Reconstructing large interaction networks from empirical time series data.

Reconstructing interactions from observational data is a critical need for investigating natural biological networks, wherein network dimensionality is usually high. However, these pose a challenge to existing methods that can quantify only small interaction networks. Here, we proposed a novel approach to reconstruct high-dimensional interaction Jacobian networks using empirical time series without specific model assumptions. This method, named "multiview distance regularised S-map," generalised the state space reconstruction to accommodate high dimensionality and overcome difficulties in quantifying massive interactions with limited data. When evaluating this method using time series generated from theoretical models involving hundreds of interacting species, estimated strengths of interaction Jacobians were in good agreement with theoretical expectations. Applying this method to a natural bacterial community helped identify important species from the interaction network and revealed mechanisms governing the dynamical stability of a bacterial community. The proposed method overcame the challenge of high dimensionality in large natural dynamical systems.

Body Size, Light Intensity, and Nutrient Supply Determine Plankton Stoichiometry in Mixotrophic Plankton Food Webs.

Trophic strategy determines stoichiometry of plankton. In general, heterotrophic zooplankton have lower and more stable C∶N and C∶P ratios than photoautotrophic phytoplankton, whereas mixotrophic protists, which consume prey and photosynthesize, have stoichiometry between zooplankton and phytoplankton. As trophic strategies change with cell size, body size may be a key trait influencing eukaryotic plankton stoichiometry. However, the relationship between body size and stoichiometry remains unclear. Here we measured plankton size-fractionated C∶N ratios under different intensities of light and nutrient supply in subtropical freshwater and marine systems. We found a unimodal body size-C∶N ratio pattern, with a maximum C∶N ratio at ∼50 µm diameter in marine and freshwater systems. Moreover, the variation in C∶N ratios is explained mainly by body size, followed by light intensity and nutrient concentration. To investigate the mechanisms behind this unimodal pattern, we constructed a size-based plankton food web model in which the trophic strategy and C∶N ratio are an emerging result. Our model simulations reproduce the unimodal pattern with a C∶N ratio of photoautotrophs ≤50 µm increasing with body size due to increase of photosynthetic carbon, whereas C∶N ratios of organisms >50 µm decrease with size due to decreasing photoautotrophic but increasing heterotrophic uptake. Based on our field observations and simulation, we extend the classic "light-nutrient" theory that determines plankton C∶N ratio to include body size and trophic strategy dependency. We conclude that body size and size-dependent uptake of resources (light, nutrients, and prey) determine plankton stoichiometry at various light and nutrient supplies.

Long-term warming destabilizes aquatic ecosystems through weakening biodiversity-mediated causal networks.

Understanding how ecosystems will respond to climate changes requires unravelling the network of functional responses and feedbacks among biodiversity, physicochemical environments, and productivity. These ecosystem components not only change over time but also interact with each other. Therefore, investigation of individual relationships may give limited insights into their interdependencies and limit ability to predict future ecosystem states. We address this problem by analyzing long-term (16-39 years) time series data from 10 aquatic ecosystems and using convergent cross mapping (CCM) to quantify the causal networks linking phytoplankton species richness, biomass, and physicochemical factors. We determined that individual quantities (e.g., total species richness or nutrients) were not significant predictors of ecosystem stability (quantified as long-term fluctuation of phytoplankton biomass); rather, the integrated causal pathway in the ecosystem network, composed of the interactions among species richness, nutrient cycling, and phytoplankton biomass, was the best predictor of stability. Furthermore, systems that experienced stronger warming over time had both weakened causal interactions and larger fluctuations. Thus, rather than thinking in terms of separate factors, a more holistic network view, that causally links species richness and the other ecosystem components, is required to understand and predict climate impacts on the temporal stability of aquatic ecosystems.

Nutrient pulses driven by internal solitary waves enhance heterotrophic bacterial growth in the South China Sea.

