LESSON 2:
LAKES OF DOMINICA

Dominica's two largest freshwater lakes, Boeri Lake and Freshwater Lake, and the world's second largest Boiling Lake, are located within Trois Pitons National Park one of the wettest areas on Dominica. Rainfall recorded over a 5-year period near Freshwater Lake averaged about 340 inches a year. Mean annual temperatures in the "lakes" region is about 23° C (73.4° F), while temperatures along the coast average 27° C (80.6° F).

What are the basic physical, chemical, and biological
characteristics of Dominica's three main lakes?

BOERI LAKE

Located approximately 2850 ft above sea level, and formed within the crater of an old volcano, Boeri Lake has the highest elevation in Dominica.
Surface area = 3.6 hectares (8.9 acres)
Depth = 130 ft (39.6 m)
Boeri Lake has a very high maximum depth in relation to the lake's diameter. This ratio, called the relative depth, works out to 18.4% for Boeri Lake. Such a high ratio ( see meromictic lakes below) indicates a very steep-sided, deep lake basin, with a narrow distance at the surface between opposite shores.
Temperature throughout the lake's water column appears to be fairly constant, between 18-19°C (64-66°F).
Main sources of water are direct rainfall and precipitation running off from surrounding land. There are no streams or rivers flowing into the lake. A low salinity level indicates the dominant influence of precipitation in affecting the lake's water chemistry.
Water levels of the lake vary with the wet and dry season by as much as 25 ft (7.6m) and at a rate of 4.5 inches (11.4 cm) per day.
Water lost from lake via evaporation and through lake bottom seepage.
Between October and December, when the lake fills to its high water mark, water outflows from lake via a single outlet at lake's eastern corner.
There are no hydrophytes (water-loving floating or emergent plants) associated with the lake.
Isolation and inaccessibility of lake to invasion has resulted in a sparce plankton community (in comparison to a mainland lake). Filamentous green algae and diatoms are the phytoplankton that dominate. Zooplankton diversity is also low and includes rotifers, a copepod, amphipod and an ostracod.
No fish live in lake, the "River Crab" or Siwik (Guinotia dentata) is found living between the rocks along the shore. (See Lesson 3 for description.)

FRESHWATER LAKE

Located at about 2,500 ft above sea level, Freshwater Lake is the largest lake on Dominica, and is man-made.
Surface area 9 acres.
Depth between 55 and 65 ft.
Difference between yearly maximum and minimum surface levels less than 1 ft.
Main sources of water are direct rainfall, underground springs (which have been estimated to supply 1.5 million gallons of water to the lake daily), and water diverted from three streams on the NE-side of Morne Macque. Slightly higher amounts of dissolved ions (in comparison to Boeri Lake) indicates the influence of ground water input (and more dissolved minerals from underlying rock) into the lake, in addition to water from precipitation.
The lake has a single outlet, through which an average of 3 million gallons of water flow out daily. Water is also lost through evaporation (most of the lake's surface is exposed to wind) and transpiration (from lake plants).
Supports colonies of hydrophytes along banks. Only emergent plants are sedges known as "Jon" (Eleocharis spp.). Free-floating plants on the surface of lake are water hyacinths (Eichornia crassipes) introduced from South America.
Tilapia, a native of Africa, inhabits lake. (See freshwater fish in Lesson 3 for description)
The "River Crab" or Siwik (Guinotia dentata) is found living around lake. (See Lesson 3 for description.)

BOILING LAKE

Located approximately 2,500 ft above sea level, Boiling Lake is believed to be a flooded fumarole ( a hole or vent from which volcanic fumes or vapors escape from the molten lava below)
Lake is approximately 200 ft (63 m) across, present depth is unknown (although past measurements have placed depth @ 195 ft (59m).
Water temperatures have been measured to range from 180° to 197° F (82° to 91.5° C) along the edges, and observed boiling (212° F/100° C) at center.
Sides of the lake are a mixture of clay, pumice and small stones.
Natural basin of the lake collects the rainfall from the surrounding hills and from two small streams that empty into the lake. The water seeps through the porous bottom to the hot lava below where it is trapped and heated to the boiling point.
Exceptionally high levels of dissolved ions in lake's water results from continual concentration by evaporation.
Water from the Boiling Lake flows into Victoria Falls and River Blanc, and enters the Atlantic Ocean by the Pointe Mulatre River.
The volcanic ash eruption in Dominica in 1880 may have come from the Boiling Lake and associated Valley of Desolation. Since that time there has been an increase in the size of the fumerole area in the Valley and subsequent destruction of the surrounding forest.
No fish live in the lake

Comparative Link #2: Compare Dominica's Boeri Lake with Lake Lacawac and Two other Lakes in the Poconos, Pennsylvania, USA

(Before comparing Boeri Lake with the Poconos Region Lakes, review how lakes can be generally classified according to their physical and biological characteristics.)

