Phytoplankton community responses to temperature fluctuations under different nutrient concentrations and stoichiometry

Constant and fluctuating high temperatures interact with Saharan dust leading to contrasting effects on aquatic microbes over time

Mediterranean lakes are facing heightened exposure to multiple stressors, such as intensified Saharan dust deposition, temperature increases and fluctuations linked to heatwaves. However, the combined impact of dust and water temperature on the microbial community in freshwater ecosystems remains underexplored. To assess the interactive effect of dust deposition and temperature on aquatic microbes (heterotrophic bacteria and phytoplankton), a combination of field mesocosm experiments covering a dust gradient (five levels, 0-320 mg L-1), and paired laboratory microcosms with increased temperature at two levels (constant and fluctuating high temperature) were conducted in a high mountain lake in the Spanish Sierra Nevada, at three points in time throughout the ice-free period. Heterotrophic bacterial production (HBP) increased with dust load regardless of the temperature regime. However, temperature regime affected the magnitude and nature of the interactive Dust×T effect on HBP. Specifically, constant and fluctuating high temperature showed opposing interactive effects in the short term that became additive over time. The relationships between HBP and predictor variables (soluble reactive phosphorus (SRP), excreted organic carbon (EOC), and heterotrophic bacterial abundance (HBA)), coupled with an evaluation of the mechanistic variable photosynthetic carbon use efficiency by bacteria (%CUEb), revealed that bacteria depended on primary production in nearly all treatments when dust was added. The %CUEb increased with dust load in the control temperature treatment, but it was highest at intermediate dust loads under both constant and fluctuating high temperatures. Overall, our results suggest that while dust addition alone strengthens algae-bacteria coupling, high temperatures lead to decoupling in the long term at intermediate dust loads, potentially impacting ecosystem function.

Intensified effect of nitrogen forms on dominant phytoplankton species succession by climate change

Nutrient proportion, light intensity, and temperature affect the succession of dominant phytoplankton species. Despite these insights, this transformation mechanism in highly turbid lakes remains a research gap, especially in response to climate change. To fill this gap, we investigated the mechanism by which multi-environmental factors influence the succession of dominant phytoplankton species in Lake Chagan. This investigation deployed the structural equation model (SEM) and the hydrodynamic-water quality-water ecology mechanism model. Results demonstrated that the dominant phytoplankton species in Lake Chagan transformed from diatom to cyanobacteria during 2012 and 2022. Notably, Microcystis was detected in 2022. SEM revealed the primary environment variables for this succession, including water temperature (Tw), nutrients (total nitrogen (TN), total phosphorus (TP), and ammonia nitrogen (NH4N)), and total suspended solids (TSS). Moreover, this event was not the consequence of zooplankton grazing. An integrated hydrodynamic-water quality-bloom mechanism model was built to explore the mechanism driving phytoplankton succession and its response to climate change. Nutrients determined the phytoplankton biomass and dominant species succession based on various proportions. High NH4N:NO3N ratios favored cyanobacteria and inhibited diatom under high TSS. Additionally, the biomass proportions of diatom (30.77 % vs. 22.28 %) and green (30.56 % vs. 23.30 %) decreased dramatically. In contrast, cyanobacteria abundance remarkably increased (35.78 % to 51.71 %) with the increasing NH4-N:NO3-N ratios. In addition, the proportion of non-nitrogen-fixing cyanobacteria was higher than that of the nitrogen-fixing cyanobacteria counterparts when TN:TP≥20 and NH4N:NO3N ≥ 10. Light-limitation phenotypes also experienced an increase with the rising NH4N:NO3N ratios. Notably, the cyanobacterial biomass reached 3-6 times that in the baseline scenario when the air temperature escalated by 3.0 °C until 2061 under the SSP585 scenario. We highlighted the effect of nitrogen forms on the succession of dominant phytoplankton species. Climate warming will increase nitrogen proportion, providing an insightful reference for controlling cyanobacterial blooms.

