We spend the day transiting the famously capricious Gulf of Tehuantepec, but today winds were calm, and we were able to cut across the bay in good time. At the southern end of the gulf is an underwater seamount, so we maneuver the Sorcerer over the seamount in hopes of encountering an upwelling. An indeed there are numerous dolphins in the area feeding on fish. We take a CTD sample just as the sun is setting, and I spend the rest of the evening processing the filters.
Two dolphins caught in mid-leap by our captain Charlie Howard.
Just south of Puerto Vallarta is Cabo Corrientes, and our satellite data indicate a large bloom extending 25 miles off the coast. As we enter the bloom the water turns an intense green, and there are numerous fish feeding in the area. Sampling conditions are ideal: bright sunshine, light winds, moderate swell. We deploy a large plankton net which rapidly fills with algae and zooplankton. Karen McNish looks at the larger diatoms and zooplankton under the scope while the rest of the crew prepares our instrumentation for deployment.
Satellite image of phytoplankton blooms along the Mexican coastline, March 2009. The Ilsa Cedros bloom is halfway down the Baja peninsula on the west side, the Cabo Corrientes bloom is the red area in the lower right corner of the image.
The CTD profile of the water column at Cabo Corrientes showing a surface phytoplankton bloom.
From the aft cockpit we deploy a CTD equipped with a sampling hose. A standard CTD measures conductivity, temperature and depth: our unit also contained a pH probe and a fluorometer for measuring chlorophyll concentration. As we lower the CTD through the water column, we generate a profile of the ocean at Cabo Corrientes down to 40 meters in depth. At left you can see the CTD plot: depth is plotted on the y-axis as a change in pressure, and pH (black) and temperature (red) are plotted on the top two x-axes, with oxygen (blue) and fluorescence (green) plotted on the bottom two x-axes. In this case, the peak fluorescence (green trace) is at 8 meters in depth, and after that, the concentration of oxygen (blue trace) falls from 90% saturated to 5% saturated. The peak fluorescence indicates the location of the chlorophyll max (or Chlmax), where most of the photosynthetic plankton are located, and the oxygen minimum (or O2 min) indicates an area of intense respiration immediately under the Chlmax. Both of these areas contain a wealth of undescribed microorganisms, and understanding the relationship between photosynthesis and respiration in the ocean is one of the keys to understanding the global carbon cycle. We took samples at 8 meters and at 35 meters before continuing our southward trip.
Sampling today starts before sunrise when we arrive at Puerto Vallarta. In conjunction with our Mexican collaborators, we are investigating the influence of coastal development, particularly intensive tourism, on marine microbiota, so we take a sample of surface water in Banderas Bay and leave the harbor with the rising sun.
Winds have picked up considerably in the last 36 hours, and tonight they are blowing in the 25 to 30 knot range, below gale force but still too strong to safely deploy our instrumentation. We sail past the plankton bloom near Cedros Island without stopping, but you can see the sparkle of the bioluminescent plankton as the Sorcerer plows on through the night.
We left under clear skies and light winds, and within hours of heading out, we were sampling the waters off of the Coronado Islands near the US/Mexican border and plotting our sampling schedule for the next few days. The team passed around the latest satellite data from SeaWiFS, NASA’s global orbiter for monitoring levels of chlorophyll in the ocean. We noted the locations of several large phytoplankton blooms along the Mexican coastline, and we made plans to sample the large bloom near Cedros Island, about halfway to Cabo San Lucas near the ‘spur’ of the Baja Peninsula.