Research Cruises Collect Measurements on the West Florida Shelf for Modeling Climate Change and Ocean Acidification
For some U.S. Geological Survey (USGS) scientists, "summer cruise" means something quite different than it does for most people. The souvenirs they bring home, for example, are hard drives filled with data. At least this was the case for two research cruises this past summer, including a day-long pilot study in July 2008 and a subsequent week of offshore transects in August. Both cruises collected various measurements of the chemical properties of seawater along the gulf coast of Florida. As part of a collaborative effort funded by the USGS and including scientists from the USGS, the University of South Florida (USF), and the National Oceanographic and Atmospheric Administration (NOAA), the crew took measurements of seawater partial pressure of carbon dioxide (pCO2), pH, dissolved-inorganic-carbon content, total carbon content, alkalinity, and atmospheric pCO2. These data will be used to help build a baseline for assessing the impact of ocean acidification on nearshore and offshore ecosystems in the Gulf of Mexico.
Why establish such a baseline? Ocean acidification has the potential to affect marine organisms severely. As the ocean absorbs increasing amounts of CO2 from the atmosphere, the pH of the ocean decreases, or becomes more acidic, and the carbonate ion concentration decreases. How this change will affect basic geochemical and biological processes is unknown, but key ecosystem organisms will likely have difficulty maintaining their calcium carbonate skeletons. The environmental consequences of such changes could be quite significant. For example, if the lower pH slows down or inhibits the production of calcium carbonate structures (such as shells, corals, and calcifying algae), less sediment may be produced. With less sediment, coastal erosion, already threatening some communities, could worsen. How fish or shellfish respond to changing water chemistry is also unknown but could have significant impact on a billion-dollar industry.
The creation of baseline maps will allow coastal managers to assess the effects of ocean acidification on the health of nearshore and offshore ecosystems, make predictions about changing conditions, and take effective and targeted protective measures to attempt to offset potential problems. Above and beyond any inherent value, Florida's marine resources contribute tens of billions of dollars annually to the State's economy. Thus, their continued well-being is in the interest of all citizens.
Although some of these parameters can be modeled using satellite remote-sensing data, such data are poorly constrained along the west Florida continental shelf and do not offer the same spatial resolution attainable from data acquired by ship-based measurements. Such spatial resolution is needed for linking seawater chemistry to high-resolution habitat and sediment maps. Although the synoptic (large scale) models being advanced by NOAA have been demonstrated to work well for the oceanic waters in this region, extending these models to coastal zones and along the west Florida shelf demands that the requisite ship-based data be acquired.
For boat-based data acquisition, the crew used the Multiparameter Inorganic Carbon Analyzer (MICA). Designed by scientists at the USF College of Marine Science, the USF Center for Ocean Technology, and SRI International (a nonprofit research institute), the system is capable of taking in four seawater channels to measure pCO2, pH, dissolved-inorganic-carbon content, and total carbon content, and one air channel to measure atmospheric pCO2. Importantly, the pumping apparatus allows all of these measurements to be taken while the boat is underway and even traveling as fast as 15 knots. Seawater conductivity, temperature, and depth are concurrently measured. Additional water samples are manually collected at various intervals for backup and comparison after laboratory analyses.
As a part of projects funded by the USGS Climate Change Research and Coastal and Marine Geology Programs, a daylong cruise west of Tampa Bay was run as a pilot study. Lisa Robbins, Paul Knorr, and Mark Hansen of the USGS and Sherwood Liu of USF tested the MICA analyzer, performing the initial calibration and working out various problems in the setup. The expedition, using a 26-ft USGS catboat, yielded 14 discrete point samples of the various parameters; more importantly, the trip prompted Liu to redesign the sampling system to sample at a higher rate of once per minute, thus allowing collection of higher-density data, which in turn more accurately capture the spatial variability of shelf chemistry. A description of the cruise and these data was presented in July 2008 at the Ocean Carbon and Biogeochemistry Summer Workshop in Woods Hole, Massachusetts.
The second cruise, involving a weeklong stay aboard a 70-ft ship of opportunity, the Here Today, sampled between Cedar Key and Cape Romano (approx 400 km) and as far west as the Florida Middle Grounds. Nate Smiley (USGS) and the ship's crew, Captain Mike Lawrence and deckhand Erik Bockman Pederson, joined Robbins, Knorr, and Liu. Braving rough seas and poor weather for part of the cruise, the group collected approximately 1,100 points over 650 km of trackline. Discrete water samples were collected at approximately 40 localities to corroborate underway data measurements. These samples were also measured for alkinity. Although failure of simple mechanical pumps (for bringing water samples up for analysis while the boat was underway) cut the collection session short by approximately 1 day, overall the cruise was a success, yielding more information over a larger spatial area than ever before acquired.
A similar mission was conducted in February, and more are planned to assess coastal and nearshore seasonal variations, as well as augment existing observations. A NOAA research vessel, the Gordon Gunter, is participating in offshore data collection in the Gulf of Mexico. The combination of USGS nearshore and NOAA offshore efforts will enable development of a large, comprehensive baseline of data.
The USGS is also working with and providing data to NOAA to help refine their region-specific algorithm in order to create large-scale maps of the aragonite- and calcite-saturation states of seawater in the greater Caribbean and Gulf of Mexico. Experiments have shown that as saturation states decline, so does the ability of such organisms as corals, foraminifera, and bivalves to produce calcium carbonate shells and skeletons. Currently, the maps cannot resolve the aragonite- or calcite-saturation state of nearshore shelf areas because satellite imagery for coastal sea-surface temperature and salinity (two of the parameters needed to calculate saturation state) are "masked out" (for example, see maps of aragonite-saturation state in the greater Caribbean). Using data from the USGS cruises, NOAA will be able to provide more accurate depictions of nearshore conditions in the regional satellite maps that they are creating.
Robbins and Dwight Gledhill (NOAA) envision the combination of remotely sensed and ship-acquired measurements of pCO2, pCO2 flux, total carbon content, alkalinity, and sea-surface aragonite-saturation state to develop into new data products that, coupled with habitat and sediment maps, will give coastal-resource managers powerful new tools to enhance their understanding of the effects of ocean acidification on the nearshore coastal environment.
For more information, contact Lisa Robbins, USGS (firstname.lastname@example.org, 727-803-8747, x3005). Also see these related publications: Royal Society Ocean Acidification report and "Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers."
in this issue:
Modeling Climate Change and Ocean Acidification