This study demonstrated the potential effects of internal waves (IWs) on heterotrophic bacterial activities for the first time. Nine anchored studies were conducted from 2009-2012 in the South China Sea areas with different physical conditions, i.e. areas subjected to elevation IWs, to depression IWs, and to weak/no IWs. The latter two areas were treated as the Control sites. Field survey results indicated that within the euphotic zone, the minima of the depth-averaged bacterial production (IBP; ∼1.0 mgC m-3 d-1 ) and growth rate (IBµ; ∼0.1 d-1 ) at all sites were similar. Except for one case, the maxima of IBP (6-12 mgC m-3 d-1 ) and IBµ (0.55-1.13 d-1 ) of the elevation IWs areas were ∼fivefolds higher than those of the Control sites (IBP 1.7-2.1 mgC m-3 d-1 ; IBµ 0.13-0.24 d-1 ). Replicate surveys conducted at the north-western area of the Dongsha atoll during spring-to-neap (NW1 survey) and neap-to-spring (NW2 survey) tide periods showed a great contrast to each other. Low variation and averages of IBµ in NW1 survey were similar to those of the Control sites, while those in NW2 were similar to the other elevation IWs sites with larger variation and higher averages of IBµ. This finding suggests that bacterial activities may be a function of the lunar fortnightly (14-day) cycle. Enrichment experiments suggested more directly that the limiting inorganic nutrients introduced by the elevation waves (EIWs) may contribute a higher IBµ within the euphotic zone.

Prokaryotic assemblages and metagenomes in pelagic zones of the South China Sea.

BACKGROUND: Prokaryotic microbes, the most abundant organisms in the ocean, are remarkably diverse. Despite numerous studies of marine prokaryotes, the zonation of their communities in pelagic zones has been poorly delineated. By exploiting the persistent stratification of the South China Sea (SCS), we performed a 2-year, large spatial scale (10, 100, 1000, and 3000 m) survey, which included a pilot study in 2006 and comprehensive sampling in 2007, to investigate the biological zonation of bacteria and archaea using 16S rRNA tag and shotgun metagenome sequencing. RESULTS: Alphaproteobacteria dominated the bacterial community in the surface SCS, where the abundance of Betaproteobacteria was seemingly associated with climatic activity. Gammaproteobacteria thrived in the deep SCS, where a noticeable amount of Cyanobacteria were also detected. Marine Groups II and III Euryarchaeota were predominant in the archaeal communities in the surface and deep SCS, respectively. Bacterial diversity was higher than archaeal diversity at all sampling depths in the SCS, and peaked at mid-depths, agreeing with the diversity pattern found in global water columns. Metagenomic analysis not only showed differential %GC values and genome sizes between the surface and deep SCS, but also demonstrated depth-dependent metabolic potentials, such as cobalamin biosynthesis at 10 m, osmoregulation at 100 m, signal transduction at 1000 m, and plasmid and phage replication at 3000 m. When compared with other oceans, urease at 10 m and both exonuclease and permease at 3000 m were more abundant in the SCS. Finally, enriched genes associated with nutrient assimilation in the sea surface and transposase in the deep-sea metagenomes exemplified the functional zonation in global oceans. CONCLUSIONS: Prokaryotic communities in the SCS stratified with depth, with maximal bacterial diversity at mid-depth, in accordance with global water columns. The SCS had functional zonation among depths and endemically enriched metabolic potentials at the study site, in contrast to other oceans.

Linking secondary structure of individual size distribution with nonlinear size-trophic level relationship in food webs.

Existing individual size distribution (ISD) theories assume that the trophic level (TL) of an organism varies as a linear function of its log-transformed body size. This assumption predicts a power-law distribution of the ISD, i.e., a linear relationship between size and abundance in log space. However, the secondary structure of ISD (nonlinear dome shape structures deviating from a power-law distribution) is often observed. We propose a model that extends the metabolic theory to link the secondary structure of ISD to the nonlinear size-TL relationship. This model is tested with empirical data collected from a subtropical reservoir. The empirical ISD and size-TL relationships were constructed by FlowCAM imaging analysis and stable isotope analyses, respectively. Our results demonstrate that the secondary structure of ISD can be predicted from the nonlinear function of size-TL relationship and vice versa. Moreover, these secondary structures arise due to (1) zooplankton omnivory and (2) the trophic interactions within microbial food webs.

Deterministic Assembly Processes Strengthen the Effects of ß-Diversity on Community Biomass of Marine Bacterioplankton.

