Modeling Lake

Lake models John Boyle

Lakes trap particles and surface-active solutes, reducing landscape fluxes

Loch of Skene

The sedimented particles provide a record of changing fluxes


Three conceptual models Naumanns classification Pearsalls natural eutrophication Wetzels ecosystem concept of lakes

And one quantitative model: nutrient capture by lakes Simple box model Issues with the simple box model

Quantifying external loading Sediment focusing and measurement of P

burial The P retention concept

Loch Grannoch

Lake models Some conceptual models Naumanns classification

Deep Low rate of primary production Clear water Low algal growth Low littoral macrophytes, low

biomass, and low diversity

Shallow High rate of primary production Clear water Low algal growth High littoral macrophytes, high

biomass, high diversity

Oligotrophic Eutrophic

Einar Naumann, University of Lund, established the notion of lake trophic status in the 1920s based on the study of algae

High rate of primary production

Turbid water High algal growth,

high biomass, low diversity

Culturally eutrophic

Now we must add

Lake models Some conceptual models Pearsalls natural eutrophication

Immediately following formation, there is

low organic matter supply from the

immature freshly exposed catchment. The

lake is ultra-oligotrophic.

As catchment soils mature, there is growing

nutrient supply. And, as the aquatic

ecosystem matures, nutrients are trapped in

the lake system, leading to increasingly

autotrophic production.

As the lake fills in with sediment (some tens

of thousands of year after formation), the

decreasing water depth leads to a

disproportionate increase in trophic status.

William Pearsall (1891-1964) developed the still influential idea that lakes and lake basins become modified as they increase in age (Pearsall, 1921)

Lake models Some conceptual models Robert Wetzels ecosystem concept of lakes

From Wetzel (1983) Limnology

Leakage of nutrients from the wetland-littoral belt drives pelagial production

Lake models

In a deep, steep-sided, lake most of

the supplied particles settle quickly to

the sediment and are lost from the

lake system.

This can also explain Pearsalls aging lakes.

In a shallow-margined lake, allochtonous

particles are trapped by the wetland littoral

zone and converted into dissolved nutrient

that can be used by algae in the lake.

Some conceptual models Robert Wetzels ecosystem concept of lakes

Wetzels conceptual model explains Naumanns observation






Loch Grannoch

Lake models A quantitative model: nutrient capture by lakes Simple box model

P fluxes (or loadings, L) in a box model

Lin

Lsed

Lout

Lin = Lsed + Lout (inflow loading = sediment loading + outflow loading)

Lin = qin TPin where qin is the water influx,

and TPin is the TP concentration of inflow water

Lout = qout TPout where qout is the water

influx, and TPout is the TP concentration of outflow

water

Lsed = SR Psed where SR is the sediment mass accumulation rate, and Psed is the sediment P concentration

Issues with the simple box model Quantifying external loading Sediment focusing and measurement of P burial The P retention concept

Lake models A quantitative model: nutrient capture by lakes Issues with the simple box model Quantifying external loading

Lewis et al., 2013 Data for River Lee,

County Cork

Fluvial P transport show great temporal variability. This makes it difficult to obtain accurate total fluvial loads. Other sources of P are harder to quantify accurately. It is therefore difficult to balance P budgets.

Lake models A quantitative model: nutrient capture by lakes Issues with the simple box model Sediment focusing and measurement of P burial

If lakes were flat bottomed, and lake water well-mixed, the mean lake sediment P load would be easy. But

So, a focusing model is required to find mean values

Lake models A quantitative model: nutrient capture by lakes Issues with the simple box model Sediment focusing and measurement of P burial

Total sediment accumulation (1900-1983) = Either, Mean with zeros Lake area Or, Mean measured accumulating area (z>2m ?)

Esthwaite Water

The way forwards? 1. Dont pick lakes with complex

shape/bathymetry 2. Make good bathymetric maps 3. Use multiple cores and interpolate using a

simple hypsometric model of sediment accumulation

4. Use 210Pb inventory as a measure of focusing

Kassjn

Lake models A quantitative model: nutrient capture by lakes Issues with the simple box model The P retention concept

A number of people, notably Richard Vollenweider and Peter Dillon, developed lake P budget models built around a phosphorus retention coefficient, Rp. Rp = (Lin Lout)/Lin = Lsed/Lin

This is very useful if we want to reconstruct the landscape P flux (Lin, seen from the lake perspective) from the sediment record (Lsed): Lin = Lsed/Rp

And, if we know, or can estimate, past values of water flux, then we can estimate mean total P concentration: TP = Lin /qin = Lsed/(qin.Rp)

So, can we predict Rp?

Lake models A quantitative model: nutrient capture by lakes Issues with the simple box model The P retention concept So, can we predict Rp?

Rp=v/(1+v) (v = P sedimentation coeff. m yr-1) (derived from Vollenweiders model)

But, v (m yr-1) = k (m3 g-1).SR (g m-2 yr-1) and SR varies greatly between lakes






Loch Grannoch

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Modeling Lake