Large Subtropical Shallow Lake Research Articles

Short-term responses of littoral phytoplankton in a large shallow subtropical lake.

The littoral zone is an essential compartment for lake biota because of its high productivity and diversity. Moreover, phytoplankton is expected to have non-equilibrium dynamics on it. The study's aimed to explore phytoplankton in the littoral zone of a shallow lake over a short-term scale. Daily sampling was conducted for 25 consecutive summer days in 2016, at two marginal points of a continuously warm, polymictic, and oligo-mesotrophic subtropical lake (Lake Mangueira, Brazil). Cyanobacteria and Chlorophyta contributed 86% of total biomass. We observed high variability in phytoplankton structure, with species turnover over diel cycles. Redundancy analysis indicated spatial differentiation for phytoplankton structure in relation to abiotic conditions. Nutrient dynamics and humic substances were significant drivers for phytoplankton variability. Phytoplankton was positively correlated with SRP and negatively with humic substances. Our results showed a non- equilibrium state for the littoral phytoplankton of Lake Mangueira, given the high variability of abiotic conditions, even at short distances. Due to its high temporal and spatial variability, the littoralzone seems to contribute to the recruitment and maintenance of phytoplankton biodiversity in shallow lakes. Further studies should consider the functional attributes of species and the complex biological interactions of phytoplankton and macrophytes along the littoral zone.

The Importance of Heat Storage for Estimating Lake Evaporation on Different Time Scales: Insights From a Large Shallow Subtropical Lake

AbstractThe change in heat storage (Gc) is an essential component of a lake's energy balance, and its importance for lake evaporation (Ew) has been widely recognized. However, the effect of Gc on Ew exhibits diversity across time dimensions. The controls on Gc and the effects of Gc on Ew estimates at different time scales remain largely unexplored. To address these gaps, we identified the primary controls on Gc at an eddy covariance site in a large shallow lake (Lake Taihu) and quantified the role of Gc in estimating Ew on three (hourly, daily, and monthly) time scales based on two energy balance‐based Ew models. Our results indicate that the diurnal variation of Gc is dominated by net radiation and peaks around noon, while the seasonal variation of Gc is mainly controlled by air temperature and peaks in spring. In contrast, the daily variation of Gc is subjective to a confluence of factors—net radiation, wind speed, and relative humidity—displaying more stochasticity than that on the other two time scales. We also found that the importance of Gc for Ew estimates decreases as the time scale extends. Compared to the two models disregarding Gc, considering the effect of Gc enhances the average Kling‐Gupta efficiency (KGE) values of the two models by 1.33, 0.42, and 0.08 on the three time scales, respectively. Overall, this study highlights the importance of time scales in evaluating the effect of Gc on Ew estimates.

Spatiotemporal diversity and community structure of cyanobacteria and associated bacteria in the large shallow subtropical Lake Okeechobee (Florida, United States).

Lake Okeechobee is a large eutrophic, shallow, subtropical lake in south Florida, United States. Due to decades of nutrient loading and phosphorus rich sediments, the lake is eutrophic and frequently experiences cyanobacterial harmful algal blooms (cyanoHABs). In the past, surveys of the phytoplankton community structure in the lake have been conducted by morphological studies, whereas molecular based studies have been seldom employed. With increased frequency of cyanoHABs in Lake Okeechobee (e.g., 2016 and 2018 Microcystis-dominated blooms), it is imperative to determine the diversity of cyanobacterial taxa that exist within the lake and the limnological parameters that drive bloom-forming genera. A spatiotemporal study of the lake was conducted over the course of 1 year to characterize the (cyano)bacterial community structure, using 16S rRNA metabarcoding, with coincident collection of limnological parameters (e.g., nutrients, water temperature, major ions), and cyanotoxins. The objectives of this study were to elucidate spatiotemporal trends of community structure, identify drivers of community structure, and examine cyanobacteria-bacterial relationships within the lake. Results indicated that cyanobacterial communities within the lake were significantly different between the wet and dry season, but not between periods of nitrogen limitation and co-nutrient limitation. Throughout the year, the lake was primarily dominated by the picocyanobacterium Cyanobium. The bloom-forming genera Cuspidothrix, Dolichospermum, Microcystis, and Raphidiopsis were highly abundant throughout the lake and had disparate nutrient requirements and niches within the lake. Anatoxin-a, microcystins, and nodularins were detected throughout the lake across both seasons. There were no correlated (cyano)bacteria shared between the common bloom-forming cyanobacteria Dolichospermum, Microcystis, and Raphidiopsis. This study is the first of its kind to use molecular based methods to assess the cyanobacterial community structure within the lake. These data greatly improve our understanding of the cyanobacterial community structure within the lake and the physiochemical parameters which may drive the bloom-forming taxa within Lake Okeechobee.