No universal mathematical model for thermal performance curves across traits and taxonomic groups

In ectotherms, the performance of physiological, ecological and life-history traits universally increases with temperature to a maximum before decreasing again. Identifying the most appropriate thermal performance model for a specific trait type has broad applications, from metabolic modelling at the cellular level to forecasting the effects of climate change on population, ecosystem and disease transmission dynamics. To date, numerous mathematical models have been designed, but a thorough comparison among them is lacking. In particular, we do not know if certain models consistently outperform others and how factors such as sampling resolution and trait or organismal identity influence model performance. To fill this knowledge gap, we compile 2,739 thermal performance datasets from diverse traits and taxa, to which we fit a comprehensive set of 83 existing mathematical models. We detect remarkable variation in model performance that is not primarily driven by sampling resolution, trait type, or taxonomic information. Our results reveal a surprising lack of well-defined scenarios in which certain models are more appropriate than others. To aid researchers in selecting the appropriate set of models for any given dataset or research objective, we derive a classification of the 83 models based on the average similarity of their fits.

Temporal-Spatial Fluctuations of a Phytoplankton Community and Their Association with Environmental Variables Based on Classification and Regression Tree in a Shallow Temperate Mountain River

The effects of environmental factors on phytoplankton are not simply positive or negative but complex and dependent on the combination of their concentrations in a fluctuating environment. Traditional statistical methods may miss some of the complex interactions between the environment and phytoplankton. In this study, the temporal-spatial fluctuations of phytoplankton diversity and abundance were investigated in a shallow temperate mountain river. The machine learning method classification and regression tree (CART) was used to explore the effects of environmental variables on the phytoplankton community. The results showed that both phytoplankton species diversity and abundance varied fiercely due to environmental fluctuation. Microcystis aeruginosa, Amphiprora sp., Anabaena oscillarioides, and Gymnodinium sp. were the dominant species. The CART analysis indicated that dissolved oxygen, oxidation-reduction potential, total nitrogen (TN), total phosphorus (TP), and water temperature (WT) explained 36.00%, 13.81%, 11.35%, 9.96%, and 8.80%, respectively, of phytoplankton diversity variance. Phytoplankton abundance was mainly affected by TN, WT, and TP, with variance explanations of 39.40%, 15.70%, and 14.09%, respectively. Most environmental factors had a complex influence on phytoplankton diversity and abundance: their effects were positive under some conditions but negative under other combinations. The results and methodology in this study are important in quantitatively understanding and exploring aquatic ecosystems.

Experimental warming promotes phytoplankton species sorting towards cyanobacterial blooms and leads to potential changes in ecosystem functioning

A positive feedback loop where climate warming enhances eutrophication and its manifestations (e.g., cyanobacterial blooms) has been recently highlighted, but its consequences for biodiversity and ecosystem functioning are not fully understood. We conducted a highly replicated indoor experiment with a species-rich subtropical freshwater phytoplankton community. The experiment tested the effects of three constant temperature scenarios (17, 20, and 23 °C) under high-nutrient supply conditions on community composition and proxies of ecosystem functioning, namely resource use efficiency (RUE) and CO2 fluxes. After 32 days, warming reduced species richness and promoted different community trajectories leading to a dominance by green algae in the intermediate temperature and by cyanobacteria in the highest temperature treatments. Warming promoted primary production, with a 10-fold increase in the mean biomass of green algae and cyanobacteria. The maximum RUE occurred under the warmest treatment. All treatments showed net CO2 influx, but the magnitude of influx decreased with warming. We experimentally demonstrated direct effects of warming on phytoplankton species sorting, with negative effects on diversity and direct positive effects on cyanobacteria, which could lead to potential changes in ecosystem functioning. Our results suggest potential positive feedback between the phytoplankton blooms and warming, via lower net CO2 sequestration in cyanobacteria-dominated, warmer systems, and add empirical evidence to the need for decreasing the likelihood of cyanobacterial dominance.

