Autotrophic Biomass Research Articles

Autotrophs are important contributors to benthic macroinvertebrate food webs in two tropical first‐order forest streams

Abstract In tropical first‐order streams, the availability of basal resources changes according to tropical seasonality, i.e., dry and rainy seasons, with consequences for the food web structure. However, the seasonal variation in autotrophic and detrital pathways remains unclear. In this study, we aimed to establish whether the dominant pathway of the benthic macroinvertebrate food web in two tropical first‐order streams in the Lacandona rainforest, Mexico, relies on autotrophic or detrital food resources. We evaluated the assimilation of autotrophic and detrital food resources through carbon and nitrogen stable isotope (δ13C and δ15N) analysis. We included the biomass of the macroinvertebrates sustained by the autotrophic and detrital pathways and the food web structure—trophic position, size structure, and degree of omnivory. The biomass sustained by the detrital pathway remained similar throughout the year in both streams. In contrast, the biomass sustained by the autotrophic pathway was higher in the dry season than in the rainy season. The benthic macroinvertebrate food web had four levels, a poorly developed size structure, and an important degree of omnivory in both streams and in both seasons. It is likely that omnivory was a response to high resource variability and a strategy to reduce intraguild competition. Autotrophs had a relatively higher importance than detritus since a much lower algae biomass sustained an equal or higher macroinvertebrate biomass, particularly in the dry season when the biomass of autotrophs increased. Seasonality influenced the macroinvertebrate food web by modifying the food resource availability and transference to higher trophic levels. However, the high frequency of omnivory reduced the seasonal changes in the food web structure. Our results suggest that the autotrophic pathway plays an important role in maintaining the benthic macroinvertebrate food web in tropical streams. The relative importance of autotrophs in the macroinvertebrate food web is associated with seasonality, the rainfall pattern, and its effect on the stream hydrology.

Acid Tolerant and Acidophilic Microalgae: An Underexplored World of Biotechnological Opportunities.

Despite their large number and diversity, microalgae from only four genera are currently cultivated at large-scale. Three of those share common characteristics: they are cultivated mainly autotrophically and are extremophiles or tolerate “extreme conditions.” Extreme growth conditions aid in preventing contamination and predation of microalgae, therefore facilitating outdoor cultivation. In search for new extremophilic algae suitable for large-scale production, we investigated six microalgal strains able to grow at pH below 3 and belonging to four genera; Stichococcus bacillaris ACUF158, Chlamydomonas acidophila SAG 2045, and Chlamydomonas pitschmannii ACUF238, Viridiella fridericiana ACUF035 and Galdieria sulphuraria ACUF064 and ACUF074. All strains were cultivated autotrophically at light intensity of 100 and 300 μmol m−2 s−1 and pH between 1.9 and 2.9. The autotrophic biomass productivities were compared with one of the most productive microalgae, Chlorella sorokiniana SAG 211-8K, grown at pH 6.8. The acid tolerant strains have their autotrophic biomass productivities reported for the first time. Mixotrophic and heterotrophic properties were investigated when possible. Five of the tested strains displayed autotrophic biomass productivities 10–39% lower than Chlorella sorokiniana but comparable with other commercially relevant neutrophilic microalgae, indicating the potential of these microalgae for autotrophic biomass production under acidic growth conditions. Two acid tolerant species, S. bacillaris and C. acidophila were able to grow mixotrophically with glucose. Chlamydomonas acidophila and the two Galdieria strains were also cultivated heterotrophically with glucose at various temperatures. Chlamydomonas acidophila failed to grow at 37°C, while G. sulphuraria ACUF64 showed a temperature optimum of 37°C and G. sulphuraria ACUF74 of 42°C. For each strain, the biomass yield on glucose decreased when cultivated above their optimal temperature. The possible biotechnological applications of our findings will be addressed.

