A Low-Cost Method of Measuring the In Situ Primary Productivity of Periphyton Communities of Lentic Waters

NOTE: Before sampling, determine the degree of replications based on the overall project needs, statistical design, or expected amount of sample variability.Five replicate pairs of light and dark incubation bottles are suggested for precise statistical analysis and to account for potential sample loss or breakage. The described floating experimental barge is designed to carry five replicates plus one pair of blank controls; see Figure 1 for a technical drawing of the experimental barge.

Figure 1: Technical drawings of the experimental barge and the side float. (A) Top view: the frame of the barge consists of four Aluminum angle L profile pieces (blue) that are joined together by four Aluminum flat bars (grey). XPS floats (pink) are mounted to the frame at two points, each one on the parallel aluminum pieces. Chains for incubation bottles are attached to the frame on both sides using snap hooks in predrilled holes (red arrows) with 550 mm of separation between them. Chains were provided with snap-hooks at 1 m and 2 m distances for incubation bottle attachment (choose the snap-hooks position according to the experimental depth). The concrete anchor is secured to the barge's bow, where an overhang of 25 mm allows for two predrilled holes (yellow arrowheads) to serve as an attachment point for the anchor's chain and research vessel. The frame is assembled or disassembled easily via the parallel joints between the four aluminum angle pieces (green arrowheads). (B) The side view shows the suspended chains with hanging incubation bottles and concrete anchor (brown square). (C) The side XPS float: Parallelaluminum angle L pieces (blue) are joined by vertical aluminum flat bars (grey). Below the crossbar section, the XPS float (pink) is mounted with the necessary hole sizes indicated (4 mm). The suspended chains are attached with snap hooks in 8 mm holes (red arrowhead). At the barge's bow, two 8 mm holes are drilled into the overhanging aluminum, one for securing the anchor to the barge (yellow arrowhead) and another for mooring the research vessel to the barge (blue). Please click here to view a larger version of this figure.

1. Construction of the experimental barge

NOTE: The floating barge consists of two equal sections mounted together, allowing easy assembly/disassembly. All used parts can be purchased at any hobby market or store selling building materials.

1. First, assemble the frame of the barge by joining four aluminum angle L profile pieces (40 mm x 40 mm x 3 mm; length of 2,000 mm) together using four aluminum flat bars (40 mm x 3 mm x 350 mm), 16 screws (4 mm x 15 mm with hexagonal nuts), and 32 washers (4 mm x 10 mm).
   NOTE: The distance and position of the flat bars are shown in the technical drawing in Figure 1A. The detailed attachment of the floats to the side flat bars is shown in Figure 1B.
2. To join the two equal sections of the frame, use four 4 mm x 15 mm screws with wing nuts and eight 4 mm x 10 mm washers to screw the aluminum angle L profiles together at the ends (Figure 1A, green arrows).
3. Use five pieces of extruded polystyrene (XPS) material (500 mm x 200 mm x 150 mm), ten 10 mm x 170 mm screws with hexagonal nuts, and twenty 10 mm x 50 mm washers to prepare five extruded polystyrene floats (500 mm x 200 mm x 150 mm each). Attach the floats to the frame at five points shown in the technical drawing (Figure 1A).
4. Drill holes into the frame (see red arrows in Figure 1A marking the positions and distances of the holes for the chains). Attach the 12 m steel chains (wire diameter of 3 mm, inside link of 5.5 mm x 26 mm) for the incubation bottles to the holes in the frame using steel carabine hooks (50 mm x 5 mm). Provide each chain with pairs of snap hooks (50 mm x 5 mm) for installing the incubation bottles to the desired depth according to the experimental design. In this case, they were seated at 1 m and 2 m depths.
5. For the anchor, fill the 15 L bucket with concrete. Insert an eye bolt into the concrete and allow it to dry undisturbed. Fasten the 5 m steel chain to the hook. Secure the anchor to the predrilled hole on the barge's bow (marked by yellow arrows in Figure 1A,B).
   ​NOTE: Technical drawings with the assembly description are shown in Figure 1A-C. Figure 2 shows the photo of the assembled experimental barge. Figure 3 shows the attachment of incubation bottles to the chain.

Figure 2: Assembled experimental barge. Photograph of the assembled experimental barge. Red arrowheads show the holes for the attachment of chains with incubation bottles. The green arrowheads point to where the two halves of the float are joined together. Please click here to view a larger version of this figure.

Figure 3: Incubation bottles. Photo of two pairs of dark and light incubation bottles hanging at a depth of 1 m. One pair of bottles contains the sample of intact microbial mats still growing on the stone (red arrowhead). The second one is the blank bottle with the lake water from the respective depth. A yellow arrowhead points to the oxygen sensor spot attached to the inner wall of the incubation bottle. Please click here to view a larger version of this figure.

