Posts Tagged 'mesocosms'

Acidification and warming affect prominent bacteria in two seasonal phytoplankton bloom mesocosms

In contrast to clear stimulatory effects of rising temperature, recent studies of the effects of CO2 on planktonic bacteria have reported conflicting results. To better understand the potential impact of predicted climate scenarios on the development and performance of bacterial communities, we performed bifactorial mesocosm experiments (pCO2 and temperature) with Baltic Sea water, during a diatom dominated bloom in autumn and a mixed phytoplankton bloom in summer. The development of bacterial community composition (BCC) followed well-known algal bloom dynamics. A principal coordinate analysis (PCoA) of bacterial OTUs (operational taxonomic units) revealed that phytoplankton succession and temperature were the major variables structuring the bacterial community whereas the impact of pCO2 was weak. Prokaryotic abundance and carbon production, and organic matter concentration and composition were partly affected by temperature but not by increased pCO2. However, pCO2 did have significant and potentially direct effects on the relative abundance of several dominant OTUs; in some cases, these effects were accompanied by an antagonistic impact of temperature. Our results suggest the necessity of high-resolution BCC analyses and statistical analyses at the OTU level to detect the strong impact of CO2 on specific bacterial groups, which in turn might also influence specific organic matter degradation processes.

Continue reading ‘Acidification and warming affect prominent bacteria in two seasonal phytoplankton bloom mesocosms’

Iron availability modulates the effects of future CO2 levels within the marine planktonic food web

Ocean acidification (OA) due to increased anthropogenic CO2 emissions is affecting marine ecosystems at an unprecedented rate, altering biogeochemical cycles. Direct empirical studies on natural communities are required to analyse the interactive effects of multiple stressors while spanning multiple trophic levels. We investigated the interactive effects of changes in CO2 and iron availability on functional plankton groups. We used mesocosms manipulating the carbonate system from the start to achieve present (low concentration, LC) and predicted future pCO2 levels (high concentration, HC). To manipulate dissolved iron (dFe), half of the mesocosms were amended with 70 nM (final concentration) of the siderophore desferoxamine B (DFB) on Day 7 (+DFB and -DFB treatments). Manipulation of both CO2 and DFB increased dFe compared to the control. During the 22 experimental days, the plankton community structure showed 2 distinct phases. In phase 1 (Days 1-10), only bacterioplankton abundances increased at elevated pCO2. In contrast, a strong community response was evident in phase 2 (Days 11-22) due to DFB addition. Biomass of the coccolithophore Emiliania huxleyi increased massively at LC+DFB. HC negatively affected E. huxleyi and Synechococcus sp., and high dFe (+DFB) had a positive effect on both. The rest of the plankton community was unaffected by the treatments. Increased dFe partially mitigated the negative effect of HC imposed on the coccolithophores, indicating that E. huxleyi was able to acclimate better to OA. This physiological iron-mediated acclimation can diminish the deleterious effects of OA on carbon export and the rain ratio, thus affecting food web dynamics and future ecosystem functioning.

Continue reading ‘Iron availability modulates the effects of future CO2 levels within the marine planktonic food web’

The influence of CO2 enrichment on net photosynthesis of seagrass Zostera marina in a brackish water environment

Seagrasses are distributed across the globe and their communities may play key roles in the coastal ecosystems. Seagrass meadows are expected to benefit from the increased carbon availability which might be used in photosynthesis in a future high CO2 world. The main aim of this study was to examine the effect of elevated pCO2 on the net photosynthesis of seagrass Zostera marina in a brackish water environment. The short-term mesocosm experiments were conducted in Kõiguste Bay (northern part of Gulf of Riga, the Baltic Sea) in June–July 2013 and 2014. As the levels of pCO2 naturally range from ca. 150 μatm to well above 1000 μatm under summer conditions in Kõiguste Bay we chose to operate in mesocosms with the pCO2 levels of ca. 2000, ca. 1000, and ca. 200 μatm. Additionally, in 2014 the photosynthesis of Z. marina was measured outside of the mesocosm in the natural conditions. In the shallow coastal Baltic Sea seagrass Z. marina lives in a highly variable environment due to seasonality and rapid changes in meteorological conditions. This was demonstrated by the remarkable differences in water temperatures between experimental years of ca. 8°C. Thus, the current study also investigated the effect of elevated pCO2 in combination with short-term natural fluctuations of environmental factors, i.e., temperature and PAR on the photosynthesis of Z. marina. Our results show that elevated pCO2 alone did not enhance the photosynthesis of the seagrass. The photosynthetic response of Z. marina to CO2 enrichment was affected by changes in water temperature and light availability.

