The accumulation rate of organic materials also depends on how productive the lake is – especially the contribution by the photosynthetic members of the microscopic plankton community (i.e., the "phytoplankton" – such as algae, diatoms, or blue-green bacteria). The VLAP data for Big PPP suggests chlorophyll-a concentrations are low enough to qualify the lake as oligotrophic (see other blog posts for a discussion of both VLAP and the oligotrophic/eutrophic questions), although the levels of cholorophyll-a and total phosphorus in surface waters of the lake show a historical trend of increase. More on that during the next year since the VLAP promises a report for the lake that will include a more extensive analysis of changes they have observed in more than 10 years of sampling data.
Meantime, another way to gain a "snapshot" view of the amount of organic matter produced within a lake (often primarily the result of phytoplankton productivity), is to observe a temperature-dissolved oxygen profile through the water column, taken when there is minimal circulation of the lake's waters. There are two times of year when this condition is met. Toward mid- to late-summer, when surface waters heat up more than deeper waters (your body is warm but your feet are cold when floating upright in the water), very little mixing of the water column occurs and deep waters stagnate. Also during the winter, when surface waters are colder than deeper waters (because freshwater at about 4 C is densest and sinks to occupy deeper parts of the lake – so water both less than AND more than 4 C are less dense and sit above 4 C water) and ice prevents wind from causing water movement and circulation, again the deep waters stagnate. In either case, without circulation, oxygen in deep waters can not be replenished from the surface, so if oxygen is consumed down there, the concentration of oxygen dissolved in the surrounding deep water declines. (Oxygen replenishment to the depths occurs primarily during spring and fall period of "turnover", when the whole water column is similar in temperature and thus equal in density, and the resistance to vertical mixing is minimal).
Some deep water oxygen is lost to various chemical reactions in bottom sediments. But the major source of oxygen depletion is from fueling the biological activities of the decomposer community – the bacteria, protists, fungi, etc. that live on and in bottom sediments and use the fall-out of organic particles (plant or animal parts, undigested wastes, biological detritus rinsed in from the surrounding landscape, etc.) as their food – their energy source. Like the rest of us "consumers", most of these decomposers use aerobic (oxygen-consuming) biochemical breakdown pathways, so to the degree that decomposition is going on, oxygen is lost from stagnant deep waters – and what is lost is not replenished until the next turn-over period.
Strictly "oligotrophic" lakes, with few nutrients and very limited phytoplankton productivity as a result, produce very little organic fall-out for the bottom-dwelling decomposers to use. As a result, not much oxygen is consumed in deep waters – even during periods of water column stagnation. Conversely, "eutrophic" lakes, loaded with nutrients and supporting dense phytoplankton growth, produce lots of organic fall-out, offering an on-going feast to the decomposers, who oblige by stripping the deep waters of their oxygen content. Comparing the actual levels of dissolved oxygen present throughout the water column with the maximal amount of dissolved oxygen possible, given the water temperature and no sources of consumption (i.e., the "saturation" level of dissolved oxygen), gives you a valuable piece of evidence about how much decomposition, and thus how much productivity, has occurred in a given lake. Little departure from saturation levels – ultra-oligotrophic, very low productivity lake. Oxygen levels falling to very low levels or perhaps to zero (= anaerobic conditions) – strongly eutrophic, very productive lake.
On Sunday, March 30, 2008, I performed such a temperature-dissolved oxygen profile on Big PPP. Since it's later in the season, I worked my way cautiously out toward the drilling site – stopping 4 times to auger my way through the snow & ice cover to be sure I was safe. In each case, there was 32-34" of consolidated, frozen snow & underlying ice present – including 10" of pretty solid ice at the base. In no case did I even begin to penetrate through the overlying consolidated snow layers, and knowing there was 10" of ice beneath, I felt okay proceeding. (Thanks to Bob Denoncourt for sharing his DO meter for this).
The graph below shows the results of this survey. You'll need to tip your head 90 degrees to the left to see the graph with data lines proceeding from the surface to the deepest spot at this particular site – about 33 ft. The temperature line (square-symbol line) is to the left and shows the less than 4 deg C (i.e., less dense waters) sitting on top of the 4 C layer toward the bottom. (If it weren't for this so-called "density anomaly" of water – being densest at 4 C (cold, but above freezing) – freshwater bodies would freeze top to bottom, and probably do in most of the creatures therein in the process). The actual dissolved oxygen curve (circle-symbol line) ranges from about 15 mg/L near the surface down to ca 5.7 mg/L at the bottom. The triangle-symbol line to the right shows what saturation dissolved oxygen levels (based solely on temperatures) would look like at each depth.
Clearly, the observed DO line doesn't closely follow the saturation curve, so we aren't as oligotrophic as would be possible. But it doesn't fall off to really low numbers or zero at the bottom, so we aren't strongly eutrophic. Using this one estimate of productivity, we might surmise that Big PPP is somewhere in between – tending toward the oligotrophic side of being "mesotrophic", since the 50% loss of saturation oxygen we see still leaves a fair bit of dissolved oxygen in the deep water to stimulate decomposition. But this does suggest to us that there is fair phytoplankton productivity in the surface waters, and that surely phytoplankton joined by zooplankton (the animal members of the plankton community) remains will have made a significant contribution to the buildup of organic-rich sediments at least in recent years.
.. Lee Pollock
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