In one of our final cruises, we had observed something quite peculiar and unexpected- the layer of hypoxia normally resting upon the seafloor bottom was raised up into the water column. The dissolved oxygen minimum was there in the middle of the column, somewhere right above 20m depth. While similar phenomenon had been observed in 2011, the reappearance of this had raised additional questions: What were the characteristics of this uplifted hypoxia and what caused it?
|The little red oval is the uplifted hypoxia found at BB6.|
Dr. Apple and I put together one last experiment to investigate the respiration of the bacterioplankton at the bottom and in this uplifted hypoxic layer. Specifically, we wanted to know whether these communities were carbon and/or temperature limited and if there was substantial differences in these limiting factors between the very bottom water and the dissolved oxygen minimum
|Draining sampled water from the CTD bottles. Picture|
courtesy of Dr. Apple
The testing procedure for respiration was much like the previous experiments I completed looking at the flow of low dissolved oxygenated waters out of the bay. It was done by titrating, only with the specific carbon and temperature treatments added to sets of samples. To provide a better understanding of how a specific variable affects the respiration, the treatments were designed in attempt to isolate the potential factors.
After nearly 48 hours of incubation, the titration marathon was started. Eventually, after 13 straight hours of work processing the samples, I had the data. The results indicated that both communities were carbon limited, while the direct effect of temperature did not substantially alter the plankton respiration.
The data provided us valuable insight the nature of these two areas of the water column during this period of uplifted hypoxia. When examining the water column during this period, the influx of cold, saline water at the very bottom suggests that mechanism for the raised layer was the penetration of ocean water from outside the bay under the hypoxia and subsequently lifting it.
|The cold, deep ocean water (shown in dark blue), snuck in under the hypoxia|
and lifted it, likely due to a difference in density.
This brief period we saw this hypoxic layer lift provided an interesting insight to the overall "story" of the summer hypoxia. From the early phytoplankton blooms, the consequent acute dissolved oxygen minimums from the increased respiration of this organic matter, to the gradual recovery of the system, Bellingham Bay is a dynamic coastal estuary. The research this summer provided new information and links valuable to the study, but also prompted new questions and opened additional avenues and directions for future research!