24.8 °C 27.9 °C
0204060Net photosynthesis [µg O2 L-1 h-1 m-2] −8−6−4−200204060
360 440 940 360 440 940
24.8 °C 27.9 °C
−1000100200300400Light calcification [µM C h-1 m-2] −500501001502000246
360 440 940 360 440 940
Respiration [µg O2 L-1 h-1 m-2] Gross photosynthesis [µg O2 L-1 h-1 m-2] Dark calcification [µM C h-1 m-2] Net calcification [mM C d-1 m-2]
pCO2 [µatm]
pCO2 [µatm]
Figure 6.4: Net photosynthesis, respiration and gross photosynthesis as well as light-, dark- and net calcification of epilithic communities after six months of temperature and acidification treatments. Data are normalized to the surface area of the epilithic communities
25.0/ 25.9 °C 26.1/ 27.9 °C 26.5/ 29.4 °C 27.7/ 30.7 °C27.7/ 30.7 °C
020406080
360 440 650 940 360 440 650 940 360 440 650 940 360 440 650 940 pCO2 [µatm]
Organism cover [%]
Peyssonnelia CCA
Figure 6.5: Percent final cover of CCA andPeyssonneliaspp. after six months exposure to temperature and acidification treatments
Effects of temperature
The observed increase in cover of the epilithic communities and the final cover of CCA under the mod-erate temperature profile (Fig. 6.3) indicates beneficial effects under future OW with GHG concentra-tions following RCP2.6-RCP4.5 (+0.9 °C). These results agree with elevated growth rates of epilithic communities and coralline algae observed in summer months, under increased temperature and irra-diance, compared to winter months (Klumpp, McKinnon, et al. 1992; Martin et al. 2006; Martin and Gattuso 2009). Potentially, enhanced physico-chemical and metabolic processes promote growth rates under moderately increased temperatures. Additionally, a higher saturation state of calcium carbonate (CaCO3) minerals at warmer temperatures may facilitate growth of calcifying organisms.
We measured significantly lower growth of epilithic communities and cover of CCA in the high temperature treatments, supporting prediction and previous data that CCA will be negatively affected by climate change (e.g. Anthony et al. 2008; Diaz-Pulido et al. 2012; Johnson and Carpenter 2012).
Our data also indicate that epilithic algal communities may already live close to their upper temperature tolerance limits, as it is the case in other coral reef organisms (Hoegh-Guldberg 1999), and an addi-tional increase of only 1.7 °C (RCP8.5) above present-day summer maxima removes the advantages of increased growth under moderate OW (+0.9 °C; RCP2.6-RCP4.5).
Peyssonneliaspp. cover was increased at highest compared to ambient and moderately increased temperature. Thus, Peyssonneliaspp. were most abundant at temperatures at which CCA were less abundant, indicatingPeyssonneliaspp. may benefit from OW under GHG concentrations at which CCA growth is impaired (i.e. RCP8.5, +1.7 °C). This hypothesis was additionally supported by significant negative correlations between cover ofPeyssonnelia spp. and CCA. Potentially, space limitation and competitive advantages of CCA over Peyssonneliaspp. at ambient and moderate temperature were removed at low and high temperature, so Peyssonneliaspp. could increase their relative abundance within the community. Thus, under future OW,Peyssonneliaspp. may increase in cover at the expense
of the more temperature sensitive CCA.
Effects of pCO2
Reduced growth and calcification of epilithic communities at elevated pCO2, as observed in the present study, has been well documented for a range of calcareous organisms including CCA species (Comeau et al. 2013; James et al. 2014; Kuffner et al. 2007; Orr et al. 2005). Decreases of light-, dark- and net calcification under increasing pCO2, as measured in the present study, may have been additionally exacerbated by a relative increase of lightly calcified Peyssonneliaover heavily calcified CCA under moderate and high pCO2 conditions. A reduction in pH, accompanied by a lowered availability of free carbonate ions, increases the energetic demands for calcification. Moreover, an altered carbonate chemistry with reducedΩis believed to impair calcification particularly for high-Mg-calcite organisms such as CCA (Kuffner et al. 2007). Reduced calcification rates under elevated pCO2 also reflect trends in growth rates of the epilithic communities and final cover of CCA, sensitive to OA, as measured in the present study.
