4: Estuaries- Elkhorn Slough

Background:

Today, we will investigate our largest local estuary, Elkhorn Slough. An estuary is a place where seawater meets freshwater, and often some sort of embayment forms at the mouth of the river. As we will discuss, there are several different types of estuaries and different ways they can be formed. Estuaries play many important roles, including providing a large ‘nursery’ for many marine organisms, water purification, sediment catchment, and wildlife utilization.

The history of Elkhorn Slough is long and very colorful, with use by humans stretching back thousands of years. The Slough has been changed considerably by human use and the natural cycle of coastal processes. From pre-historical times, the Ohlone Indians used the area for food, resources, and dwellings. The infiltration of the Spanish missions and Mexican cattle ranchers drastically changed the nature of the Slough and began a long period of large modifications, including the filling in of some areas and changes to the flow of water. During the 1700s, the Russian Sea Otter trade flourished and brought the near extinction of the species. In 1811 alone, over 9,000 otters were killed in Elkhorn Slough! Hunting, logging, whaling, farming, and aquaculture were all large industries in the area and brought with them many important changes to the Slough, including many invasive species both on land and at sea. More modern uses of the Slough include the railroad, a hunting club, and large dairies, all of which still show their scars on the Slough’s fragile ecosystem.

Of all of these changes, one of the biggest was a natural event. A large storm in 1908 broke the Salinas River through the dunes at its current location between Marina and Castroville and thereby cut off the main source of freshwater flowing into the Slough. At this point, the Slough changed from a full-time estuary to a seasonal estuary, only getting large amounts of freshwater during the rainy months.

In the 1950s, the power plant and refractories were built, both of which used large amounts of seawater. The Moss Landing Harbor was constructed in 1946 and has drastically changed the flow patterns of the Slough, causing some areas of the Slough to fill in with sediment and other areas to be scoured away.

Field Explorations:

We are going to examine two different areas of Elkhorn Slough and assess the biological and physical characteristics at each location. Our first visit will be at the estuary mouth near the Hwy 1 bridge in Moss Landing at the floating boat launch to talk about the physical and biological properties of estuaries and sloughs. These include: types of estuaries, plant systems, animal systems, soft-bottom communities, and salt/fresh issues. We will examine land use, watershed factors, and the history of the Elkhorn Slough.

Access the Elkhorn Slough Data Table here. (Go to "File" and "Make a copy" to create a version you can type into)

Site 1: Harbor Mouth

We will start by collecting some information on the physical and biological characteristics of this location.

1. We will record the Depth, Turbidity, Temp, Salinity, Dissolved Oxygen, Phosphate levels, and others, from the floating boat launch and then compare these with the same measurements from a second location inside the slough.

We will use several different digital sensors to collect most of the data, and a Secchi disc to examine water clarity. A Secchi disk is an 8-inch (20 cm) disk with alternating black and white quadrants (sometimes all white). It is lowered into the water until it can no longer be seen by the observer. This depth of disappearance, called the Secchi depth, is a measure of the transparency of the water. The Secchi disk measures the transparency of the water. Transparency can be affected by the color of the water, algae concentration, and suspended sediments.

2. Next, we will try to assess what animal and algal life may be in the area. Start by looking at the animals and algae found on the sides of the dock. This is a good indication of what might be found in other areas of the harbor. We will also use the bottom grab to see what is on the sea floor in this location. We can see how deep it is at this location, what the bottom is made of, and what larger animals we might find. We’ll take the grab and empty it into the sieve and see what we can find. If the tide is out, we will also look for organisms in the mud.

Site 2: South Slough

Lastly, we will visit the South Slough area, where the old Salinas River drained into the Slough before 1908, and look at the biological and physical environment of this back-slough area and compare it with the harbor mouth. Examine the life on the sides of the shoreline. What types of animals and plants do you see? What types do you not see?

1. Redo the profiles of Depth, Turbidity, Temp, Salinity, DO, Phosphate, etc., from both the North and the South sides of the bridge.
2. Compare how the values were similar or different between the three locations.
3. We will also attempt to examine the densities of an invasive species, the Japanese mud snail.

Assignment:

The assignment is to choose 3 different physical measurements (like temperature, salinity, and phosphate) and compare them between all three of the sites to analyze and "tell a story" about the numbers. How did things change from one site to another? Why do you hypothesize that might have been?

1. In your analysis, use graphs to help tell a story with the numbers and look for patterns.

2. Write a short paragraph that explains what the numbers are showing and why you think the data shows what it does.

Put the graph and paragraph together into a document, and then turn that in here.

If you need some help getting started on how to make your graphs, check out this video.

Lab Thumbnail image: “Elkhorn Slough” by Brad Parrett, CC BY-ND 2.0