Ocean Acidification in the Northwest Atlantic: Exploring Effects on Marine Life and Ecosystem Processes
Every year, the ocean's surface absorbs about one third of all carbon dioxide emissions. When the atmospheric carbon dissolves in the ocean, it reacts with water molecules and produces carbonic acid. That's what is called "ocean acidification". Scientists use the pH scale to measure just how acidic the surface waters are getting. Lower numbers on the pH scale indicate an increase in the acidity of the water. Since the beginning of the Industrial Revolution, the ocean has absorbed a lot of carbon dioxide from greenhouse emissions – causing about a 30 percent increase in ocean acidity relative to historic pH measurements. This trend is expected to continue over the next century, intensifying ocean acidification near the surface where much of the marine life that humans depend on live.
Acidification and Marine Life
The most direct biological effects of acidification are expected to be on marine "calcifiers" — organisms that form shells and skeletons of calcium carbonate (CaCO3) — including phytoplankton, zooplankton and other invertebrates such as molluscs, crustaceans, sea urchins and corals. As acidity increases, the saturation stateFootnote 1 of calcium carbonate (CaCO3) declines, making it more difficult for these organisms to form their protective outer shells. Decreases in calcification, or a softening of shells, could cause considerable financial losses for commercial fisheries for lobster, Northern Shrimp, Snow Crab and other calcifying species.
Ocean acidification can also reduce the growth rate and size of marine organisms, increase abnormalities and death rates in sensitive life stages, and affect reproduction to varying degrees. Adverse environmental conditions are likely to have more impacts on early life stages, which could affect populations and ultimately the number of adults.
"The consequences of acidification are more varied and complex than previously thought, since the impacts will vary based on species sensitivity, habitat and their potential to adapt," says Fisheries and Oceans Canada research scientist Dr. Edward Trippel of the St. Andrews Biological Station. Differences in the response of organisms could have a considerable impact on marine biodiversity, community structure, and the availability of marine resources.
To address this issue, the international scientific community, including Fisheries and Oceans Canada, has undertaken a variety of research activities to monitor ocean acidification and assess its potential impacts. This research will aid in the development of mitigations for the marine resources that are likely to be affected by this global threat.
Exploring Impacts on Northwest Atlantic Marine Life
Dr. Trippel and research scientist Dr. Michel Starr of the Maurice Lamontagne Institute are co-leading research into the effects of ocean acidification on marine fauna and ecosystem processes in the Northwest Atlantic. The research is currently exploring the impacts on phytoplankton, zooplankton (particularly copepods), American Lobster and Atlantic Cod and Northern Shrimp, and plans are underway to include studies on Snow Crab and possibly other species.
American Lobster Research
"Lobster is an important industry in Atlantic Canada and it is one of the animals that are considered to be very susceptible to acidification because calcium carbonate is a larger component of its exoskeleton," says Dr. Trippel.
In August 2012, researchers at the St. Andrews Biological Station carried out a preliminary experiment that exposed the larvae and juveniles of American Lobster to four CO2 levels — 400 parts per million (ppm), which is approximately present day levels, as well as 800, 1,400 and 4,000 ppm — reflecting different pH levels. The findings are preliminary and more experiments will be undertaken to assess carapace length, larval stage and survival following exposure.
"Since the shells of lobster may soften in acidified water, we are also planning to investigate the molting cycle and the physiological challenges lobsters will face at lower pH levels," says Dr. Trippel.
The response of lobsters to acidification is expected to be complex since they "translocate" calcium each time they shed or molt their shell as they grow. Before molting, the lobster absorbs the calcium in the shell it is about to shed and stores it in the gastroliths, stony organs in the stomach. This calcium is later used to quickly harden the mouthparts and walking legs so the lobster can begin to feed and move. As soon as the mouthparts harden, it eats the old shell, enabling it to begin hardening the new exoskeleton and leave its shelter for food.
Future research will explore how much calcium lobsters store in their gastroliths at different CO2 levels, the rate at which the new shells of lobster calcify or harden after molting, as well as malformations, growth changes and death rates. Research scientist Dr. Kumiko Azetsu-Scott of the Bedford Institute of Oceanography and a graduate student at Dalhousie University are also planning research into the impacts of acidification on lobsters.
Other research at St. Andrews Biological Station, in collaboration with scientists and a graduate student from the University of Keil, Germany, is exploring the adaptability of Atlantic Cod to withstand low pH (more acidic) conditions. In 2012, the offspring of different parents (seven males and one female) from the Northwest Atlantic cod stock were exposed to four different CO2 levels from fertilization until hatch. The cod were monitored for a range of characteristics including embryonic development, malformation, hatching success, and larval size of different families.
"The study found that hatching success was lower for cod exposed to higher concentrations of CO2 . Adverse effects on hatching success were noticeable at 800 ppm, double the current level," says Dr. Trippel. "By comparison, in a similar experiment on cod from the Baltic Sea, there were no effects on the hatching success at all CO2 levels, likely because the Baltic Sea is naturally more acidic."
Another very significant finding: the offspring of one male in particular were better able to withstand lower pH than the offspring of the other six males.
"This indicates, for the first time, that there is some individual variability and thereby genetic capacity in the current population of Northwest Atlantic cod to adapt to changes in pH," says Dr. Trippel.
Ocean acidification is just one stressor associated with climate change and may act in combination with other environmental factors. Marine life will also be exposed to warming, hypoxia (a shortage of dissolved oxygen), and potential biological shifts in the ecosystem. A follow-up study on cod is being carried out to assess the combined impact of various CO2 levels at different temperatures.
Mesocosm Research at Maurice Lamontagne Institute
In 2012, Dr. Starr and Dr. Michael Scarratt, also a research scientist at the Maurice Lamontagne Institute, conducted the first major Canadian study on ocean acidification in mesocosms (experimental water enclosures), in collaboration with the Marine Sciences Institute of the Université du Québec à Rimouski and Université Laval. The experiments used sea water from the St. Lawrence Estuary containing a natural community of phytoplankton to explore how acidity may alter phytoplankton communities and productivity, nutrients, and several other key ecosystem processes. Twelve different tests were performed involving various pH levels, nutrients and temperature conditions.
The findings of these experiments, which integrate the sensitivities of many species to pH and the indirect effects throughout the food web, will be used to define the factors associated with sensitivity to acidification in numerical ecosystem and biogeochemical models that are under development at the Institute.
Also at IML, research scientist Dr. Denis Chabot is exploring the physiological response of adult Northern Shrimp to four different CO2 levels. In particular, changes in pH can influence metabolism and the ability of an animal to supply oxygen to its tissues. This can ultimately affect the growth rate, reproduction and survival of organisms, as well as reduce their tolerance to other stress factors such as temperature or hypoxia (a shortage of dissolved oxygen in the water). Dr. Stéphane Plourde will also study the impacts of the same four CO2 levels on reproduction of the copepod Calanus hyperboreus. Both species live in the acidic, hypoxic bottom waters of the St. Lawrence Estuary.
"Canada is in the very early stages of studying the biological impacts of ocean acidification," says Dr. Trippel. "We have a lot to learn still and the importance of the research is reflected by the wide variety of species that may be adversely affected by acidification with some impacts at lower levels of the food web perhaps creating a cascading effect throughout the food web."
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