Daily responses of the planktonic food web to environmental dynamics in the vegetated littoral zone of a large shallow lake

Littoral zones are the most productive areas of shallow lakes and make important contributions to energy transfer to higher trophic levels in aquatic ecosystems. Great habitat heterogeneity characterizes the littoral ecosystems, often encompassing macrophyte covered areas that provide microhabitats with diverse structural complexity, and several effects over the structure and function of biological communities. The effects of macrophytes on trophic interactions are more complex in subtropical lakes than in temperate lakes. The relationships between planktonic food web components and the effects of a highly dynamic environment in structuring plankton communities were studied in a large subtropical shallow lake. The study was conducted for twenty-five consecutive days in the summer of 2016 at two sites (Station 1 and Station 2) in the well-developed littoral zone of Lake Mangueira hosting different plant life forms. Strong spatial variation between sampling stations was evidenced in the density of most of the communities, with the highest means of bacterioplankton, phytoplankton, and ciliates density being recorded at the station surrounded mainly by submerged macrophytes (Station 1). These plants particularly modify microhabitat complexity by increasing the availability and functional diversity of niches. Furthermore, submerged macrophytes tend to accumulate more periphyton biomass, a quality food resource for many consumers. Important grazing relationships, nutrient demands, and trophic structure were assessed through carbon biomass of the communities. Phytoplankton accounted for a greater proportion of the entire plankton community than bacteria, while zooplankton had low carbon biomass of crustaceans and a high dominance of ciliates and rotifers, with a greater grazing impact on bacterioplankton than on phytoplankton. According to RDA and multiple-regression analysis, nutrients and humic substances were the main predictors of variability in the littoral food web components over a short-sampling interval. The carbon biomass of bacteria and metazooplankton were both positively related to humic substances and nutrients at Station 1, as well as the biomass ratio of metazooplankton to phytoplankton and metazooplankton to ciliates. This suggests that humic substances have been metabolized by bacteria and transferred to higher trophic levels through predation. On the other hand, humic substances negatively affected the carbon biomass of bacteria and cyanobacteria at Station 2, suggesting a mostly refractory DOC around the emergent plants. Results indicate that despite the greater importance of bacterial biomass in the energy transfer through zooplankton predation, the efficiency of carbon transfer in algal and bacteria-based food webs was both very low. The effects of macrophytes on the quantity and/or quality of nutrients and substrates for bacteria uptake and phytoplankton requirements, explained the spatial variability of plankton food web components, even over the short distance between stations, as well as the inefficient microbial loop found in the littoral zone inhabited by macrophytes.

Meteorological drivers and ENSO influence on phytoplankton biomass dynamics in a shallow subtropical lake.

Meteorological features influence the dynamics of aquatic ecosystems and consequently their biotas. This study aimed to identify the meteorological drivers of phytoplankton biomass (chlorophyll a), sampled seasonally over a period of 12years (2001-2013) in Lake Mangueira, a large shallow subtropical lake in southern Brazil. The lake is 90km long and 3-10km wide with a mean depth of 3m and is oligo-mesotrophic and highly affected by wind action. In general, non-parametric multiplicative regression analysis identified wind direction, radiation, and the Oceanic Niño Index as the main drivers of variation in chlorophyll a. Notably, ENSO periods caused changes in physical, chemical, and meteorological parameters, including conductivity, total suspended solids, total and dissolved nitrogen, alkalinity, soluble reactive silica, wind speed, and precipitation. Phytoplankton biomass showed significant differences between ENSO periods and the periods without events, occurring in the highest values during La Niña years. This study showed that meteorological variables can significantly influence productivity patterns, indicating the importance of including them in limnological studies.

