Trophic Gradient Research Articles

Molecular-level composition of dissolved organic matter in distinct trophic states in Chinese lakes: Implications for eutrophic lake management and the global carbon cycle

Dissolved organic matter (DOM) is an abundant and mobile part of the aquatic environment and plays important roles in aquatic biogeochemical cycles and the global carbon cycle. Recently, eutrophication has become an important environmental issue in global lakes, but how eutrophication drives changes in the molecular composition of DOM along trophic gradients remains poorly understood. We thus characterized 67 DOM isolates from 11 lakes along a trophic gradient in China by using a combined approach including absorption spectroscopy, excitation-emission matrix fluorescence and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Our results indicated that dissolved organic carbon and absorption coefficients at 350 nm increased with increasing trophic status index. The ultraviolet absorbance at 254 nm and fluorescence intensity of all fluorescent components were higher in eutrophic lakes than in oligotrophic lakes. DOM in high trophic state lakes tended to be dominated by higher molecular weight, unsaturation degree, greater abundance of S-containing compounds, and condensed or polycyclic aromatic compounds than oligotrophic lakes. Additionally, autochthonous DOM characterized by more aliphatic compounds increased with the increasing trophic state. We concluded that nutrient input along with allochthonous DOM favors the lake eutrophication and subsequently increases the release and accumulation of autochthonous DOM. Consequently, eutrophication modifies the structure of the organic matter into more complex materials with increased input of allochthonous DOM and increased release of autochthonous DOM, which could accelerate global carbon cycle processes. Our results here have potential to contribute significantly to future studies of DOM dynamics in eutrophic lakes.

Internal phosphorus loading and its driving factors in the dry period of Brazilian semiarid reservoirs

This study investigated the relationships between physical, limnological and climatic drivers with the internal total phosphorus (TP) loading produced over the dry period in 30 water supply reservoirs of the Brazilian semiarid. Improvements in the understanding of sedimentary TP fluxes in reservoirs of dryland regions are pressing as they usually have serious water quality related issues, remaining mostly eutrophic especially under frequent drought events. Gross daily fluxes and net seasonal average release rates were calculated from mass balance and regression equations considering water and sediment TP concentrations, anoxic duration, water temperature and fish contribution. Additionally, the ratio of wind speed to reservoir volume was proposed as a new surrogate and then applied as explanatory variable to predictive models. The results indicated TP release rates higher than reported for non-semiarid lakes/reservoirs with average gross fluxes ranging from 17.64 to 35.99 mg m−2 day−1. This may be attributed to the enriched sediments (1029.49 ± 552.49 mg kg−1) allied with warmer water temperature, high trophic state, and prolonged anoxic periods (average duration of about 60 days). The average release rates were negatively correlated with water transparency and water depth, and positively correlated with Chl-a, wind speed and trophic state. The release rates increased across the trophic gradient (p < 0.05), about 10-fold higher under hypertrophic conditions than in oligotrophic ones. As anoxia is linked with eutrophication, phosphorus release is more likely in eutrophic ecosystems. Regarding the new surrogate, a strong predictive ability for TP release (R2: 0.26–0.93) was observed. Similarly, the proposed models presented a physically consistent behavior with a stabilizing releasing pattern suggesting the achievement of equilibrium in nutrient exchange between sediment-water interface. This research advanced by combining and proposing methods to assess and quantify sedimentary fluxes in data-scarce regions balancing accuracy and transferability, in order to be replicable to other dryland environments globally.

Diversity and distribution of sediment bacteria across an ecological and trophic gradient.

The microbial communities of lake sediments have the potential to serve as valuable bioindicators and integrators of watershed land-use and water quality; however, the relative sensitivity of these communities to physio-chemical and geographical parameters must be demonstrated at taxonomic resolutions that are feasible by current sequencing and bioinformatic approaches. The geologically diverse and lake-rich state of Minnesota (USA) is uniquely situated to address this potential because of its variability in ecological region, lake type, and watershed land-use. In this study, we selected twenty lakes with varying physio-chemical properties across four ecological regions of Minnesota. Our objectives were to (i) evaluate the diversity and composition of the bacterial community at the sediment-water interface and (ii) determine how lake location and watershed land-use impact aqueous chemistry and influence bacterial community structure. Our 16S rRNA amplicon data from lake sediment cores, at two depth intervals, data indicate that sediment communities are more likely to cluster by ecological region rather than any individual lake properties (e.g., trophic status, total phosphorous concentration, lake depth). However, composition is tied to a given lake, wherein samples from the same core were more alike than samples collected at similar depths across lakes. Our results illustrate the diversity within lake sediment microbial communities and provide insight into relationships between taxonomy, physicochemical, and geographic properties of north temperate lakes.