Seasonality of phytoplankton community and dynamics of autotrophic carbon in a cold temperate port (Argentina)

The changes within and between seasons in phytoplankton composition and abundance determine the carbon biomass available for upper levels of the food web. Temporal changes in phytoplankton community and environmental parameters in a port in Puerto Madryn, Southwest Atlantic were analysed. During an annual period (2011–2012), samples of surface seawater were collected approximately monthly. We determined phytoplankton community structure (species composition and abundance) and biomass (determined by carbon content and chlorophyll a (Chl a)). Water temperature, salinity and transparency were measured when sampling the surface water and local meteorological data were considered. The main groups observed were diatoms (Bacillariophyta; the most abundant during the concentration peaks of Chl a), dinoflagellates (Dinophyta) and flagellates, which mainly included species of Cryptophyta and Chlorophyta. Diatoms exhibited blooms in March (summer–autumn) and September (spring), represented by Skeletonema costatum and Pseudo-nitzschia spp. respectively. Dinoflagellates contributed to the highest carbon biomass, with peaks in January (summer) and April (autumn), exemplified by Prorocentrum micans and Scrippsiella acuminata, respectively. Temporal differences in community composition were related to the seasonal changes in temperature, solar irradiance, precipitation, salinity and wind velocity. The environmental conditions seem not only to determine the species composition but also cell size distribution: nanoplanktonic (≤ 20 µm) species dominated mainly during late spring, summer and early winter while microplanktonic species (> 20 µm) during late winter and early spring. Our results showed within-season changes and show that not only Chl a, but also carbon content can be considered, as the former is a biased estimator of phytoplankton biomass. This study provides the first seasonally resolved estimation in the area of the carbon biomass available for upper levels of the food web and a necessary information for future scenarios prediction. HIGHLIGHTS• Two diatom blooms were followed by an increase in dinoflagellate abundance.• Annual environmental conditions drive the phytoplankton community structure.• First seasonally estimation of autotrophic biomass availability for Patagonian trophic web.

Impact of dust addition on Mediterranean plankton communities under present and future conditions of pH and temperature: an experimental overview

Abstract. In low-nutrient low-chlorophyll areas, such as the Mediterranean Sea, atmospheric fluxes represent a considerable external source of nutrients likely supporting primary production, especially during periods of stratification. These areas are expected to expand in the future due to lower nutrient supply from sub-surface waters caused by climate-driven enhanced stratification, likely further increasing the role of atmospheric deposition as a source of new nutrients to surface waters. Whether plankton communities will react differently to dust deposition in a warmer and acidified environment remains; however, an open question. The potential impact of dust deposition both in present and future climate conditions was investigated in three perturbation experiments in the open Mediterranean Sea. Climate reactors (300 L) were filled with surface water collected in the Tyrrhenian Sea, Ionian Sea and in the Algerian basin during a cruise conducted in the frame of the PEACETIME project in May–June 2017. The experiments comprised two unmodified control tanks, two tanks enriched with a Saharan dust analogue and two tanks enriched with the dust analogue and maintained under warmer (+3 ∘C) and acidified (−0.3 pH unit) conditions. Samples for the analysis of an extensive number of biogeochemical parameters and processes were taken over the duration (3–4 d) of the experiments. Dust addition led to a rapid release of nitrate and phosphate, however, nitrate inputs were much higher than phosphate. Our results showed that the impacts of Saharan dust deposition in three different basins of the open northwestern Mediterranean Sea are at least as strong as those observed previously, all performed in coastal waters. The effects of dust deposition on biological stocks were different for the three investigated stations and could not be attributed to differences in their degree of oligotrophy but rather to the initial metabolic state of the community. Ocean acidification and warming did not drastically modify the composition of the autotrophic assemblage, with all groups positively impacted by warming and acidification. Although autotrophic biomass was more positively impacted than heterotrophic biomass under future environmental conditions, a stronger impact of warming and acidification on mineralization processes suggests a decreased capacity of Mediterranean surface plankton communities to sequester atmospheric CO2 following the deposition of atmospheric particles.

Reduced N‐Limitation and Increased In‐Stream Productivity of Autotrophic Biofilms 5 and 15 Years After Severe Wildfire

AbstractThe ecosystem services provided by forests are under threat as wildfire frequency and severity increase throughout the western US. Severe wildfire can change physical environments and biogeochemical processes in watersheds with lasting effects on watershed nutrient cycling. For example, nitrate‐nitrogen (NO3‐N) export often increases following wildfire and can remain elevated for decades in severely burned watersheds. In this study, we investigated the effects of wildfire on stream biotic processing and watershed nutrient balance following two wildfires that burned along the Colorado Front Range. We evaluated stream water chemistry, nutrient limitation, benthic biomass, and stream metabolism along stream reaches within three burned and three unburned watersheds from July 26 to August 16, 2017. Although the two high‐severity wildfires occurred 5 and 15 years prior to the study, the streams draining burned watersheds still had 23‐times higher NO3‐N concentrations than unburned watersheds, a trend that is consistent across seasons and throughout the 15‐year post‐fire record. Autotrophic nitrogen (N) limitation was reduced in the nitrate‐rich burned streams. Consequently, autotrophic biomass and primary productivity were 2.5 and 20‐times greater, respectively, in burned relative to unburned streams. Together, these data suggest that N supply from burned uplands exceeded the increase in stream N demand and was the primary cause of chronic, elevated NO3‐N export from these severely burned watersheds. Accordingly, aquatic ecosystems within or downstream of burned watersheds may be susceptible to eutrophication and harmful algal blooms until vegetation recovery and plant nutrient demand reduce N supply to streams.

