2: Physical Oceanography- Measurement, Density, and Buoyancy

Physical Oceanography- Measurement, Density, and Buoyancy

Purpose:
Explore many of the tools and techniques used to study and measure the physical world. You will learn about taking multiple samples and the importance of averages and error in data collection. We will explore the concepts of salinity, temperature, and density, and how they interact in the lives of sea creatures.

Tasks:
Practice using several different tools and record your results in the lab manual. Create three water layers with density differences significant enough to allow for separation and no mixing. Lastly, you will try to build a model of a plankton that sinks as slowly as possible through the water.

Criteria:
Your recorded numbers will be shared in class so we can talk about class averages. Your water layers will be examined in class and shared with other groups. Your plankton will compete with other plankton to see who sinks slowest. A homework assignment will be turned in next week that explores the idea of density and buoyancy in greater detail with three specific examples.

Introduction
This course looks at the marine realm through the eyes of ecology. Ecology is the scientific study of the interactions between organisms and their environment. This endeavor is empirical, being dependent on evidence that is observable by the senses. This evidence can include purely descriptive data, but more often it tries to make sense of the world through the collection of numerical, quantitative data. Being able to say a snail was 4.2 cm long is much more informative than saying it is “small”, or stating that the water is 3o°C and has a salinity of 33 is much more helpful than stating that the water is “cold and salty.”

There are two main components studied in ecology: the biotic, or living world, and the abiotic, or non-living world. These components are quite different, and likewise so are the methods and tools that allow us to collect empirical, numerical data on these two areas. Within the biotic realm, we often concern ourselves with measures of an organism’s abundance, density, and spacing. We will learn about these techniques and tools in later labs. In addition, we often try to quantify predation, competition, and other biotic factors like behavior (which are often not easily done!). Abiotic factors can include both physical and chemical factors, such as location, temperature, light, pH, wave force, wind, rock, sand, etc. The almost endless list of abiotic factors that a scientist could think to measure is measured by a large variety of different tools, many of which can measure the same thing in many different ways, with sometimes different units. A trend in recent years with the miniaturization of sensors, probes, and computer technology is the creation of multipurpose tools, which can measure many different abiotic factors, sometimes all at once. Using some of these tools, we will get some hands-on experience measuring common abiotic factors and practice graphing the results!

pH determination

1. You will spend time examining pH, a measure of the acidity of a solution in terms of the proportion of hydrogen ions (H+).
2. Record the pH values of the three solutions A, B, and C using the pH paper and the digital pH meter. Do the values agree?

Salinity
There are several different methods of testing the amount of dissolved solids, or salinity, in solutions. You can determine salinity using the difference in the refraction of light through a sample. You can also measure the density of the solution with a floating hydrometer. Electrical conductance is also commonly used; the higher the salinity in the water, the more easily current can flow. Salinity is usually measured in “parts per thousand (‰).

1. Using the hand-held refractometer, determine the salinity of the three solutions. Place a drop of the solution on the glass plate and put the cover down on the drop. While holding it up to the light, look in the scope, and you should see a shadow line, which is where you read the sample. Make sure you read the number from the right side of the meter (‰).

2. Using the digital conductivity meter, measure the salinity of the three samples again. How do the three values compare? Note: you need to pay close attention to the flashing lights and green button! The instructor will explain.
3. Take a look at the glass, floating hydrometer. Your instructor will demonstrate how to use the device by letting it float until still and then reading the measurement off the scale inside the glass. You don’t need to do this yourself.

Refractometer View inside Hydrometer Pasco unit

Temperature
Several different temperature measuring devices have been put into two beakers of liquid.

1. Record the temperature on each device for each liquid. Which device is most accurate? How do you know?

Density

One of the most fundamental abiotic properties of water is its density. The density of water is mainly determined by two things: salinity and temperature. The formula for density is:

D=M/V

Where: D is the object's density (measured in kilograms per cubic meter), M is the object's total mass (measured in kilograms), V is the object's total volume (measured in cubic meters)

For a given volume of water, the only things that determine density right now are the mass (M) and volume (V), and the two things that do that are temperature and salinity.

Lowering the temperature squeezes the same number of water molecules into a smaller volume (they are closer together), thereby increasing the density. Raising the temperature does the opposite.

So: Colder Temp = Lower Volume = ­Higher Density

Salinity adds dissolved salts into the volume of water, raising the mass, but doesn’t add much to the volume, and therefore the density will go up as the salinity increases.