Assessment of genetic variation in Schizothorax Esocinus Heckel, 1838 from Dal and Manasbal lakes of Kashmir

Schizothorax esocinus (Churru snow trout) Heckel, 1838 is a key freshwater species in the valley of Kashmir. The present study aimed to evaluate the genetic variability in the fish from the two eminent lakes of Kashmir valley namely Dal and Manasbal lakes, having different trophic gradients using biochemical (genetic) marker (SDSPage). The electrophoretic analysis revealed 6 bands of molecular weights ranging from 13 to 150 kDa for Dal lake and 8 bands of molecular weights ranging from 15 to 150 kDa for Manasbal lake. The Rf value ranged from 0.15321 to 0.70625 for sampled fishes of Dal lake and from 0.14231 to 0.63559 for sampled fishes of Manasbal lake. The study provided some basic information about the genetic variation of S. esocinus populations in the Dal and Manasbal lakes. Electropherogram studies revealed that the studied populations of Schizothorax esocinus show some degree of variation in the electrophoretic migration of muscle proteins. Polymorphism in muscle protein is clearly demonstrated among this group of fishes from the two lakes.

Influence of land use and occupation on water quality and on the diatom community of reservoirs in northeast Brazil

Historically, the reduction in water quality has been associated with changes in land use, especially changes in urbanization and expansion of agricultural frontiers. The objective of this study was to understand the characteristics of the diatomological community and the use and occupation of the soil around three reservoirs (Arara, Rio do Canto and Araçagi) and natural lagoon (Lagoa do Paó) (Northeast Brazil). Land use was determined using high spatial resolution satellite images. Abiotic variables of water and total phosphorus, organic matter and diatoms in surface sediments were analyzed. Abiotic variables and assemblages of diatoms were analyzed using Principal Component Analysis (PCA). Four types of land use have been identified, associated with anthropogenic change and intensive use of the landscape. The PCA separated the reservoirs by trophic status based on the total phosphorus in the water. 24 taxa of diatoms distributed in 12 genera and 10 families were identified in the sediment. Our results indicate the occurrence of planktonic, epiphytic and benthic diatoms adapted to different trophic gradients

Comparing microscopy and DNA metabarcoding techniques for identifying cyanobacteria assemblages across hundreds of lakes

Accurately identifying the species present in an ecosystem is vital to lake managers and successful bioassessment programs. This is particularly important when monitoring cyanobacteria, as numerous taxa produce toxins and can have major negative impacts on aquatic ecosystems. Increasingly, DNA-based techniques such as metabarcoding are being used for measuring aquatic biodiversity, as they could accelerate processing time, decrease costs and reduce some of the biases associated with traditional light microscopy. Despite the continuing use of traditional microscopy and the growing use of DNA metabarcoding to identify cyanobacteria assemblages, methodological comparisons between the two approaches have rarely been reported from a wide suite of lake types. Here, we compare planktonic cyanobacteria assemblages generated by inverted light microscopy and DNA metabarcoding from a 379-lake dataset spanning a longitudinal and trophic gradient. We found moderate levels of congruence between methods at the broadest taxonomic levels (i.e., Order, RV=0.40, p < 0.0001). This comparison revealed distinct cyanobacteria communities from lakes of different trophic states, with Microcystis, Aphanizomenon and Dolichospermum dominating with both methods in eutrophic and hypereutrophic sites. This finding supports the use of either method when monitoring eutrophication in lake surface waters. The biggest difference between the two methods was the detection of picocyanobacteria, which are typically underestimated by light microscopy. This reveals that the communities generated by each method currently are complementary as opposed to identical and promotes a combined-method strategy when monitoring a range of trophic systems. For example, microscopy can provide measures of cyanobacteria biomass, which are critical data in managing lakes. Going forward, we believe that molecular genetic methods will be increasingly adopted as reference databases are routinely updated with more representative sequences and will improve as cyanobacteria taxonomy is resolved with the increase in available genetic information.

