Coral and mollusc resistance to ocean acidification adversely affected by warming

Increasing atmospheric carbon dioxide (CO₂) concentrations are expectedto decrease surface ocean pH by 0.3–0.5 units by 2100, lowering the carbonate ion concentration of surfacewaters. This rapid acidification is predicted to dramatically decrease calcification in many marine organisms. Reduced skeletal growth under increased CO₂ levels has already been shown for corals, molluscs and many other marine organisms. The impact of acidification on the ability of individual species to calcify has remained elusive, however, as measuring net calcification fails to disentangle the relative contributions of gross calcification and dissolution rates on growth. Here, we show that corals and molluscs transplanted along gradients of carbonate saturation state at Mediterranean CO₂ vents are able to calcify and grow at even faster than normal rates when exposed to the high CO₂ levels projected for the next 300 years. Calcifiers remain at risk, however, owing to the dissolution of exposed shells and skeletons that occurs as pH levels fall. Our results show that tissues and external organic layers play a major role in protecting shells and skeletons from corrosive sea water, limiting dissolution and allowing organisms to calcify. Our combined field and laboratory results demonstrate that the adverse effects of global warming are exacerbated when high temperatures coincide with acidification.

Biodiversity and ecosystems: Acid ocean cover up

The response to ocean acidification varies widely among, and even within, calcifying taxa. A study sheds light on this perplexing variability by quantifying the role of external organic layers in protecting calcified structures from corrosive sea water.

Monitoring pH of seawater in the Adriatic sea. Results from a regional observing effort

Ocean acidification in the Mediterranean Sea is expected to have a rapid response to the climate change, being its hydrological balance deficient and anthropogenic forcing high. So the basin can be regarded as a key site very sensitive to the climate variability, especially in dense cold water formation areas. In this frame, the Adriatic Sea can play a very relevant role for the entire Eastern Mediterranean Sea, since in winter is exposed to cold dry winds eventually producing dense waters (NAdDW, ADW), which are important contributors to the Eastern Mediterranean deep circulation. Here are presented results of two repeated surveys, on mesoscale, and those of two short timeseries, one representative of a coastal environment and the other of open sea. Preliminary results, based on spectrophotometric analysis of pH (total scale, precision of ± 0.001 pHT units), indicate the two times series reflect dynamics and processes of their respective environments.

Dissolved carbon dioxide, nutrients and oxygen in the Adriatic sea. A regional observing effort

The Adriatic Sea can be strongly affected by changes in climate and weather, it can play an important role in biological productivity and air-sea CO2 fluxes,. In general the carbon cycling knowledge in these region is still limited., Regarding the atmospheric CO2 sequestration mechanism, through the Continental Shelf Pump, the Adriatic Sea can play a very relevant role for the entire Eastern Mediterranean, since in winter is exposed to cold dry winds eventually producing dense waters (NAdDW and ADW), which are important contributors to the Eastern Mediterranean deep circulation. Here are presented the results (fCO2, AOU, dissolved inorganic nutrients) of two repeated surveys on basin scale, and one short time series in the Gulf of Trieste, representative of the coastal environment.

Calcification and organic production on a Hawaiian coral reef

Net ecosystem calcification rates (NEC) and net photosynthesis (NP) were determined from CO₂ seawater parameters on the barrier coral reef of Kaneohe Bay, Oahu, Hawaii. Autosamplers were deployed to collect samples on the barrier reef every 2 hours for six 48-hour deployments, two each in June 2008, August 2009, and January/February 2010. NEC on the Kaneohe Bay barrier reef increased throughout the day and decreased at night. Net calcification continued at low rates at night except for six time periods when net dissolution was measured. The barrier reef was generally net photosynthetic (positive NP) during the day and net respiring (negative NP) at night. NP controlled the diel cycles of the partial pressure of CO₂ (pCO₂) and aragonite saturation state (Ω_arag) resulting in high daytime Ω_arag levels when calcification rates were at their peak. However, the NEC and NP diel cycles can become decoupled for short periods of time (several hours) without affecting calcification rates. On a net daily basis, net ecosystem production (NEP) of the barrier reef was found to be sometimes net photosynthetic and sometimes net respiring and ranged from − 378 to 80 mmol m^-2 d^-1 when calculated using simple box models. Daily NEC of the barrier reef was positive (net calcification) for all deployments and ranged from 174 to 331 mmol CaCO₃ m^-2 d^-1. Daily NEC was strongly negatively correlated with average daily pCO₂ (R² = 0.76) which ranged from 431 to 622 μatm. Daily NEC of the Kaneohe Bay barrier reef is similar to or higher than daily NEC measured on other coral reefs even though Ω_arag levels (mean Ω_arag = 2.85) are some of the lowest measured in coral reef ecosystems. It appears that while calcification rate and Ω_arag are correlated within a single coral reef ecosystem, this relationship does not necessarily hold between different coral reef systems. It can be expected that ocean acidification will not affect coral reefs uniformly and that some may be more sensitive to increasing pCO₂ levels than others.