2. Installation in the field

1. Use of an inflatable kayak is suggested for barge placement and conducting experiments as it is easily transportable.
2. Select a place with an ideal depth to anchor the float. Choose the depth so that the lower incubation bottles are at least 2 m above the bottom to avoid disturbing the sediment into the water column around the incubation bottles.
3. Attach the assembled barge behind the stern of the boat. Carefully lower the anchor along the side of the boat and align it so that it hangs slightly below the water surface so that the float can be easily towed along with the anchor to the spot with the required depth.
4. Untie the anchor from the boat, lower it to the bottom, and secure the barge to the anchor chain.
5. Secure the chains for attaching the incubation bottles to the barge.

3. Incubation bottle preparation

1. Use the transparent wide-necked 0.5 L bottles with gas-tight seals.
   NOTE: It is possible to adjust the size of the bottles but remember to increase the floatation of the barge with more polystyrene sheets as well. The barge described here can reliably carry 24 0.5 L glass bottles.
2. Attach the oxygen optical sensor spots to the inner wall of each bottle.
3. Add an opaque layer to the dark treatment bottles by wrapping them with black electrical tape.
4. Cut a tiny hole in the spot with the optical sensor. To prevent light from entering the bottle, make the hole slightly smaller than the diameter of the sensor.
   ​NOTE: Any opaque layer that prevents light from entering the bottle will also work. The advantage of black electrical tape is that it resists abrasion and does not peel off in water.

4. Sample collection and handling

NOTE: Divers carry out the manual collection of samples in deeper water. In shallow water, it can be done by snorkeling or wading.

1. Place the incubation bottles in the portable box.
2. Dive with the box to the respective depth. Avoid disturbing the sediment in the surrounding water.
3. Fill the incubation bottles with the samples carefully. Try to disturb the biomass of the sample as little as possible, for example, by using long tweezers. If the microbial mats grow on a solid surface, such as a small stone, carefully transfer the whole stone with intact biomass into the bottle.
   NOTE: Avoid collecting big stones when sampling mats grow on stones-the glass bottles can be broken during further manipulation.
4. Fill one pair of light/dark bottles with clean water from the respective depths to serve as blank controls.
   NOTE: The bottles without the periphyton sample serve as a control determining the oxygen production/consumption of ambient water organisms. It ensures that the calculated net or gross primary productivity of periphyton is unbiased.
5. Ensure that the water in all incubation bottles is clean and contains no disturbing sediment.
6. Close the bottles and bring them to the boat anchored to the floating barge.

5. Measuring the primary productivity

NOTE: The person sitting in the boat takes the box from the diver and performs the following steps.

1. Attach the first two pairs of incubation bottles to the snap-hooks on the first chain.
2. Measure the initial oxygen concentration in each bottle using the fiber-optic oxygen meter. Attach the optical cable of the meter to the oxygen sensor mounted inside the bottle and immediately (within a few seconds) read out the O₂ concentration contactless (through the bottle wall). Record the measured value.
   NOTE: The handling time is short; from taking the bottles from the divers to the initial setting to the respective depth, it takes just a few minutes.
3. Immediately afterward, carefully lower the chain with the attached bottles back into the water. Ensure that the incubation bottles are placed at the same depth as the biomass that was placed in them was sampled.
4. Make another measurement from the ship after 1 h (see NOTE below). Carefully pull each chain with the bottles into the boat, read the oxygen value by attaching the optical cable to the sensor, and lower the samples into the water again.
   NOTE: Adjust the time between individual measurements during the incubation according to the intensity of O₂ productivity/consumption of samples to avoid oversaturation of bottles.
5. Repeat this procedure at least four or five times with all pairs of bottles.
   ​NOTE: The whole setup of the experiment in the field is shown in Figure 4.

Figure 4: Schema of the experimental setup in the field. Illustration of the anchored experimental barge on the lake surface. The incubation bottles (0.5 L) with microbial mat biomass are hung at two different depths (1 m and 2 m). The divers collected samples of microbial mats directly into the incubation bottles at the appropriate depths. Oxygen concentration in individual bottles is measured from the ship. The bottles are pulled out of the water. The oxygen concentration value is measured in a few seconds by attaching an optical cable to the oxygen sensor. The bottles are then carefully lowered back into the water. The whole procedure of measuring two pairs of incubation bottles from two depths takes ~2 min. Please click here to view a larger version of this figure.

6. Sample analyses

1. After the end of the measurements, take the samples directly from the bottles, and transfer the biomass of the microbial mat to the small plastic flasks. If the mats grow on solid substrates (e.g., stones), scrub them with a toothbrush or a small knife.
2. In the laboratory, filter each replicate through pre-weighed glass fiber filters to determine the dry weight⁹.