The presence of more species in the community of a sampling site (α diversity) typically increases ecosystem functions via nonrandom processes like resource partitioning. When considering multiple communities, we hypothesize that higher compositional difference (ß diversity) increases overall functions of these communities. Further, we hypothesize that the ß diversity effect is more positive when ß diversity is increased by nonrandom assembly processes. To test these hypotheses, we collected bacterioplankton along a transect of 6 sampling sites in the southern East China Sea in 14 cruises. For any pairs of the 6 sites within a cruise, we calculated the Bray-Curtis index to represent ß diversity and summed bacterial biomass as a proxy to indicate the overall function of the two communities. We then calculated deviation of observed mean pairwise phylogenetic similarities among species in two communities from random to represent the influences of nonrandom processes. The bacterial ß diversity was found to positively affect the summed bacterial biomass; however, the effect varied among cruises. Cross-cruise comparison indicated that the ß diversity effect increased with the nonrandom processes selecting for phylogenetically dissimilar species. This study extends biodiversity-ecosystem functioning research to the scale of multiple sites and enriches the framework by considering community assembly processes. IMPORTANCE The implications of our analyses are twofold. First, we emphasize the importance of studying ß diversity. We expanded the current biodiversity-ecosystem functioning framework from single to multiple sampling sites and investigated the influences of species compositional differences among sites on the overall functioning of these sites. Since natural ecological communities never exist alone, our analyses allow us to more holistically perceive the role of biodiversity in natural ecosystems. Second, we took community assembly processes into account to attain a more mechanistic understanding of the impacts of biodiversity on ecosystem functioning.

Allometric scaling of interspecific RNA transcript abundance to extend the use of metatranscriptomics in characterizing complex communities.

Metatranscriptomics allows profiling of community mRNA and rRNA transcript abundance under certain environmental conditions. However, variations in the proportion of RNA transcripts across different community size structures remain less explained, thus limiting the possible applications of metatranscriptomics in community studies. Here, we extended the assumptions of the growth-rate hypothesis (GRH) and the metabolic theory of ecology (MTE) to validate the allometric scaling of interspecific RNA transcript (mRNA and rRNA) abundance through metatranscriptomic analysis of mock communities consisting of model organisms. The results suggest that body size imposes significant constraints on RNA transcript abundance. Interestingly, the relationship between the total mitochondrial transcript abundance (mRNA and rRNA slopes were -0.30 and -0.28, respectively) and body size aligned with the MTE assumptions with slopes close to -», while the nuclear transcripts displayed much steeper slopes (mRNA and rRNA slopes were -0.33 and -0.40, respectively). The assumed temperature dependence was not observed in this study. At the gene level, the allometric slopes range from 0 to -1. Overall, the above results showed that larger individuals have lesser RNA transcript abundance per tissue mass than smaller ones regardless of temperature. Analyses of field-collected microcrustacean zooplankton samples demonstrated that the correction of size effect, using the allometric exponents derived from the model organism mock community, explains better the patterns of interspecific RNA transcripts abundance within the metatranscriptome. Integrating allometry with metatranscriptomics can extend the use of RNA transcript reads in estimating ecological processes within complex communities.

Trade-Offs between Competitive Ability and Resistance to Top-Down Control in Marine Microbes.

Trade-offs between competitive ability and resistance to top-down control manifest the "kill-the-winner" hypothesis that explains how mortality caused by protists and viruses can promote bacterial diversity. However, the existence of such trade-offs has rarely been investigated in natural marine bacterial communities. To address this question, we conducted on-board dilution experiments to manipulate top-down control pressure (protists only or protists plus viruses [protists+viruses] combined) and then applied 16S rRNA gene high-throughput sequencing techniques to assess the responses of each bacterial taxon. Dilution experiments enabled us to measure the top-down-control-free growth rate as the competitive ability and top-down-control-caused mortality as the reverse of resistance to top-down control. Overall, bacterial taxa with higher top-down-control-free growth rates were accompanied by lower top-down-control-caused resistance. Furthermore, competition-resistance trade-offs were stronger and more consistent when top-down control was caused by protists+viruses combined than by protists only. When protists+viruses were diluted, the bacterial rank abundance distribution became steepened and evenness and richness were decreased. However, when protists were diluted, only richness decreased. Our results indicate the existence of competition-resistance trade-offs in marine microbes and demonstrate the positive impacts of such trade-offs on bacterial diversity. Regardless, the strength of the competition-resistance trade-offs and the impacts on bacterial diversity were contingent on whether top-down control was caused by protists+viruses combined or protists only. IMPORTANCE We addressed the "kill-the-winner" hypothesis from the perspective of its principle (the competition-resistance trade-off) in marine bacterial communities incubated in situ. Our results supported the existence of competition-resistance trade-offs and the positive effect on bacterial community diversity. The study linked theoretical expectations and complex natural systems and provided new knowledge regarding how top-down controls and competition trade-offs shaped natural bacterial communities.

Effects of Tropical Cyclone Passage on Plankton Community Respiration in a Phosphate-Limited Freshwater Ecosystem.