Winter Climate Shapes Spring Phytoplankton Development in Non‐Ice‐Covered Lakes: Subtropical Lake Taihu as an Example

AbstractWinter warming plays a vital role in spring phytoplankton community succession in temperate lakes due to variation of ice cover duration in winter. How winter conditions affect spring phytoplankton in subtropical lakes without winter ice cover is, however, largely unknown. In this study, covering 26 years, we elucidated the effects of both climate conditions and nutrient levels in winter on the phytoplankton community (expressed as Morpho‐Functional Groups) in spring in Lake Taihu, a large shallow subtropical lake in China. During this period, wind speed declined significantly in both winter and spring from 1992 to 2017 and the sunshine hours increased slightly from 2000 to 2017. The biomass of eight dominant phytoplankton groups showed increasing trends from 1992 to 2017, while it declined for one group, which mainly included filamentous green algae such as Ulothrix sp. Nonmetric multidimensional scaling (NMDS) results indicated that wind speed and sunshine duration in winter and spring were the most important factors affecting the spring phytoplankton community in Lake Taihu. Also, partial least squares path modeling (PLS‐PM) suggested that the spring phytoplankton community was strongly affected by winter conditions. Our study indicated that the “climate memory” effects on phytoplankton, resulting from winter and acting on the following spring, not only occur in north temperate winter ice‐covered shallow lakes but also in subtropical lakes without ice‐covered in winter.

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.

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.

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.

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.

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).

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.

Recovery of plankton from hurricane impacts in a large shallow lake

Abstract We quantified recovery of plankton in a large subtropical shallow lake from the catastrophic impacts of three successive major hurricanes. This assessment was possible because hurricanes passed directly over the lake amid an ongoing long‐term sampling programme that included nearly all components of the plankton, from bacteria to crustacean zooplankton. We compared attributes of plankton 5 years after the hurricanes to a pre‐hurricane period and to a period immediately after the storms. We evaluated both community‐level properties (biomass, biomass ratios, diversity, and dominance of major plankton groups) and species‐level properties (species absolute and relative biomass) at four sites in the lake representing different ecological zones. The hurricanes strongly affected water quality and plankton community structure. The lake experienced a regime shift, losing its submerged aquatic vegetation and becoming homogenous and turbid at all sampled sites. Five years after the storms, chemical and physical conditions recovered across the lake, with a few exceptions. Between 35 and 93 plankton species were lost at the sampling sites, with greatest losses in the phytoplankton. Relative species biomass displayed substantive changes too. Daphnia ambigua did not recover at three of the sites, Polyarthra vulgaris greatly increased lake‐wide, and at a central pelagic site, there was a total loss of heterotrophic nano‐flagellates and a much higher biomass of diatoms than before the hurricanes, despite recovery of irradiance, depth, nutrient levels and other attributes. Most community‐level properties were resilient, returning to pre‐hurricane conditions of total biomass, ratios of autotrophs to heterotrophs and ratios of protozoa to metazoa. This likely happened because of species compensation in the biodiverse community. The exception was at a central pelagic site, where the community‐level properties did not recover and nearly 50 per cent of species were lost. The community resilience, despite a regime shift, may have occurred because of a controlled lowering of water levels in the lake for flood protection, which led to regrowth of lost submerged aquatic vegetation (SAV) and migration of mud sediments back towards mid‐lake. In this lake and others with a history of high nutrient inputs, shallow depth and flocculent sediments, resilience may be low unless counter‐acting forces are able to push the system back after a regime shift.

