Part 6: Bacterial Concentrations at the Beach

Objective: Interpret a complex plot of bacterial concentrations at different locations on the beach. Determine whether different types of bacteria are likely to come from the same source, and which type could come from the sewage outfall. (Reference: Rosenfeld, L.K., C.D. McGee, G.L. Robertson, M.A. Noble, and B.H. Jones, 2006.  Temporal and spatial variability of fecal indicator bacteria in the surf zone off Huntington Beach, CA.  Marine Environmental Research., volume 61, pages 471-493)

For this assignment, you must first print this plot and this worksheet. Now look at the map above. Find the locations of the Santa Ana River, Talbert Marsh and the OCSD sewage treatment plant.

Every day, the OCSD collects water samples in the surfzone, at locations all along the beach. They travel 39,000 ft, or 10 km along the coast in each direction, starting at the mouth of the Santa Ana river. This map does not have a scale, but I estimate that they are sampling almost from Sunset Beach to Corona Del Mar. You can check my estimate by clicking "view larger map", but be sure you know how to get back to this page.

Now look at the plot you printed out. The title is "Surfzone Bacterial Concentrations", so you know that it relates somehow to measurements taken on the beach. The y-axis of the plot is distance along the beach (not onshore/offshore as many students assume). The x-axis of the plot is time, from May through October of 2001. The lines of dots represent locations and times when bacterial measurements were made. Note that the dots are in horizontal lines, indicating that bacterial samples were made repeatedly at the same location on the beach. Every dot indicates a measurement. If that sample contained enough bacteria to close the beach, a shape is centered on the dot.

Diamonds represent the concentration of enterococcus bacteria. Each diamond is centered at a specific time and location, and the size of the diamond shows how many enterococcus were found at that location and that time. Squares represent the concentration of fecal coliforms at each location and time. So, when and where do you see large squares indicating significant concentrations of fecal bacteria? When and where do you see diamonds indicating significant concentrations of enterococcus? Do the two types of bacteria appear at the same place or the same time?

Building up the plot

Now get a full-sized piece of paper, and draw on it the axes for this plot. You don't need all the details, but make sure you've got the information shown below, left. Look at the Enterococcus distribution - are they all in one place or all over the beach? I chose to represent the enterococcus distribution using long, thin blue ellipses, to show how it spreads along the beach (below). How would you indicate that distribution on your plot? Now consider the Coliform distribution - how would you indicate that pattern? I chose to use green squares (below). When the two patterns are overlaid, do they look the same or different (below, right)?

Just the axes of the plot	Enterococcus distribution spread along the beach	Coliform distribution at a single lcoation	Enterococcus and Coliform

There is very important information on the right hand side of the plot as well. The square and diamond there represent the bacterial concentration in the outfall plume. Look at the diamond representing the concentration of enterococcus bacteria in the plume; is it larger or smaller than the diamond representing the highest enterococcus concentration on the beach? Imagine that the small diamond is a little drop of paint at one edge of a pan of water -- could this little drop become a really big paint spill on the other side of the pan? By this analogy, is there any way the outfall plume could account for the enteroccus contamination on the beach? Now, paint cannot reproduce, and bacteria can, so this analogy does not completely exhonerate the outfall, but it does make the outfall seem less like a "smoking gun" when it comes to enterococcus contamination. Now look at the square representing the concentration of fecal coliform in the plume. Is it larger or smaller than the square representing fecal coliforms on the beach? Could part of this large source of coliforms account for the small beach contamination?

Enterococcus concentration beach > plume	Coliform concentration plume > beach	Complete plot from Rosenfeld et al, 2006

Assignment 6:

1. Describe the basics of the bacterial concentration plot, such as axes and what the shapes represent.
2. Is it likely that the enterococcus and coliform contamination on the beach come from the same source? Why or why not?
3. By comparing the bacterial concentrations in the plume and on the beach, can you tell which bacteria are more likely to come from the OCSD outfall?

1. Describe the basics of the bacterial concentration plot, such as axes and what the shapes represent. The x-axis is time, and the y-axis is distance along the beach (not across the beach!). Each diamond or square marks a time when the beach was contaminated, and the size of the shapes indicates the severity of the contamination
2. Is it likely that the enterococcus and coliform contamination on the beach come from the same source? Why or why not? The enterococcus and coliform bacteria appear in different places: entero is spread along the northern half of the beach, while coliform appears only near the Santa Ana River. They also appear at different times, so they are not likely to come from the same source.
3. By comparing the bacterial concentrations in the plume and on the beach, can you tell which bacteria are more likely to come from the OCSD outfall? The concentration of enterococcus bacteria in the outfall plume is much smaller than the largest concentrations seen on the beach, so it is unlikely that the entero came from the outfall. By contrast, the concentration of fecal coliforms in the plume is much greater than that on the beach, so the coliform could have come from the outfall.