Heavy metal inhibition on an alternating activated sludge system and its comparison to conventional methods: case study of Cu2+

Abstract In order to understand the behaviour of wastewater treatment plants with heavy metal presence, this study evaluates the treatment process in the presence of heavy metals (Cu2+ as a case study) and compares it with the absence of heavy metals. An activated sludge model is improved by means of incorporating other novel inhibitory kinetic and settler models for this evaluation. To achieve this goal, a simulation algorithm is developed using the MATLAB code to detect any heavy metal influence on the aerobic and anoxic growth of heterotrophic and autotrophic biomass. The code also allows for a comparison of treatment plant performance with and without Cu2+ in both conventional and alternating systems. The results reveal that the presence of heavy metals, in this study for Cu2+ at 0.5 mg/L, in a biological treatment system, has an inhibitory effect on the heterotrophic bacteria but more so on the autotrophic bacteria growth and it prevents nitrification and denitrification, thus having a negative effect on the nitrogen removal in the alternating systems.

The biogeochemical role of a microbial biofilm in sea ice

A paradox is commonly observed in productive sea ice in which an accumulation in the macro-nutrients nitrate and phosphate coincides with an accumulation of autotrophic biomass. This paradox requires a new conceptual understanding of the biogeochemical processes operating in sea ice. In this study, we investigate this paradox using three time series in Antarctic landfast sea ice, in which massive algal blooms are reported (with particulate organic carbon concentrations up to 2,600 µmol L–1) and bulk nutrient concentrations exceed seawater values up to 3 times for nitrate and up to 19 times for phosphate. High-resolution sampling of the bottom 10 cm of the cores shows that high biomass concentrations coexist with high concentrations of nutrients at the subcentimeter scale. Applying a nutrient-phytoplankton-zooplankton-detritus model approach to this sea-ice system, we propose the presence of a microbial biofilm as a working hypothesis to resolve this paradox. By creating microenvironments with distinct biogeochemical dynamics, as well as favoring nutrient adsorption onto embedded decaying organic matter, a biofilm allows the accumulation of remineralization products (nutrients) in proximity to the sympagic (ice-associated) community. In addition to modifying the intrinsic physicochemical properties of the sea ice and providing a substrate for sympagic community attachment, the biofilm is suggested to play a key role in the flux of matter and energy in this environment.

Scale-Specific Patterns of the Production of the Charophyte Chara aspera in the Brackish Baltic Sea: Linking Individual and Community Production and Biomass Growth

An understanding of the nature of scale-dependence in macroalgal production and to quantify how primary production is incorporated into autotrophic biomass requires an assessment of primary production at multiple scales. In this study we experimentally estimated seasonal variability in individual photosynthesis, community production and growth in biomass of the charophyteChara asperain the Baltic Sea together with the key environmental variables known to regulate the production of macroalgae. The experiments suggested that the production and growth ofC. asperawas defined largely by light and temperature at all studied scales. However, the algal production at the same light levels was systematically lower at the community than an individual level, suggesting the importance of shelf-shading and below-ground processes in natural plant assemblages. Moreover, the observation scale also defined response types between the environment, plant production and growth. This implies that the patterns of variability of the production and growth of macrophytes should always be interpreted in the context of scale and any multiscale model development should involve experimental validation at all important scales.

Eutrophication effects on CH4 and CO2 fluxes in a highly urbanized tropical reservoir (Southeast, Brazil).