So: ­Higher Salt = ­Higher Mass = ­Higher Density

In this lab, we will investigate the density of liquid layers and how they interact (or don’t) by carefully combining a few different liquids in two different experiments. Using any combination of salinity (fresh water all the way to super-saturated with salt), temperature (very cold to very hot), and some food coloring, you will mix up and dye water ‘layers’ that will interact when you put them into a small plastic tub. If you are careful about not mixing the water layer (don’t pour them roughly into the tank, etc.), you can expect your water layers to sit on top of each other, depending on their density. See if you can make a three-layered ocean in your tub and experiment with making waves on the surface and in the lower layers by blowing with a straw. Additional instructions and tips will be provided during the lab.

Here is a good website to calculate Density with Salinity and Temperature: http://www.csgnetwork.com/h2odenscalc.html

Buoyancy

Plankton are organisms that drift; they cannot swim against a current any stronger than about 1 knot (1.15 miles/hour). Usually, plankton are very small, microscopic organisms, but some larger animals, like certain jellyfish, are also considered plankton. Plankton are divided into two groups: plant-like (phytoplankton) and animal-like (zooplankton). Phytoplankton make their food through photosynthesis using sunlight to combine carbon dioxide and water into sugar. Creatures that make their own food are called autotrophs (self-feeders). Zooplankton must ingest, or eat, food made by other creatures, so they are heterotrophs (other-feeding). Plankton are usually heavier than water. This is important because if a planktonic organism just floated on the surface of the water, it might not be able to get to food sources below it, or it might get too warm or too much light from the sun (even phytoplankton can be "burnt" by the sun!). So, plankton will tend to sink in the water column. But phytoplankton do need to stay where sunlight penetrates in the ocean. Zooplankton feed on phytoplankton, so the zooplankton want to stay where the phytoplankton are in the water column. One important note is that zooplankton are usually able to swim upward in the water column very slowly to maintain their position. But if they sink too quickly or are too heavy, they will go straight to the bottom of the ocean and not be able to get back up! Therefore, planktonic organisms will have adaptations that prevent them from sinking too quickly.

These adaptations include the following:
1) small size (small things sink slower than large things)

2) long spines or projections that increase drag (drag is sort of like friction)

3) long, thin, or flattened shape - also increases drag

4) contain small amounts of oil (which is lighter than water)

The Great Plankton Race

Start to think about how you would build a plankton (smaller than 3 inches across). What would you try to do to minimize its sinking? Using random stuff around your house, try to build a plankton that sinks, but sinks as slowly as possible. The “perfect” plankton would drop down under the surface and then remain motionless. That is VERY hard to make! Use a large, clear container of some sort with at least a foot of water depth to be your test tank. Water pitchers are often good.

The rules are simple:
1) You need to start 1 cm below the water’s surface.
2) If it floats up to the surface, it is disqualified.
3) We will time how long it takes to hit the bottom of the container.

Laboratory Write-up Instructions

All organisms in the aquatic environment must deal with the challenges of having their living tissues and their cells be denser than the surrounding water. Proteins, organelles, muscle tissue, and most other things inside cells are denser than the surrounding fluid (seawater). We’ll see later in the lecture that this difference between the insides of organisms and the outside environment is quite important on all sorts of levels, but for now, let’s limit our attention to the issues of density, sinking, and adaptations that creatures might have to deal with this challenge. Many creatures must remain in a specific area of the ocean to be successful and can't just sink into the deep ocean or float at the surface.

For this lab write-up, you are to investigate the strategies used by three different organisms that live out at sea (not on the bottom) and explain their adaptations and/or behaviors used in dealing with the specific problem of buoyancy. The three organisms will cover a wide size range, as the strategies involved tend to be quite different depending on the organism’s size! Use any combination of the book, lectures, web, or other sources.

Your three organisms will fall into each of the following three size classes:

1. Smaller than 1 cm in length
2. Between 1 cm and 10 cm
3. Bigger than 10 cm

Please write up one paragraph on each of the organisms. Include the following information in each paragraph:

1. Define the species’ particular challenges as they relate to buoyancy.
2. Explain how its shape, size, insides, and/or behavior might be used to deal with the problem of sinking or floating.
3. List the sources you used using a full citation format (do a web search for MLA for examples of what to include).

Please turn this in ONLINE, typed, next week before the beginning of the next lab.

Thumbnail: “Moving waters” by Ishrona, CC BY 2.0