phytoplankton bloom in the spring. This then lifts the mixed-layer depth to 10 meters. The changes in oceanic biology and vertical mixing between winter and summer in the North Atlantic Gyre seasonally alter the total amount of carbon dioxide in the seawater. Interannual trends have established that carbon dioxide concentrations within this gyre are increasing at a similar rate to that occurring in the atmosphere. This discovery concurs with that made in the North Pacific Gyre. The North Atlantic Gyre also undergoes temperature changes via atmospheric wave patterns. The North Atlantic Oscillation (NAO) is one such pattern. During its positive phase, the gyre warms. This is due to a weakening of the westerly winds, resulting in reduced wind stress and heat exchange, providing a greater period of time for the gyre water temperatures to rise. Lead contamination Measured samples of aerosols, marine particles, and water in the gyre from 1990–92 include examining lead isotope ratios. Certain isotopes are hallmarks of pollution essentially from Europe and the near Middle East by trade winds; other contamination was primarily caused by American emissions. The surface layers of the Sargasso Sea were read for such concentrations. 42–57% of the contamination came from American industrial and automotive sources, despite the reduction in the production and use of leaded gasoline in the United States. Since 1992 lead has clearly reducing concentrations – this is theorised to hold true across the Atlantic in surface layers. Garbage patch See also Ocean current Oceanic gyre Volta do mar References External links Ocean currents Currents of the Atlantic Ocean Oceanic gyres