Shallow urban polluted reservoirs at tropical regions can be hotspots for CO2 and CH4 emissions. In this study, we investigated the relationships between eutrophication and GHG emissions in a highly urbanized tropical reservoir in São Paulo Metropolitan Area (Brazil). CO2 and CH4 fluxes and limnological variables (water and sediment) were collected at three sampling stations classified as hypereutrophic and eutrophic. Analysis of variance (ANOVA) and the principal component analysis (PCA) determined the most significant parameters to CO2 and CH4 fluxes. ANOVA showed significant differences of CO2 and CH4 fluxes between sampling stations with different trophic state. The hypereutrophic station showed higher mean fluxes for both CO2 and CH4 (5.43 ± 1.04 and 0.325 ± 0.167 g m-2 d-1, respectively) than the eutrophic stations (3.36 ± 0.54 and 0.060 ± 0.005 g m-2 d-1). The PCA showed a strong relationship between nutrients in the water column (surface and bottom) and GHG fluxes. We concluded that GHG fluxes were higher whenever the trophic state increases as observed previously in temperate and tropical reservoirs. High concentrations of nutrients in the water column in the studied area support the high production of autotrophic biomass that, when sedimented, ends up serving as organic matter for CH4 producers. These outcomes reinforce the necessity of water quality improvement and eutrophication mitigation in highly urbanized reservoirs in tropical regions.

Influence of road salt thawing peaks on the inflow composition and activated sludge properties in municipal wastewater treatment.

Operational data over 2 years from three large Austrian wastewater treatment plants (WWTPs) with design capacities of 4 million, 950,000 and 110,000 population equivalent (PE) were examined. Salt peaks, due to thawing road salt were detected and quantified by electrical conductivity, temperature and chloride measurement in the inflow of the WWTPs. Daily NaCl inflow loads up to 1,147 t/d and PE-specific loads of 0.26-0.5 kg NaCl/(PE · y) were found. To mimic the plants' behaviour in a controlled environment, NaCl was dosed into the inflow of a laboratory-scale activated sludge plant. The influence of salt peaks on important activated sludge parameters such as sludge volume index, settling velocity and floc size were investigated. Influent and effluent were sampled extensively to calculate removal rates. Respiration measurements were performed to quantify activated sludge activity. Particle size distributions of the activated sludge floc sizes were measured using laser diffraction particle sizing and showed a decrease of the floc size by approximately two-thirds. The floc structure was examined and documented using light microscopy. At salt concentrations below 1 g/L, increased respiration was found for autotrophic biomass, and between 1 and 3 g NaCl/L respiration was inhibited by up to 30%.

Nitrate-Enrichment Structures Phytoplankton Communities in the Shallow Eastern Mediterranean Coastal Waters

Submarine groundwater discharge (SGD) has been shown to be an important source of nutrients in coastal environments, especially nitrogen and silica, and thereby relive nutrient limitation to phytoplankton. Here, we followed autotrophic microbial biomass, activity, and community composition at a site strongly influenced by SGD and a nearby nutrients-poor reference site at the oligotrophic Israeli shallow rocky coast [southeastern Mediterranean Sea (SEMS)] between 2011 and 2019. The surface water at the SGD-affected area had significantly higher NO3 + NO2 (∼10-fold) and Si(OH)4 (∼2-fold) levels compared to the reference site, while no significant differences were observed for PO4 or NH4. This resulted in a significant increase in algae biomass (∼3.5-fold), which was attributed to elevated Synechococcus (∼3.5-fold) and picoeukaryotes (∼2-fold) at the SGD-affected site, and in elevated primary production rates (∼2.5-fold). Contrary to most SGD-affected coastal areas, diatoms biomass remained unchanged between sites, despite the elevated N and Si, suggesting the dominance of picophytoplankton over microphytoplankton at the SEMS. DNA sequencing of the 16S and 18S rDNA supported these findings. These results highlight the influence of SGD on shallow-water microbial populations. Our observations are consistent with recent studies showing that phytoplankton along the Israeli coast are likely nitrogen + silica limited, and may have important ecological and regulatory implications for environmental policy and management of coastal aquifers.

