Speaker Info

Walter Boynton
Chesapeake Biological Laboratory

Email: boynton@umces.edu


Walt is a professor with the Chesapeake Biological Laboratory. His research interests include: coastal marine ecology, with emphasis on nutrient processes, especially the sediment-water interface; ecosystem scale and modeling; Eutrophication; food web dynamics.

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Seminar Abstract

We have done a great job at figuring out where all the nitrogen comes from, but we are a little weak on where it goes. Is it going away or will it result in longer term problems? There has been a 7-Fold increase in N since John Smith's arrival to Bay Area. 50% increase during first 360 yrs and 50% increase in last 40 yrs. There are quite a few hotspots in the land and sea-scape for nitrogen sources, but also areas that remove nitrogen. A mass balance for the Patuxent tidal marshes (which represent only 2% of the basin landscape) showed 48% removal of all the nitrogen coming into the system. Rates of removal are equivalent to all the sewage treatment plants on the Patuxent. Population in the basin is going up, but the area of impervious surfaces has increased even more. Historically the Chesapeake region had significantly greater wetlands area, promoted in part by beaver activity. This has resulted in significant reductions in the rates of denitrification. The bay has nutrient obesity - too much of a good thing. Restoration goals should include fostering wetland areas.

Seminar Transcript

[Walter Boynton:] This is a sprint seminar. It has to do with, literally, where has all the nitrogen gone. The motivation for this talk is that it has become clearer and clearer to me that we have done a great job in knowing where things like nitrogen come from, but that we are a little bit weaker on the idea of where the heck does this stuff go. And the reason that is important is that if it doesn't go away, it could be back to bite us. In other words, induce time delays and lags that might be serious in restoration. So, we need to know about this. So, that is one of the reasons why I decided to give this little talk about nitrogen. We have done some research, we have coupled with the monitoring people, with the USGS, with the modelers, to try to investigate this question. One of the things we found is I think there are some real hot spots in the seascape and the landscape that remove nitrogen. I want to tell you a story about that. I also want to leave a few take home messages and because this is such a short seminar, I am not reading any of these graphics. You read them. So, let's talk a little bit about what we know about nitrogen loads in the Bay. This is from the Patuxent River, three time periods, covers 360 years, and the take home message here is that it took about 350 years to double the nitrogen load in the Patuxent and then in the last 50 years, we think we have doubled it again. So, the number I carry around in my head for pristine bay/ current bay is enriched by about a factor of six to eight. There is a neat story behind how we got the John Smith's stuff, but I can't tell you that story right now. So, big load increases. There are also some long records around here of loads. This is from Norb Jaworski, about 100 year record of load from various sources to the head of the Potomac River estuary. One of the points I make here is that climate makes a difference. This is the East Coast drought of the 1960's. That tells us one thing right away and that is climate variability is pretty important. Never mind climate change. Climate variability is really important. We struggle with that all the time. The second thing it tells us is that diffuse sources here are doggone important. After all, we don't go through toilet flushing droughts. I mean, we all flush whether it is raining or not. However, the landscape doesn't release things if it doesn't rain so much, so interesting point there. And then the Bay Program. One of the gripes I have had with the Bay Program-- there is nobody here from the Bay Program, is there? Is that-- I am an academic I can say anything, right? Okay, is that the world was created sometime in August of 1984 and we have progressed forward since then. The Darwin fit in there somewhere and so forth and so on, but that is when the world was created, August 1984. That is not actually the case and we can learn something from the earlier history of the Bay and I just showed you a couple of those. And in fact one thing that I am pretty impressed with, our friends at the USGS put together this, so not only do we have some really fine records now over long periods of time of places where nutrients come from like the fall lines, but also spread out across the landscape, so we see here the flip side of what my talk is about. These are the hot spots for nutrient sources and we need to ask the question, where are the hot spots for where these stuff goes? And that is what I am going to tell you about. So, this is a bit of a story, it takes place in the Patuxent River estuary and primarily in the upper part of the Patuxent River estuary which I will show you a little bit about so that you will feel like you have lived