Interestingly, net calcification rates were not lowered under past pCO2condition which is coherent with regards to carbonate chemistry (i.e. increased Ω), but is contrary to findings of lowered long-term growth of the epilithic communities as well as lowered final cover of CCA andPeyssonneliaspp.
in this particular treatment. The observation of reduced growth rates under past pCO2 is novel and unexpected, particularly for CCA, since the carbonate chemistry (i.e. elevatedΩ) in this treatment should facilitate calcification and consequently growth rates and cover. Results from a short-term study (two weeks) by Comeau et al. (2013) did not observe any effects of reduced CO2 on Hydrolithon onkodes orLithophyllum flavencens, while results from Diaz-Pulido et al. (2011) indicate mineral changes in CCA under reduced CO2conditions after an eight week experiment. So far, most studies, investigating the effects of OA on calcareous organisms, considered present-day and future (i.e. elevated) pCO2, but did not account for past (i.e. reduced) pCO2 conditions. As shown by Johnson et al. (2014) CCA are able to acclimatize to oscillating pCO2 conditions. But previous studies disregard the potential for acclimatization of organisms that may have already taken place from past to present-day conditions and the possibility that they may have potential to further acclimatize under rising pCO2.
Here, epilithic communities and CCA performed best under present-day pCO2 conditions, despite it being a less favorable condition for calcification, compared to past pCO2treatments (Table 6.1). As shown by Albright et al. (2013) reef water from the GBR has substantial fluctuations in pCO2due to diur-nal changes in net carbon production of organisms and due to seasodiur-nal changes in seawater temperature with values well above global average (∼397 µatm; Tans and Keeling (2015)). Moreover, particularly at inshore locations of the GBR, pCO2 concentrations increased two- to three times faster and are
al-ready higher than the global average, potentially due to anthropogenically induced trophic changes in the water column and benthos, as suggested by Uthicke et al. (2014). Thus, CCA may already have physiologically acclimatized to higher pCO2in their natural habitat. Potentially, CCA prefer a DIC op-timum for photosynthesis, calcification and growth which appears to be in the range of the present-day treatment of this study. Changes in carbonate chemistry diverging from present-day condition may have implications on other cellular processes than calcification and may negatively influence growth rates, as observed here. Nevertheless, epilithic communities are likely to reduce calcification, carbon fixation, and carbonate production in future high pCO2 environments following RCP8.5. Consequently, high pCO2 may have implications on carbonate production by epilithic communities, on structural proper-ties of future coral reefs and their resilience against- as well as their recovery after disturbances such as tropical cyclones. The question remains to which extent CCA are capable to acclimatize to further increasing pCO2.
Non-linear calcification responses of CCA have been reported elsewhere (Johnson and Carpenter 2012; Ries et al. 2009) and CCA may be able to acclimatize and to protect themselves from decreasing Ωand associated dissolution under OW and OA conditions by increasing the more stable dolomite, over the more soluble high-Mg-calcite, as their skeletal mineral (Diaz-Pulido et al. 2014). Dolomite-rich CCA have been shown to better resist OA conditions (Nash et al. 2013). These findings agree with the observed enhanced performance and thus potential acclimatization of CCA from past to present-day pCO2 conditions, in the present study. However, findings from tropical CO2 seeps indicate reduced abundance of CCA in OA conditions of∼750 µatm pCO2, suggesting no acclimatization after life-long exposure to high levels of seawater CO2(Fabricius et al. 2011). Results from the latter study, combined with findings of the present study, indicate acclimatization is possible to a certain extent (i.e. past to present-day and potentially to moderate pCO2), but not to high pCO2. These results emphasize the efforts to reduce anthropogenic GHG emissions in order to keep future GHG concentrations below the RCP2.6–RCP4.5 to preserve CCA on future coral reefs.
Because Peyssonneliaspp. cover was not significantly lowered at high compared to present-day pCO2, they may be less impacted under pCO2 concentrations following RCP8.5, compared to CCA.
Potential reasons for the advantage gained by Peyssonellia spp. over CCA include the overcoming of space limitation and interspecific competition. Additionally, the deposition of lightly compared to heavily calcified skeletons, as well as the more stable aragonite compared to the more soluble high-Mg-calcite of CCA, may lead to competitive advantages ofPeyssonneliaspp. over CCA in a high pCO2 world.
