Q1. Lake Hinge is a fictitious, shallow lake with a surface area of 93 hectare (ha) and

mean depth of 1.2 m. Key data for the lake are summarized in Table Q1 below. A

site survey has shown that there is no point source discharge into the lake. The flux

of phosphorous into and out of the lake via groundwater is assumed to be

negligible.

i.

ii.

iii.

Table Q1. Data adapted from M Søndergaard, R Bjerring, E Jeppesen (2013) Persistent

internal phosphorus loading during summer in shallow eutrophic lakes. Hydrobiologia

710: 95-107. All values are mean values over summer months.

variable

value

mean total phosphorus concentration in lake water

mean total nitrogen concentration in lake water

total phosphorus in sediment of lake

atmospheric deposition rate

total external loading of total phosphorus to lake

including atmospheric input

volumetric flow rate of water out of lake

mean hydraulic retention time

mean Secchi depth

mean chlorophyll a concentration in lake water

0.150 mg/L

1.8 mg/L

3.29 mg per g of dry

sediment

20 kg P/ha/yr

3 g/m²/yr

23.4 x 106 m³/yr

18 days

0.58 m

112 μg/L

From the information given in Table Q1, would you classify the lake as

eutrophic, mesotrophic, or oligotrophic? Justify your answer concisely.

[4 marks]

Draw a schematic of the lake depicting the pathways of phosphorus into and

out of the lake. Write the corresponding mass balance equation.

[12 marks]

Assume that phosphorus content in the lake water remains constant during

the summer and calculate the net transport of phosphorus between the

sediment at the bottom of the lake and the overlying water. Report your

answer in units of kg/yr to the nearest integer. Is the sediment a net source

or sink of phosphorus for the lake?

[9 marks]