Posts Tagged 'abundance'



In situ response of Antarctic under-ice primary producers to experimentally altered pH

Elevated atmospheric CO2 concentrations are contributing to ocean acidification (reduced seawater pH and carbonate concentrations), with potentially major ramifications for marine ecosystems and their functioning. Using a novel in situ experiment we examined impacts of reduced seawater pH on Antarctic sea ice-associated microalgal communities, key primary producers and contributors to food webs. pH levels projected for the following decades-to-end of century (7.86, 7.75, 7.61), and ambient levels (7.99), were maintained for 15 d in under-ice incubation chambers. Light, temperature and dissolved oxygen within the chambers were logged to track diurnal variation, with pH, O2, salinity and nutrients assessed daily. Uptake of CO2 occurred in all treatments, with pH levels significantly elevated in the two extreme treatments. At the lowest pH, despite the utilisation of CO2 by the productive microalgae, pH did not return to ambient levels and carbonate saturation states remained low; a potential concern for organisms utilising this under-ice habitat. However, microalgal community biomass and composition were not significantly affected and only modest productivity increases were noted, suggesting subtle or slightly positive effects on under-ice algae. This in situ information enables assessment of the influence of future ocean acidification on under-ice community characteristics in a key coastal Antarctic habitat.

Continue reading ‘In situ response of Antarctic under-ice primary producers to experimentally altered pH’

Analyzing the impacts of elevated-CO2 levels on the development of a subtropical zooplankton community during oligotrophic conditions and simulated upwelling

Ocean acidification (OA) is affecting marine ecosystems through changes in carbonate chemistry that may influence consumers of phytoplankton, often via trophic pathways. Using a mesocosm approach, we investigated OA effects on a subtropical zooplankton community during oligotrophic, bloom, and post-bloom phases under a range of different pCO2 levels (from ∼400 to ∼1480 μatm). Furthermore, we simulated an upwelling event by adding 650 m-depth nutrient-rich water to the mesocosms, which initiated a phytoplankton bloom. No effects of pCO2 on the zooplankton community were visible in the oligotrophic conditions before the bloom. The zooplankton community responded to phytoplankton bloom by increased abundances in all treatments, although the response was delayed under high-pCO2 conditions. Microzooplankton was dominated by small dinoflagellates and aloricate ciliates, which were more abundant under medium- to high-pCO2 conditions. The most abundant mesozooplankters were calanoid copepods, which did not respond to CO2 treatments during the oligotrophic phase of the experiment but were found in higher abundance under medium- and high-pCO2 conditions toward the end of the experiment, most likely as a response to increased phyto- and microzooplankton standing stocks. The second most abundant mesozooplankton taxon were appendicularians, which did not show a response to the different pCO2 treatments. Overall, CO2 effects on zooplankton seemed to be primarily transmitted through significant CO2 effects on phytoplankton and therefore indirect pathways. We conclude that elevated pCO2 can change trophic cascades with significant effects on zooplankton, what might ultimately affect higher trophic levels in the future.

Continue reading ‘Analyzing the impacts of elevated-CO2 levels on the development of a subtropical zooplankton community during oligotrophic conditions and simulated upwelling’

Changing structure of benthic foraminiferal communities due to declining pH: results from laboratory culture experiments

The ocean absorbs large amounts of CO2 emitted from human activities, which results in a decrease in seawater pH. Marine calcifying organisms such as foraminifera, are most likely to be affected by this declining pH. In this study, we collected sediments from five stations of different depths (34–73 m) in a continental shelf of the Yellow Sea. The entire benthic foraminiferal communities together with sea sediments were cultured under three constant pHs (8.3, 7.8, and 7.3) for 6 and 12 weeks in the laboratory to study their responses to pH or incubation time. The microcosm’s experimental results obtained showed that most of the foraminiferal community parameters (abundance, species richness, Margalef index, and Shannon-Wiener diversity) decreased significantly (p<0.05) with the decline in pH in all the tested stations. The responses of foraminifera to the decline in pH were species-specific, for instance, Protelphidium tuberculatum and Cribroelphidiumfrigidum were highly sensitive to declining pH and were finally eliminated at low pH, while some species (e.g., Lagenammina atlantica, Verneuilinulla advena, V. propinqua, Haplophragmoides applanata, and H. canariensis) could tolerate low pH and acted as pH-tolerant species. In addition, the proportion of hyaline taxa showed a significant (p<0.05) positive correlation with pH, while agglutinated type showed a negative response. Furthermore, different incubation times (6 and 12 weeks) showed significant effects on the nearshore communities other than the offshore treatments, which were, however, entirely declined after 6 weeks’ incubation under low pH manipulation. Our results indicated that nearshore foraminiferal communities showed rather a resilience to the declining pH and the offshore foraminifera, especially those in the central area of the Yellow Sea Cold Water Mass were found to be more sensitive to the decline in pH in the continental shelf sediments of the Yellow Sea.