The structuring role of submerged macrophytes in a large subtropical shallow lake: Clear effects on water chemistry and phytoplankton structure community along a vegetated-pelagic gradient

It is well known that submerged macrophytes exert positive feedback effects that enhance the water transparency, stabilizing the clear-water state in shallow temperate lakes. However, the structuring effect of macrophytes on the food web of subtropical and tropical ecosystems is still poorly understood. In this study we investigated the influence of dense submerged vegetation beds on the water chemistry and phytoplankton structure along a littoral-pelagic gradient of large subtropical shallow lake in southern Brazil. Seasonal monitoring was carried throughout one year following along a submerged vegetated-pelagic transect in order to analyze the effects of macrophyte’s coverage (percentage of volume infested- PVI) on the water chemistry and phytoplankton community structure. Clear variations on nutrient concentration and phytoplankton biomass/composition could be observed permanently along the transect. Nutrients as orto-phosphate (PO4−) and bicarbonate increased linearly towards the pelagic zone, whereas dissolved organic carbon and humic substances decreased linearly as PVI decreased. Concomitantly, a significant increase in the phytoplankton biomass was observed outwards from the submerged vegetation bed. In the vegetated area, small species (C-R strategists), unicellular flagellates were selected; whereas in the pelagic zone, larger (K-selected) species of cyanobacteria occurred, especially representatives of the functional groups M, LO, SN, S1 and K. Such results indicate that the macrophytes and inherent metabolism, such as potential excretion of dissolved organic compounds with allelochemicals and nutrient uptake from water column influence the structure of the phytoplankton community reducing also significantly the biomass of cyanobacteria within the dense submerged vegetated zone. Because of the continuous growth of macrophytes over the year in low latitude systems, their feed-back effect pattern tends to also dictate a different role in ecosystem dynamics and structure of the food web. These findings contribute to the management and conservation of subtropical and tropical lakes.

Environmental dissimilarity over time in a large subtropical shallow lake is differently represented by phytoplankton functional approaches

The aim of the present study was to understand how different phytoplankton functional approaches responded to environmental variability in a large shallow lake, namely Lake Mangueira, in southern Brazil. This coastal lake has a maximum depth of 7m and is ~90km long and ~3–10km wide. Physical and chemical variables, maximum linear dimension, life forms, functional and morphofunctional groups of phytoplankton were analysed. The results showed that the phytoplankton were primarily comprised Cyanobacteria (63.9%), followed by middle-sized algae (21–50μm; 46.7%) and colonial non-flagellated taxa (63%). The highest percentage of total biomass was accounted for by the functional group ‘K’ (as classified by Reynolds et al. 2002; 36.3%), large mucilaginous colonies (46.79%) according to morphologically based functional classification and other large colonies (mostly non-vacuolated; 36.7%) according to morphofunctional classification. Dissimilarity analysis indicated a significant correlation between abiotic data and functional approaches (P=0.001). The dissimilarity in the functional compositions of phytoplankton was related to nutrient and light conditions, especially to silicon content and water transparency, in all functional approaches. The highest correlation with abiotic variables was seen for morphologically based functional group community structure, although the functional group composition of phytoplankton sensu Reynolds et al. seemed to be the most effective system in describing environmental variability in Lake Mangueira over the long term.

Influence of long-term inundation and nutrient addition on denitrification in sandy wetland sediments from Poyang Lake, a large shallow subtropical lake in China

Wetlands at aquatic/terrestrial transition zones may play an important role in nitrogen removal due to the denitrification during inundation events. In this study, air-dried sandy sediment cores from Poyang Lake’s wetlands were continuously inundated in the laboratory to investigate the rate, efficiency, and pattern of denitrification at the sediment-water interface (SWI). Denitrification rates responded to inundation in three stages: an adaptive period, a rapid growth period, and a stable period. The average denitrification rates during these stages were 52.62 ± 11.33 μmol N2 m−2 h−1, 115.74 ± 58.40 μmol N2 m−2 h−1 and 187.74 ± 16.44 μmol N2 m−2 h−1, respectively. Multiple regression analysis showed that inundation duration was the most important factor controlling denitrification rates at the SWI rather than nitrate concentrations, which were also positively correlated with denitrification rates. Changes in water content, oxidation-reduction potential (Eh), and pH in the surface sediments, which were consequences of inundation, significantly influenced the denitrification rate (p < 0.05). The N2O/(N2O + N2) flux ratio in denitrification production varied from 1.61% during the adaptive period to 0.03% in the stable period, suggesting that continuous inundation of sediments in aquatic-terrestrial transition zones of Poyang Lake could decrease the proportion of greenhouse gases (N2O). Since inputs of bioavailable nitrogen and phosphorus to Poyang Lake have increased in recent decades, the influence of elevated nutrients on denitrification under continuous inundation were also evaluated by simulation experiments. The results showed that increased nitrate or phosphate loading stimulate denitrification rates in sandy sediments during the inundation process. The addition of nitrate plus phosphate showed a stronger influence on denitrification than single nutrient additions. This study facilitates understanding of the influence of continuous inundation and elevated nutrients on denitrification in sandy sediments in aquatic/terrestrial transition zones.