there all your life. So, the upper Patuxent has got lots of tributaries like many of our systems. They tend to-- it is a narrow sort of little river. It is flashy. When it rains, the flow goes up right away. In the more developed streams there is more discharge, the peaks are higher, the valleys are lower. In the more forested ones, the peaks are lower, the valleys are higher. So, we can see the human signature even in the hydrology very rapidly. It also serves as a water supply and it is as is true now of much of that basin, it is developing rapidly. The middle Patuxent shown on the yellow here, covers above a little more than a third of the basin and when we get down to the estuary, it is tidal. It has more marsh area than open area, and as I am about to tell you it is a key element in the nutrient economy of this whole ecosystem. The tidal marshes is in the Patuxent, and this is interesting, they are very productive and they are keeping pace with sea level rise. Almost 70-percent of the marshes is in the Chesapeake are erosional. These are not. They have maintained about the same acreage and about the same configuration at least since 1917 and it is probable that they were building prior to that, so they haven't been eroding. Whenever I go into these marshes which I try to avoid almost at all cost, I am reminded of Humphrey Bogart and Katharine Hepburn in the African Queen, you know, I am always looking around the boat to see if there are any gin bottles around you know or whether somebody's hidden those over the side. It is-- the point is, they are-- you can almost feel the photosynthesis. I mean, they are-- it is a huge photosynthetic factory and it comes at all kinds of forms. Okay, this is where monitoring and science collide in a productive way, and so this is an important little slide. Some folks at Jug Bay started measuring nitrate concentrations on the inflowing tide, water flowing into the tidal marshes and then they measured nitrate when the water was flowing out of the tidal marshes. And you can see here that there were gigundo differences. We are talking 200 micromolar nitrate going into these marshes and we are tracking 20 molar nitrate, that is a factor of 10, I mean, that is a huge difference. All of this apparently occurring in something like six, seven or eight hours, so I mean, this is enough to get your attention. And you know as a science guy, you know, we all said, well, wow, what is causing that? And so people came up with all kinds of hypothesis, none of which seem to be very quantitative. They all seemed to have things wrong with them. So, what we did was we reverted to one of the things that people in many branches of science including ecologists resort to and that is a mass balance, a budget. It is one of the things we believe in. And what this really means is in a budget, you need to know the inputs, the outputs, and the changes in storage. If you understand the system that is roughly at steady state, they need to balance. So, it is a way to hold our feet, as uncomfortable as that is, hold our feet to the fire relative to understanding. So, in this little budget that we produced to try to understand the role of these marshes as potential hot spots in the landscape, we put together a budget. So, what we did was we needed to measure those black arrows coming out from the pinkish circles. The monitoring program, the USGS, MDE (Maryland Department of Environment), they measure those things. We needed to also measure the black arrows coming out the bottom of the box and monitoring programs typically do not measure rate processes. So, we went out and measured the B's, that is long term burial; the D's, that is denitrification; and the T, the transport, that is the movement of materials from the upper estuary into the lower estuary, okay? So, we needed to get those measured to see if we could balance this budget. And that doesn't mean we understand the system, but it tends to move us in that direction. If a budget is wildly out of balance, you know one thing, something is missing. There are bad measurements in here. Conceptually, you are missing something. So, we started to put it together. Denitrification was very important, just a little quick tutorial here. Denitrification is a real end source for nitrogen. It transforms nitrate, NO3 into N2 gas. Nitrate: plants love it. N2 gas is basically biologically inert, so it is a real loss term. And this is the reaction sequence that has to occur for denitrification. So, we went out all over these marshes and the creeks, collected sediment cores, used the technique to measure denitrification, did that for a year in the tidal fresh and the oligohaline marshes. We also took these cores in areas represented by high marsh, low marsh, creek side marsh so that-- and we took the number of cores in proportion to the area that those habitats represented. And then we took cores in the sediment and used what is called the Lead-210 technique to estimate accretion rates, then we looked at the concentrations of nitrogen and phosphorus and so forth down in the core, and from that we could estimate what we call a long-term burial rate. This is the stuff that is on its way to