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Subtidal Macro-Algae Sampler

UC San Diego MAE 156B

David Leung - Tanima Shukla - Joseph To - Bridget Zeiger

Sponsored by: 

An Executive Summary can be found here

Background

Estuaries often go through a process called eutrophication. This process occurs when a body of water acquires a high concentration of nitrates and phosphates, normally due to erosion, agricultural run-off, and discharge of sewage. Consequently, this process stimulates the growth of macro algae. Figure 1 shows the typical effect eutrophication has on the amount of macro algae in a body of water.

Figure 1: Macro Algae in Weeks Bay Estuary due to Eutrophication

Since an excess amount of macro algae will deplete the amount of oxygen in seawater and thus have negative impacts on water quality, a way to assess water quality is the amount of algae in a body of water. Typical measurements are by mass of algae per unit area (g/m2) and by percent cover. The mass of algae is taken by both wet and dry samples. Percent cover is taken by visually inspecting the percent of algae that covers a unit area. As seen in Figure 2, a quadrat is typically used to measure percent cover. Quadrats have a square frame with a grid pattern on it to help measure the amount of algae that is covering the frame.

Figure 2: Quadrat showing 6-25 Percent Cover

The Energy and Environmental Sciences Group at SPAWAR SSC Pacific is currently concerned about the impact macro algae has on water quality. They currently have acoustic and electronic methods to measure such impacts, but need another method to verify their results. SPAWAR has therefore asked UC San Diego’s Mechanical and Aerospace Engineering Department to build a sub-tidal macro algae sampler to manually collect macro algae in estuaries and use this data to verify their electronic and acoustic methods.

Objectives:

The goal of this project is to develop a sub-tidal macro algae sampler with the following functional requirements:

• Must fit on a standard kayak (4.26 meters long, 0.509 meters wide)

  • Must be less than 9.07 kg (20 pounds)

• Must be operable by a 1 – 2 person team on a small boat

  • Must have workable depths of 3.048 – 6.096 meters (10 – 20 feet)

• Must be either reusable or very low cost

• Must be able to sample in various sediments (i.e. various grain sizes, density of plant growth, etc.)

• Must capture consistent sample sizes

• Must capture macro algae through the water column to the sediment bed

• Can mount and actuate a camera on the device for visualization purposes (i.e. take a picture for percent of algae coverage

Final Design:

    The final design consists of a cylindrical sampler that is lowered by high density polyethylene (HDPE) tubing into the water, as seen in Figure 3. The cylindrical capsule housing is made of brass mesh. Two circularly formed sheet metal pieces are mounted on the top and bottom of this cylindrical brass housing. Sheet metal columns are attached to the circular metal pieces for support.

Figure 3: Final Design

    To lower the sampling capsule into the water, modular HDPE tubing was cut in lengths of 0.3048, 0.6069, and 1.524 meters. This tubing can be attached to and detached from one another using quick connect union connectors to create the desired dispensing length. 

    Algae samples are cut using a steel wire cutting mechanism. The cutting mechanism consists of a brass block and steel music wire. One piece of wire runs inside along the top and bottom circular sheet metal pieces in an `O’ shape. There are two holes in each of the brass blocks. One end of the wire goes through one hole while the other end goes through the other hole. One end of the wire is secured by a nut and bolt, while the other end is pulled to close the wire shut. Algae is cut between the brass block and cutting wire.

    In order to ensure that both the top and the bottom of the sampler close at the same time, a pulley is placed in the middle of the cutting wire. To actuate the cutting mechanism, the pulley is pulled. The pulley is connected to a steel cord through two swaged loops, as seen in Figure 4. A carabiner connects these loops. The steel cord runs within the tubing and is pulled by the user to actuate the cutting mechanism.

                            Figure 4: Close-Up Parts of Sampler/Dispenser

   After the algae is cut, a cylindrical mesh bag made of tulle closes to secure the algae, similar to how the cutting wire closes. Wire runs along the top and bottom of the cylindrical mesh bags. This wire is held in place in an `O’ shape until actuated by the user, in the same manner as the cutting wire.

    The wires in the bag and for cutting are held in place by pieces of Delrin with slits in them, as seen in Figure 5. Four Delrin pieces are mounted by four bolts and nuts around the perimeter of both top and bottom sheet metal pieces. Wires from the bag and cutting wire are inserted into these slits and held by friction until enough force is exerted upward to start the cutting mechanism and bag closing, whose forces are directed inward.

    Overall, a macro-algae sampler that has the potential to reliably sample algae manually was made. While there is still some testing needed to determine just how well this sampler works, due to pool testing there is a good reason to believe that the sampler will work well with minor adjustments.

                                                                                                                                                                                                                                                                             Figure 5: Exploded CAD of Sampler

Prototype Testing and Results:

The prototype test consisted of using HDPE tubing to lower the final design to the bottom of a pool (2.133 meters). Afterwards, the cutting mechanism and mesh bag wire were actuated to close both of the wires. Finally, the HDPE tubing was used to raise the sampler back up to the surface. Swimmers mimicked current from an estuary.

Results Included:

• The sampler stands upright against water flow

• The Delrin Pieces held wire and bag in an open configuration

• The mesh bag wire close properly

• The cutting wire closed properly

• The setup took longer than expected (about 5 minutes)

• HDPE tubing and quick connects can hold weight without coming apart

• Sampler sinks with its own weight

  Figure 6: Final Testing

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