Benthos of Green Bay, Lake Michigan revisited after 40 years: A temporal update and assessment of environmental associations

Benthos of southern Green Bay, Lake Michigan have not been comprehensively examined since 1978. Since then, invasive species appeared, urbanization intensified, and restoration efforts were implemented, which likely altered the benthic macroinvertebrate community. Further, current benthos are subjected to dynamic factors including eutrophication, sedimentation, and periodic hypoxia. Understanding community responses to these anthropogenic stressors and natural habitat gradients is imperative to preserving biological integrity within Green Bay. Therefore, the objectives of this project were to describe the current macroinvertebrate community, examine changes since 1978, and assess the roles of productivity, substrate type, water depth, and hypoxia in structuring macroinvertebrate communities. Benthos were sampled at 197 stations, including 97 also sampled in 1978 by Markert (1982) and 100 that were added to increase spatial resolution. We collected 93 macroinvertebrate taxa in southern Green Bay with the community dominated by Chironomus and immature tubificid worms. Nonmetric multidimensional scaling (NMDS) ordination distinguished present and historical communities. Although oligochaete worms and chironomids remained dominant over time, Chironomus abundance increased and characterized the present community, whereas benthos were historically more diverse. The magnitude of temporal change varied spatially among zones of Green Bay, with larger differences concentrated in the Middle Bay and the Inner Bay remaining comparable to 1978. Present-day assemblages were most associated with the trophic gradient driven by Green Bay’s southernmost tributary, the Fox River, but also differed with substrate type and had similar structures in areas subjected to frequent hypoxia. Routine monitoring should continue to track changes while accounting for spatial effects.

Joint analysis of macrofaunal and meiofaunal assemblages improves the assessment of lagoonal environmental heterogeneity

Matching datasets of macrofaunal and meiofaunal assemblages were jointly analyzed for the first time in a coastal lagoon (Cabras Lagoon, Sardinia) in relation to the lagoonal environmental heterogeneity. Within the common faunal list, indicator species/taxa for different environmental conditions were found evenly for both assemblages, with the two faunal components responding distinctly to the putative environmental gradients according to their autecology. The relationship between the number of individuals and species/taxa richness of both assemblages showed an exponential increase at increasing salinity and DO concentrations in water as expected basing on the confinement theory. Along the trophic gradient, the two metrics of both macrofaunal and meiofaunal assemblages showed a similar belt-shaped curve in agreement with theoretical predictions and quantitative field studies, with a peak at intermediate OM values followed by a sharp decrease at OM above 12% at the most confined site. While not a causality measure, our results indicate that these simple benthic-OM relationships applied to both faunal components represent a valuable screening-level indicator for evaluating the likelihood of reduced sediment quality and associated bioeffects in eutrophic lagoons. The present study demonstrates that macrofaunal and meiofaunal assemblages in coastal lagoons, as commonly shown in marine waters, are coherent and distinct entities and that their joint use represents a more comprehensive and robust tool to assess the environmental heterogeneity and quality of these highly variable systems.

Microbial Communities Mediating Net Methylmercury Formation Along a Trophic Gradient in a Peatland Chronosequence

Microbial Communities Mediating Net Methylmercury Formation Along a Trophic Gradient in a Peatland Chronosequence

Spatiotemporal Optical Properties of Dissolved Organic Matter in a Sluice-Controlled Coastal Plain River with Both Salinity and Trophic Gradients

Spatiotemporal Optical Properties of Dissolved Organic Matter in a Sluice-Controlled Coastal Plain River with Both Salinity and Trophic Gradients

Towards a quantitative reconstruction of lake trophic state in temperate lakes using subfossil cladocera and diatoms: Composition of a training set from NE Poland

We present a training set, the database involving physical-chemical water parameters together with the subfossil Cladocera and diatoms community composition in the surface sediments of 64 postglacial lakes in NE Poland sampled along a wide trophic gradient (from oligo- to highly eutrophic). The most important water parameters measured in water were chlorophyll-a, electrical conductivity (EC) and oxygen concentration. In addition, total phosphorus (TP) and Secchi depth (SD) were determined for the surface water layer. The data collected will be used to calculate a transfer-function for quantitative reconstruction of trophic state in freshwater temperate lakes.