Spatial Distribution of Phytoplankton Community Composition and Their Correlations with Environmental Drivers in Taiwan Strait of Southeast China

Large-scale dinoflagellate blooms have appeared in recent decades in the Taiwan Strait, Southeast China. To study spatial variability of phytoplankton community composition, physical and chemical environmental drivers in surface seawater of the Taiwan Strait, we conducted cruises in May and July 2019. Cell numbers of dinoflagellates were significantly higher than that of diatoms in most sampling stations during the cruise in May, whereas diatoms were the major contributor to autotrophic biomass in July. Phytoplankton community shifted from a dinoflagellate- and diatom-dominated system in May to diatom dominance in July. The dominant phytoplankton species (genera) were the harmful algal bloom dinoflagellates Prorocentrum donghaiense and Scrippsiella trochoidea and the diatoms Coscinodiscus in May, and Rhizosolenia, Pseudo-nitzschia, and Guinardia in July. Cell densities of dinoflagellates and P. donghaiense reduced exponentially with increasing seawater temperature and salinity and decreasing dissolved inorganic nitrogen (DIN) concentrations. Based on the results of our work and previous studies, it becomes obvious that harmful dinoflagellate blooms are likely to be a major component of the planktonic food web in the Taiwan Strait at a temperature of 17.0–23.0 °C, a salinity of 29.0–33.0 psu, and a DIN concentration higher than 2.0 μmol L–1.

Kinetic study and pyrolysis: GC/MS products analysis of Spirulina platensis cultivated under a different growing medium

Microalgae have a great potential to produce biofuels, but the cost is still too high mainly due to the biomass production. Mixotrophic cultivation has been pointed as microalgae cultivation mode for biomass/bioenergy production with lower cost. The proposals of this work were to cultivate S. platensis in autotrophic and mixotrophic medium using molasses as source of organic carbon and investigate the thermal behavior of obtained biomass by means of thermogravimetric analysis and pyrolysis (Py-GC/MS). The kinetics models proposed by Flynn and Wall and model-free kinetic were used to determine the activation energy. These data are important to projection of design, operation and modeling of thermochemical conversion system for microalgae. The use of molasses as supplement in culture medium for the growth of S. platensis was a good way to increase the biomass productivity and decrease the protein content. The Flynn–Wall and model-free kinetic methods were adequate to calculate activation energy on the conversion range of 0.20 to 0.80. The activation energies were 168–229 kJ mol−1 (Flynn and Wall), 177–238 kJ mol−1 (model-free kinetic) for mixotrophic biomass and 174–220 kJ mol−1 (Flynn and Wall), 181–229 kJ mol−1 (model-free kinetic) for autotrophic biomass. The compositions of volatile compounds produced by S. platensis biomass pyrolysis were different due to cultivation method that influenced the composition of biomass. Hydrocarbons, oxygenated and nitrogenated compounds were produced with different quantities. The volatile compounds content of phenols, oxygenated and nitrogenated increased and non-aromatic and aromatic hydrocarbons content decreased for pyrolysis of mixotrophic biomass. However, the mixotrophic cultivation has a great influence on the microalgae biomass production and should be a factor considered in thermal degradation project for microalgae.

Seasonal Variability in the Microbial Plankton Community in a Semienclosed Bay Affected by Upwelling: The Role of a Nutrient Trap

The Ria de Vigo is a semienclosed bay in northwestern Iberia that is affected by seasonal upwelling (spring to autumn) that triggers high levels of primary productivity. Fisheries and aquaculture coexist with other human activities, such as shipbuilding and tourism, in this area. The microbial plankton community in this system was studied to understand how this community is structured and progresses seasonally. Autotrophic biomass (AB) dominated from spring to autumn when upwelling prevailed, while AB and heterotrophic biomass (HB) covaried throughout the year as HB:AB = 0.26 ± 0.04 (r2 = 0.7), indicating a link between the two communities. Diatoms and autotrophic nanoflagellates accounted for 80 ± 18% of the AB, whereas the HB was mainly composed of heterotrophic bacteria (36  11%) and heterotrophic nanoflagellates (30  12%). The AB and HB, as well as their main components, showed a continuous increase during the upwelling season that was attributed to growth. However, a sudden increase occurred in autumn, when downwelling caused the biomass to accumulate before leading to the low values in winter. As upwelling promotes the export of materials to the open ocean, the increase in biomass was possibly due to the existence of a nutrient trap that fueled phytoplankton growth through recycling at the sediment-water interface a substantial portion of the organic matter previously synthetized in the water column. The reorganization of the community that followed the winter decrease in biomass began with the proliferation of small plankton forms.

