Marine Science (0697) | Investigating ecosystems

Investigating Ecosystems: Your Toolkit for Marine Science (Syllabus Section 5.2)

Welcome to the practical side of Marine Ecology! In the previous chapter, we learned all the technical terms about ecosystems, like species, population, and community. But how do scientists actually go out and measure these things in the real (and often wet!) marine environment?

This chapter gives you the essential fieldwork skills needed to investigate marine ecosystems, especially coastal areas like rocky shores and sandy beaches. Don't worry if this seems tricky—we will break down the methods step-by-step!

Why We Investigate Marine Ecosystems

When studying a marine habitat, we usually want to know two main things about the organisms living there:

• Population Size (How many of a certain species are there?)
• Species Richness (How many *different* species are there?)
• Distribution (Where exactly are the organisms found, and why?)

1. Measuring Population Size and Species Richness (Random Sampling)

To estimate how many organisms live in a large area, we can't count them all. Instead, we use a small, manageable section called a sample. To make sure our sample is fair and represents the whole area, we use random sampling with quadrats.

What is a Quadrat?

A quadrat is simply a square frame of a known size (e.g., 0.25 m² or 1 m²). They are essential tools for counting sessile (non-moving) organisms like barnacles, mussels, or macroalgae (seaweed).

The Process of Random Sampling

Random sampling ensures that every part of the habitat has an equal chance of being sampled, which prevents researcher bias.

1. Define the area: Measure the total area (e.g., 10m x 10m) you want to sample.
2. Generate Random Coordinates: Use a random number generator or draw numbers from a hat to select coordinates (X and Y axis) within your defined area.
3. Place the Quadrat: Place the quadrat at the exact coordinates chosen.
4. Count or Measure: Record the number of individuals of the species you are interested in (for population size) or simply list all the different species present (for species richness).
5. Repeat: Repeat this process many times (e.g., 20 or 30 times) to collect enough data.

Calculating Population Size (Density)

If you count the number of limpets in 20 quadrats, you can estimate the total population. First, calculate the average density:

$$ \text{Density} = \frac{\text{Total number counted}}{\text{Total area of all quadrats used}} $$

Then, estimate the total population:

$$ \text{Estimated Population Size} = \text{Density} \times \text{Total area of habitat} $$

Quick Takeaway: Quadrat sampling is used for counting organisms (population size) and identifying the variety of life (species richness) in a habitat, using *random placement* to ensure the results are reliable.

2. Investigating Distribution (Systematic Sampling)

Sometimes, we don't just want to know *how many* organisms there are, but *where* they are and *why* their numbers change across the environment. This is called investigating distribution.

This is common on shorelines, where conditions (like exposure to air, or wave action) change dramatically with the tide.

Line Transects vs. Belt Transects

To study distribution, we use systematic sampling, which means placing our samples in a straight line along a known environmental gradient (like the gradient from the low-tide mark up to the high-tide mark).

1. Line Transect:

This is the simplest form. You lay a long tape measure (the line) from one point to another (e.g., perpendicular to the sea). You then record every species that touches the line at fixed intervals (e.g., every 0.5 m or 1 m).

• Pros: Quick and easy.
• Cons: Only tells you if a species is present, not how dense the population is.

2. Belt Transect:

This is more detailed and combines transect sampling with quadrat sampling. You lay the tape measure as the central line, but at fixed intervals along the line, you place a quadrat (the "belt") beside the line. You then count or estimate the abundance of species within that quadrat.

• Pros: Provides data on both distribution (where) and population density (how many) along the gradient.
• Cons: Takes much longer than a simple line transect.

Analogy: Think of a line transect as just drawing a line across a map and noting the towns exactly on the border. A belt transect is noting all the towns and their populations within a strip of land along that border.

Memory Trick:
*R*andom sampling gives you *R*ichness and population size.
*S*ystematic sampling (Transects) shows you *S*patial distribution.

3. Measuring the Profile of Shore or Slope

The profile is the change in height and slope across the shore. It is crucial because the height above sea level determines the length of time an organism is exposed to air (desiccation) during low tide. Different species are adapted to different zones (supratidal, intertidal, subtidal).

A Suitable Method for Measuring Slope Profile

You need to measure the distance moved horizontally and the change in height vertically as you move up the shore:

1. Start Point: Place the first metre rule (or ranging pole) upright at the lowest point you can safely reach (e.g., the water's edge).
2. Horizontal Distance: Measure a set horizontal distance (e.g., 1 metre) up the shore from your first rule.
3. Using the Level: Use a second metre rule and a spirit level (or a simple sighting level/Abney level) to find the point that is exactly horizontal to the height mark on the first pole.
4. Measure Vertical Change: Measure the vertical height from the horizontal sighting point down to the ground. This is your height change (or "drop").
5. Record and Repeat: Record the horizontal distance (e.g., 1 m) and the vertical height change. Move the first rule to the location of the second rule and repeat the process all the way up the shore.

By plotting these horizontal and vertical changes, you can draw an accurate profile of the shore and relate it to where different species are found.

4. Analyzing Sediments (Particle Profiles and Moisture Content)

On sedimentary shores (sandy or muddy beaches), the organisms living beneath the surface (infauna) are hugely affected by the physical properties of the sediment itself.

Measuring Particle Profiles (Sediment Size)

The size of the sediment grains (sand, silt, or clay) determines how well water and oxygen can penetrate. Sandy shores have larger particles, allowing better drainage and oxygen availability compared to muddy shores with tiny particles.

Suitable Method: Sieve Analysis

1. Collect and Dry: Collect a sample of sediment from the specific point being investigated. Dry the sample completely, usually in a low-temperature oven, until the mass is constant.
2. Weigh: Weigh the total dried sample mass.
3. Sieving: Pass the sample through a stack of nested sieves. Each sieve has progressively smaller mesh size (e.g., 2 mm, 1 mm, 0.5 mm, 0.25 mm, etc.).
4. Weigh Fractions: Weigh the sediment retained on each sieve separately.
5. Calculate: Calculate the percentage of the total sample mass that belongs to each size fraction. This gives you the particle profile.

Measuring Moisture Content of Sand or Sediments

High moisture content in sediments often means low oxygen levels (anaerobic conditions), which is a key factor affecting which organisms can burrow there.

Suitable Method: Drying

1. Weigh Wet Sample: Take a sediment sample and weigh it immediately (Mass wet).
2. Dry Sample: Place the sample in a drying oven (usually around \(100^\circ\text{C}\)) for several hours until all the water has evaporated.
3. Weigh Dry Sample: Weigh the sample again (Mass dry).
4. Calculate Moisture Content: The difference in mass is the mass of the evaporated water.

The moisture content (%) is calculated using the formula (you need to know how to use this!):

$$ \text{\% Moisture} = \frac{\text{Mass wet} - \text{Mass dry}}{\text{Mass wet}} \times 100 $$

Example: If the wet sample weighed 50 g and the dry sample weighed 40 g:
Mass of water lost = 10 g.
% Moisture = \((10 / 50) \times 100 = 20\%\)