Plankton community respiration (CR) in aquatic ecosystems varies with environmental factors, which could be altered during tropical cyclones (TCs). A potential increase in CR resulting from the effects of TCs is generally understudied. Here, we examined the relationship between plankton CR and environmental factors, including during TC-affected periods, in a phosphate-limited freshwater ecosystem. We conducted an intensive in situ sampling in Fei-Tsui Reservoir (FTR) from January 2010 to December 2015 during TC periods and non-TC periods. Our results showed a consistent temporal pattern that plankton CR increased between March to October and declined between November to February. These changes in plankton CR, primarily supported by bacterial biomass, were positively influenced by euphotic depth-averaged temperature. The CR also significantly increased with euphotic depth-averaged NO2-concentrations and decreased with euphotic depth-averaged NO3-concentrations. These results indicated that these factors typically influenced CR dynamics in the FTR. During TC periods, plankton CR was increased further due to a higher and ideal euphotic depth-averaged temperature (23-27°C) and increased supply of limiting nutrient resources via stream runoff. Overall, this study showed that a TC positively influences plankton CR by creating favorable water conditions. Notably, with a higher frequency of intense TCs projected for the Western North Pacific in most climate change scenarios, the impact of TCs on CR may increase in the near future.

Transition of carbon-nitrogen coupling under different anthropogenic disturbances in subtropical small mountainous rivers.

The commonly observed inverse relationship between dissolved organic carbon (DOC) and nitrate (NO3-) concentrations in aquatic systems can be explained by stoichiometric and thermodynamic principles regulating microbial assimilation and dissimilation processes. However, the interactive effects of human activities and dissolved oxygen (DO) on the DOC and DIN (dissolved inorganic nitrogen, mainly composed of NO3--N and NH4+-N) relations are not well identified, particularly in subtropical small mountainous rivers (SMRs). Here, we investigated the exports and relations of DOC-DIN in 42 Taiwan SMRs under different anthropogenic disturbances. Results showed that the island-wide mean concentrations of the three solutes in streams are generally low, yet the abundant rainfall and persistent supply contrarily lead to disproportional high DOC and DIN yields. The inverse DOC-NO3--N relation does not appear under well­oxygenated conditions, regardless of low or high human disturbance. However, a significant inverse relationship between DOC-NO3--N would emerge in highly-disturbed watersheds under low-oxygenated conditions (mean annual DO <6.5 mg L-1), where excess N accumulates as NH4+-N rather than NO3--N. The controlling mechanism of DOC-DIN relations would shift from energetic constraints to redox constraints in low-oxygenated conditions. Although riverine concentrations of DOC, NO3--N, and NH4+-N could be elevated by human activities, the transition of DOC-DIN relation pattern is directly linked to DO availability. Understanding the mechanism that drives CN coupling is critical for assessing the ecosystem function in the delivery and retention of DOC and DIN in aquatic ecosystems.

New index of functional specificity to predict the redundancy of ecosystem functions in microbial communities.

An ecosystem function is suggested to be more sensitive to biodiversity loss (i.e. low functional redundancy) when focusing on specific-type functions than broad-type functions. Thus far, specific-type functions have been loosely defined as functions performed by a small number of species (facilitative species) or functions involved in utilizing complex substrates. However, quantitative examination of functional specificity remains underexplored. We quantified the functional redundancy of 33 ecosystem functions in a freshwater system from 76 prokaryotic community samples over 3 years. For each function, we used a sparse regression model to estimate the number of facilitative Amplicon Sequence Variants (ASVs) and to define taxon-based functional specificity. We also used Bertz structural complexity to determine substrate-based functional specificity. We found that functional redundancy increased with the taxon-based functional specificity, defined as the proportion of facilitative ASVs (= facilitative ASV richness/facilitative ASV richness + repressive ASV (ASVs reducing functioning) richness). When using substrate-based functional specificity, functional redundancy was influenced by Bertz complexity per se and by substrate acquisition mechanisms. Therefore, taxon-based functional specificity is a better predictive index for evaluating functional redundancy than substrate-based functional specificity. These findings provide a framework to quantitatively predict the consequences of diversity losses on ecosystem functioning.

Viral shunt in tropical oligotrophic ocean.

