Measuring and Recording your Results

-----> After several months in the water, the surface of a square settling plate might begin to look something like this:

How could you measure and interpret results such as these?

Your first step would probably be to decide what animal groups or taxa are present. Using reference books, keys, or the photos on this website, you might decide that the following organisms were present:

	Ciona savignyi
	Other solitary tunicates, perhaps Styela gibbsii
	Other colonial tunicates, perhaps Distaplia occidentalis
	Tube worms, Eudistylia vancouveri
	Nudibranch, Hermissenda crassicornis

Even if you can't identify an organism by its full scientific name, you may still be able to place it in a group of similar organisms that can be measured or counted. For example, groups names such as "colonial tunicates," or "polychaete worms," or "filamentous red algae" are still useful to measure or count even if you can't decide exactly what species are present.

Other difficult questions are raised in trying to count or measure animals and plants that cover a surface, especially when they are colonial organisms. For example:

Is the organism pictured here one animal, or is it hundreds of animals? How would you measure an organism that grows randomly in all directions?

Point Count Sampling Here is a method used by biologists to measure organisms that cover surfaces. It uses a grid which can be overlaid over the settling plate. The grid can be simply printed on transparency paper, or it can be made from a wooden or plastic frame, strung with a series of parallel rubber bands or strings.

Each intersection of lines on the grid serves as a sample point. The organism found below each intersection is recorded. All sample points where no organism was found are recorded as well. Because there are 100 intersections on this grid, it is possible to record 100 sample points from this settling plate.

If a biologist has many plates to sample, he or she may decide not to sample all 100 intersections, but instead select a smaller number, or subset to sample. The best practice in choosing a subset is to use random sampling, that is, to generate random numbers between 1 and 100 and sample those points only. Computer programs such as Microsoft Excel can generate random numbers, but random numbers can also be pulled out of a hat. Randomizing reduces the chance that the researcher will bias the study by using a pattern that doesn't represent all parts of the surface equally.

Percent Cover Estimation

A grid with 100 squares such as the one above can also serve as a tool for estimating percent cover, another useful measurement technique. On the plate shown above, can you estimate how much of the surface was not colonized?

Total Population Counts In some circumstances a biologist might choose to count the full number of individuals in certain species. For example, it might be interesting to know the numbers of Ciona savignyi that have colonized plates at hanging at different depths. The data shown in this graph were obtained by counting all individuals growing at each of the depths shown.

Some questions to explore . . .

• What organisms are the first to settle? Does their abundance change as more organisms appear?
• Does biodiversity of marine plants change with depth? Of marine animals?
• Are different organisms more abundant in shady places or sunny places?
• If an herbivore (such as a limpet) is placed on a settling plate, what species growing on the plate are affected? What species are affected if a carnivore (such as a nudibranch) is placed on the plate?

The number of interesting questions that can be investigated using Random Point Counts, Percent Cover Estimates, and Total Population Counts as tools is endless.

Contact us! We're eager to hear from you. Send us your results and your ideas for further investigations. If you find the invasive tunicate, Ciona savignyi, let us know, and we will add your site to our map. We will post results you share with us on this website, and provide links any to websites you create that relate to ecology of floating docks. Good Luck!