Dynamics of planktonic prokaryotes and dissolved carbon in a subtropical coastal lake

To understand the dynamics of planktonic prokaryotes in a subtropical lake and its relationship with carbon, we conducted water sampling through four 48-h periods in Peri Lake for 1 year. Planktonic prokaryotes were characterized by the abundance and biomass of heterotrophic bacteria (HB) and of cyanobacteria (coccoid and filamentous cells). During all samplings, we measured wind speed, water temperature (WT), pH, dissolved oxygen (DO), precipitation, dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and carbon dioxide (CO2). DOC was higher in the summer (average = 465 μM - WT = 27°C) and lower in the winter (average = 235 μM - WT = 17°C), with no significant variability throughout the daily cycles. CO2 concentrations presented a different pattern, with a minimum in the warm waters of the summer period (8.31 μM) and a maximum in the spring (37.13 μM). Daily trends were observed for pH, DO, WT, and CO2. At an annual scale, both biological and physical-chemical controls were important regulators of CO2. HB abundance and biomass were higher in the winter sampling (5.60 × 109 cells L-1 and 20.83 μmol C L-1) and lower in the summer (1.87 × 109 cells L-1 and 3.95 μmol C L-1). Filamentous cyanobacteria (0.23 × 108-0.68 × 108 filaments L-1) produced up to 167.16 μmol C L-1 as biomass (during the warmer period), whereas coccoid cyanobacteria contributed only 0.38 μmol C L-1. Precipitation, temperature, and the biomass of HB were the main regulators of CO2 concentrations. Temperature had a negative effect on the concentration of CO2, which may be indirectly attributed to high heterotroph activity in the autumn and winter periods. DOC was positively correlated with the abundance of total cyanobacteria and negatively with HB. Thus, planktonic prokaryotes have played an important role in the dynamics of both dissolved inorganic and organic carbon in the lake. © 2013 Fontes, Tonetta, Dalpaz, Antônio and Petrucio

Nutrient distribution in a shallow subtropical lagoon, south Brazil, subjected to seasonal hypoxic and anoxic events

© 2017, Universidade de Sao Paulo. All rights reserved. The Conceição Lagoon, located in south Brazil, is a semi-enclosed coastal ecosystem that has seasonal hypoxic and anoxic conditions in its vertically stratified central region, characterized as a site of retention and mineralization of organic matter. This study investigates water column dynamics in the central region of the Conceição Lagoon (CCL) and its relation to physical and chemical variables, in order to understand the hypoxic and anoxic events. Surface, halocline and bottom waters were evaluated at three sampling sites along the CCL. The samples were collected in triplicate during the summer, fall and winter of 2014. Hypoxic and/or anoxic events occurred in the summer (1/21) at the halocline (3 m) and bottom (4 m) waters, and in the fall (2/5) in the bottom water (4.5 m). Positive values of apparent oxygen utilization showed mineralization processes in the halocline and bottom waters. The lowest vertical stratification index was recorded in August (southern winter), which was associated with wind speed (14.7 m.s-1) and direction (southern quadrant). Nutrient concentrations were higher in winter, related to increasing of water mixing. This was the first study to evaluate the dynamics of hypoxic and anoxic events in the CCL and how nutrients respond to the physical structure of the water column

A Vigorous Specialized Microbial Food Web in the Suboxic Waters of a Shallow Subtropical Coastal Lagoon

To examine the extent of the microbial food web in suboxic waters of a shallow subtropical coastal lagoon, the density and biomass of bacteria and protozooplankton were quantified under different dissolved oxygen (DO) levels. In addition, bottom waters of a stratified site were compared with bottom waters of a homogeneous site under periods of high and low biological oxygen production/consumption in the lagoon. At the stratified site, microbial biomass decreased with oxygen decline, from oxia to suboxia, with a recovery of the initial total biomass after a 20-day period of persistent suboxia. A peak in density and biomass of purple sulfur bacteria (PSB) (90 μg C L-1) occurred in the suboxic waters 20 days prior to the peak in biomass of ciliates >50 μm (Loxophyllum sp. of 150 μm) (160 μg C L-1), demonstrating a top down biomass control. Ciliates >50 μm were positively correlated with PSB and bacteriochlorophyll a (photosynthetic pigment of PSB). Total protozoan biomass reached 430 μg C L-1 in the suboxic waters of the stratified site, with ciliates >50 μm accounting for 90% of the total ciliate biomass and of 55 % of biomass of protozoa. At the homogeneous site, total protozoan biomass was only 66 μg C L-1, where flagellates and ciliates <25 μm were the dominant microorganisms. Therefore, as light is available for primary producers in the bottom waters of shallow stratified coastal lagoons or estuaries, one can expect that high primary production of PSB may favor a specialized microbial food web composed by larger microorganisms, accessible to zooplankton that tolerate low DO levels. © 2012 Springer Science+Business Media, LLC

