Scientific Posting #20

Mid-Summer Temperature-DO Profiles

Earlier, we posted results from winter and spring water column surveys of temperature and dissolved oxygen (a posting from April 4, 2008 and Scientific Posting Numbers 15 respectively). Accompanying those results are discussions of how this information can help us understand the circulation and productivity of Big Pea Porridge Pond.

This posting continues the sequence by adding temperature and oxygen profiles from summer (August 3, 2008), when the temperature stratification or layering of the water column is maximal. The warmed top layer or epilimnion of the lake shows temperature in the 23-25 C range (73.4-77 F) extending some 4 meters (13 ft) into the water column. The deep waters of the lake, i.e., the hypolimnion waters, are in the 7-8 C (44.5-46.5 F) range. The intervening zone, from 4-8 meters depth, in which the temperature falls by at least 1 degree/meter, is the metalimnion or thermocline.

Larger, "first class" lakes hold all of the summer-added heat in surface waters, leaving the deep hypolimnion waters constant at 4 C (39.2 F) year-round. The hypolimnion depths in smaller lakes like Big Pea Porridge Pond incorporate some "excess" surface heat defining our lake as a "second class" lake (an unfortunate term with ABSOLUTELY NO bearing on its importance to us!).

Oxygen enters freshwater lakes in one of three ways: imported -- via diffusion from the surrounding atmosphere or as dissolved oxygen in inlet streams, or in-situ -- as a by-product of photosynthesis (6 CO2 + 6 H20 → C6H12O6 (glucose) + 6 O2). The August 4th oxygen profile (circles) provides interesting results. A curve of this sort, with the oxygen maximum in the metalimnion (thermocline) is known as a "positive heterograde oxygen curve". Large, cold water inlet streams could theoretically bring higher levels of oxygen in as a colder, denser water mass at such depths. But there are no such inlet streams entering Big Pea Porridge Pond. The only other logical source for high levels of oxygen 6 meters down in the metalimnion would be productivity by single-celled or colonial phytoplankton located there. Because high light levels (including damaging UV light) found in surface waters are actually inhibitory to most phytoplankton, maximal photosynthetic activity by phytoplankton is usually found somewhat deeper. But a deep positive heterograde peak like that seen here in early August results from dominance by cold-adapted phytoplankton that do best in waters away from the sun-heated epilimnion. Often less-desirable blue-green photosynthetic bacteria such as the filamentous Oscillatoria are responsible for such curves. (Blue-greens are less desirable because they tend to produce chemical byproducts that other organisms find toxic). We need to follow up with a deep plankton tow to determine what phytoplankters are found at depth here in our pond.

Observed dissolved oxygen readings can be compared to the line in the figure with triangle symbols that represents what would have been saturation levels of dissolved oxygen based on the actual temperature reading alone (recall the inverse relationship between temperature and the solubility of all gases -- meaning that the higher the temperature, the less dissolved oxygen would be available). The supersaturation levels within the metalimnion are clear. But also note that observed dissolved oxygen falls short of saturation levels toward the bottom. Typically, loss of dissolved oxygen in deep waters results from oxygen consumption by the bacterial and fungal decomposers found in bottom sediments. The degree of their activity, and thus the degree of oxygen consumption through their respiration, is a reflection of the amount of organic (biologically produced) matter available. Little productivity (oligotrophy) would result in little decomposer activity and little oxygen depletion below saturation levels. High productivity (eutrophy) would provide enough organic fall-out to fuel extensive decomposer action and a steep depletion of deep oxygen perhaps even to producing anaerobic (oxygen-free) conditions. Our situation matches what we observed in our March profiles – about one half the available oxygen has been consumed, suggesting a mesotrophic condition in Big Pea Porridge Pond. .. Lee

Scientific Information Posting No. 19

NOTICE

UPCOMING PROJECT INFORMATION PROGRAMS

There are 2 informational programs concerning the project scheduled in the next several days for those interested who want to get caught up on latest research developments and plans.

The first is a slide and video illustrated presentation at 7:00 PM, on Thursday, August 7th, at the Tin Mountain Conservation Center's Headquarters, on Bald Hill Road, in Albany, NH. Specific program details can be obtained by contacting the TMCC at 447-6991.

