The Madison-Hills Paleoecology Project ("MPEP")

Introduction

The MPEP is a privately funded endeavor that will drill and sample the layers of soft sediment that have accumulated in the deepest part of Big Pea Porridge Pond ("BPPP") in Madison, New Hampshire during the past +/- 14,000 years. The purpose of the work is to scientifically analyze, technically describe/catalogue, and radiocarbon/proxy date these progressively deposited materials to establish the ecologic change-sequence history of the Pond's basin since the departure of the last ice sheet. The work described above will begin in late January or early February 2008 and be completed by late Spring or Summer 2008.

Scientific Basis of the MPEP

Lake-bottom sediments represent the most continuously detailed records of post-glacial (Pleistocene to Holocene) climate and environmental change available, and such records provide the best long term context for the dramatic physical and biological/ecological changes that have occurred during what has become to be known as the "Anthropocene" period (time since the beginning of extensive human habitation).

Who's Involved

The scientific staff of MPEP includes the following individuals, all of whom are donating their professional expertise to the project:

P. Thompson Davis, Ph.D., Dept. of Natural & Applied Sciences, Bentley College.
Brian Fowler, Quaternary Scientist, Project Director.
Lee Pollock, Ph.D., Dept. of Biology, Drew University.
Lisa Doner, Ph.D., Center for the Environmental, Plymouth State University



Friday, May 9, 2008

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
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