Interactive effects of temperature and pCO2
The increase in community net calcification at high temperature/present-day pCO2and the decrease in low temperature/high pCO2agree with theoretical and observed optima of carbonate chemistry (i.e. Ω) for calcification (Table 6.1). Community net calcification is improved under higher temperature and lower pCO2levels compared to lower temperatures and higher pCO2. Similarly, dark calcification rates were lowest at low temperature/high pCO2 and highest at high temperature/past pCO2, as anticipated from the particular carbonate chemistry in these treatments. Additional respiratory CO2release from the organisms in the dark may have exacerbated the negative effects of reducedΩdue to low temperature and OA, leading to drastic declines in calcification if the epilithic communities.Peyssonneliaspp. cover was promoted by high temperature and affected by past, but not increased pCO2, indicatingPeyssonneliaspp.
may benefit in future environments under increasing OW and OA following RCP8.5. ThusPeyssonnelia spp. may better acclimatize and subsequently have competitive advantages over other organisms, such as CCA, under future conditions. In addition, decreased cover of CCA under both factors, OW and OA, suggest additive negative effects on CCA in future environmental conditions following RCP4.5-RCP8.5, resulting in decreased abundance of CCA. The observed decrease in gross photosynthetic rates of the epilithic community under high temperature and present day pCO2 treatment was unexpected and potentially derived from normalization to relative surface area of organisms with relatively high community cover of these substrates. This pattern was not seen in absolute gross photosynthetic rates (Fig. 6.6 in supplementary material).
Epilithic communities were affected by temperature, pCO2and the interaction of both and are likely to drastically change under future environmental conditions. Acclimatization of CCA andPeyssonnelia spp. may have been happening from past to present-day environmental conditions, as suggested in the present experiment, but it is uncertain to what extent acclimatization may occur in the future. Heavily calcified CCA were performing best under present-day (pCO2) and moderate (temperature) environmen-tal conditions and may transitionally gain advantages by warming in the near future, but they are likely to be negatively impacted under high GHG concentrations in the long term. Considerable implications on their calcification rates and abundance will additionally reduce future reef resilience and recovery.
Lightly calcified Peyssonneliaspp. showed higher resilience to changing conditions and may benefit under high GHG concentrations in the long term. However, it is unlikely thatPeyssonneliaspp. will be able to fill ecological gaps emerging from decreasing CCA.Peyssonneliaspp. have been shown to induce settlement and metamorphosis of larvae from some coral species (Heyward and Negri 1999), but studies also showed thatPeyssonnelia are less suitable as settlement substrate compared to CCA (Diaz-Pulido et al. 2010). Peyssonneliamay constitute a less stable substrate for larvae settlement and may impede access of settled corals to the calcified reef substrate (Littler and Littler 1984). Lowered
absolute calcification and productivity (see Figure 6.6 in supplementary material) of the epilithic com-munities under high temperature and high pCO2 indicates reduced reef stability, productivity, nutrient recycling and consequently biodiversity on future coral reefs.
Acknowledgments
We want to thank Jordan Hollarsmith, Emmett Clarkin, Camille Domy, Cassy Thompson, Patrick Buerger, Kathryn Berry, Laura Arthur and Caroline Assailly for their great help in maintaining the aquaria system. Many thanks to the staff at the SeaSim facility and the AIMS workshop, Andrea Severati, Tom Barker, Paul Boyd, Craig Humphrey, Eneour Puill-Stephan, Grant Milton, Justin Hochen, Niall Jeeves, Michael Kebben and Gary Brinkman who contributed to the aquarium design, control sys-tems and monitoring of the experimental conditions. Thanks to Lindsay Harrington for her help with the identification of CCA and epilithic organisms. Thanks to Michelle Liddy and Florita Flores for their help with the incubation experiments and general assistance. This study was funded by the Australian Institute of Marine Science, the Australian Government’s National Environmental Research Program and a Super Science Fellowship grant from the Australian Research Council.