Continue reading ‘The influence of CO2 enrichment on net photosynthesis of seagrass Zostera marina in a brackish water environment’

Special edition of Estuarine, Coastal and Shelf Science – “Ocean acidification in the Mediterranean Sea: pelagic mesocosm experiments”

The topic of ocean acidification has received extensive attention in a recently published special edition of the journal Estuarine, Coastal and Shelf Science. Volume 186, Part A presents a series of 12 research papers focusing on pelagic mesocosm experiments conducted in the Mediterranean Sea in 2012 and 2013. Plankton plays a key role in the global carbon cycle. It is therefore important to project the evolution of plankton community structure and function in a future high-CO2 world. Several results from experiments conducted at the community level have shown increased rates of community primary production and shifts in community composition as a function of increasing pCO2. However, the great majority of these – experiments have been performed under high natural or nutrient-enriched conditions and very few data are available in areas with naturally low levels of nutrient and chlorophyll i.e. oligotrophic areas such as the Mediterranean Sea, although they represent a large and expanding part of the ocean surface. In the frame of the European Mediterranean Sea Acidification in a changing climate project (MedSeA; http://medsea-project.eu), large-scale in situ mesocosms (9 x 50 m3, 12 m deep) have been used to quantify the potential effects of CO2 enrichment in two coastal areas of the Mediterranean Sea: the bay of Calvi (Corsica, France) in June/July 2012 and the bay of Villefranche (France) in February/March 2013. These two experiments gathered the expertise of more than 25 scientists from 7 institutes and 6 countries (France, Greece, Spain, UK, Italy, Belgium, US).

Continue reading ‘Special edition of Estuarine, Coastal and Shelf Science – “Ocean acidification in the Mediterranean Sea: pelagic mesocosm experiments”’

Influence of ocean acidification on plankton community structure during a winter-to-summer succession: An imaging approach indicates that copepods can benefit from elevated CO2 via indirect food web effects

Plankton communities play a key role in the marine food web and are expected to be highly sensitive to ongoing environmental change. Oceanic uptake of anthropogenic carbon dioxide (CO2) causes pronounced shifts in marine carbonate chemistry and a decrease in seawater pH. These changes–summarized by the term ocean acidification (OA)–can significantly affect the physiology of planktonic organisms. However, studies on the response of entire plankton communities to OA, which also include indirect effects via food-web interactions, are still relatively rare. Thus, it is presently unclear how OA could affect the functioning of entire ecosystems and biogeochemical element cycles. In this study, we report from a long-term in situ mesocosm experiment, where we investigated the response of natural plankton communities in temperate waters (Gullmarfjord, Sweden) to elevated CO2 concentrations and OA as expected for the end of the century (~760 μatm pCO2). Based on a plankton-imaging approach, we examined size structure, community composition and food web characteristics of the whole plankton assemblage, ranging from picoplankton to mesozooplankton, during an entire winter-to-summer succession. The plankton imaging system revealed pronounced temporal changes in the size structure of the copepod community over the course of the plankton bloom. The observed shift towards smaller individuals resulted in an overall decrease of copepod biomass by 25%, despite increasing numerical abundances. Furthermore, we observed distinct effects of elevated CO2 on biomass and size structure of the entire plankton community. Notably, the biomass of copepods, dominated by Pseudocalanus acuspes, displayed a tendency towards elevated biomass by up to 30–40% under simulated ocean acidification. This effect was significant for certain copepod size classes and was most likely driven by CO2-stimulated responses of primary producers and a complex interplay of trophic interactions that allowed this CO2 effect to propagate up the food web. Such OA-induced shifts in plankton community structure could have far-reaching consequences for food-web interactions, biomass transfer to higher trophic levels and biogeochemical cycling of marine ecosystems.