Phytoplankton in headwater streams: spatiotemporal patterns and underlying mechanisms

Phytoplankton are key members of river ecosystems wherein they influence and regulate the health of the local environment. Headwater streams are subject to minimal human activity and serve as the sources of rivers, generally exhibiting minimal pollution and strong hydrodynamic forces. To date, the characteristics of phytoplankton communities in headwater streams have remained poorly understood. This study aims to address this knowledge gap by comparing phytoplankton communities in headwater streams with those in plain rivers. The results demonstrated that within similar watershed sizes, lower levels of spatiotemporal variability were observed with respect to phytoplankton community as compared to plain rivers. Lower nutrient levels and strong hydrodynamics contribute to phytoplankton growth limitation in these streams, thereby reducing the levels of spatiotemporal variation. However, these conditions additionally contribute to greater phytoplankton diversity and consequent succession towards Cyanophyta. Overall, these results provide new insights into the dynamics of headwater stream ecosystems and support efforts for their ecological conservation.

Does water temperature influence in microcystin production? A case study of Billings Reservoir, São Paulo, Brazil

We investigated the relationship between some water quality parameters and microcystin, chlorophyll-a, and cyanobacteria in different conditions of water temperature. We also proposed to predict chlorophyll-a concentration in the Billings Reservoir using three machine learning techniques. Our results indicate that in the condition of higher water temperatures with high density of cyanobacteria, microcystin concentration can increase severely (>10² μg/L). Besides the magnitude observed in higher concentrations, in water temperatures above 25.3 °C (classified as high extreme event), higher frequencies of inadequate values of microcystin (87.5%), chlorophyll-a (70%), and cyanobacteria (82.5%) compared to cooler temperatures (<19.6 °C) were observed. The prediction of chlorophyll-a in Billings Reservoir presented good results (0.76 ≤ R² ≤ 0.82; 0.17 ≤ RMSE≤0.20) using water temperature, total phosphorus, and cyanobacteria as predictors, with the best result using Support Vector Machine.

Predicting effects of multiple interacting global change drivers across trophic levels

Global change encompasses many co-occurring anthropogenic drivers, which can act synergistically or antagonistically on ecological systems. Predicting how different global change drivers simultaneously contribute to observed biodiversity change is a key challenge for ecology and conservation. However, we lack the mechanistic understanding of how multiple global change drivers influence the vital rates of multiple interacting species. We propose that reaction norms, the relationships between a driver and vital rates like growth, mortality, and consumption, provide insights to the underlying mechanisms of community responses to multiple drivers. Understanding how multiple drivers interact to affect demographic rates using a reaction-norm perspective can improve our ability to make predictions of interactions at higher levels of organization-that is, community and food web. Building on the framework of consumer-resource interactions and widely studied thermal performance curves, we illustrate how joint driver impacts can be scaled up from the population to the community level. A simple proof-of-concept model demonstrates how reaction norms of vital rates predict the prevalence of driver interactions at the community level. A literature search suggests that our proposed approach is not yet used in multiple driver research. We outline how realistic response surfaces (i.e., multidimensional reaction norms) can be inferred by parametric and nonparametric approaches. Response surfaces have the potential to strengthen our understanding of how multiple drivers affect communities as well as improve our ability to predict when interactive effects emerge, two of the major challenges of ecology today.