7. Data analyses

1. During the incubation period, measure the oxygen concentration in the light and dark bottles and compare it with the oxygen concentration in the water column when the bottles are filled.
   NOTE: The change in oxygen in the light bottle over time is the combined result of gross ecosystem productivity (GEP) and respiration by all the organisms in the bottle (autotrophs and heterotrophs from the ambient water and the periphytic community). The decrease in oxygen in the dark bottle measures the respiratory losses of both autotrophs and heterotrophs. The change in oxygen concentration in the control (i.e., bottles without the periphyton) is only the product of heterotrophic or autotrophic organisms in the ambient water. Periphyton productivity and respiration were estimated by subtracting ambient water productivity and respiration measured in blank incubation bottles.
2. Note down the O₂ concentration in percent oxygen saturation (i.e., calibrate the oxygen sensors to 0% and 100% oxygen saturation). Before estimating the primary productivity, convert the raw data () to some reasonable unit.
   NOTE: In this study, the data are converted to mmol of O₂ per gram of organic matter (OM) of the periphyton mass at the dry weight basis, according to Benson and Krause¹⁰.
   1. Calculate the conversion using equation (1):
      (1)
      Where C_p is O₂ concentration in water (mg (O₂) L-1) when it is fully saturated by O₂, V_Bottle is the volume of the bottle in mL, V_Stones is the volume occupied by the stone in mL, is the weight of the periphyton mass in g, and 32 is the molar weight of O₂.
   2. Calculate C_p using equation (2):
      (2)
      Where C_s is the standard O₂ concentration, P is the atmospheric pressure at the lake surface, P_w is the partial pressure of the water vapor at the lake surface, and ω is water density.
   3. Calculate C_s, ω, P, and P_w from previously defined empirical equations (3-6) when lake elevation (h in km) and water temperature at the lake surface (t in °C) are known:
      (3)
      (4)
      (5)
      (6)
      NOTE: From equations (1-6), it is evident that the calculated O₂ concentration is most accurate for shallow depths. As the depth increases, the calculated concentration gets more biased in terms of absolute concentration. It is optimal when the rate of change of oxygen concentration along the depth is known for each lake so that the absolute O₂ concentration can be corrected if necessary. Once the O₂ concentration is calculated, its change over time can be used to calculate two different fluxes of O₂ at two different conditions. Under light conditions, net ecosystem productivity (NEP) is directly proportional to the change in O₂ concentration over time (see below). The term "ecosystem" is used here to denote that the periphyton is composed of autotrophic and heterotrophic organisms. In the dark, the change in O₂ concentration over time is proportional to the sum of respiratory losses of autotrophic and heterotrophic organisms, thus defining ecosystem respiration (RE). The difference between NEP and RE defines gross ecosystem productivity (GEP). If the respiratory losses of the heterotrophic part of the community are negligible, GEP becomes equal to gross ecosystem productivity.
3. Determine the rate of change of O₂ concentration over time by third-degree polynomial regression, as shown in equation (7).
   (7)
   Where A₀ is O₂ concentration at time zero and A₁–A₂ are coefficients of polynomial regression.
   NOTE: The polynomial function is used because it can serve as an approximation of any differential equation. Thus, it is not necessary to know the accurate functional relationship between O₂ concentration and time. Therefore, any assumption associated with the functional relationship (e.g., linearity) does not need to be controlled. By definition, the second term of polynomial regression, A₁ defines the rate of change of O₂ concentration at time zero (i.e., instant rate), which is independent of A₀ and, thus, the absolute O₂ concentration at time zero. For that reason, the estimate of O₂ flux is not affected by the bias in absolute O₂ concentration calculations caused by pressure change across the depth gradient. A₁ has units O₂ (mmol g(OM)^-1) per time (1 h in this study).
   1. Calculate A₁, calculated separately for bottles with and without exposure to light and containing microbial mat biomass (i.e., V_Stones > 0) and for control bottles with and without exposure to light and containing free water (i.e., V_Stones = 0).
      NOTE: Assigning these different regression coefficients notations , , , and , respectively, the equation (8) for productivity GEP calculation can be written as:
      (8.1)
      (8.2)
      (8.3)
      The term defines net ecosystem productivity (Figure 5A; i.e., net oxygen productivity ), and the term represents the sum of autotrophic and heterotrophic respiration (Figure 5B; RE, i.e., assuming both respirations are similar under dark and light conditions).
   2. Subtract R from NEP to obtain GEP (Figure 5C).
      NOTE: Equation (8) implicitly assumes that and are both positive, and and are both negative. If is positive, check the raw data carefully for outliers . can be theoretically negative because the respiratory losses caused by heterotrophic activity can be higher than photosynthetic activity.