Continue reading ‘Influence of ocean acidification on plankton community structure during a winter-to-summer succession: An imaging approach indicates that copepods can benefit from elevated CO2 via indirect food web effects’

Estimates of ikaite export from sea ice to the underlying seawater in a sea ice–seawater mesocosm

The precipitation of ikaite and its fate within sea ice is still poorly understood. We quantify temporal inorganic carbon dynamics in sea ice from initial formation to its melt in a sea ice–seawater mesocosm pool from 11 to 29 January 2013. Based on measurements of total alkalinity (TA) and total dissolved inorganic carbon (TCO2), the main processes affecting inorganic carbon dynamics within sea ice were ikaite precipitation and CO2 exchange with the atmosphere. In the underlying seawater, the dissolution of ikaite was the main process affecting inorganic carbon dynamics. Sea ice acted as an active layer, releasing CO2 to the atmosphere during the growth phase, taking up CO2 as it melted and exporting both ikaite and TCO2 into the underlying seawater during the whole experiment. Ikaite precipitation of up to 167 µmolkg−1 within sea ice was estimated, while its export and dissolution into the underlying seawater was responsible for a TA increase of 64–66 µmolkg−1 in the water column. The export of TCO2 from sea ice to the water column increased the underlying seawater TCO2 by 43.5 µmolkg−1, suggesting that almost all of the TCO2 that left the sea ice was exported to the underlying seawater. The export of ikaite from the ice to the underlying seawater was associated with brine rejection during sea ice growth, increased vertical connectivity in sea ice due to the upward percolation of seawater and meltwater flushing during sea ice melt. Based on the change in TA in the water column around the onset of sea ice melt, more than half of the total ikaite precipitated in the ice during sea ice growth was still contained in the ice when the sea ice began to melt. Ikaite crystal dissolution in the water column kept the seawater pCO2 undersaturated with respect to the atmosphere in spite of increased salinity, TA and TCO2 associated with sea ice growth. Results indicate that ikaite export from sea ice and its dissolution in the underlying seawater can potentially hamper the effect of oceanic acidification on the aragonite saturation state (Ωaragonite) in fall and in winter in ice-covered areas, at the time when Ωaragonite is smallest.

Continue reading ‘Estimates of ikaite export from sea ice to the underlying seawater in a sea ice–seawater mesocosm’

Impact of environmental factors on bacterioplankton communities

Aquatic bacteria are main drivers of biogeochemical cycles and contribute predominantly to organic matter and nutrient recycling. As a high biodiversity is assumed to stabilize ecosystem functioning, it is necessary to understand the bacterial community dynamics and their structuring factors. It is known that different taxa are dominant across different habitats and seasons. This indicates an occurrence of species sorting by community structuring environmental factors. A first attempt for the understanding of bacterial distribution is to test for a correlation between microbial composition and measured environmental variables. In order to get further insights into the impact of environmental factors on bacterial communities, this thesis assessed the influence of major structuring drivers by using 16S rRNA gene amplicon sequencing, bacterial bulk parameters and interdisciplinary approaches in laboratory experiments and field studies.

In a field study in the Benguela upwelling system, the influence of different levels of primary production and the planktonic succession on bacterial community composition and its development was investigated. Community analysis revealed a clustering of different microbial assemblages along aging upwelled water. This zonation was mainly driven by phytoplankton composition and abundance and the spatial differences were comparable with a temporal succession that occurs during phytoplankton blooms in temperate coastal waters. A dominance of Bacteroidetes and Gammaproteobacteria was observed during algal blooming and high abundance of “Pelagibacterales” was found in regions with low algal abundance. Overall, this study highlightes the strong impact of quality and quantity of phytoplankton and nutrients on the bacterial communities.

A laboratory experiment with Baltic Sea water was performed to better understand the potential impact of rising temperature and CO2 on planktonic bacteria. The development of the bacterial community composition was followed in bifactorial mesocosm experiments during a diatom bloom in autumn and a phytoplankton bloom in summer. The results confirmed that phytoplankton succession and temperature were the major variables structuring the bacterial community. The impact of CO2 on the broad community was weak but high-resolution community analyses revealed a strong effect on specific bacterial groups, which might play important roles in specific organic matter degradation processes.

The response of bacterial communities to a disturbance by a saline intrusion could be investigated during a major Baltic inflow event. Community structuring factors were dominated by mixing of the inflow water with the former bottom water. Although the inflow had a selecting effect on the bacterial community, some immigrated taxa showed increased potential activity and seem to profit from changing environmental conditions. These results suggest a potential impact of inflow events on bacterial functions and therefore on biogeochemical processes.

Altogether, the results confirm the strong structuring effects of environmental conditions on bacterial community composition. Furthermore, high-resolution sequencing enabled an identification of specific affected taxa, which in turn give first clues for the impact of the investigated factors on specific bacterial functions.

Continue reading ‘Impact of environmental factors on bacterioplankton communities’


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Ocean acidification in the IPCC AR5 WG II

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