Phytoplankton competition and resilience under fluctuating temperature

Environmental variability is an inherent feature of natural systems which complicates predictions of species interactions. Primarily, the complexity in predicting the response of organisms to environmental fluctuations is in part because species' responses to abiotic factors are non-linear, even in stable conditions. Temperature exerts a major control over phytoplankton growth and physiology, yet the influence of thermal fluctuations on growth and competition dynamics is largely unknown. To investigate the limits of coexistence in variable environments, stable mixed cultures with constant species abundance ratios of the marine diatoms, Phaeodactylum tricornutum and Thalassiosira pseudonana, were exposed to different temperature fluctuation regimes (n = 17) under high and low nitrogen (N) conditions. Here we demonstrate that phytoplankton exhibit substantial resilience to temperature variability. The time required to observe a shift in the species abundance ratio decreased with increasing fluctuations, but coexistence of the two model species under high N conditions was disrupted only when amplitudes of temperature fluctuation were high (±8.2°C). N limitation caused the thermal amplitude for disruption of species coexistence to become lower (±5.9°C). Furthermore, once stable conditions were reinstated, the two species differed in their ability to recover from temperature fluctuations. Our findings suggest that despite the expectation of unequal effect of fluctuations on different competitors, cycles in environmental conditions may reduce the rate of species replacement when amplitudes remain below a certain threshold. Beyond these thresholds, competitive exclusion could, however, be accelerated, suggesting that aquatic heatwaves and N availability status are likely to lead to abrupt and unpredictable restructuring of phytoplankton community composition.

Effects of dredging activities and seasonal variation on coastal plankton assemblages: results from 10 years of environmental monitoring

Coastal zones support the most productive marine ecosystems, yet they are increasingly threatened by anthropogenic stressors such as dredging. In this study, we investigated how seasonal variation and dredging activities conducted during the construction of a harbor and submarine base (Sepetiba Bay, RJ, Brazil) affected the phytoplankton and zooplankton assemblages. The observed temporal variability at five different sites over 10 years revealed that dredging exceeds the expected influence of dry and rainy seasons on plankton abundance and diversity. In general, the abundance of both groups increased during dredging due to the resuspension of nutrients and benthic organisms. This increase was particularly evident in the dinoflagellate Scrippsiellaa cuminata, the diatoms Thalassiosira rotula and Nitzschia longissima, and the herbivorous zooplankton Acartia clausii and Pseudevadne tergestina. Moreover, season and dredging activities synergistically influenced plankton assemblages, resulting in larger seasonal variations during dredging activities. After the end of the harbor construction, plankton abundance decreased and remained low until the end of the monitoring, which may indicate persistent changes in the biodiversity and ecosystem functioning of impacted areas.

Fluctuation at High Temperature Combined with Nutrients Alters the Thermal Dependence of Phytoplankton

The Metabolic Theory of Ecology (MTE) predicts that the temperature increases exert a common effect on organisms stimulating metabolic rates, this being stronger for a heterotrophic than for an autotrophic metabolism. However, no available studies within the MTE framework have focused on organisms' response under fluctuation at high temperature interacting with factors such as nutrient availability, or how this interaction could affect the coexistence between mixotrophic and strict autotrophic phytoplankton. Hence, we assess how the phytoplankton metabolism and species composition are affected under scenarios of high temperature and fluctuation at high temperature, and how nutrients alter the direction and magnitude of such impact. For that, we use a mixed culture composed of two phytoplankton species: a strict autotrophic species and a mixotrophic species. Our results indicate that, in agreement with the MTE, only fluctuation at high temperature treatment registered a greater activation energy (E_a) value for respiration than for primary production and stimulated mixotrophic over strict autotrophic species abundance compared to control treatment. Remarkably, fluctuation at high temperature had a strong negative impact on the total abundance of the mixed-culture. The interaction between nutrient enrichment and fluctuation at high temperature increased abundance of the strict autotrophic species and overall species abundance, and led to Ea values that were higher in primary production than in respiration. Changes in community composition, enhanced by nutrient enrichment, could be behind this response, which can have implications in ecosystem functioning in a changing world.