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Supplementary material
24.8 °C 27.9 °C
010203040Net photosynthesis [µg O2 L-1 h-1 m-2] −6−4−20010203040
360 440 940 360 440 940 µatm
24.8 °C 27.9 °C
050100150200Light calcification [µM C h-1 m-2] −2002040600123
360 440 940 360 440 940
Respiration [µg O2 L-1 h-1 m-2] Gross photosynthesis [µg O2 L-1 h-1 m-2] Dark calcification [µM C h-1 m-2] Net calcification [mM C d-1 m-2]
µatm
Figure 6.6: Net photosynthesis, dark respiration, gross photosynthesis and light-, dark- and net calci-fication of epilithic communities after six months of temperature and acidicalci-fication treatment. Data are normalized to the total surface area of the substrate
General Discussion
The overall objective of this thesis was to fill current knowledge gaps in ocean acidification (OA) research in order to gain a better understanding of future coral reef ecosystems under environmental change. To achieve this, several field- and laboratory-based studies were conducted to investigate the effects of OA and the effects of OA in combination with other stressors, namely decreased light availabil-ity, increased dissolved inorganic nutrients (DIN) and ocean warming (OW), on photosynthesizing and calcifying coral reef organisms. The summarized results (Table 7.1 and 7.2) of individual and interactive environmental stressors are utilized to predict effects on coral reef ecosystems in this rapidly changing environment and to provide suggestions for future research and global and coastal management plans.
The particular objectives of this thesis were to investigate:
1. how photosynthesizing and calcifying coral reef organisms are affected by future ocean acidifica-tion scenarios, and whether they respond differently to ocean acidificaacidifica-tion condiacidifica-tions.
2. how decreased light availability affects the response of photosynthesizing and calcifying coral reef organisms to ocean acidification.
3. whether increased dissolved inorganic carbon and -nitrogen have interactive effects on photosyn-thesizing and calcifying coral reef organisms.
4. how photosynthesizing and calcifying coral reef organisms and their communities respond to com-binations of past and future ocean acidification and warming scenarios.
7.1 Responses of coral reef organisms to ocean acidification
As shown in the studies of the present thesis, the organisms of the different experiments varied in their responses to OA conditions between, but also within organism groups (Table 7.2). CoralAcropora mille-porawas negatively affected by OA in its growth rates and net calcification (Chapter 4), while corals
Table 7.1: Summary of experimental results from the different studies presented in this thesis
Ocean acidification Ocean acidification and decreased light
availability
Ocean acidification and eutrophication
Ocean acidification and warming
Foraminifera:
- increased growth under OA in M. vertebralis - no effects of OA on photobiology of foraminifera investigated
Halimeda spp.:
- no effect of OA on net calcification
- increased light
calcification under OA in H. digitata and H.
opuntia - reduced dark
calcification under OA in H. opuntia
- increased Cinorg content under OA in H. digitata - reduced Cinorg content in H. opuntia
- reduced δ13C under OA in H. digitata and H.
opuntia
Corals:
A. millepora
- reduced growth and net calcification under OA and low light (additive effects) - no effect of OA on light calcification
- reduced light
calcification in low light - reduced dark calcification under OA and low light (additive effects) - reduced net- and gross photosynthesis and respiration in low light Algae:
H. opuntia
- no OA effect on growth and net calcification - reduced growth in low light
- no effect of OA on calcification in light - reduced dark calcification under OA
- reduced net- and gross photosynthesis and - increased pigment content in low light
Corals:
A. tenuis - no effects on calcification and photosynthetic rates - increased dark and net NOx uptake in DIN treatments
- increased pigment content in DIN treatments
S. hystrix - no effects on calcification rates - increased photosynthetic rates in DIN treatments - increased NOx uptake in light and dark in DIN treatments
- increased pigment content in DIN treatments - increased protein content in DIN treatments Algae:
H. opuntia - increased net photosynthesis in DIN treatments
- increased light NOx
uptake in DIN treatments - increased pigment content in DIN treatments - increased Corg and N content and reduced C:N ratio in DIN treatments
Epilithic community:
- highest growth under moderate temperature increase
- lowest growth under pCO2