Viruses cause massive bacterial mortality and thus modulate bacteria-governed carbon transfer and nutrient recycling at global scale. The viral shunt hypothesis states the crucial role of viral lysis in retaining microbial carbon into food web processes, while its applicability to nature has not been well identified for over two decades. Here, we conducted nine diel surveys in the tropical South China Sea and suggested that the time scale adopted in sampling and system trophic status determine the "visibility" of the viral shunt in the field. Specifically, viral abundance (VA), bacterial biomass (BB), and bacterial specific growth rate (SGR) varied synchronously and presented the significant VA-BB and VA-SGR linkages at an hourly scale, which reveals direct interactions between viruses and their hosts. The differential responses of the viral shunt to temperature, i.e., looser VA-SGR coupling in warm and tighter VA-SGR coupling in cold environments, imply an altered carbon cycling in tropical oceans under climatic warming.

Using metatranscriptomics to estimate the diversity and composition of zooplankton communities.

DNA metabarcoding is a rapid, high-resolution tool used for biomonitoring complex zooplankton communities. However, diversity estimates derived with this approach can be biased by the co-detection of sequences from environmental DNA (eDNA), nuclear-encoded mitochondrial (NUMT) pseudogene contamination, and taxon-specific PCR primer affinity differences. To avoid these methodological uncertainties, we tested the use of metatranscriptomics as an alternative approach for characterizing zooplankton communities. Specifically, we compared metatranscriptomics with PCR-based methods using genomic (gDNA) and complementary DNA (cDNA) amplicons, and morphology-based data for estimating species diversity and composition for both mock communities and field-collected samples. Mock community analyses showed that the use of gDNA mitochondrial cytochrome c oxidase I (mtCO1) amplicons inflates species richness due to the co-detection of extra-organismal eDNA. Significantly more amplicon sequence variants, nucleotide diversity, and indels were observed with gDNA amplicons than with cDNA, indicating the presence of putative NUMT pseudogenes. Moreover, PCR-based methods failed to detect the most abundant species in mock communities due to priming site mismatch. Overall, metatranscriptomics provided estimates of species richness and composition that closely resembled those derived from morphological data. The use of metatranscriptomics was further tested using field-collected samples, with the results showing consistent species diversity estimates among biological and technical replicates. Additionally, temporal zooplankton species composition changes could be monitored using different mitochondrial markers. These findings demonstrate the advantages of metatranscriptomics as an effective tool for monitoring diversity in zooplankton research.

Causal networks of phytoplankton diversity and biomass are modulated by environmental context.

Untangling causal links and feedbacks among biodiversity, ecosystem functioning, and environmental factors is challenging due to their complex and context-dependent interactions (e.g., a nutrient-dependent relationship between diversity and biomass). Consequently, studies that only consider separable, unidirectional effects can produce divergent conclusions and equivocal ecological implications. To address this complexity, we use empirical dynamic modeling to assemble causal networks for 19 natural aquatic ecosystems (N24◦~N58◦) and quantified strengths of feedbacks among phytoplankton diversity, phytoplankton biomass, and environmental factors. Through a cross-system comparison, we identify macroecological patterns; in more diverse, oligotrophic ecosystems, biodiversity effects are more important than environmental effects (nutrients and temperature) as drivers of biomass. Furthermore, feedback strengths vary with productivity. In warm, productive systems, strong nitrate-mediated feedbacks usually prevail, whereas there are strong, phosphate-mediated feedbacks in cold, less productive systems. Our findings, based on recovered feedbacks, highlight the importance of a network view in future ecosystem management.

Utilization of Δ17O for nitrate dynamics in a subtropical freshwater reservoir.

Feitsui Reservoir, a freshwater body in Taiwan with minimal anthropogenic stress, meets the water demand for the population of more than five million living in Taipei city. In view of the biogeochemical processes controlling the long-term trophic status of this socio-economically and ecologically important aquatic system, probing the nitrogen cycle and its dynamics is essential. Here, we monitored the concentration and stable isotopic compositions (δ15N, δ18O, and Δ17O) of nitrate in the Feitsui Reservoir and in the atmospheric wet deposition at intervals of 1-2 weeks for a year, along with measurements of environmental data such as chlorophyll a, dissolved oxygen, and community respiration. Emphasis was laid on Δ17O (= δ17O - 0.52 × Î´18O) because of the mass-conservative behavior of Δ17O during partial assimilation and denitrification. The present approach offered an effective method to quantify the gross nitrification and removal/uptake rates of nitrate in the reservoir. The atmospheric nitrate exhibited elevated Δ17O values ranging from 12.6‰ to 30.1‰ (23.3 ± 5.0‰), compared to the lower Δ17O values of ~0 to 4.6‰ (1.1 ± 0.7‰) recorded in the reservoir nitrate. Utilizing Δ17O for dissolved nitrates, we observed a seasonal trend of higher nitrification and removal rates during the summer than in the winter. Our estimates showed annually-averaged nitrification rate of 55 ± 11 mmol m-2 d-1 and removal/uptake rate of 57 ± 11 mmol m-2 d-1 (or a nitrate turnover time of ~2.5 months), representing the active nature of nitrogen cycling in this preserved subtropical reservoir.

