Ocean Acidification can Mediate Biodiversity Shifts by changing Biogenic Habitat

by Elizabeth Rodarte

Ocean acidification is the process in which the pH of the world’s oceans decreases due to the production of atmospheric CO2. The increase of CO2 and decrease in pH leads to changes in calcification, growth, and abundance of species such as coral reefs, mussels, seagrass, and macroalgae. Habitats experience the indirect effects of such CO2 increases. They must remain resistant to sudden changes in pH and CO2 in order to benefit the organisms they support. By modeling the effects of lowering pH in habitats with corals, mussels, seagrass, and microalgae, we can determine the costs to these species. Coral reefs and mussels are calcifying organisms that are negatively affected by the pH which limits survival and stunts, or even stops, growth and development. Lower pH decreases the species complexity of corals and mussels and ultimately the species richness in habitats. Mytilus mussels, for example, require specific pH to function. The species of mussels, other than Mytilus, that survive decreases in pH lack “structural complexity” to support dense surrounding vegetation. Therefore, the loss of Mytilus mussels due to ocean acidification allows for a more stable yet less diverse habitat. Continue reading

The Other Ocean Acidification Problem: CO2 as a Resource

by Dawn Barlow

This study addresses the effects of enhanced CO2 levels in the ocean by looking at how increased acidity might indirectly cause phase shifts in community structure of coral reef and kelp forest ecosystems in temperate and tropical waters. Under elevated acidity and temperature conditions, productivity of certain photosynthetic organisms such as mat-forming algae (low-profile ground-covering macroalgal and turf communities) can increase, making CO2 not only a direct stressor but also an indirect stressor by being a resource for certain competitive organisms, creating enormous potential for shifts in species dominance. Additionally, ocean acidification acts together with other environmental stressors and primary consumers, and these factors also influence community response to acidic conditions. Connell et al. (2013) investigate the prevalence of mat-forming algae in three different scenarios where CO2 levels were either ambient or elevated: in the laboratory, in mesocosms in the field, and at naturally occurring CO2 vents that locally alter the seawater chemistry. They find that in all the scenarios, the algae mats respond positively to the elevated conditions, increasing growth rate and cover to so that the algae became a majority space holder regardless of any herbivory. This is likely because the new environmental conditions favor species with fast growth and colonization rates and short generation times, and these are the species that are capable of completely… Continue reading