being coal. In other words it is not the labile bits of organic matter that are going to get recycled. These are the pieces of organic matter containing nitrogen and phosphorus that really do get buried in the accreting sediment column. So, here is the story and we are getting close to the end. Does it balance? So, watch the little question marks. About 5,500-- 5,400 kilograms per day get into the Patuxent River estuary. Our estimates indicated about 1,100 get denitrified in the marsh and in the creeks; about 1,400 get buried, so in these accreting marshes burial is really important; about 3,000 gets exported, and the amount we could not account for is 46. So, we think we get a balanced budget here. These are decade averaged inputs, so we are favoring neither wet nor dry years. Now, this doesn't proved we absolutely know what is going here, but it certainly helps us in that direction. And while we summarize all this, we have got this 5,400 kilograms per day coming in from all the sources, sewage treatment plants, diffuse loads, atmospheric deposition, about 2,800 goes down the estuary, 2,600 is lost. And here is the key point, in this marsh creek complex represents 2 percent of the landscape above Benedict, Maryland, above, upstream of the mesohaline estuary, 2 percent of the whole system, it represents about 1.3 percent of the total area, it removes 48 percent of all the nitrogen that gets in here. That is called a hot spot. So, we ought to think about not paving these marshes. To put this in another context, the sewer, there are 9 or 10 sewage treatment plants on the Patuxent that discharge more than a million gallons per day. All of them have nitrogen removal capabilities. They remove about 0.8 million kilograms of nitrogen per year. The marshes remove about 0.9 million kilograms per year. So, we are about the same over all. Certainly, within the same order of magnitude, one of which we have to pay for, the other one which we don't, but it does take up space, okay, 2 percent of the basin above there are salt marshes. Now, nitrogen removal is not the only benefit, but it is a big number. Let's look at some restoration activities and before doing that, here is a scary graph. This one scares me. Population in the basin is going up, but the amount of impervious surface is going up even faster. What we really need to see is, you know population perhaps leveling in impervious surface diving; that is not the case. So, we see a lot of this kinds of stuff, which does not help anything. So, now I am almost in the end and I think I got a minute to go, so what do we do about this, how do we deal with this? And Grace Brush wrote an interesting essay a couple years ago. She is a paleoecologist and she said the following things in this essay. That is a beaver by the way. The pre-Colonial landscape was covered with forest and many wetlands. Pollen grain analysis indicated there were micro-swamps everywhere, okay. Over the past 300 years, particularly the last 50, we have done away with a lot of that and increased the loads. And there is, in her mind, a net loss in the denitrifying capability of the landscape and it appears to her that beavers were the key engineers in this. And so I got busy on this and likely in the Chesapeake Basin there were 5 million beavers running around, gnawing on things when John Smith showed up. By the time the human population got to 5 million which was in about 1940, there were no beavers. I mean, at least not four-legged flat tailed things, you know, there were us. There were 5 million of us, now there is 20 million of us. So what she argues is to try to restore as much as possible this pre-Colonial wet marshy condition, mimic the beavers and create environments that really favor denitrification. And those environments are places where oxidized and reduced surfaces are close together. Okay, it is that simple. So, people who are working on this, this is a photo of one of them. Some of these wet ponds, some sand-bottomed wet ponds and different configurations of these, you know, are pushing up around 30-percent removal. I have seen some documentation where they are up around 60 percent removal. That is the kind of thing I think would be really useful and help us with this nutrient reduction. So restoration, it is tough in the face of high growth rates. The Bay clearly is overly enriched and I call it nutrient obesity. It is too much of a good thing. That is a better problem than the having too much of a toxic thing. It is still a problem. Diffuse sources are incredibly important and I think we are going to need lots of creativity. I am not recommending that the State of Maryland have an oyster hatchery, a shad hatchery, and a beaver hatchery, but I think we ought to mimic some of the activities of beavers in the landscape. Make it wetter, moister and have strong oxidized and reduced interfaces that are close together. And I think I have showed you an example of a very hot spot that is removing both nitrogen and I did not tell you the phosphorus story, but it removes a horrendous amount of phosphorus as well. We need to make these hot spots more common and learn how to use them. Done! [Bill Dennison:] Thank you.

