Phosphorus

Introduction

Phosphorus is a major factor in determining the growth of algae and weeds in a lake. The big culprit is the soluble phosphorus introduced into a water course by humans. eg, phosphates such as in detergents, fertilizers, and sewage. Suffice it to say, every effort must be made to minimize such anthropogenic sources. Chandos is fortunate in that it is not downstream of other lakes/rivers, nor has towns and sewage plants along its shores. More regulation over septic systems would help, eg, perhaps a requirement to have a system certified to a certain standard before a sale. (similar to a safety cert for a car)
With reference to the trophic status chart of the last section, we do not wish to see total phos above 12 µg/L, and preferably closer to 8. It does appear that phos levels have been on the decline over the last 20 years, probably due to education, improved septic systems, and the use of phosphate-free products.

  From Mike White’s 2006 paper: MikeWhite klsa phos report
Phosphorus is found in both soluble and insoluble forms, which together account for the total phosphorus (TP) in a lake ecosystem.   The insoluble forms occur predominantly from dead or decaying organisms (leaf litter, aquatic macrophytes, phytoplankton, zooplankton, etc.) and eventually falls to the lake bottom, while the  soluble forms stay suspended in the lake water column. Soluble phosphorus is comprised of numerous complex compounds; however, a proportion is soluble reactive phosphorus (SRP). SRP is readily used (absorbed) by phytoplankton and macrophytes and thus increases lake productivity. Almost all natural sources of phosphorus (~90%) enter a lake system in the insoluble form, whereas, phosphorus from anthropogenic (human induced) sources are predominately of the soluble form(~90%) (Mackie, 2001). This means that phosphorus entering aquatic systems from human sources is immediately available for primary production. The insoluble phosphorus, which has fallen to the lake sediment, can be converted to soluble form and is not trapped there permanently. The mobilization process can be quite complex, but in its simplest form insoluble phosphorus can be reduced to a soluble state at the sediment-water interface through decreasing redox potential and pH levels. These conditions exist when lake sediment oxygen levels decrease and become anoxic (depleted of oxygen). This anoxic condition occurs in lakes when algae die and fall to the lake bottom. As the dead algae are decomposed bacteria consume oxygen and favourable conditions for phosphorus mobilization occur (Mackie, 2001). Thus, once a lake becomes eutrophic (turbid algal state) this negative feedback loop can make restoration efforts challenging. So what does this tell us? It is possible to limit anthropogenic sources of phosphorus, creating an initial decrease in levels; however, long-term reduction may take many years as the insoluble phosphorus is mobilized and absorbed by plant species. The easiest way to restore a lake is to prevent it from becoming eutrophic in the first place.” (think of trying to “uncook an egg”)

Phosphorus Cycle

(from Wikipedia) The phosphorus cycle is the biogeochemical cycle that describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere. Unlike many other biogeochemical cycles, the atmosphere does not play a significant role in the movement of phosphorus, because phosphorus and phosphorus-based compounds are usually solids at the typical ranges of temperature and pressure found on Earth.

Humans are extreme disrupters of the natural phosphorus cycle, as we mine it extensively  and redistribute it widely. The initial source of phosphorus is rocks and minerals.  It is a very long cycle, and one can hardly conjure the geochronological time and path it would take for a phos atom to make it from living algae back to sedimentary rock and back to algae again.  And along the way this atom may be stuck in some seemingly endless biological sub-cycle before finally escaping and continuing its pilgrimage back to rock.

If you have the time (9 mins),  an entertaining and educational video on the nitrogen and phos cycles can be found here: Khan Academy N & P cycles.   (phos cycle at 5:15)

Phosphorus in South Bay

The role of phosphorus in the trophic state of fresh water lakes is well known.

The chart below shows South Bay phosphorus readings from the epilimnion (top waters) from various sources back to 1981.

Some observations and interpretation:

The 1981-82 data with the line through it is MOE data.  See their report chandos-study-1986 .  (1981 table station: CH-3 comp. and the 1982 table “for station CH3 composite Sample”)
It is instructive to notice how much variation there is in  the phos readings over the course of 1981 and 82.  This is a caution not to read too much into limited amounts of phos data.

It does seem like the phos situation is trending down, which is good, and it seems not unreasonable to say that the phos levels in South Bay are in the 8-12 range.  Nevertheless, we should continue to focus on lowering the anthropogenic component.

The lone point (#28 – 5 µg/L ) for the 2014 paleo study at CH3 in South Bay seems low.  The reading at another location in South Bay was 9 µg/L.

Phosphorus in Gilmour Bay

Because of the anoxic conditions at the bottom of Gilmour Bay, there is a marked difference in phosphorus levels from the top to the bottom.  (much higher at the bottom)  This is because in the absence of oxygen, the redox conditions change, allowing certain chemical reactions to take place that result in phosphorus being released from the sediment.

  • In  1981 the bottom phosphorus ranged from 12 to 163 with a mean of 44 µg/L.
  • In 1982 the bottom phos ranged from 15 to 230 with a mean of 94.8 µg/L.
  • In 2014 the hypolimnion (bottom)  phos reading was 140 µg/L.

The following graph shows the data for the Gilmour Bay top waters…

As with South Bay, the Gilmour Bay top water phosphorus can be said to be in the 8-12 µg/L range.

There have been some excellent phosphorus testing and analysis performed on the TSW chain of Kawartha Lakes.  See  Kawartha Lakes Stewards Report 2015

During-season phosphorus variation

Spring phos levels tend to be lower if there is a flushing effect from in-flowing rivers.  Chandos, being a head lake, likely does not receive as great a benefit from its relatively small watershed as do those lakes on the Trent Severn Waterway.

The KLSA have an excellent paper by Mike White  (2006) regarding phosphorus in the Kawartha Lakes:  White report KLSA