Eutrophication causes microbial community homogenization via modulating generalist species

Eutrophication substantially influences the community structure of aquatic organisms and has become a major threat to biodiversity. However, whether eutrophication is linked to homogenization of microbial communities and the possible underlying mechanisms are poorly understood. Here, we studied bacterial and fungal communities from water and sediments of 40 shallow lakes in the Yangtze-Huaihe River basin, a representative area characterized by intensifying eutrophication in China, and further examined the beta diversity patterns and underlying mechanisms under eutrophication conditions. Our results indicate that eutrophication generally caused biotic homogenization of bacterial and fungal communities in both habitats showing decreased community variations for the sites with a higher trophic state index (TSI). In the two habitats, community dissimilarities were positively correlated with TSI changes for both taxonomic groups, while the local contribution to beta diversity (LCBD) remarkably declined with increasing TSI for the fungal community. These phenomena were consistent with the pivotal importance of the TSI in statistically accounting for beta diversity of bacterial and fungal communities in both habitats. In addition, we found that physicochemical factors such as water temperature and pH were also important for bacterial and fungal communities in water, while heavy metal elements were important for the communities in sediments. Interestingly, generalist species, rather than specialist species, were revealed to more dominantly affect the variations in beta diversity along the trophic gradient, which were quantified by Bray-Curtis dissimilarity and LCBD. Collectively, our findings reveal the importance of generalist species in contributing to the change of beta diversity of microbial communities along trophic gradients, which have profound implications for a comprehensive understanding of the effects of eutrophication on microbial community.

Factors regulating lake periphyton biomass and nutrient limitation status across a large trophic gradient

Abstract Because of the historical focus of limnology on pelagic processes, the factors controlling lake periphyton growth and nutrient limitation are understudied compared to the phytoplankton. We deployed nutrient‐diffusing substrata at 28 sites spanning a wide trophic status gradient in Lakes Superior and Michigan to assess periphyton biomass accrual on control substrata and the response of periphyton to single and combined phosphorus (P) and nitrogen (N) additions. Periphyton growth was unimodally related to a composite metric of site trophic status, with highest biomass at mesotrophic sites and lower growth at oligotrophic and highly eutrophic sites. Contrary to expectations, P limitation was rare. Instead, several lines of evidence pointed to primary N or N + P co‐limitation of periphyton. Limitation extent was negatively related to site trophic status, with stronger nutrient limitation at oligotrophic sites. Our results support the hypothesis that phytoplankton and periphyton biomass respond differently to nutrient enrichment and suggest that different nutrients may limit pelagic and benthic primary production, even in the same system. Our findings also support the use of periphyton as an early warning indicator of nutrient pollution and help explain why large, oligotrophic lakes may be especially susceptible to localised benthic algal blooms.

Environmental gradients and physical barriers drive the basin‐wide spatial structuring of Mediterranean Sea and adjacent eastern Atlantic Ocean prokaryotic communities

AbstractThe Mediterranean Sea is a miniature ocean divided by the Sicily Strait into two basins with a marked west to east trophic gradient and separated of the nearby eastern Atlantic Ocean by the Strait of Gibraltar. Here, we test the hypothesis that these physical and environmental barriers favor the development of specific prokaryotic assemblages, leading to changes in community structure both in the vertical and horizontal spatial scales. By analyzing taxonomic and phylogenetic diversity using amplicon sequence variants (ASVs) of the 16S rRNA gene, we show that there is indeed marked vertical segregation of prokaryotic groups, similar to that found in other areas of the ocean, but also a clear horizontal structuring among the two Mediterranean basins and the adjacent Atlantic waters. Prokaryotic diversity increased with depth and toward the Atlantic, whereas the easternmost stations displayed more phylogenetically diverse phylotypes, despite harboring globally less diverse communities. Basin‐indicator taxa (ASVs) accounted for a large fraction of the community (between 50% and 80%) in each of the basins at the surface and bathypelagic layers, being associated with different environmental variables. The existence of biogeographic and environmental barriers in the Mediterranean Sea is likely related to the trophic gradient at the surface and the isolation of water bodies in depth due to the Gibraltar and Sicily straits. Our work highlights the importance of studying microbial regional biogeography and provides the basis for future studies on the impact of this regionalization in the function of Mediterranean Sea prokaryotic communities.