Seminar Discussion

Visit the blog post for the complete discussion with images, or to leave a comment.

This blog post discusses the seminar given by Walter Boynton of the Chesapeake Biological Laboratory, at the IAN Seminar Series on July 29, 2010.

The discussion focused on the bioavailability of nitrogen in its different forms. The dissolved inorganic nitrogen forms (e.g., nitrate and ammonium) were contrasted with dissolved organic nitrogen and particulate nitrogen. It is recognized that some fraction of dissolved organic nitrogen is bioavailable, but generally is not as immediately available as inorganic forms. Particulate nitrogen is only available for uptake by phytoplankton after decomposition.

Walter was asked about his method of estimating the nitrogen loading in pre-colonial times. His approach was to ask terrestrial ecologists what the nitrogen export was from unaltered mixed deciduous forests that they were studying. He was careful to select forests unaffected by acid rain, since anthropogenic inputs of nitrous oxides due to fossil fuel burning would inflate the nitrogen values. The 3-4 estimates converged to around 1 lb of nitrogen per acre per year. Walter then took the forested area of the watershed estimated by Grace Brush using palentological records and extrapolated accordingly.

The issue of the mid-Patuxent marshes holding their own against relative sea level rise was discussed. Walter noted that the Patuxent marshes have been intact since at least 1917. The salt marshes of the lower Patuxent River and Chesapeake Bay overall, in contrast, are eroding as a result of relative sea level rise. The difference in the tidal freshwater marshes could be due to the sediment accretion from both upstream but also local sources. Another factor was the sediment sizes--larger, coarse grained sediments are better for marsh accretion than fine, silt sized particles. In addition, it was noted that the undeveloped region surrounding the tidal freshwater marshes of the Patuxent River allows for channels to migrate and promotes natural hydrologic processes. In contrast, the salt marshes of the lower Patuxent River are largely constrained by development (e.g., hardened shorelines).

Walter discussed how the Patuxent compares to other Chesapeake tributaries. The Patuxent is one of few tributaries that did not have highly developed regions at the head of tide. In contrast, the Potomac River head of tide marshes have largely been replaced by what is now Washington, D.C. Walter likened the removal of these marshes to the removal of a nitrogen kidney. The Choptank River on the Eastern Shore has an undeveloped region at the head of tide similar to the Patuxent River. Both the Patuxent and Choptank Rivers have tidal freshwater marshes which remove substantial amounts of nitrogen, resulting in only 20 and 17% of the nitrogen inputs to be exported downstream. The Potomac River without these marshes exports 40% of the nitrogen inputs, twice as much as rivers with intact marshes. Walter is searching for nutrient budget data from other river systems with 'globs' of intact marshes to see if this nitrogen removal by marshes is generic.

The data Walter presented with a large divergence in nitrogen concentrations between incoming and outgoing tides was discussed. The rapid uptake (within one tidal cycle) was noted and contrasted to the relationship that Walter indicated between residence time (in days) and net denitrification. The relationship between residence time and denitrification is not necessarily a causal relationship--longer residence times could simply be regions that promote marsh accretion. Walter talked about the large areas of shallow water with dense vegetation with prolific bacteria and algae (where you can 'feel' photosynthesis) with the three conditions that promote denitrification: 1) abundant dissolved nitrate, 2) anaerobic (without oxygen) conditions adjacent to aerobic (with oxygen) conditions, and 3) labile organic material. The recommendation by Grace Brush to 'replumb' the watershed to emulate the wetter conditions that occurred when there were an estimated 5 million beavers in pre-colonial times was discussed. Walter related his experience in Calvert County when the first beavers returned and the news reports were about 'nature's engineers' and 'cute fur-bearing creatures', but after some time and more beavers causing local flooding, the reports became about the 'miserable overgrown rodents'. Beavers are now found in many protected areas on the western and eastern shores of Chesapeake Bay, but various measures apart from beaver dams need to be taken to emulate pre-colonial water retention. Several approaches were discussed, including stormwater ponds designed as wetlands and maintained properly. Walter noted that a group from University of New Hampshire tests various stormwater treatment technologies, with particular reference to technologies with longer lifetimes and less maintenance requirements. Walter indicated that he was unable to account for the high denitrification rates touted by floating island technologies.

The importance of light reaching the bottom, a topic of Walter's previous IAN seminar, was noted as helpful for denitrification--enhancing the oxidation of ammonium to nitrate which could then be denitrified. One of the features of tidal freshwater marshes is the very shallow water, allowing the aerobic/anaerobic interface in the sediments to be capped by benthic microalgae, which absorb dissolved nutrients in the sediments.