Mechanistic insights into the role of large carnivores for ecosystem structure and functioning

Large carnivores can exert top–down effects in ecosystems, but the size of these effects are largely unknown. Empirical investigation on the importance of large carnivores for ecosystem structure and functioning presents a number of challenges due to the large spatio‐temporal scale and the complexity of such dynamics. Here, we applied a mechanistic global ecosystem model to investigate the influence of large‐carnivore removal from undisturbed ecosystems. First, we simulated large‐carnivore removal on the global scale to inspect the geographic pattern of top–down control and to disentangle the functional role of large carnivores in top–down control in different environmental contexts. Second, we conducted four small‐scale ecosystem simulation experiments to understand direct and indirect changes in food‐web structure under different environmental conditions. We found that the removal of top–down control exerted by large carnivores (> 21 kg) can trigger large trophic cascades, leading to an overall decrease in autotroph biomass globally. Furthermore, the loss of large carnivores resulted in an increase of mesopredators. The magnitude of these changes was positively related to primary productivity (NPP), in line with the ‘exploitation ecosystem hypothesis’. In addition, we found that seasonality in NPP dampened the magnitude of change following the removal of large carnivores. Our results reinforce the idea that large carnivores play a fundamental role in shaping ecosystems, and further declines and extinctions can trigger substantial ecosystem responses. Our findings also support previous studies suggesting that natural ecosystem dynamics have been severely modified and are still changing as a result of the widespread decline and extinction of large carnivores.

Dispersal mitigates bacterial dominance over microalgal competitor in metacommunities.

Ecological theory suggests that a combination of local and regional factors regulate biodiversity and community functioning in metacommunities. The relative importance of different factors structuring communities likely changes over successional time, but to date this concept is scarcely documented. In addition, the few studies describing successional dynamics in metacommunity regulation have only focused on a single group of organisms. Here, we report results of an experimental study testing the effect size of initial local community composition and dispersal between local patches on community dynamics of benthic microalgae and their associated bacteria over community succession. Our results show that over time dispersal outweighed initial effects of community composition on microalgal evenness and biomass, microalgal β-diversity, and the ratio of bacteria to microalgae. At the end of the experiment (ca. 20 microalgae generations), dispersal significantly decreased microalgal evenness and β-diversity by promoting one regionally superior competitor. Dispersal also decreased the ratio of bacteria to microalgae, while it significantly increased microalgal biomass. These results suggest that the dispersal-mediated establishment of a dominant and superior microalgae species prevented bacteria from gaining competitive advantage over the autotrophs in these metacommunities, ultimately maintaining the provision of autotrophic biomass. Our study emphasizes the importance of time for dispersal to be a relevant community-structuring mechanism. Moreover, we highlight the need for considering multiple competitors in complex metacommunity systems to properly pinpoint the consequences of local change in dominance through dispersal for metacommunity function.

Dynamics of environmental conditions during the decline of a <i>Cymodocea nodosa</i> meadow

Abstract. The dynamics of the physicochemical and biological parameters were followed during the decline of a Cymodocea nodosa meadow in the northern Adriatic Sea from July 2017 to October 2018. During the regular growth of C. nodosa from July 2017 to March 2018, the species successfully adapted to the changes in environmental conditions and prevented H2S accumulation by its reoxidation, supplying the sediment with O2 from the water column and/or leaf photosynthesis. The C. nodosa decline was most likely triggered in April 2018 when light availability to the plant was drastically reduced due to increased seawater turbidity that resulted from increased terrigenous input, indicated by a decrease in salinity accompanied with a substantial increase in particulate matter concentration, combined with resuspension of sediment and elevated autotrophic biomass. Light reduction impaired photosynthesis of C. nodosa and the oxidation capability of belowground tissue. Simultaneously, a depletion of oxygen due to intense oxidation of H2S occurred in the sediment, thus creating anoxic conditions in most of the rooted areas. These linked negative effects on the plant performance caused an accumulation of H2S in the sediments of the C. nodosa meadow. During the decay of aboveground and belowground tissues, culminating in August 2018, high concentrations of H2S were reached and accumulated in the sediment as well as in bottom waters. The influx of oxygenated waters in September 2018 led to the re-establishment of H2S oxidation in the sediment and remainder of the belowground tissue. Our results indicate that if disturbances of environmental conditions, particularly those compromising the light availability, take place during the recruitment phase of plant growth when metabolic needs are at a maximum and stored reserves minimal, a sudden and drastic decline of the seagrass meadow occurs.