Extreme Weather Events Enhance DOC Consumption in a Subtropical Freshwater Ecosystem: A Multiple-Typhoon Analysis.

Empirical evidence suggests that the frequency/intensity of extreme weather events might increase in a warming climate. It remains unclear how these events quantitatively impact dissolved organic carbon (DOC), a pool approximately equal to CO2 in the atmosphere. This study conducted a weekly-to-biweekly sampling in a deep subtropical reservoir in the typhoon-prevailing season (June to September) from 2004 to 2009, at which 33 typhoons with distinctive precipitation (<1~362 mm d-1) had passed the study site. Our analyses indicated that the phosphate (i.e., DIP; <10~181 nMP) varied positively with the intensity of the accumulated rainfall 2-weeks prior; bacteria growth rate (0.05~3.68 d-1) behaved as a positive function of DIP, and DOC concentrations (54~119 µMC) changed negatively with bacterial production (1.2~26.1 mgC m-3 d-1). These implied that the elevated DIP-loading in the hyperpycnal flow induced by typhoons could fuel bacteria growth and cause a significant decline of DOC concentrations. As the typhoon's intensity increases, many mineral-limited lentic freshwater ecosystems might become more like a CO2 source injecting more CO2 back to the atmosphere, creating a positive feedback loop that might generate severer extreme weather events.

Impact of upwelling on phytoplankton blooms and hypoxia along the Chinese coast in the East China Sea.

This study evaluates the rarely observed phenomenon of the simultaneous occurrences of phytoplankton blooms, hypoxia, and upwelling along the Zhejiang coast in the East China Sea. Results show that the upwelling uplifted bottom water to 5-10 m below the surface. In the upwelling region, phytoplankton blooms (Chl a = 10.9 µg L-1) occurred and hypoxia or low-oxygen appeared below the surface water. High concentrations of nitrate and phosphate were regenerated in the hypoxic regions, corresponding with mean values (± SD) of 16.9 (± 1.5) and 0.90 (± 0.14) µM, respectively. The upwelling expanded the region of hypoxic water, which nearly reached the surface, thereby increasing the threat to marine life. In addition to fluvial nutrients, the upwelling of water with high nutrient levels, especially phosphates, can enhance phytoplankton blooms. The results suggest that hypoxia can become more severe due to further decomposition of bloom-derived organic matter after blooms crash.

Hypoxia in autumn of the East China Sea.

Hypoxia (O2 ≤ 2 mg L-1) can severely threaten the survival of marine life and alter the biogeochemical cycles of coastal ecosystems. Its impacts are dependent on its duration. In the present study, hypoxia was observed in autumn at the end of October 2011. It may be one of the latest recorded annual hypoxic events in the East China Sea (ECS). In the hypoxic regions, a large amount of nutrients and dissolved inorganic carbon were observed to regenerate. Also, acidification (low pH) was observed. On the other hand, hypoxic dissipation may be due to the destratification caused by the upwelling of the hypoxic regions in the ECS. These results suggest that hypoxia may occur for longer periods of time than expected and, accordingly, the effects of hypoxia on the ECS ecosystems should be reconsidered and further evaluated.

Evolutionary constraints on species diversity in marine bacterioplankton communities.

Variation in microbial species diversity has typically been explained as the outcome of local ecological factors driving species coexistence, overlooking the roles of evolutionary constraints. Here, we argue that macro-evolutionary niche conservatism and unequal diversification rates among phylum-level lineages are strong determinants of diversity-environment relationships in bacterial systems. That is, apart from stochasticity, environmental effects operate most strongly on phylum composition, which in turn dictates the species diversity of bacterial communities. This concept is demonstrated using bacterioplankton in the surface seawaters of the East China Sea. Furthermore, we show that the species richness of a local bacterioplankton community can generally be estimated based on the relative abundances of phyla and their contributions of species numbers in the global seawater pool-highlighting the important influence of evolutionary constraints on local community diversity.