Assessing the role of pH in determining water column nitrification rates in a coastal system

Ocean acidification is predicted to impact the nitrogen cycle in a variety of ways. Specifically, manipulations of water column pH have shown that nitrification, the microbial conversion of ammonium to nitrate, is inhibited at low pH. A decrease in nitrification may impact phytoplankton composition and production, denitrification, and the production of nitrous oxide. We compiled an existing unique data set of concurrent water column nitrification rates and water column pH values from a temperate New England estuary (Narragansett Bay, RI, USA). Contrary to the current hypothesis, we found that nitrification rates were highest at low pH and significantly (P = 0.0031) lower at high water column pH. In this study, pH varied up to 0.85 units, 20% more than the maximum predicted ocean pH decrease of 0.7 units. These results highlight that nitrifying organisms in coastal systems tolerate a wide range of pH values. Moreover, the degree of negative correlation with pH may depend on site-specific environmental conditions. Combined, these findings indicate that the current hypothesis of the negative impacts of ocean acidification on nitrification, at least for the coastal ocean, might need reevaluation.

Novel methodology for assessing phytoplankton response to pCO2 enrichment in fresh and saltwater

Atmospheric CO2 emissions are on the rise and are expected to reach 780 parts per million by the year 2100. Research investigating the impacts of increasing CO2 is a relatively new field and the response of phytoplankton communities is largely unknown, especially in coastal and freshwater ecosystems where no CO2 manipulation studies have completed. The present study attempts to encourage uniformity in methods utilized in CO2 perturbation studies and identifies changes in phytoplankton abundance in freshwater (James River) and coastal ocean (Atlantic, Cape Hatteras) sites. A novel bubbling method to manipulate pCO2 was compared with the classic method of acid addition in conjunction with laboratory and in situ experiments. The novel and classic methods were equally effective at manipulating carbonate chemistry to predicted levels. However, the laboratory experiment saw greater variation in both pCO2 levels and chlorophyll-a concentrations throughout the four-day incubation period. The results from the present study encourage use of the novel methodology in combination with in situ experimental setup to assess changes in phytoplankton communities as a result of pCO2 enrichment. This pairing will allow greater replication of small volume incubations without introducing new abiotic conditions such as temperature and light. Additionally this study found no significant treatment effect on phytoplankton communities in either freshwater James River or coastal Atlantic.

Divergent ecosystem responses within a benthic marine community to ocean acidification

Ocean acidification is predicted to impact all areas of the oceans and affect a diversity of marine organisms. However, the diversity of responses among species prevents clear predictions about the impact of acidification at the ecosystem level. Here, we used shallow water CO2 vents in the Mediterranean Sea as a model system to examine emergent ecosystem responses to ocean acidification in rocky reef communities. We assessed in situ benthic invertebrate communities in three distinct pH zones (ambient, low, and extreme low), which differed in both the mean and variability of seawater pH along a continuous gradient. We found fewer taxa, reduced taxonomic evenness, and lower biomass in the extreme low pH zones. However, the number of individuals did not differ among pH zones, suggesting that there is density compensation through population blooms of small acidification-tolerant taxa. Furthermore, the trophic structure of the invertebrate community shifted to fewer trophic groups and dominance by generalists in extreme low pH, suggesting that there may be a simplification of food webs with ocean acidification. Despite high variation in individual species’ responses, our findings indicate that ocean acidification decreases the diversity, biomass, and trophic complexity of benthic marine communities. These results suggest that a loss of biodiversity and ecosystem function is expected under extreme acidification scenarios.

COMPASS Webinar: CGBD polling results re: public perception of ocean acidification

Many of you who attended the OAPI meeting at WHOI in March of this year heard a brief yet thought-provoking presentation by Lisa Dropkin of Edge Research, who discussed the results of her polling research on public perceptions and attitudes toward ocean acidification in the US and UK. While the presentation raised many important questions, we at COMPASS feel the information provides a useful window into the cultural landscape that is likely to inform media and policy communications, especially as ocean acidification research gains momentum and traction in the coming months and years.