Life in the Dead Zone

The July 19th Scanfish survey in Chespeake Bay.

OK, not the most scientific title, but it certainly describes the deep water out here.  The low oxygen water extend the entire length of our transect, from the Bay Bridge all the way down to the Rappahannock Shoals (see figure). That in itself is not entirely unexpected, though the fact that it is really, truly zero for almost that entire distance is pretty interesting to me. I am not a data historian for the Bay, but for most of the data I’ve looked at from the last 15 years, I’ve not seen zero, nil, nada, oxygen extending this far south. That certainly doesn’t mean it didn’t happen, but our original plan had been to use this southern station as an “oxic control”, to compare with our more northern section. That won’t be possible this cruise, because we couldn’t really afford the time and effort of driving around the bay and poking holes in the water to see what it smells like. We will have some interesting contrasts between the stations on this cruise I think. Temperature stratification is slightly different between our two stations, as is salinity. But I think one big difference will be the zooplankton predators. We’re catching quite a few Ctenophores out here – comb jellies – and at last look there weren’t many in the Choptank River near our northern station. This could be a good opportunity to do a comparison between hypoxic/anoxic stations with different predators to really address the question of what controls the copepod populations in the summer: is it the low oxygen water or the predators. More on that another time.

One other interesting thing about the Scanfish data I plotted up is that there is a hint of more oxygenated water at the bottom than in the middle of the water column. This shows up in our CTD casts we’re doing hourly while out here. I think it suggests that there is oxygenated deep water coming in over the Rappahannock shoals from offshore, and it’s mixing with the anoxic water to produce strongly hypoxic, but not anoxic deep water. This causes the lowest oxygen water to occur in the mid depths, a condition often called and OMZ – oxygen minimum zone – when it’s observed in deeper waters offshore and in enclose seas like the Black Sea. It could make for some interesting dynamics; there may be very different communities in the deep water that are stressed because of the low oxygen, and that want to move up towards oxygenated water but run into this anoxic water above them. I’m speculating, but it has certainly been seen in deeper systems with OMZs.

A quick note about the equipment: Most of my blog posts contain some misery about equipment failures, but so far we’ve had good success with the instruments on board. The RV Sharp technical teams found water in the cable we use to tow the Scanfish and MOCNESS, and they traced it back 100 m up the cable, so they cut that all off, reterminated it, and so far so good.  We’ve a lot of science yet to accomplish but things are moving well so far.

A quick note about people: We have a lot of green folks out here on this trip, and I don’t mean environmentally friendly (though they may be that too), or seasick (which they are definitely not considering the hot but spectacularly calm weather we’ve got at the moment). This summer we’re partnering with COSEE Coastal Trends to develop some curricular materials related to the types of science we’re doing, and to make this successful we’ve got two undergrads from Hampton University and two teachers (one from Rockville, MD and the other from Texas) who are making this happen with some input from me and a lot of guidance from our grad student and postdoc. I’ll try to get some guest blog postings from them, so be on the look out for exciting new perspectives.  We’ve also got a visitor from East Carolina University and an REU student, not to mention a summer intern from our local community out here. It makes for a more crowded ship than we’re used to but there’s plenty of ice cream and it’s a phenomenal team to work with. Speaking of, they’re out on deck sweating while I’m blogging away in the cool conference room of the ship.

Stay tuned!

After a quick 30 minute visit to Solomons MD to get our fisheries friends from CBL, we are on our way back to the Scanfish survey. Details to follow, but *spoiler alert* we aren’t seeing much oxygen…

We are ready to cast off lines for our second cruise this year. Updates to follow, but we should be sampling by dawn.

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.

Scanfish and CTD data collected on 25 May 2011, during DeZoZoo 1101.

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.

Scanfish data collected on 30 May 2011, during DeZoZoo 1101.

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.

Arriving in Cambridge.

The Scanfish is back in the water for the last operation on this cruise!

We’ve been dealt some tough hands on this cruise, but as I write this we just finished another series of net tows and wrapped up a few experiments in bright sunshine and light winds. The Memorial Day festivities appear to be in full swing on the pleasure craft, as evidenced by the sails dotting the horizon and clusters of sport fishing boats to the north of us. As nice as it would be to be attending a barbecue with the family or “sampling” the nekton with a rod and reel, it’s awfully nice to be out here with a great crew doing cool science.