How are lakes classified?

Every lake exists under varying conditions, and so each is unique in its own way. Yet, certain common characteristics can be used to group lakes into general categories to help with comparisons.

TIMING OF MIXING

The annual temperature of a lake's water will vary with latitude, altitude, and water depth. Because water's density corresponds with temperature (greatest density at 4° C, lowest density as ice at 0°C, and generally less dense as a liquid with increasing temperature), a lake's water will form layers according to its temperature and corresponding density. Cooler liquid water, being more dense, will sink towards the bottom of a lake. Warmer water, being less dense, will float as the top layer in a lake. This layering of lake water according to differences in temperature, is called thermal stratification.

If the temperature and density of a lake's water column varies enough between the top and bottom, it will set-up into three fairly distinct thermal layers. The epilimnion — the warmer, less dense upper layer. The hypolimnion — a cooler, more dense, lower layer; and the metalimnion — which is found between these two layers. Where the temperature change in the lake's water column is most rapid with depth, is a region called the thermocline, which is often associated with the metalimnion.

1. Dimictic Lake:
In temperate, midlatitude regions, temperature changes seasonally between hot summers and cold winters. As a result, lakes can become thermally stratified during the summer, when the lake's top layer warms (to create an epilimnion) and a deeper layer remains cold (to create a hypolimnion). The metalimnion (thermocline) occurs where the temperature difference between the two layers drops most rapidly.

With the arrival of cooler, autumn temperatures, the epilimnion cools to make the lake's water column the same temperature and density through out, top to bottom, allowing winds blowing over the lake's surface to mix the lake's water.

In winter, when the temperature of the lake has cooled down to about 4° C, and water density is at its maximum, an inverse stratification takes place. Water colder than 4° C , and approaching freezing, is less dense and forms a top layer, eventually becoming ice, while the 4° C water sinks to the lake's bottom. When ice forms on the lake surface, it prevents any wind-induced mixing.

In spring, another mixing of the lake occurs when the top layer warms enough to melt the ice, and eventually make the lake's water temperature the same throughout, allowing winds to once again mix the lake's waters. Lakes that have winter ice cover, and that mix twice a year (in autumn before ice cover and in the spring between ice melt and the onset of thermal stratification) are called dimictic. Most temperate lakes are dimictic.

2. Monomictic Lake:
Monomictic lakes are rarely ice-covered and circulate all through the winter. (Examples: The Great Lakes except Lake Erie, Lake Tahoe)

3. Polymictic Lake:
Polymictic Lake: Polymictic lakes are shallow, wind-exposed lakes in warmer regions, where stratification can set-up and then be destroyed repeatedly by daily temperature fluctuations, or storms, causing mixing to take place many times a year.

DEPTH OF MIXING

1. Holomictic Lake:
During the annual cycle, the lake's whole (holo) water column is mixed during circulation, top to bottom.

2. Meromictic Lake:
Lakes with typically a large depth, where wind energy may be insufficient to stir up the bottom, will not have their bottom layers involved in the annual mixing. Meromictic lakes tend to have high relative depths (ratio of maximum depth/lake diameter). Some meromictic lakes tend to accumulate a denser, salty water near the bottom, and do not have a water column that mixes entirely. These lakes are chemically meromictic, and have a warm bottom layer called a monimolimnion (where the increased density of the dissolved salts counters the natural upward buoyancy of the warm water). Salts may accumulate from an influx of salty ground water, from storm surges or wind blown salt if the lake is near the sea, or become concentrated from evaporation. The upper layer of meromictic lakes that does mix is called the mixolimnion.

NUTRIENT LEVELS

Lakes can be classified according to their nutrient and productivity (net photosynthesis) levels.

1. Oligotrophic Lakes (from Greek: oligotrophus = low nutritious) have low nutrient and productivity levels.

Nutrients: Low levels and low supply rates of at least one major nutrient (e.g. nitrogen, phosphorus, silica)
Oxygen levels: Does not vary much from saturation in epilimnion or hypolimnion.
Biota: Low primary productivity (plant photosynthesis). Low densities and yields of phytoplankton and zooplankton, zoobenthos and fish.
Light: Transparent water, light penetration often below thermocline. Secchi depth 8-40 m.
Basin shape and watershed: Deep lakes with steep sides. Infertile soils and undisturbed, rocky watershed. WL ration low (e.g. 1:1) (WL = ratio of watershed area to lake surface area)

2. Eutrophic Lakes (from Greek: eutrophus = high nutritious) have high supplies of nutrients and high productivity levels.

