong cold air events totally happened 4, 11 and 33 times, and lasted for 14, 31 and 78 days, respectively. The sensible and latent heat exchanges between lake and atmosphere were accelerated significantly by the passage of cold air. The sensible heat flux increased by 10.3, 6.0 and 4.3 times during cold wave, severe cold air events and strong cold air events, respectively. The latent heat flux was increased by 4.0, 2.1 and 2.7 times, respectively. Although cold air passage only occupied 16.4% of entire cool season, the cold air events contributed 34.9% and 51.7% of the total latent and sensible heat fluxes, respectively. Moreover, the strong cold air events were the biggest contributor. During cold air events, the temperature gradient between the air and water is the most significant factor governing the sensible heat exchange rate. While, the latent heat flux is mostly dominated by wind speed. Compared to deep lakes, shallow lakes response faster to cold air activities. Therefore, the latent and sensible heat fluxes of shallow lakes increase much more, especially during cold waves.

The structuring role of macrophytes on plankton community composition and bacterial metabolism in a large subtropical shallow lake

Abstract Aim We aimed at investigating changes in major groups of plankton (bacterioplankton, phytoplankton and zooplankton) and bacterioplankton metabolism along a transect covering a littoral, macrophyte dominated zone to a pelagic, phytoplankton dominated zone in the large subtropical shallow lake Mangueira, Rio Grande do Sul State, Brazil. Methods The 3.2 km littoral-pelagic transect (7 points) was sampled in October 2007 in the southern section of the lake. Major taxonomic groups of phytoplankton and zooplankton were counted and identified by optic microscopy. The bacterioplankton was identified by FISH (Fluorescent in situ Hybridization), and bacterial abundance, biovolume and biomass were quantified by epifluorescence microscopy. Primary and bacterial productions were measured by incorporation of radioactive bicarbonate and leucine, respectively. Bacterial respiration was measured by experiments of oxygen consumption, and an extensive data set of limnological variables were measured through standard methods. Results Sharp changes in chemical/physical variables, as well as in bacterial density, biovolume, biomass and bacterial growth efficiency occurred on average at 30% of macrophyte coverage, with most of them increasing towards pelagic zones. The composition of major groups of zooplankton, phytoplankton, bacterial morphotypes and phylotypes indicated that different planktonic assemblies live apart in these two zones. Conclusions Our results demonstrate that macrophytes are a structuring driver of the plankton community and bacterial metabolism in this large subtropical shallow lake.

Responses of the phytoplankton functional structure to the spatial and temporal heterogeneity in a large subtropical shallow lake

Abstract Aim Studies on biological communities that take into account only the species composition and abundances (or biomass) and their relative contributions, most of the time, do not reflect their ecological functions, especially considering the wide spatial and temporal variation of large shallow lakes. This paper aimed at evaluating the influence of environmental spatial and temporal heterogeneity on the functional structure of phytoplankton in a subtropical large shallow lake. Methods Seasonal samplings were carried out in 2010 and 2011, in 19 sampling sites distributed along the entire length (90 km) and width (3-10 km) of Lake Mangueira, a large (820 km2 ) and shallow lake (zmean = 2.6 m), comprising the littoral and pelagic zones of the north, central and southern regions. Abiotic variables and phytoplankton functional traits (volume, maximum linear dimension, life forms) and functional groups were analyzed as measures of functional structure. Results The results showed that there was no spatial organization of phytoplankton functional traits during the study. Colonial non-flagellated organisms, organisms with cellular volume between 103 and 104 μm3 and greater than 104 μm3, and with maximum linear dimension between 21 and 50 μm prevailed in all zones and regions. Phytoplankton functional groups and traits responded to resource variation, especially increasing their variety and contribution during spring and summer periods. Conclusions The functional structure of the phytoplankton community in Lake Mangueira, here accessed by functional traits and RFGs, was more conditioned by its environmental temporal variability rather than by the spatial variation, indicating that the resources and life conditions seasonal variation strongly influence the phytoplankton in this ecosystem.

Effects of climatically-modulated changes in solar radiation and wind speed on spring phytoplankton community dynamics in Lake Taihu, China.

Many studies have focused on the interactive effects of temperature increases due to global warming and nutrient enrichment on phytoplankton communities. Recently, non-temperature effects of climate change (e.g., decreases in wind speed and increases in solar radiation) on large lakes have received increasing attention. To evaluate the relative contributions of both temperature and non-temperature effects on phytoplankton communities in a large eutrophic subtropical shallow lake, we analyzed long-term monitoring data from Lake Taihu, China from 1997 to 2016. Results showed that Lake Taihu’s spring phytoplankton biovolume and composition changed dramatically over this time frame, with a change in dominant species. Stepwise multiple linear regression models indicated that spring phytoplankton biovolume was strongly influenced by total phosphorus (TP), light condition, wind speed and total nitrogen (TN) (radj2 = 0.8, p < 0.01). Partial redundancy analysis (pRDA) showed that light condition accounted for the greatest variation of phytoplankton community composition, followed by TP and wind speed, as well as the interactions between TP and wind speed. Our study points to the additional importance of non-temperature effects of climate change on phytoplankton community dynamics in Lake Taihu.

