St. Petersburg Coastal and Marine Science Center
pH of sea water and familiar liquids
A pH of seven is neutral. Values below seven are increasingly acidic and values above seven are increasingly basic, or alkaline. Human blood has a pH of ~7.5 and seawater has a pH around 8.2. Rainwater is naturally acidic with a pH of about 5.6. Acid rain can be as low as 4.2. A lower pH increases the erosive capability of the water.
The Florida shelf, including the state of Florida, is an ancient calcium carbonate platform. Many shellfish, plankton, coral, and even some macroalgae (Halimeda) produce calcium carbonate from elements in seawater. Seawater is a buffered solution, meaning that it has a higher capacity to absorb carbon dioxide (CO2) without changes in pH. Another way of saying it is that seawater has the ability to neutralize excess acid. However, an increasingly higher saturation of CO2 will eventually result in lower seawater pH. Lower seawater pH may inhibit calcium carbonate production and dissolve existing shells. Ocean acidification could affect the marine food chain and the ability of marine life to adapt to climate change.
Historically, the Florida shelf has supported rich and productive populations of fish, and other marine and benthic organisms; however, climate change may threaten these valuable resources. Although government and media focus has largely been on temperature increases and sea-level rise associated with climate change, ocean acidification and its consequences are also important. Declining ocean pH and carbon concentrations are well-known consequences of increased atmospheric and surface-ocean pCO2. Although the oceans are natural carbon sinks, ocean acidification may alter calcifying organisms and the ocean’s role in carbon sequestration/fluxes. The impacts of changes in seawater carbonate chemistry on bio-calcification and survival of marine organisms require experimental and field study.
Customers and cooperators are interested in how economically significant resources such as fish and shellfish are responding to ocean acidification. This project will examine the baseline carbon and carbonate chemistry data (including carbonate saturation state) and the fundamental process of marine biogenic calcification in nearshore waters of the Florida shelf and Florida Keys. These data are used in conjunction with satellite data to provide a synoptic view of how the shelf is responding yearly and seasonally to environmental changes. Fieldwork on the west Florida shelf is used with historic data, to provide snapshots of the transition between photosynthetic and non-photosynthetic bottom dwelling organisms (plants and animals) that produce carbonate sediments and the oceanographic conditions associated with them. Research will emphasize ocean chemistry and where geochemical transitions occur.
The coastal shelf of Florida spans temperate-to-subtropical latitudes and extensive shallow coasts-to-deep pelagic zones with a variety of sediment layers. The west Florida shelf is characterized by a gradation of dominantly quartz sand deposits to the north to entirely carbonate sediments in the south. The calcium carbonate sediments originate from calcifying (i.e., carbonate shell forming) organisms. One of the environmental factors that control carbonate shell development and sediment production is the concentration of CO2 in seawater. Increased CO2 concentration in the atmosphere has led to an increase in CO2 in seawater, resulting in ocean acidification, so called because of a decrease in pH. The ability of calcifying organisms to form their shells has a dramatic impact on sediment production on the west Florida shelf and on the type of sediment (e.g., calcium carbonate vs. silica sands) that dominates the ecosystem.
The continental shelves of Florida provide a natural laboratory to test shifts in habitat ranges of calcifying organisms. Both the east and west Florida shelves extend from temperate to subtropical latitudes. Along this gradient, carbonate sedimentation changes from organism produced shell-hash, known as “heterozoan” carbonates, in cool temperate zones to carbonate sediments produced in association with photosynthesis, known as “photozoan,” in warmer subtropical zones. Our research is focusing on changes in distributions of these benthic assemblages on the Florida shelves to provide evidence of ecosystem-level effects of ocean acidification on calcifying organisms.