Nutrients: High winter levels and supply rates of all major and minor nutrients
Oxygen levels: Great variation from saturation. Depleted in the hypolimnion during summer stratification. Supersaturated in epilimnion due to high rate of photosynthesis.
Biota: High primary productivity (plant photosynthesis). High densities and yields of phytoplankton and zooplankton, zoobenthos and fish.
Light: Water cloudy, light penetration relatively low, often not reaching thermocline or lake bed. Secchi depth 0.1-2 m
Basin shape and watershed: Shallow lakes (usually lesss than 10m deep) with gently sloping sides.Often unstratified. Found in cultivated, disturbed, or naturally fertile watershed. WL ratio high (e.g. 100:1)

3. Mesotrophic Lakes: Intermediate between oligotrophic and eutrophic lakes. Secchi depth 2-8 m.

Compare data from Boeri Lake with Three Temperate Lakes in the Poconos Region, Pennsylvania, USA

Comparison 1: RELATIVE DEPTH

Goal 1: CALCULATE the RELATIVE DEPTHS for each lake.
Goal 2: Using the relative depth as morphometric evidence (measurement of shape) to determine which lakes show a potential for being meromictic.
Goal 3: Answer questions about the lakes' calculated relative depth values.

GOAL 1:CALCULATION OF THE RELATIVE DEPTHS FOR EACH LAKE
Formula: Relative Depth = lake's maximum depth / lake's diameter

To find each lake's diameter:

STEP 1: Using : Find and record the Lake Area in Hectares for each of the three lakes (Giles, Lacawac, Waynewood).

STEP 2: Convert each lake's area in hectares to m2 by multiplying hectares by 10,000. (1 hectare = 10,000 m2)

Example: 15.2 hectares x 10, 000 = 152, 000 m2

Note: Go to Morphometric Parameters for Boeri Lake, find surface area for Boeri Lake which is already in m2

STEP 3: Find the diameter of each lake (in m) by dividing the lakes area in m2 by ¶(3.14). Take the square root of that number, and then multiply that answer by 2. This answer is the diameter of the lake in meters. Round the diameter figure to the nearest .1

Example: 152,000 m2 / 3.14 = 484076.643 of which the square root is 220.0 x 2 = 440 m

To find each lake's Max. Depth:

STEP 1: Using : Find the Max. Depth (in m) for the three Pocono lakes and record. Using Find the Max. Depth (in m) for Boeri Lake and record.

To find each lake's Relative Depth

STEP 1: Using the formula (RD = MD/LD) for Relative Depth divide the max. depth by the lake's diameter. Multiply this number by 100. This will give the relative depth as a percentage.

Example: 50m/440m = 0.11 x 100 = 11%

STEP 2: Record the relative depth for each lake

GOAL 2: USE THE RELATIVE DEPTH TO PROVIDE SUGGESTIVE EVIDENCE OF MEROMIXIS.
Lakes with high relative depth values would have the morphometric evidence (large depth in relation to surface diameter) to suggest they are potentially meromictic (where the lake does not mix completely throughout its deep water column). The upper end of relative depth values for meromictic lakes is 22.8%

STEP 1: Compare the calculated relative depths of Boeri Lake with Lake Giles, Lake Lacawac, and Lake Waynewood. Rank the lakes using their relative depths from highest to lowest.

GOAL 3: ANSWER QUESTIONS ABOUT THE RELATIVE DEPTH VALUES
Using your calculated relative depth values and lake classification background information provided, answer the following questions. See how your ideas match the answers that will be provided Friday, Jan 21, 2000.

Question 1: Which of the lakes, based on relative depth value, shows the greatest morphometric evidence of being a meromictic lake?

Question 2: How do the relative depth values of the four lakes rank in comparison to the upper end value of 22.8% for meromictic lakes?

Question 3: Although their are several factors that determine meromixis, why would a high relative depth be an important variable?

Question 4: What further evidence might be gathered to prove a lake was meromictic?

Comparison 2: THERMAL STRATIFICATION

Goal 1: Create a graph for both Boeri Lake and Lake Lacawac using data tables provided for each lake.

See solution to Goal 1. Boeri and Lacawac

Goal 2: Use the graphs to determine the presence or absence of thermal layers in Boeri Lake and Lake Lacawac.
Goal 3: Answer questions about the thermal profiles of each lake.

GOAL 1: GRAPH TEMPERATURE TO DEPTH FOR BOTH BOERI LAKE AND LAKE LACAWAC
STEP 1: Set up a graph for each lake using the graph template as a model (Note: The depth range for Boeri Lake should be extended to 40 m.)