Physiological Consequences of Oceanic Environmental Variation: Life from a Pelagic Organism's Perspective

To better understand life in the sea, marine scientists must first quantify how individual organisms experience their environment, and then describe how organismal performance depends on that experience. In this review, we first explore marine environmental variation from the perspective of pelagic organisms, the most abundant life forms in the ocean. Generation time, the ability to move relative to the surrounding water (even slowly), and the presence of environmental gradients at all spatial scales play dominant roles in determining the variation experienced by individuals, but this variation remains difficult to quantify. We then use this insight to critically examine current understanding of the environmental physiology of pelagic marine organisms. Physiologists have begun to grapple with the complexity presented by environmental variation, and promising frameworks exist for predicting and/or interpreting the consequences for physiological performance. However, new technology needs to be developed and much difficult empirical work remains, especially in quantifying response times to environmental variation and the interactions among multiple covarying factors. We call on the field of global-change biology to undertake these important challenges.

Phytoplankton biodiversity is more important for ecosystem functioning in highly variable thermal environments

The 21st century has seen an acceleration of anthropogenic climate change and biodiversity loss, with both stressors deemed to affect ecosystem functioning. However, we know little about the interactive effects of both stressors and in particular about the interaction of increased climatic variability and biodiversity loss on ecosystem functioning. This should be remedied because larger climatic variability is one of the main features of climate change. Here, we demonstrated that temperature fluctuations led to changes in the importance of biodiversity for ecosystem functioning. We used microcosm communities of different phytoplankton species richness and exposed them to a constant, mild, and severe temperature-fluctuating environment. Wider temperature fluctuations led to steeper biodiversity-ecosystem functioning slopes, meaning that species loss had a stronger negative effect on ecosystem functioning in more fluctuating environments. For severe temperature fluctuations, the slope increased through time due to a decrease of the productivity of species-poor communities over time. We developed a theoretical competition model to better understand our experimental results and showed that larger differences in thermal tolerances across species led to steeper biodiversity-ecosystem functioning slopes. Species-rich communities maintained their ecosystem functioning with increased fluctuation as they contained species able to resist the thermally fluctuating environments, while this was on average not the case in species-poor communities. Our results highlight the importance of biodiversity for maintaining ecosystem functions and services in the context of increased climatic variability under climate change.

Responses of Marine Diatom-Dinoflagellate Competition to Multiple Environmental Drivers: Abundance, Elemental, and Biochemical Aspects

Ocean-related global change has strongly affected the competition between key marine phytoplankton groups, such as diatoms and dinoflagellates, especially with the deleterious consequency of the increasing occurrence of harmful algal blooms. The dominance of diatoms generally shifts toward that of dinoflagellates in response to increasing temperature and reduced nutrient availability; however, contradictory findings have also been observed in certain sea areas. A key challenge in ecology and biogeochemistry is to quantitatively determine the effects of multiple environmental factors on the diatom-dinoflagellate community and the related changes in elemental and biochemical composition. Here, we test the interplay between temperature, nutrient concentrations and their ratios on marine diatom-dinoflagellate competition and chemical composition using bi-algal competition experiments. The ubiquitous diatom Phaeodactylum tricornutum and dinoflagellate Prorocentrum minimum were cultivated semi-continuously, provided with different N and P concentrations (three different levels) and ratios (10:1, 24:1, and 63:1 molar ratios) under three temperatures (12, 18, and 24°C). The responses of diatom-dinoflagellate competition were analyzed by a Lotka-Volterra model and quantified by generalized linear mixed models (GLMMs) and generalized additive models (GAMs). The changes in nutrient concentrations significantly affected diatom-dinoflagellate competition, causing a competitive superiority of the diatoms at high nutrient concentrations, independent of temperature and N:P supply ratios. Interestingly, the effect amplitude of nutrient concentrations varied with different temperatures, showing a switch back toward a competitive superiority of the dinoflagellates at the highest temperature and at very high nutrient concentrations. The ratios of particulate organic nitrogen to phosphorus showed significant negative correlations with increasing diatoms/dinoflagellates ratios, while lipid biomarkers (fatty acids and sterols) correlated positively with increasing diatoms/dinoflagellates ratios over the entire ranges of temperature, N and P concentrations and N:P ratios. Our results indicate that the integration of phytoplankton community structure and chemical composition provides an important step forward to quantitatively understand and predict how phytoplankton community changes affect ecosystem functions and biogeochemical cycles in the ocean.

