Going Green to Blue

As we round the southern most point on our trip we notice that the water has gone from blue to green, and that there appear to be surface current and eddies in the water. We decide to stop and have a look with the CTD. As we lower the instrument from the aft cockpit, we encounter a layer of chlorophyll so thick that it actually coats the lines and hoses with a green layer of algae. At right is the CTD profile - the thermocline here is particularly sharp, and the temperature (red line) drops from 29ºC at the surface to 14.9ºC at 50 meters depth, indicating that cool, nutrient rich waters are relatively close to the surface. As expected, oxygen (blue) peaks at 110% saturated at the chlorophyll max, and drops to less than 10% saturated at 50 meters deep, indicating a switch from photosynthesis to respiration. Equally telling, the pH rises slightly at the Chlmax, indicating the consumption of CO2, and then plummets as carbon dioxide is produced from respiration. This represents an increase in acidity of over 50% in the space of 30 meters, which is quite amazing to see. This is by far the largest bloom we have encountered on our trip, and we take samples at 50 and 20 meters before continuing to the Panama Canal.

Below are images of the three sets of filter racks, corresponding to the 3.0 micron filters (top pair), the 0.8 micron filters (middle pair), and the 0.1 micron filters (the bottom pair).  The filters on the left side all came from the layer of active respiration at 50 meters, where the primary processes of metabolism are bacterially driven.  The filters on the right are taken from the CHLmax layer, where much of the primary production is occurring.  All three of the CHLmax filters were heavily pigmented, with the top filter almost a millimeter thick in green algae.