other than present-day conditions
- reduced photosynthesis under high temperature - reduced photosynthesis at present-day compared to past pCO2
- reduced calcification under high pCO2
- highest net calcification at high temperature/present-day pCO2
- lowest net calcification at low temperature/high pCO2
- changes in community composition
CCA:
- highest cover under moderate temperature increase
- reduced cover under pCO2
other than present-day conditions
Peyssonnelia spp.:
- increased cover at high temperature
- reduced cover at past pCO2
- cover negatively correlated to CCA
Acropora tenuisandSeriatopora hystrixdid not show any negative effects on the response parameters measured (Chapter 5). Similarly, previous literature indicates varying responses of coral calcification to OA. Some studies showed negative effects (e.g. Renegar and Riegl 2005; Marubini et al. 2008; Comeau et al. 2013b), while others showed no impacts of OA on different coral species (e.g. Cohen and Fine 2012; Comeau et al. 2013a; Comeau et al. 2014b). These different effects of OA on corals indicate species-specific responses. As previously shown, branching corals were particularly reduced and thus impacted by OA conditions at tropical CO2 seeps, while massive species were less affected (Fabricius et al. 2011; Fabricius et al. 2014). This could also indicate that the growth form/morphology influences
Table 7.2: Summary of significant treatment effects on response variables of experimental species from the different studies presented in this thesis. Green arrows indicate an increase, red arrows a decrease and circles no significant treatment effect on response parameters compared to control conditions
Organism/s Response variable OA Low light High DIN OW Corals:
A. millepora Growth ↓ ↓ - -
Net calcification ↓ ↓ - -
Light calcification o ↓ - -
Dark calcification ↓ ↓ - -
Net photosynthesis o ↓ - -
Dark respiration o ↓ - -
Gross photosynthesis o ↓ - -
A. tenuis Dark NOx flux o - ↑ -
Net NOx flux o - ↑ -
Pigment content o - ↑ -
S. hystrix Net photosynthesis o - ↑ -
Dark respiration o - ↑ -
Gross photosynthesis o - ↑ -
Dark NOx flux o - ↑ -
Pigment content o - ↑ -
Protein content o - ↑ -
Halimeda spp.:
H. digitata Light calcification ↑ - - -
Ctot ↓ - - -
Corg ↓ - - -
Cinorg ↑ - - -
Corg:Cinorg ↓ - - -
δ13C ↓ - - -
H. opuntia Growth o ↓ o -
Light calcification ↑/o o o -
Dark calcification ↓/o o o -
Net photosynthesis ↑/o ↓ ↑ -
Dark respiration o ↓ o -
Gross photosynthesis o ↓ ↑ -
Light NOx flux o - o -
Dark NOx flux o - ↑ -
Net NOx flux o - ↑ -
Pigment content o ↑ ↑ -
Corg o - ↑ -
Cinorg ↓ - ↓ -
N o - ↑ -
C:N o - ↓ -
δ13C ↓ - - -
Epilithic algae
Community Growth ↓ - - ↓
Light calcification ↓ - - o
Dark calcification ↓ - - ↑
Net calcification ↓ - - o
CCA Final cover ↓ - - ↓
Peyssonneliaspp. Final cover o - - ↑
Foraminifera
M. vertebralis Growth ↑ - - -
the responses of corals to OA. Branching species have a higher surface area to volume ratio compared to massive species and thus are more exposed to their physical environment. Higher exposure may make
them more susceptible to OA conditions and explain the different responses among corals. Yet, studies also showed different responses within corals of the same genera and similar morphology. While A.
tenuisandS. hystrixwere unaffected by OA conditions in the study of the present thesis (Chapter 5), Acropora cervicorniswas negatively affected in a study by Renegar and Riegl (2005), and S. hystrix was impacted by OA at tropical CO2 seeps (Strahl et al. pers. comm.). This indicates that other fac-tors may also contribute to the responses of corals to OA. The differences in the experimental setup implemented between the present study and the study by Renegar and Riegl (2005) indicate environ-mental/experimental factors, such as temperature, or seawater supply, contributed to the different results of the two experiments. Both, generally higher calcification rates and higherΩarin warmer temperatures (28.4 vs. 25.3 °C) may have resulted in no observed effects of OA in the present compared to the other study (Renegar and Riegl 2005). The continuous supply of nutrition by flow-through conditions may have increased the nutritional status of the corals in the present study and thus lowered its responses to OA compared to the other study (Renegar and Riegl 2005). Moreover, other experimental factors such as duration, light regimes, or the method used to change the carbonate chemistry, potentially contribute to organisms’ responses to OA. Thus, it is important to choose experimental conditions which closely resemble natural environments in order to extrapolate experimental results to coral reef ecosystems.
