It has taken me a little longer this year than it did last year to get the hydrographic data together from our May cruise in the Chesapeake Bay, but I’ve finally gotten some of it up. The results of our two surveys are below. We ran into some unexpected problems with the Scanfish so the first survey was completed using data from the Scanfish as well as a series of CTD casts approximately 4 miles apart. By the end of the cruise we found we had a little extra time and everything was working well so we completed an abbreviated survey from our northern station south to near the mouth of the Potomac. This was a survey of convenience, but it seemed to work well with our cruise plan so we will try to incorporate it into subsequent cruises.
One big reason to incorporate a second survey at the end of the cruise is evidenced by the data we generated, and the differences between the two surveys, taken just 6 days apart. It may not be abundantly clear looking at these graphs on the computer, but there were some fairly substantial changes in temperature and dissolved oxygen between these surveys. The surface water temperature warmed up a few degrees between them, and while some of the increase was likely due to the hot weather we had during the cruise, much of it was also likely due to advection of some water masses out of the Bay in the surface outflow. But we will save that for the physical oceanographers to tell us. Another reason I think it might have been water moving is because the salinity looks like it changed a little bit too- saltier in the northern part of the transect on the second survey.
The two other data plots are what we might call “non-conservative” tracers. This means that they vary in different ways. Physical oceanographers use this term for things that are affected by biology, including dissolved oxygen and chlorophyll fluorescence (of course – since it’s found in algae!). We wouldn’t use these measurements alone to trace water masses, but along with the “conservative” tracers like temperature and salinity, we can learn something about where the water is moving. In this case I do not think that the dissolved oxygen tells us much about the movement of water, because this is the time of year we would expect the biological activity of the bacteria in the deep water to be drawing down the oxygen. Similarly, the fluorescence is likely changing as much due to growth and death of the algae as it is by water movement, so we wouldn’t conclude much about the currents from the fluorescence data.
However there are some interesting patterns in those data plots, if we compare between the first and second surveys. First, the dissolved oxygen seems to have dropped slightly over 6 days. It was already lower than I expected, and certainly lower than the same time last year. Unfortunately we were unable to do a survey all the way south, so we do not know how far the hypoxic water (less than 2 mg per litter) penetrates. The fluorescence values are plotted as relative to the maximum fluorescence because we have not yet calibrated these sensors with actual chlorophyll measurements for this cruise. This means we can’t say anything about whether or not the fluorescence increased or decreased between surveys, but we can look at the changes in distribution between them. In the first survey chlorophyll fluorescence was high in the north and most of the high values were in the surface waters. By the second survey the maximum chlorophyll fluorescence was found right on the pycnocline – or the region of the water where density changes. If you look at the other three plots from the second survey, you will see that somewhere between 8 and 12 m deep, each variable – temperature, salinity, and dissolved oxygen – change, and right in this same region we see the maximum chlorophyll values. This is the kind of pattern we often think of as occurring in summer.
So what will our July cruise have in store for us? I am not sure, but I would guess that we’ll see very strong evidence of hypoxia and perhaps even anoxia. The salinity is still low near our northern station (I have it from good authority from other researchers sampling near the mouth of the Choptank River), and that may affect the jellyfish populations by delaying them a bit. Temperatures should not be a problem for bay anchovy spawning or jellyfish growth, but the combination of very low oxygen and salinity could mean something different from what we saw last summer.
Should be interesting, so stay tuned! We’ll be out in late July and perhaps get some updates from our colleagues that are out there in the next two weeks.