Phytoplankton, periphyton, and zooplankton patterns in the pelagic and littoral regions of a large subtropical shallow lake

This study aimed at understanding the patterns among size structure of the phytoplankton, periphyton, and zooplankton considering the spatial variability (pelagic and littoral zones) in a subtropical large shallow polymictic lake (Lake Mangueira, southern Brazil). Water samples were gathered at intervals of 3, 5, and 15 days, for 60 days during summer 2012, for physical, chemical, and biological analyses (abundance and maximum linear dimension of subsurface zooplankton, phytoplankton, and glass slide-colonized periphyton) in both lake zones. A summer storm on the 20th day disturbed the periphyton succession in the littoral and pelagic zones. Phytoplankton size classes varied similarly to the smaller size of zooplankton in the pelagic zone, whereas most of periphyton size classes coincided with the larger zooplankton in the littoral zone, evidencing that zooplankton in Lake Mangueira may exploit both communities and both compartments (littoral and pelagic zones). The study demonstrated that the patterns of those communities are dynamic and closely related to the environmental variability in Lake Mangueira.

Coupling large-scale hydrological and hydrodynamic modeling: Toward a better comprehension of watershed-shallow lake processes

Changes in hydrological processes in large watersheds may heavily impact the dynamics of downstream aquatic ecosystems such as lakes and reservoirs. In general, simple approaches that neglect the spatial heterogeneity of watersheds impede understanding of the many processes occurring in these downstream aquatic ecosystems. In most cases, observed river discharges are used as the boundary condition for lake/reservoir modeling, which limits analyses of different situations and of areas that are ungauged or water bodies with little data available. Therefore, coupling hydrological models with hydrodynamic models seems to be a promising approach to allow one to understand how catchment features (e.g., land use, topography) and river discharges impact the dynamic patterns, including local circulation patterns, water levels and water quality in large water bodies in response to basin-wide forcing factors. In this study, we used a complex, spatially explicit, integrated approach to assess the hydrological responses to external forcing factors such as wind and river discharges, in the hydrodynamics of a large shallow subtropical lake. A large-scale integrated system composed of several sub-watersheds and a lake was analyzed by coupling a large-scale hydrological model with a hydrodynamic model. Our findings indicated that (i) the integrated model improved the capacity for representing the spatial and temporal variability of lake water-surface levels; (ii) the main hydrodynamic processes (water levels and flow structure) are controlled on a seasonal scale (months) by the river discharges, and on a short time scale (days) by wind influence (intensity and direction); and (iii) the modeling strategy allowed evaluation of anthropogenic stressors such as irrigation withdrawals on lake levels. This approach is an attempt to develop a physically based management model to simulate complex systems as a whole, allowing one to understand the main factors controlling the hydrodynamic processes and water levels, and accounting for internal (e.g., recirculation, horizontal mixing) and external factors (e.g., anthropogenic stressors, irrigation).

Eutrophication, Research and Management History of the Shallow Ypacaraí Lake (Paraguay)

Ypacaraí Lake is the most renowned lake in landlocked Paraguay and a major source of drinking and irrigation water for neighbouring towns. Beyond its socioeconomic and cultural significance, it has great ecological importance, supporting a rich biodiversity. Rapid growth of human presence and activities within its basin has led to its environmental degradation, a heartfelt matter of high political concern that compels intervention. Here, by reconstructing the history of scientific and management-oriented research on this system, we provide a comprehensive assessment of current knowledge and practice to which we contribute our recent, novel findings. An upward trend in total phosphorus concentration confirms ongoing eutrophication of an already eutrophic system, evidenced by consistently high values of trophic state indices. Downward trends in water transparency and chlorophyll-a concentration support the hypothesis that primary production in this lake is fundamentally light limited. Statistical and other analyses suggest high sensitivity of the system to hydraulic, hydro-morphological and hydro-meteorological alterations arising, respectively, from engineering interventions, land use and climate change. By discussing knowledge gaps, opportunities for research and challenges for management and restoration, we argue that this case is of high scientific value and that its study can advance theoretical understanding of shallow subtropical lakes.

Succession Characteristics of Phytoplankton Functional Groups and Their Relationships with Environmental Factors in Dianshan Lake, Shanghai

Dianshan Lake, a subtropical shallow lake, is one of the main drinking water sources of Shanghai City. In order to explore the characteristics of phytoplankton functional groups (PFGs) and their relationships with environmental factors in Dianshan Lake, phytoplankton and water samples collected from January 2015 to December 2016 were characterized. Results suggested that there were 26 PFGs during our study period. The biomasses of 15 PFGs (B, C, D, F, G, H1, J, Lo, M, MP, P, W1, W2, X1, and X2) were dominant and had marked annual succession patterns: D/C Lo/C/MP MP/J/P/W1 P in 2015 and P W2/Lo W2/G/M/H1 P in 2016. To elucidate the relationships of PFGs with the environmental factors, canonical correspondence analysis (CCA) was conducted. Results showed that water level fluctuation (WLF), dissolved oxygen (DO), water temperature (WT), total phosphorus (TP), electrical conductivity (EC), transparency (SD), and total nitrogen (TN) were the main impact factors associated with the composition variations of PFGs in Dianshan Lake. From this study, it is proposed that the effects of nitrogen, phosphorus, and WLF on the PFGs in Dianshan Lake should be paid more attention.