Continue reading ‘Changing structure of benthic foraminiferal communities due to declining pH: results from laboratory culture experiments’

Response of the Arctic marine inorganic carbon system to ice algae and under‐ice phytoplankton blooms: a case study along the fast‐ice edge of Baffin Bay

Past research in seasonally ice‐covered Arctic seas has suggested that ice algae play a role in reducing dissolved inorganic carbon (DIC) during spring, preconditioning surface waters to low dissolved CO2 (pCO2sw), and uptake of atmospheric CO2 during the ice‐free season. The potential role of under‐ice phytoplankton blooms on DIC and pCO2sw has not often been considered. In this study we examined the inorganic carbon system beneath landfast sea ice starting midway through a bottom ice algae bloom and concluding in the early stages of an under‐ice phytoplankton bloom. During most of the ice algae bloom we observed a slight increase in DIC/pCO2sw in surface waters, as opposed to the expected reduction. Biomass calculations confirm that the role of ice algae on DIC/pCO2sw in the study region were minor and that this null result may be widely applicable. During snow melt, we observed an under‐ice phytoplankton bloom (to 10 mg/m3 Chl a) that did reduce DIC and pCO2sw. We conclude that under‐ice phytoplankton blooms are an important biological mechanism that may predispose some Arctic seas to act as a CO2 sink at the time of ice breakup. We also found that pCO2sw was undersaturated at the study location even at the beginning of our sampling period, consistent with several other studies that have measured under‐ice pCO2sw in late winter or early spring. Finally, we present the first measurements of carbonate saturation states for this region, which may be useful for assessing the vulnerability of a local soft‐shelled clam fishery to ocean acidification.

Continue reading ‘Response of the Arctic marine inorganic carbon system to ice algae and under‐ice phytoplankton blooms: a case study along the fast‐ice edge of Baffin Bay’

Ocean acidification regulates the activity, community structure and functional potential of heterotrophic bacterioplankton in an oligotrophic gyre

Ocean acidification (OA), a consequence of increased global carbon dioxide (CO2) emissions, is considered a major threat to marine ecosystems. Its effects on bacterioplankton activity, diversity and community composition have received considerable attention. However, the direct impact of OA on heterotrophic bacterioplankton is often masked by the significant response of phytoplankton due to the close coupling of heterotrophic bacterioplankton and autotrophs. Here, we investigated the responses of a heterotrophic bacterioplankton assemblage to high pCO2 (790 ppm) treatment in warm tropical western Pacific waters by conducting a microcosm experiment in dark for 12 days. Heterotrophic bacterioplankton abundance and production were enhanced by OA over the first 6 days of incubation, while the diversity and species richness were negatively affected. Bacterioplankton community composition in the high pCO2 treatment changed faster than that in the control. The molecular ecological network analysis showed that the elevated CO2changed the overall connections among the bacterial community and resulted in a simple network under high CO2 condition. Species‐specific responses to OA were observed and could be attributed to the different life strategies and to the ability of a given species to adapt to environmental conditions. In addition, high‐throughput functional gene array analysis revealed that genes related to carbon and nitrogen cycling were positively affected by acidification. Together, our findings suggest that OA has direct effects on heterotrophic bacterioplankton in a low‐latitude warm ocean and may therefore affect global biogeochemical cycles.