The calcifying green algaHalimeda opuntiawas not negatively impacted in its growth or net calcifi-cation, under OA conditions, in the studies of the present thesis (Chapter 3, 4 and 5). In addition, several otherHalimedaspecies were still able to grow and calcify under OA conditions at tropical volcanic CO2 seeps in Papua New Guinea (PNG) (Chapter 3). While these results concur with some previous literature (e.g. Comeau et al. 2013b; Hofmann et al. 2014), other studies suggest negative impacts of OA on some Halimedaspecies (e.g. Hall-Spencer et al. 2008; Price et al. 2011; Sinutok et al. 2011). As discussed above, experimental factors may contribute to the variable responses of organisms to OA, suggesting thatHalimedaspp. respond differently in artificial compared to natural experimental conditions. Yet, the finding thatHalimedaspp. were absent at temperate CO2seeps in the Mediterranean (Hall-Spencer et al. 2008), while they were unaffected at tropical CO2 seeps in PNG (Chapter 3), indicates regional-specific responses ofHalimedaspp. to OA may occur. Warmer and more constant water temperatures throughout the year in the tropics may contribute to the observed differences. Overall, these results suggest that several tropicalHalimedaspecies are unlikely to be impacted by OA alone in the future.
Previous results from manipulative experiments, which showed negative OA effects on theseHalimeda species, may have to be re-evaluated with respect to their extrapolation potential to natural environ-ments. Nevertheless, comparisons between similar studies also indicated regional-specific responses of Halimedaspp. towards OA.
Epilithic communities that were investigated in the present thesis (Chapter 6) showed negative
ef-fects of OA on growth and calcification rates. Particularly, crustose coralline algae (CCA) showed reduced cover, while other red algae of the genus Peyssonneliawere unaffected by high pCO2 condi-tions. The few studies available on CCA responses to OA suggest decreased calcification at high pCO2 (Diaz-Pulido et al. 2012; Comeau et al. 2013b), but also non-linear responses with higher calcification rates at moderately increased pCO2 (Ries et al. 2009; Johnson and Carpenter 2012), or even signs of acclimatization to elevated pCO2 conditions (Nash et al. 2013; Diaz-Pulido et al. 2014). The fact that CCA were impacted by high pCO2, whilePeyssonneliaspp. were not, may be explained by utilization of the less stable mineral for calcification (high-Mg-calcite vs. aragonite) or the generally higher calci-fication intensity of CCA compared toPeyssonneliaspp. Notably, growth of the community and cover of CCA were reduced under lowered pCO2as well as under increased pCO2 compared to present-day conditions. The observed reduced growth in reduced pCO2was novel and unexpected since the carbon-ate chemistry in this treatment should have facilitcarbon-ated calcification and growth of calcareous organisms.
This observation indicates: (1) the community and CCA are acclimatized to ambient pCO2 conditions, and alterations in seawater carbonate chemistry (regardless if reduced or increased) lead to decreased growth/calcification rates; (2) potential acclimatization has been happening from past to present-day pCO2 conditions; (3) there are optimal conditions for the interplay of photosynthesis and calcification in slightly elevated pCO2 environments, as experienced in the present-day compared to the past pCO2 treatment. These findings are supported by other studies that observed non-linear calcification responses (Johnson and Carpenter 2012) and acclimatization of CCA by changes in the carbonate mineralogy (Nash et al. 2013; Diaz-Pulido et al. 2014). Yet, observations from CO2seeps in PNG indicate reduced CCA abundance at high CO2. Overall, the results from the present thesis and literature suggest that ac-climatization of CCA may have been happening from past to present-day pCO2and that it may happen to some extend in the future. But acclimatization to high pCO2, as projected under the RCP6.0-RCP8.5 by the end of this century, is unlikely.
The large benthic foraminiferal species were not negatively affected by elevated pCO2 (Chapter 2).
In the present study, the investigated species experienced positive shell growth and thus were still able to calcify under future OA conditions after short-term (six weeks) exposure to elevated pCO2. Contrary to expectations,M. vertebraliscould even increase its growth rates under elevated pCO2which agrees with another study that showed increased growth of several other foraminiferal species in intermediate pCO2treatments (Fujita et al. 2011). But the ability of these foraminifera to withstand or benefit from short-term exposure to elevated pCO2 is no guarantee for their survival in the long term. Results from field studies showed decreasing diversity of foraminifera under lowered pH at temperate CO2 seeps in the Mediterranean (Dias et al. 2010) and absence of foraminifera at tropical CO2seeps in PNG (Uthicke et al. 2013). Thus, results from short-term and long-term studies have to be distinguished.