Ecology of peatland testate amoebae in Svalbard and the development of transfer functions for reconstructing past water-table depth and pH

Peatlands are valuable archives of information about past environmental conditions and represent a globally-important carbon store. Robust proxy methods are required to reconstruct past ecohydrological dynamics in high-latitude peatlands to improve our understanding of change in these carbon-rich ecosystems. The High Arctic peatlands in Svalbard are at the northern limit of current peatland distribution and have experienced rapidly rising temperatures of 0.81 °C per decade since 1958. We examine the ecology of peatland testate amoebae in surface vegetation samples from permafrost peatlands on Spitsbergen, the largest island of the Svalbard archipelago, and develop new transfer functions to reconstruct water-table depth (WTD) and pH that can be applied to understand past peatland ecosystem dynamics in response to climate change. These transfer functions are the first of their kind for peatlands in Svalbard and the northernmost developed to date. Multivariate statistical analysis shows that WTD and pore water pH are the dominant controls on testate amoeba species distribution. This finding is consistent with results from peatlands in lower latitudes with regard to WTD and supports work showing that when samples are taken across a long enough trophic gradient, peatland trophic status is an important control on the distribution of testate amoebae. No differences were found between transfer functions including and excluding the taxa with weak idiosomic tests (WISTs) that are most susceptible to decay. The final models for application to fossil samples therefore excluded these taxa. The WTD transfer function demonstrates the best performance (R2LOO = 0.719, RMSEPLOO = 3.2 cm), but the pH transfer function also performs well (R2LOO = 0.690, RMSEPLOO = 0.320). The transfer functions were applied to a core from western Spitsbergen and suggest drying conditions ~1750 CE, followed by a trend of recent wetting and increasing pH from ~1920 CE. These new transfer functions allow the reconstruction of past peatland WTD and pH in Svalbard, thereby enabling a greater understanding of long-term ecohydrological dynamics in these rapidly changing ecosystems.

Taxonomic and functional differences between winter and summer crustacean zooplankton communities in lakes across a trophic gradient

AbstractDespite increasing interest in winter limnology, few studies have examined under-ice zooplankton communities and the factors shaping them in different types of temperate lakes. To better understand drivers of zooplankton community structure in winter and summer, we sampled 13 lakes across a large trophic status gradient for crustacean zooplankton abundance, taxonomic and functional community composition and C/N stable isotopes. Average winter zooplankton densities were one-third of summer densities across the study lakes. Proportionally, cladocerans were more abundant in summer than winter, with the opposite pattern for calanoids and cyclopoids. In green (eutrophic) lakes, zooplankton densities were higher under the ice than in brown (dystrophic) and blue (oligotrophic) lakes, suggesting better conditions for zooplankton in productive lakes during winter. Overall, zooplankton communities were more similar across lakes under the ice than during the open water season. Feeding group classification showed a decrease in herbivore abundance and an increase in predators from summer to winter. C/N stable isotope results suggested higher lipid content in overwintering zooplankton and potentially increased reliance on the microbial loop by winter zooplankton. Our results show substantial variation in the seasonality of zooplankton communities in different lake types and identify some of the factors responsible for this variation.

Multiple-Facet Diversity Patterns of Aquatic Vegetation in Lakes along a Trophic Gradient

The EU Water Framework Directive foresees the ecological assessment of surface waters against identified pressures. Nutrient loading is the main pressure impairing the ecological quality of lake ecosystems, and aquatic macrophytes are considered good indicators of ecological response. In this study, we statistically assessed different aspects of aquatic plant (macrophyte) diversity in response to different trophic levels in Mediterranean lakes. We used 5690 relevés of aquatic vegetation, distributed over 305 transects, sampled in 18 freshwater lake ecosystems during 2013–2016. Our results show a significant decrease in taxonomic alpha diversity in lakes with a total phosphorus content above 100 μg/L. Syntaxonomic diversity followed the species richness pattern as well. Functional richness decreased along the trophic gradient, while functional dispersion was higher in lakes with high trophic levels. Taxonomic and functional beta partitioning presented changes in assembly processes leading to greater community homogeneity in lakes with higher trophic levels. In summary, we found no redundancy between taxonomic and functional diversity indices. These results provide novel insights into aquatic plant assembly processes of impacted freshwater lakes needed to forward conservation and restoration practices.