The second is an informal "Display and Q & A Session" to be held in conjunction with the Green Mountain Conservation Group's Annual Celebratory Evening & Fundraising Dinner, beginning at 4:30 PM, on Saturday, August 9th, at the Province Lake Golf Club, in Parsonfield, Maine. Event information can be obtained by contacting the GMCG at 539-1859.

We look forward to seeing you.

Scientific Information Posting No. 18

LARGER PHOTOGRAPHS AVAILABLE

Several folks have asked if there are larger versions of the photographs included in Scientific Post No. 17 available(?). Yes, there are.

Bob Christiansen emplaced a link near the bottom of Post No. 17 which, if you click on it as instructed, will lead you to a selection of enlarged images for easier viewing.

Scientific Information Posting No. 17

Splitting the Cores

Sorry to offer this posting a little out of order. With all the flurry of follow-up investigation of material sampled from the core, we neglected to offer a glimpse of the core splitting session itself.

A group of us met at Plymouth State University on Saturday, May 3 to begin the process. To start with, we retrieved the deeper core segments from the 4 C coldroom where they had been stored since the original sampling date. The foil and cling wrap covering of the core segment was carefully unwrapped and details regarding its appearance and texture were noted.




Then the segment was split longitudinally into halves to reveal the interior material.
Having laid a tape measure along side it, we photo-documented the segment and, using a standard soil color chart, we noted its appearance before subtle color tones could change as these sediments were exposed to air for the first time in thousands of years.



Most core segments (such as the one below) have a brownish-gray appearance produced by the remains of biologically produced organic materials (plant, animal, and microorganismal remains) mixed with inorganic mineral materials. Brownish organic deposits in lake bottoms is known as "gyttja" (a Swedish word, pronounced "yit-yah").



Then at specific depth intervals along the core, subsamples of sediment were extracted for detailed analysis. All subsamples were removed from one side of the split core so that the opposite side could be stored intact for archival purposes. Subsamples taken






will be examined for the remains of midge larvae (or chironomids – useful in reconstructing past temperatures), pollen (useful in reconstructing a picture of changes in the surrounding landscape), the degree of sediment compaction (useful in analyzing the impact of overlying geological forces) and organic content (using a "loss on ignition" or LOI technique – useful in characterizing changes in overall biological productivity over time). The significance of each of these studies will be presented as results become available in the months ahead.

As we had seen in the field, in the 11th segment we collected, a dramatic change in appearance was noted between the brownish (gyttja) sediments and the gray mineral silts that apparently lack such organics. The illustration below shows that transition point between 23 and 24 meters below the Pond surface. A subsample of sediment from this transition point was used for dating purposes (described in Scientific Posting 16).

Click on this image for short slideshow of larger images!

Lee Pollock

WHERE ARE THE PICTURES?

Several people have recently asked where/how to find the pictures and slide show taken during the actual drilling out on the ice last March. It's easy.

Simply scroll all the way down to the last the postings on the blog as it exists today and then click on "Older Postings" in the lower right-hand corner. Once you're into these older postings, scroll down until you reach the post that includes the photos/slide show.

Scientific Information Posting No. 16

WHEN DID THE GLACIER LEAVE THE BIG PEA PORRIDGE POND BASIN?

Well folks, sorry for the long delay in getting information to you here on the blog. Since we finished the first round of sample splitting and sampling in early May, we've been awaiting laboratory results to report and to use to plan the next steps in our investigation. Now, the first of these results is "in", and here it is - the one you've all been waiting for.

The Carbon-14 (AMS) age of the likely oldest organic material at the bottom of the Pond is 12,150 +/- 50 years. If you correct this to calender years, it lies between 13,910 and 14,100 years, or just about 14,000 years.

This is the age of the first organic material to be deposited on the surface of the very compact silt and clay basin left by the glacier when it departed, and it represents the starting date for the rest of the paleoecological history of the Pond our investigation will gradually reveal over the coming months. This date represents the point in time when the local climate had warmed sufficiently from very cold glacial conditions so that the first types of pond vegetation and critters could sustain themselves in and around the Pond. Our investigation from here will document just what these life forms were and how they changed over time to the present. Suffice to say for now, though, you can rely on the fact that we are all part of a history here on the Pond that started 14,000 years ago.