Can chlorophyll-a in meso-oligotrophic shallow waters be estimated using statistical approaches and empirical models from MODIS imagery?

ABSTRACT Accurate estimation of chlorophyll-a (Chl-a) concentration in inland waters through remote-sensing techniques is complicated by local differences in the optical properties of water. In this study, we applied multiple linear regression (MLR), artificial neural network (ANN), nonparametric multiplicative regression (NPMR) and four models (Appel, Kahru, FAI and O14a) to estimate the Chl -a concentration from combinations of spectral bands from the MODIS sensor. The MLR, NPMR and ANN models were calibrated and validated using in-situ Chl -a measurements. The results showed that a simple and efficient model, developed and validated through multiple linear regression analysis, offered advantages (i.e., better performance and fewer input variables) in comparison with ANN, NPMR and four models (Appel, Kahru, FAI and O14a). In addition, we observed that in a large shallow subtropical lake, where the wind and hydrodynamics are essential factors in the spatial heterogeneity (Chl-a distribution), the MLR model adjusted using the specific point dataset, performed better than using the total dataset, which suggest that would not be appropriate to generalize a single model to estimate Chl-a in these large shallow lakes from total datasets. Our approach is a useful tool to estimate Chl -a concentration in meso-oligotrophic shallow waters and corroborates the spatial heterogeneity in these ecosystems.

Composición funcional de invertebrados en los detritos de macrófitas sumergidas: efectos del hábitat y del tiempo de degradación

El objetivo de este estudio fue determinar si el tiempo de degradacion y el habitat provisto por dos macrofitas sumergidas afectan la estructura de la comunidad de invertebrados en un lago somero subtropical. Se evaluo la descomposicion de detrito en 60 bolsas. Los detritos fueron asignados al azar a cuatro tratamientos: Potamogeton pectinatus en descomposicion dentro de su propio stand (PP) y en el interior de un stand de Chara zeylanica (PC), y C. zeylanica dentro de su propio stand (CC) y en el interior de un stand de P. pectinatus (CP). Durante el experimento de degradacion (20 dias) se evaluo la fauna asociada tanto a los detritos como a las macrofitas vivas. Las bolsas de detrito se lavaron, se secaron y luego se determino el coeficiente de degradacion. Estimamos la riqueza y la abundancia de los taxa de invertebrados asociados y evaluamos la preferencia de las comunidades de invertebrados comparando la fauna de las macrofitas vivas y de sus detritos. Los invertebrados asociados se clasificaron en grupos funcionales de alimentacion. Chara zeylanica mostro una perdida de peso cuatro veces mas rapida que P. pectinatus. La mayor abundancia de invertebrados se produjo en el tratamiento PP y la mayor riqueza de invertebrados en los tratamientos de CC y PP. Los colectores y depredadores presentaron las mayores abundancias. El tratamiento PP tuvo el mayor numero de colectores y PC una distribucion mas equitativa de los grupos funcionales. Cinco taxones mostraron preferencia por detritos de P. pectinatus en lugar de C. zeylanica o P. pectinatus vivos, y los detritos de la carofita fueron preferidos por seis taxones dentro de las plantas vivas, y por dos taxones cuando se incubaron en el interior del stand de P. pectinatus. Nuestro estudio demostro el efecto del sitio de incubacion en el coeficiente de descomposicion y en la estructura de las comunidades de invertebrados, y revelo que la abundancia de invertebrados en detritus se debe principalmente a su abundancia en las plantas vivas. Ademas, la ausencia de trituradores sugiere el uso de detritos principalmente como refugio en lugar de como recurso alimentario.

Revealing Biotic and Abiotic Controls of Harmful Algal Blooms in a Shallow Subtropical Lake through Statistical Machine Learning.

Harmful algal blooms are a growing human and environmental health hazard globally. Eco-physiological diversity of the cyanobacteria genera that make up these blooms creates challenges for water managers tasked with controlling the intensity and frequency of blooms, particularly of harmful taxa (e.g., toxin producers, N2 fixers). Compounding these challenges is the ongoing debate over the efficacy of nutrient management strategies (phosphorus-only versus nitrogen and phosphorus), which increases decision-making uncertainty. To improve our understanding of how different cyanobacteria respond to nutrient levels and other biophysical factors, we analyzed a unique 17 year data set comprising monthly observations of cyanobacteria genera and zooplankton abundances, water quality, and flow in a bloom-impacted, subtropical, flow-through lake in Florida (United States). Using the Random Forests machine learning algorithm, an ensemble modeling approach, we characterized and quantified relationships among environmental conditions and five dominant cyanobacteria genera. Results highlighted nonlinear relationships and critical thresholds between cyanobacteria genera and environmental covariates, the potential for hydrology and temperature to limit the efficacy of cyanobacteria bloom management actions, and the importance of a dual nutrient management strategy for reducing bloom risk in the long term.