STEP 2: Place temperature data from the data table for Boeri Lake and Lake Lacawac at the corresponding depths.

GOAL 2: DETERMINE PRESENCE OR ABSENCE OF THERMAL STRATIFICATION
STEP 1: Look at your Lake Lacawac and Boeri Lake graphs to locate any rapid temperature changes in the lake's water column. If there are any noticeable changes: Draw a line at the depth where the most rapid temperature change begins. (The temperature profile will appear the most horizontal during this change.) Then, draw a line at the depth where the most rapid temperature change ends (begins to become more vertical again).

STEP2: If there are any rapid temperature changes, there should be three layers, made distinguishable on the graph by the two lines you have drawn to designate the beginning and end of the rapid temperature change. Label the top layer EPILIMNION; the middle layer METALIMNION; and the bottom layer HYPOLIMNION. The presence of these layers indicate thermal stratification.

GOAL 3: ANSWER QUESTIONS ABOUT THE THERMAL STRATIFICATION PROFILES FOR LAKE LACAWAC AND BOERI LAKE
Using your graph information and lake classification background information provided, answer the following questions. See how your ideas match the answers that will be provided Friday, Jan 21, 2000.

Question 1: Does the thermal profile of Lake Lacawac show thermal stratification?

Question 2: Should thermal stratification be present in Lake Lacawac given the lake's latitude, and the time of year the measurements where taken? Explain why or why not? (Refer to Timing of Mixing background information to help with this answer.)

Question 3: Does the thermal profile of Boeri Lake show thermal stratification?

Question 4: Should thermal stratification be present in Boeri Lake given the lake's latitude, and the lake's relative depth? Explain why or why not? (Refer to Depth of Mixing background information and the information provided for Boeri Lake's physical characteristics to help with this answer.)

Comparison 3: LAKE PRODUCTIVITY

The secchi disk is a circular plate, divided into quarters, painted alternately black and white, that can be lowered on a rope into a lake's water until it is no longer visible. This depth is called the "secchi disk depth'" which can be used to assess the lake's water clarity. The clearer the lake, the further down the disk can be lowered, and the higher the secchi disk depth. High amounts of algae, often associated with higher nutrient levels and lake productivity, can cloud the water, reducing the secchi depth. Therefore, a lower secchi depth reading generally indicates higher lake productivity. High productivity is associated with eutrophic lakes. Low amounts of algae, making for clearer water and higher secchi depth readings, indicates low productivity, and is associated with oligotrophic lakes. Medium clarity and secchi depth readings are associated with mesotrophic lakes. Although only an indicator, the "low tech" secchi disk is one of the most effective tools to estimate a lake's productivity.

Goal 1: Estimate a secchi depth average for the three Pocono lakes using .
Goal 2: Use the secchi depth average to generally classify each lake according to their estimated productivity levels.
Goal 3: Answer questions about the secchi depth readings.

GOAL 1: ESTIMATION OF SECCHI DEPTH AVERAGE
STEP 1: Using record the highest and lowest secchi depths (in m) from the secchi depth graphs for each lake using the darker, solid, middle line. Add the two numbers together and divide by two. This will give a quick average of the monthly mean secchi depth for each lake.

GOAL2: ESTIMATION OF A GENERAL PRODUCTIVITY CLASSIFICATION FOR EACH LAKE USING THE SECCHI DEPTH AVERAGE.
STEP 1: Refer to the background information in Nutrient Levels and the secchi depth ranges listed there for the three productivity classifications of lakes.

STEP 2: Compare the secchi depth average for each lake to the secchi depth ranges used for classification and assign a general productivity classification to each lake.

GOAL 3: ANSWER QUESTIONS ABOUT SECCHI DEPTH READINGS
Answer the following questions concerning secchi depth. See how your ideas match the answers that will be provided Friday, Jan 21, 2000.

Question 1: The secchi depth readings for lakes varies with the season. What factors would influence this variation?

Question 2: In many lakes, the secchi depth is approximately one-third the depth of the photic zone (where light penetrates to a depth still sufficient for photosynthesis). Estimate the average depth of the photic zone for Lake Lacawac using your calculated secchi depth average.

Comparison 4: COMPARE FIELD DATA FROM BOERI LAKE COLLECTED BY SVHS TO BOERI LAKE DATA GATHERED BY SYRACUSE UNIVERSITY

SVHS Boeri Lake Field Data Table (data gathered with probe at lake, sent back via internet) Compare SVHS data with Boeri Lake data gathered by Syracuse Univ.
All Dominica lessons and photography © 1999 Lance Leonhardt