Spatial and temporal variability of zooplankton–phytoplankton interactions in a large subtropical shallow lake dominated by non-toxic cyanobacteria

The size-specific and composition relationships between zooplankton and phytoplankton were evaluated in a large subtropical lake, as well as the influence of environmental variability on spatial scale considering temporal variation. Seasonal sampling was conducted for 2 years, covering three areas (south, centre and north) and the pelagic and littoral zones in Mangueira Lake (southern Brazil). The zooplankton to phytoplankton biomass ratio, usually, was very low, indicating a weak top-down control on phytoplankton; however, the strength of this interaction varied with zooplankton composition and temporal or spatial variability of the environmental factors. Environmental, bottom-up and probably top-down forces involved the structure of zooplankton, whereas phytoplankton was mainly controlled by nutrients. The phytoplankton predominant biomass consisted of colonial forms of non-toxic cyanobacteria. Rotifers (90–150µm) were more able to benefit from phytoplankton production, whereas large-bodied zooplankton, when present in higher biomass, were not related to phytoplankton. High contribution of small ciliates and significant positive correlations between zooplankton and total phosphorus presuppose that microbial food webs primarily sustain the macro-zooplankton production in this system. The environmental variability induced by wind action or diversification of niches also played a substantial role in the structure of the plankton community, and the strength of zooplankton–phytoplankton interactions.

Contrasting factors drive within-lake bacterial community composition and functional traits in a large shallow subtropical lake

Lakes are considered as “islands” for assessing microbial biogeography, but bacterial community composition (BCC) and function may vary significantly within lakes, with the roles of scale and connectivity still unclear. This study investigated the spatial and temporal heterogeneity of the BCC (automated ribosomal intergenic spacer analysis) and functional traits (FT, carbon-source utilization), and the contribution of: (i) environmental variables, (ii) phytoplankton, (iii) season, and (iv) space, through variance partitioning in the large and well-mixed Lake Mangueira. The BCC and FT differed in time and space, with BCC being explained by environmental variables and phytoplankton, whereas FT was explained only by space. The smallest scale of variability detected by the BCC and FT (~49 km) was larger than scales found in the other studies, suggesting an effect of lake size (fetch and connectivity). Our results indicate that barriers to bacterial dispersal due to long distances are overcome by high connectivity, reinforcing the role of species sorting for BCC. FT were probably driven by gene dispersal and/or the effects of local conditions on migrant bacterial taxa and resuspended bacteria. Our results highlight the role of within-lake heterogeneity for ecosystem functioning and the implications for the appropriate scale to use in sampling bacterial communities.

Process-based modeling of shallow lake metabolism: Spatio-temporal variability and relative importance of individual processes

A process-based model was used to evaluate distributed estimates of aquatic metabolism and the importance of different individual processes related to dissolved-oxygen dynamics in a large shallow subtropical lake, Lake Mangueira, on the southern coast of Brazil. In order to assess spatial differences in metabolism estimates, the lake was divided into three geographical areas (North, Central, and South) and each area was subdivided into two biological zones (littoral and pelagic). A well-marked littoral to pelagic gradient was observed, with significant differences in the metabolism estimates between the biological zones of each compartment. In addition, a significant longitudinal difference in metabolism between the North and other lake areas was also apparent. Temporal dynamics featured continuous switching between net autotrophic and net heterotrophic conditions. Phytoplankton primary production and respiration were the main individual processes controlling gross primary production and ecosystem respiration, respectively, for the entire lake. Our findings indicated that spatially distributed estimates of lake metabolism led to different conclusions than did overall metabolism estimates based on single points. Also, individual processes must be taken into account in order to fully understand their relative importance on different spatial and temporal scales for the overall metabolism of aquatic ecosystems.