Phytoplankton and dimethylsulfide dynamics at two contrasting Arctic ice edges

Abstract. Arctic sea ice is retreating and thinning and its rate of decline has steepened in the last decades. While phytoplankton blooms are known to seasonally propagate along the ice edge as it recedes from spring to summer, the substitution of thick multiyear ice (MYI) with thinner, ponded first-year ice (FYI) represents an unequal exchange when considering the roles sea ice plays in the ecology and climate of the Arctic. Consequences of this shifting sea ice on the phenology of phytoplankton and the associated cycling of the climate-relevant gas dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) remain ill constrained. In July–August 2014, two contrasting ice edges in the Canadian High Arctic were explored: a FYI-dominated ice edge in Barrow Strait and a MYI-dominated ice edge in Nares Strait. Our results reveal two distinct planktonic systems and associated DMS dynamics in connection to these diverging ice types. The surface waters exiting the ponded FYI in Barrow Strait were characterized by moderate chlorophyll a (Chl a, <2.1 µg L−1) as well as high DMSP (115 nmol L−1) and DMS (12 nmol L−1), suggesting that a bloom had already started to develop under the markedly melt-pond-covered (ca. 40 %) FYI. Heightened DMS concentrations at the FYI edge were strongly related to ice-associated seeding of DMS in surface waters and haline-driven stratification linked to ice melt (Spearman's rank correlation between DMS and salinity, rs=-0.91, p<0.001, n=20). However, surface waters exiting the MYI edge at the head of Nares Strait were characterized by low concentrations of Chl a (<0.5 µg L−1), DMSP (<16 nmol L−1), and DMS (<0.4 nmol L−1), despite the nutrient-replete conditions characterizing the surface waters. The increase in autotrophic biomass and methylated sulfur compounds took place several kilometers (ca. 100 km) away from the MYI edge, suggesting the requisite for ice-free, light-sufficient conditions for a phytoplankton bloom to fully develop and for sulfur compound dynamics to follow and expand. In light of the ongoing and projected climate-driven changes to Arctic sea ice, results from this study suggest that the early onset of autotrophic blooms under thinner, melt-pond-covered ice may have vast implications for the timing and magnitude of DMS pulses in the Arctic.

Ultraphytoplankton community composition in Southwestern and Eastern Mediterranean Basin: Relationships to water mass properties and nutrients

The composition, biomass, and spatial and vertical distribution of ultraphytoplankton (< 10 μm) were investigated at the single-cell level by flow cytometry during the INCOMMET oceanographic cruise carried out during the fall of November 2013, covering eleven stations spread out along coastal Tunisian waters at less than 100 m depth. Flow cytometry analysis of samples taken at different stations and depths allowed the determination of five major groups of ultraphytoplankton; among them two populations of pico-cyanobacteria Synechococcus and Prochlorococcus, picoeukaryotes, nanoeukaryotes and cryptophytes. In addition to these autotrophic groups, three unknown groups were resolved and were distinguished by their fluorescence and size signals. The unknown groups' integrated biomass was larger than that of the other ultraphytoplankton components. The abundance of picoeukaryotes, nanoeukaryotes and cryptophytes increased gradually southwards towards the Gulf of Gabès, whereas Prochlorococcus were more abundant in the Gulf of Hammamet at more than 60 m depth. Structural equation modeling analysis showed weak links between most of the cell groups and nutrients concentrations. Clustering analysis based on the abundance of the phytoplankton groups resulted in aggregation into three clusters and two major zones, discriminating the Gulf of Gabès from the more northern Bays. The results suggest that the physical features, essentially the water mass properties, were the main drivers shaping the ultraphytoplankton community composition and that the autotrophic biomasses were not determined by nutrient control.

Phytoplankton associated with seasonal oxygen depletion in waters of the western continental shelf of India

Abstract We report here results of a time series study carried out to understand seasonal variability of phytoplankton in relation to changes in dissolved oxygen based on observations at the Candolim Time Series (CaTS) station located in coastal waters of Goa, Central West Coast of India. The water column remains well oxygenated during the non-southwest (SW) monsoon period (November–May). However, upwelling of water already depleted in dissolved oxygen during the SW monsoon, coupled with degradation of copious amounts of locally produced organic matter, leads to severe oxygen depletion including sulphate reduction beneath a thin surface layer during late summer-early autumn (August–October). The onset of oxygen-deficient conditions greatly alters the community structure of water-column phytoplankton. Pico-autotrophs and pennate diatoms were found to dominate the autotrophic biomass in such oxygen depleted waters.