Natural events of anoxia and low respiration index in oligotrophic lakes of the Atlantic Tropical Forest

Hypoxia is a well-recognized condition reducing biodiversity and increasing greenhouse gas emissions in aquatic ecosystems, especially under warmer temperatures of tropical waters. Anoxia is a natural event commonly intensified by human-induced organic inputs in inland waters. Here, we assessed the partial pressure of O2 (pO2) and CO2 (pCO2), and the ratio between them (represented by the respiration index, RI) in two oligotrophic lakes of the Atlantic Tropical Forest, encompassing dry and rainy seasons over 19 months. We formulated the hypothesis that thermal stratification events could be coupled to natural hypoxia in deep waters of both lakes. Our results indicated a persistence of CO2 emissions from these tropical lakes to the atmosphere, on average ± standard error (SE) of 17.4 mg Cm-2h-1 probably subsided by terrestrial C inputs from the forest. Additionally, the thermal stratification during the end of the dry season and the rainy summer was coupled to anoxic events and very low RI in deep waters, and to significantly higher pO2 and RI at the surface (about 20 000 μatm and 1.0, respectively). In contrast, the water mixing during dry seasons at the beginning of the winter was related to a strong destratification in pO2, pCO2 and RI in surface and deep waters, without reaching any anoxic conditions throughout the water column. These findings confirm our hypothesis, suggesting that lakes of the Atlantic Tropical Forest could be dynamic, but especially sensitive to organic inputs. Natural anoxic events indicate that tropical oligotrophic lakes might be highly influenced by human land uses, which increase organic discharges into the watershed. © 2012 Author(s). CC Attribution 3.0 License

Linking summer conditions to CO<inf>2</inf> undersaturation and CO<inf>2</inf> influx in a subtropical coastal lake

© 2015, The Japanese Society of Limnology. In this study, we tested the hypothesis that pCO2 and air–water CO2 fluxes in the surface waters of a subtropical lake vary on two time scales (diel and seasonally) and that CO2 concentrations would decrease during the day and in summer. We estimated the variability of pCO2 and the air–water CO2 flux from pH-alkalinity in four 48-h periods that were representative of each subtropical season. There was high variability in pCO2 and the air–water gas flux over 48 h, but there was no clear pattern between day and night. CO2 concentrations showed a strong positive correlation with heterotrophic bacterial biomass and a negative correlation with dissolved organic carbon concentrations and water temperature. The lake was predominantly diel and seasonally CO2 supersaturated; the highest CO2 efflux was observed in the spring and a CO2 influx was observed in summer. Our hypothesis was confirmed; pCO2 was lowest in summer and during the daytime in spring and summer due to physical and biological conditions that favoured photosynthetic activities. These findings suggest that temporal shifts in the microbial community and meteorological variables, which are indirectly related to temperature, may be important drivers of CO2 concentrations in Peri Lake. In conclusion, pCO2 and the air–water CO2 flux vary temporally (diel and seasonally) in the littoral zone of this subtropical coastal lake, with shifts between CO2 influx and efflux throughout the sampling periods

Inter- and intra-annual variations of pCO<inf>2</inf> and pO<inf>2</inf> in a freshwater subtropical coastal lake