Cyanobacterial blooms in oligotrophic lakes: Shifting the high‐nutrient paradigm

Abstract Freshwater cyanobacterial blooms have become ubiquitous, posing major threats to ecological and public health. Decades of research have focused on understanding drivers of these blooms with a primary focus on eutrophic systems; however, cyanobacterial blooms also occur in oligotrophic systems, but have received far less attention, resulting in a gap in our understanding of cyanobacterial blooms overall. In this review, we explore evidence of cyanobacterial blooms in oligotrophic freshwater systems and provide explanations for those occurrences. We show that through their unique physiological adaptations, cyanobacteria are able to thrive under a wide range of environmental conditions, including low‐nutrient waterbodies. We contend that to fully understand cyanobacterial blooms, and thereby mitigate and manage them, we must expand our inquiries to consider systems along the trophic gradient, and not solely focus on eutrophic systems, thus shifting the high‐nutrient paradigm to a trophic‐gradient paradigm.

Priority effects of stream eutrophication and assembly history on beta diversity across aquatic consumers, decomposers and producers

Priority effects are stochastic processes that consider the influence of the order of arrival of species on community dynamics and structure. We evaluated the short-term effects of stream eutrophication and colonization time in freshwater benthic communities (primary producers - periphytic algae, decomposers – fungi, and consumers - macroinvertebrates) to test whether (i) beta diversity is higher in eutrophic streams due to priority effects driven by stochastic community formation processes (ecological drift or random dispersal), and (ii) in the early stages of colonization, priority effects drive the history of the formation and the initial establishment of the community in the stream, resulting in higher beta diversity. The present study was conducted in situ over 28 days in temperate streams along a trophic gradient, with colonization being evaluated every seven days. The study identified 84 species of alga, 43 families of macroinvertebrates, and 44 species of aquatic fungi. Our results demonstrated that deterministic processes were responsible for the formation of aquatic producers, while priority effects (stochasticity) were more important for the aquatic decomposers and consumers. In the case of the producers, beta diversity was highest in the hypertrophic stream, but did not vary significantly over colonization time. The beta diversity of the decomposers was highest in the hypertrophic stream and in the later stages of succession, due primarily to mechanisms of facilitation. The beta diversity of the consumers was lowest in the hypertrophic stream due primarily to the priority and inhibitory effects of the predominant groups, and highest at seven and 21 days of colonization. As these three taxonomic groups differ in their intrinsic biological characteristics, and in their functional role in the ecosystem, our short-term field study demonstrated that both stochastic and deterministic processes combine to influence the configuration of the community, and that the relative importance of the two processes varies systematically along a trophic gradient.

Homogenization of diatom assemblages is driven by eutrophication in tropical reservoirs

Eutrophication is one of the most widespread causes of biotic homogenization in freshwater ecosystems. Biotic homogenization can be characterized as reductions in local diversity (alpha) and occupation of available niches by more generalist species. Beta diversity is expected to decrease in more homogeneous communities, however, there is no consensus on how it responds to eutrophication. We used a space-for-time approach to analyze the process of biotic homogenization on diatom assemblages in response to eutrophication in tropical reservoirs ranging from oligotrophic to hypereutrophic conditions. Diatom assemblages were analyzed in phytoplankton and surface sediment from 12 reservoirs with different trophic levels. We calculated total beta diversity and turnover and nestedness components and used regressions to analyze their relationships with productivity differences (without distance effects). Total beta diversity had a positive influence of the trophic gradient, whereas turnover was not related to eutrophication. However, we found that eutrophication and lower species richness (alpha diversity) led to increasing rates of the nestedness component. We also observed that the homogenization process was not characterized by invasion of new species, but, on the contrary, by filtering nutrient-rich tolerant species also present in oligo-mesotrophic reservoirs and able to occupy available niches in the eutrophic reservoirs. These findings (increase in nestedness, decrease in alpha diversity, and development of tolerant species) suggest that biotic homogenization is leading to a simplification of diatom assemblages in tropical reservoirs, making assemblages from eutrophic and hypereutrophic reservoirs a subset of assemblages from oligotrophic and mesotrophic ones.