It is Memorial Day, and it’s a good time to reflect on the sacrifices that have allowed our free society to flourish, and as result so has our science. It’s also worth noting that this project is funded by the National Science Foundation, which was created in 1950 and has its roots in cold war politics. The agency was formed “To promote the progress of science; to advance the national health, prosperity, and welfare; and to secure the national defense.” (Wikipedia)

As far as our science goes this lovely Monday morning, we seem to be seeing sulfide laden deep water. I’m not a marine chemist, but I’m pretty sure this is somewhat early for sulfidic water to be present, particularly this far south. We’ll try to get some samples to determine the amount of sulfides present, but we weren’t planning to do that so it may take some creative thinking.

So why do we care about that? Well, low oxygen can really stress organisms but sulfides are toxic. Our colleagues who work on these things are interested in what ions organisms use as electron receptors for energy. In simplest terms, everything but the tiniest microbes rely on oxygen because it has the highest energy per molecule. As oxygen gets used up, certain organisms can switch to lower energy compounds like nitrates and things like that. Once all the oxygen is used up there is a bit of a lag and then some of the organisms that can do it begin to switch to using sulfides as their terminal electron receptor. The byproduct of that chemical process is H2S, or hydrogen sulfide which is lethal to most organisms. Some microbes have mechanisms to deal with it, but metazoans (multi-cellular organisms including plants and animals) do not. In fact, even low oxygen is lethal to many metazoans before the switch to sulfide occurs. Hence the title of this post. Our deepest samples are nearly devoid of the copepods, fish larvae, and other zooplankton we are catching just a meter or so above the deep samples. So there might be microbes, but not much else alive down there.

I’ll try to get some more pictures up this afternoon.  We’re going to finish up the net tows around dinner time and then try to get back some of our aborted Scanfish survey before heading home. Then we’ll hopefully get to enjoy some of those barbecue leftovers tomorrow evening. We did have some spectacular, and I mean truly scrumptious, brisket with fingerling potatoes and collard greens last night that seemed awfully appropriate. Our cook on this trip is sailing as relief for the regular cook, and her full time position is on a California research vessel, but the interesting part is that she has a blog about her life as a research vessel cook, food, and travels. Check it out here.

Onward!

Just when things were moving along swimmingly, the captain pulled the anchor so we could start our net tows and SURPRISE! Old gill nets and crab pots are tangled on the anchor. The crew have been trying to free it using boat hooks an other long handled implements, but it looks like they will have to put in the small boat. One concern of the captain’s is that we can’t just leave it for the next poor sailor to be waylaid. Our plankton will be there when this sorted out. Safety first of course.

Our beloved and temperamental Scanfish did not cooperate on our first day, so we don’t have the nice images of conditions that I usually make on the cruise. Instead we did CTD casts every 4 miles or so. A Scanfish is pretty much just a CTD that swims up and down behind the ship and can be towed at 5-6 knots, whereas a CTD is simply lowered from the ship to near the bottom and retrieved. The CTD can collect water though, so each instrument has its advantages. But I digress. The reason I’m describing the differences is because I finally was able to plot up some of the data from this cruise and I think it tells an interesting story. Instead of focusing on the CTD casts from our transect that took the place of the Scanfish survey I plotted up all the casts from our southern station – the one that is supposed to be more oxic. That data is shown in the plot below. The four panels with contour plots show temperature, salinity, dissolved oxygen, and chlorophyll fluorescence. Time is on the x-axis and depth is on the y-axis. The colors for all plots range from low values in blue to high values in red. On the right is a map with the station location of these samples clearly marked.

CTD contour plots from the south station in May 2011 (DeZoZoo cruise 1101), including temperature (upper left), salinity (upper middle), dissolved oxygen (lower left), fluorescence (lower middle), and a station map (right).

What I see in these graphs is that during the two days at our station the surface water warmed considerably – from about 20C to nearly 24C – while the dissolved oxygen in the deep water decreased. Salinity did not seem to change too much, though the depth of the interface between low and high salinity water shoaled (or became shallower). Fluorescence was patchy, with evidence of a rather large bloom at one point.

The really good news is that things are going pretty well so far, and we’ve got about 24 hours left out here, and we’re a little ahead of schedule. That means we’ll actually put the Scanfish back in the water for a 20 mile run of the mid-Bay tomorrow night. We’ve missed quite a large swatch of what we hoped to get because of Scanfish problems, but this way we will get some of it back, and we might even gain some insight onto the changes we’ve seen the last few days out here. Once back on land I’ll be doing some comparisons between the CTDs from the first survey and the up coming Scanfish survey. But now, it’s almost time to put the MOCNESS in the water, after I swing by the ice cream freezer…