© International Society of Limnology 2015. Inland waters emit significant amounts of carbon dioxide to the atmosphere, but tropical and subtropical lakes are underrepresented in current assessments. Here we present results of a 6-year study of the dynamics of surface partial pressures of carbon dioxide and oxygen (pCO2 and pO2) in a subtropical lake, Lake Peri, Brazil, to determine how temperature, rainfall, and wind moderate surface concentrations. Both pCO2 and pO2 tended to increase during the transitions between seasons when rainfall increased, with pCO2 averaging 2.5-3-fold higher than atmospheric values. Occasionally during autumn/winter, pCO2 similarly increased and pO2 decreased. We infer that the increases in both gases during the transition periods resulted from increasing inputs of allochthonous material into the lake. Those in winter resulted from near-bottom intrusions that intermittently reach the depth of measurement. In autumn/winter, pCO2 was 3-fold higher (average 1700 μatm) compared to spring/summer (550 μatm), whereas changes in pO2 did not have a clear seasonal pattern. Overall median net CO2 evasion was 11 mg C m-2 d-1. Variability in the extent of rainfall and the associated high intra- and inter-annual variability in CO2 and CO2 emissions are in part controlled by atmospheric processes related to the South American Monsoon System and to El Niño Southern Oscillation cycles

Vulnerability of seagrass blue carbon to microbial attack following exposure to warming and oxygen

© 2019 Seagrass meadows store globally-significant quantities of organic ‘blue’ carbon. These blue carbon stocks are potentially vulnerable to anthropogenic stressors (e.g. coastal development, climate change). Here, we tested the impact of oxygen exposure and warming (major consequences of human disturbance) on rates of microbial carbon break-down in seagrass sediments. Active microbes occurred throughout seagrass sediment profiles, but deep, ancient sediments (~5000 yrs. old) contained only 3% of the abundance of active microbes as young, surface sediments (<2 yrs. old). Metagenomic analysis revealed that microbial community structure and function changed with depth, with a shift from proteobacteria and high levels of genes involved in sulfur cycling in the near surface samples, to a higher proportion of firmicutes and euraracheota and genes involved in methanogenesis at depth. Ancient carbon consisted almost entirely (97%) of carbon considered ‘thermally recalcitrant’, and therefore presumably inaccessible to microbial attack. Experimental warming had little impact on carbon; however, exposure of ancient sediments to oxygen increased microbial abundance, carbon uptake and sediment carbon turnover (34–38 fold). Overall, this study provides detailed characterization of seagrass blue carbon (chemical stability, age, associated microbes) and suggests that environmental disturbances that expose coastal sediments to oxygen (e.g. dredging) have the capacity to diminish seagrass sediment carbon stocks by facilitating microbial remineralisation

Primary production in a subtropical stratified coastal lagoon contribution of anoxygenic phototrophic bacteria

Anaerobic anoxygenic phototrophic bacteria can be found in the suboxic waters of shallow stratified coastal systems, and may play important roles in the total primary production of subtropical stratified coastal lagoons. We investigated the spatiotemporal variability of light CO2 fixation and net oxygen production in the stratified Conceição Lagoon (Brazil) in summer and fall of 2007, as well as the contribution of bacteriochlorophyll a (BChl a)- containing bacteria to photosynthetically driven electron transfer. Both chlorophyll a (Chl a) and BChl a varied in space, while only BChl a varied in time (three-fold increase from summer to fall). In summer, net oxygen production and light CO2 fixation were correlated, with both having higher rates with higher Chl a concentrations in the enclosed region of the lagoon. In fall, CO2 fixation was decoupled from oxygen production Denaturing gradient gel electrophoresis revealed that bacterial communities of oxic site 12 and suboxic site 33 formed one cluster, different from other oxic samples within the lagoon. In addition, BChl a/Chl a ratios at these sites were high, 40% and 45%, respectively. Light acted as the main factor controlling the BChl a concentration and CO2 fixation rates. High turbidity within the enclosed area of the lagoon explained high BChl a and decoupling between CO2 fixation and oxygen production in oxygenated waters. Contribution of purple sulfur bacteria to total bacterial density in suboxic waters was 1.2%, and their biomass contributed to a much higher percentage (12.2%) due to their large biovolume. Our results indicate a significant contribution of anaerobic anoxygenic bacteria to the primary production of the “dead zone” of Conceição Lagoon