Continue reading ‘Ocean acidification regulates the activity, community structure and functional potential of heterotrophic bacterioplankton in an oligotrophic gyre’

Contrasting effects of acidification and warming on dimethylsulfide concentrations during a temperate estuarine fall bloom mesocosm experiment

The effects of ocean acidification and warming on the concentrations of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) were investigated during a mesocosm experiment in the Lower St. Lawrence Estuary (LSLE) in the fall of 2014. Twelve mesocosms covering a range of pHT (pH on the total hydrogen ion concentration scale) from 8.0 to 7.2, corresponding to a range of CO2 partial pressures (pCO2) from 440 to 2900 µatm, at two temperatures (in situ and +5 ∘C; 10 and 15 ∘C) were monitored during 13 days. All mesocosms were characterized by the rapid development of a diatom bloom dominated by Skeletonema costatum, followed by its decline upon the exhaustion of nitrate and silicic acid. Neither the acidification nor the warming resulted in a significant impact on the abundance of bacteria over the experiment. However, warming the water by 5 ∘C resulted in a significant increase in the average bacterial production (BP) in all 15 ∘C mesocosms as compared to 10 ∘C, with no detectable effect of pCO2 on BP. Variations in total DMSP (DMSPt = particulate + dissolved DMSP) concentrations tracked the development of the bloom, although the rise in DMSPt persisted for a few days after the peaks in chlorophyll a. Average concentrations of DMSPt were not affected by acidification or warming. Initially low concentrations of DMS (<1 nmol L−1) increased to reach peak values ranging from 30 to 130 nmol L−1 towards the end of the experiment. Increasing the pCO2 reduced the averaged DMS concentrations by 66 % and 69 % at 10 and 15 ∘C, respectively, over the duration of the experiment. On the other hand, a 5 ∘C warming increased DMS concentrations by an average of 240 % as compared to in situ temperature, resulting in a positive offset of the adverse pCO2 impact. Significant positive correlations found between bacterial production and concentrations of DMS throughout our experiment point towards temperature-associated enhancement of bacterial DMSP metabolism as a likely driver of the mitigating effect of warming on the negative impact of acidification on the net production of DMS in the LSLE and potentially the global ocean.

Continue reading ‘Contrasting effects of acidification and warming on dimethylsulfide concentrations during a temperate estuarine fall bloom mesocosm experiment’

Spatio-temporal distribution of physicochemical and bacteriological parameters in the north area of Monastir bay, eastern coast of Tunisia

Temporal characterization of physicochemical and bacteriological parameters of the Monastir bay was conducted out on 12 stations, during six sampling periods in 2014. Results showed a seasonal variation on the physicochemical parameters of the water masses (temperature, salinity, oxygen, pH, and turbidity) and well-oxygenated waters. Results indicated the absence of mineral phosphorus and the presence of low concentration of organic phosphorus in the stations close the coastline. Mineral nitrogen represented completely by nitrate, and organic nitrogen was detected everywhere during all sampling periods without any particular distribution. Chlorophyll-a concentrations present at low ratio characterizing an oligotrophic ecosystem showed two peaks, one during spring (April, May) and second in fall (September), and were significantly correlated with temperature (R2 = 0.82). Statistical analysis of different physicochemical parameters showed a correlation between temperature pH and oxygen. ANOVA tests showed a significant difference inter-sampling periods and between stations. Bacterial flora is dominated by halotolerant germs, which showed higher concentrations in the southern part of the studied area and are inversely correlated with salinity, turbidity, oxygen, and organic nitrogen (respectively R2 = − 0.62; − 0.79; − 0.84; − 0.72). The same evolution pattern was observed in mesophilic non-halo-obligate microflora. The Vibrionaceae concentration was correlated with water temperature and was within the standards for marine waters. Fecal coliform bacteria are absent in the studied area during all sampling periods. No particularity in water quality was noticed in this ecosystem, which characterized a good state. However, one can say that the collected data on physicochemical and bacteriological evolution can provide baseline information for assisting management of the Monastir bay, which represented a typical and important model of south Mediterranean Sea.

Continue reading ‘Spatio-temporal distribution of physicochemical and bacteriological parameters in the north area of Monastir bay, eastern coast of Tunisia’


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