Meta-analysis of the response of marine phytoplankton to nutrient addition and seawater warming

As an indispensable part of the marine ecosystem, phytoplankton are important prey for zooplankton and various marine animals with important commercial value. The influence of seawater warming and eutrophication on phytoplankton communities is well known, but few studies have explained the effects of the interaction between temperature and nutrients on marine phytoplankton. Through meta-analysis and meta-regression, the phytoplankton responses to the effects of nutrient addition and seawater warming were evaluated in this study. Nitrogen (N) addition led to an increase in phytoplankton biomass, while phosphorus (P) had no significant effect on phytoplankton biomass. However, this result may be biased by the uneven distribution of the research area. N limitation is widespread in the areas where these collected studies were conducted, including many parts of North and South Atlantic and West Pacific Oceans. The key limiting nutrient in other areas lacking corresponding experiments, however, remain unclear. The effect of seawater warming was not significant, which indicates the uncertainty about the effect of temperature on phytoplankton. The results of ANOVA show that nutrient addition and seawater warming had similar effects in various marine habitats (coastal regions, estuaries and open seas), while salinity could have caused the difference in the N effects among the three habitats. Furthermore, our results showed that the impact of temperature depends on nutrient conditions, especially N status, which has rarely been considered before. This result demonstrated the importance of evaluating nutrient limitation patterns when studying climate warming. The impact of rising temperatures may need to be reevaluated because N limitation is common.

Beyond nitrogen: phosphorus - estimating the minimum niche dimensionality for resource competition between phytoplankton

The niche dimensionality required for coexistence is often discussed in terms of the number of limiting resources. N and P limitation are benchmarks for studying phytoplankton interactions. However, it is generally agreed that limitation by small numbers of resources cannot explain the high phytoplankton diversity observed in nature. Here, we parameterised resource competition models using experimental data for six phytoplankton species grown in monoculture with nine potential limiting resources. We tested predicted species biomass from these models against observations in two-species experimental mixtures. Uptake rates were similar across species, following the classic Redfield ratio. Model accuracy levelled out at around three to five resources suggesting the minimum dimensionality of this system. The models included the resources Fe, Mg, Na and S. Models including only N and P always performed poorly. These results suggest that high-dimensional information about resource limitation despite stoichiometric constraints may be needed to accurately predict community assembly.

Anti-Fouling Effects of Saponin-Containing Crude Extracts from Tropical Indo-Pacific Sea Cucumbers

Sea cucumbers are bottom dwelling invertebrates, which are mostly found on subtropical and tropical sea grass beds, sandy reef flats, or reef slopes. Although constantly exposed to fouling communities in these habitats, many species are surprisingly free of invertebrate epibionts and microfouling algae such as diatoms. In our study, we investigated the anti-fouling (AF) activities of different crude extracts of tropical Indo-Pacific sea cucumber species against the fouling diatom Cylindrotheca closterium. Nine sea cucumber species from three genera (i.e., Holothuria, Bohadschia, Actinopyga) were selected and extracted to assess their AF activities. To verify whether the sea cucumber characteristic triterpene glycosides were responsible for the observed potent AF activities, we tested purified fractions enriched in saponins isolated from Bohadschia argus, representing one of the most active anti-fouling extracts. Saponins were quantified by vanillin-sulfuric acid colorimetric assays and identified by LC-MS and LC-MS/MS analyses. We were able to demonstrate that AF activities in sea cucumber extracts were species-specific, and growth inhibition as well as attachment of the diatom to surfaces is dependent on the saponin concentration (i.e., Actinopyga contained the highest quantities), as well as on the molecular composition and structure of the present saponins (i.e., Bivittoside D derivative was the most bioactive compound). In conclusion, the here performed AF assay represents a promising and fast method for selecting the most promising bioactive organism as well as for identifying novel compounds with potent AF activities for the discovery of potentially novel pharmacologically active natural products.