Daniel Lee and I got up at 0430 to look at the scanfish data. Daniel processed the data with Seabird software, and plotted oxygen concentration and salinity using Surfer software. Yesterday we scanfish-ed through the “hydraulic control point” south of the Potomac R. mouth at station CB5.5, and found in that region that bottom water had a slightly higher oxygen concentration than water at middle depth (see the yellow blip of low oxygen at ~9 meters depth at latitude -37.6 on the figure). This seemed strange until we realized this region of the Bay where there is “three layer flow.” The Bay gets shallow to the south, and water moving north off of the shallows flows between the surface water layer and bottom water layer. It looks like this water is unusually low in oxygen. Daniel and I agreed that this was too confusing for our project, so we selected station CB5.4 about 10 miles to the north to be our “southerly” station. At this station bottom waters have ~1mg/l oxygen, and would be considered “hypoxic.”
We steamed back up the Bay to station CB5.4, and selected an anchoring site ~1 mile south of the station. At that station we cast the CTD at ~0800 and filled niskin bottles at four depths (2 bottles per depth) for Ali Barba and Aiden Fisher who screened one of each pair of bottles to collect copepods and nauplii. They incubated the other bottles in a deck incubator for 6 hours to see if the copepods can survive in the water they were captured in. Their hypothesis is that copepods in the Bay swim in and out of low-oxygen water to feed. The idea of this experiment is that if copepods were captured in water that they would normally have to swim out of, they will die during the incubation. Ali and Aiden also collected copepods with two vertical net tows off the stern of the ship.
Next we steamed to a shallow site to the east of the main channel (about 8.5m deep), did a quick CTD cast, and then collected a small box core sample. The sediments were too sandy, so we moved to a deeper site where we collected a box core sample and cast the CTD. We then collected cores at an intermediate depth site (14m) and a deep site (27m). Jeff Cornwell and Mike Owens subsampled these box cores to incubate for measurements of sediment sulfate respiration (35-S sulfate reduction), aerobic respiration (change in O2), and total respiration (change in CO2). After their incubations they will sample the top 1 cm of their cores for DNA to assess the microbial community composition.
After lunch we began our big CTD cast to collect water at 8 depths. This is the standard profile we do on our day trips to our main sampling station. We lowered the CTD to the bottom, looked at the profile of oxygen and salinity, and selected depths so that three were in the lower layer, three in the upper layer, and two in between. Then the team sprung into action. We turned on our on-deck diaphragm pump and pumped water from each depth up to our sampling manifold. The tubing on our submersible impeller pump detached from the CTD frame during the cast, so we collected all our water samples with the diaphragm pump. Ian Hewson’s team collected large volume water samples (~20L) at the bottom depth and at three meters for metatranscriptomic analysis. My group collected DNA and RNA samples from all eight depths for microbial community composition, collected water in BOD bottles for bacterial production rate measurements, and fixed water in lugol’s solution (iodine and formaldehyde) and gluteraldehyde for microscope counts of protists, bacteria, and viruses. Jeff’s group collected water for a suite of nutrient and geochemistry measurements, and for measurements of aerobic and total respiration. It took a while to get organized in the fairly crowded wet lab, but we got our routine figured out, and we finished pumping after 2 hours. Then everyone dispersed into their lab spaces to process, process, process. We also continued regular CTD casts at 1500, 1700, 1900, 2100, and 2300 hours. At 1500 and 2300 the copepod team collected niskin bottles at four depths to continue their survey. Then we pulled anchor and cruised up the Bay to our next sampling station.