Bear in mind that this date is just the first in what we expect will be a long line of such information (along with more professional analyses) that will come out of our investigations over the next several months. Please feel free to send us your comments and questions via the Commenting Feature here on the blog. We'd enjoy the chance to talk with you about these exciting developments.

Also, keep on eye here for "social postings" about the one or more of information meetings we hope to conduct this summer on the Pond to describe what we're finding and discuss what it all means about the paleoclimate and past ecology of the Pond's basin. As things proceed along with our work and we learn more and more about what went on here in the past 14,000 years, "the Pond will never be the same" to any of us.

More later and as it becomes available...

Scientific Information Posting No. 15

Follow-up DO-Temperature Profile

Earlier, we posted a temperature-dissolved oxygen profile for Big Pea Porridge Pond taken March 30, 2008. Its purpose was to show:

1) the winter temperature profile, with densest (4 C) water filling the lake bottom and underlying colder (therefore less dense) water beneath the ice-covered surface. With less dense water "capping off" more dense deep water, the water column is stabilized, i.e., little vertical mixing or replenishment of deep waters occurs in winter. Also, ice cover prevents wind (the major force behind lake water mixing) from accessing the water surface. Winter stagnation of the column results.

2) the dissolved oxygen curve dropping off with depth. Loss of oxygen in deeper waters is primarily the result of the bacterial/fungal decomposition of organic (i.e., biologically derived) matter that has sunk to accumulate on the bottom. Without circulation to replenish lost oxygen, the extent of oxygen loss is a rough measure of how much organic matter is produced in the lake – in other words, its level of productivity.

As warmth returns in spring and the ice melts, the < 4 C surface waters heat up to match the 4 C deep waters. At that point, temperatures and therefore density of waters within the entire water column are equal. As a result, the water column loses its "stability". As the wind blows across surface waters, pushing them toward the leeward side of the lake, surface waters are driven downward, while elsewhere in matching volume, the lake deep waters are forced to the surface. This creates a "turnover" during which the water column becomes uniformly mixed. Temperatures and dissolved oxygen levels (replenished as oxygen-stripped deeper waters mix to the surface and make atmospheric contact) are equal top to bottom.

This process is the "spring turnover". Full lake vertical mixing like this returns to sun-lit surface waters the nutrients that were freed during the winter as decomposers worked over the organic matter there. These vital nutrients were trapped until the turnover by the stagnant winter water column. With sunlight and nutrients finally available in surface waters, the single-celled and colonial phytoplankton (the plant component of the plankton community – algae, diatoms, dinoflagellates, bluegreen bacteria, etc.) – can surge into photosynthetic action, producing a peak in lake's annual productivity known as the "spring bloom".

The graph below shows the temperature-dissolved oxygen profile observed on May 9, 2008 at the "deep spot" in Big Pea Porridge Pond. If you tip your head 90 degrees to the left, you can see the results from top (left) to bottom. As you will see, we missed catching the turn-over period exactly. While the water column is still pretty uniform with regard to dissolved oxygen levels, the temperature profile is no longer uniform. Understandably, the top 3 meters of surface waters are capturing more heat than deeper waters. By doing that, the warming surface waters, referred to as the "epilimnion", are also becoming progressive less dense than the deeper, cold waters of the "hypolimnion". (There is an inverse relationship between temperature and density – the higher the temperature, the lower the density of water). The intermediate zone including the steep temperature gradient separating these two layers, e.g., between 2-3 meters on this graph, forms the "metalimnion" or "thermocline". As the temperature-density contrast builds between epilimnion and hypolimnion waters, the stability of the water column builds. Deeper, colder, denser waters resist wind-driven vertical mixing of warmed surface waters. The whole lake is no longer involved in mixing – just the wind-driven, warmer epilimnion waters continue to circulate. As the vertical depth of mixing becomes more limited, lower-density, surface epilimnion waters become warmer and warmer, and the stability of the water column increases. This leads to a summer-time stratification/stagnation of the water column that isolates deep, darker, colder hypolimnion from the sunny, warm low density epilimnion. The buildup of the summer temperature stratification will be the subject of our next temperature-dissolved oxygen profile later on in the season.



Lee