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

Detailed in-situ measurement of the gross productivity of periphyton or any water microorganism in a suitable site can improve current knowledge of the processes controlling primary productivity dynamic in lentic waters. The major improvements in the proposed methods are non-invasiveness, cost-effectiveness, and the possibility of simultaneous measurements in various location. It also enables the collection of large data assets without additional expenses.

Replacing the barge in conducting the experiments, use an inflatable kayak as it's easily transportable. Select a place with an ideal depth to anchor the float. Next, attach the assembled barge behind the stern of the boat and carefully lower the anchor along the side of the boat.

Align the anchor by hanging it slightly below the water surface so that the float can be easily towed with the anchor to the spot with the required depth. After reaching the spot, untie the anchor from the boat and lower it to the bottom, then secure the barge to the anchor chain and the chains for attaching the incubation bottles to the barge. To prepare the incubation bottles, attach oxygen optical sensor spots to the inner wall of transparent, wide-neck 0.5-liter bottles with gas-tight seals, then make an opaque layer for the dark treatment bottles by wrapping them with black electrical tape.

Cut a tiny hole in the spot of the placed optical sensor and ensure that the cut hole is slightly smaller than the diameter of the sensor to prevent light from entering the bottle. Place the incubation bottles in the portable box and dive with the box to the respective depth without disturbing the sediment in the surrounding water. Next, using long tweezers, carefully fill the samples into the incubation bottles, taking care not to disturb the biomass of the sample too much.

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. Fill one pair of light and dark bottles with clean water from the respective depths without any sediments to serve as blank controls. After ensuring that the water in all incubation bottles is clean and contains no disturbing sediment, bring the closed bottles to the boat anchored to the floating barge.

Attach the first two pairs of incubation bottles to the snap hooks on the first chain, then 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 within a few seconds, read out the oxygen concentration in the meter and record the measured value. Immediately after measurement, carefully lower the chain with the attached bottles back into the water, ensuring that the incubation bottles are placed at the same depth from which the biomass placed in them was sampled.

After one hour, measure the oxygen concentration again by carefully pulling each chain with the bottles into the boat. Reading the oxygen value was demonstrated previously and lowering the samples into the water again. After completing all the measurements, take the samples out from the incubation bottles and scrub the microbial mat grown on the surface of a hard substance like stone with a toothbrush or small knife, then transfer the contents to plastic flasks.

An increase in the oxygen concentration over time is apparent in both the control and the sample bottles exposed to light indicating net ecosystem productivity. Nevertheless, the slope of increase is significantly higher in bottles with microbial mat samples. The change in the oxygen concentration over time in the dark bottles is the sum of autotrophic and heterotrophic respiration.

In this case, the slope of oxygen concentration in the control bottle is not significantly different from zero. Subtracting the respiration rates from net ecosystem productivity yields the rates of gross ecosystem productivity and the data complies well with the definition. A practical application of this method was achieved in the fields of three post-mining lakes that show the gross ecosystem productivity of the periphytic community during the vegetation season.

During the biomass sampling process underwater, it is important to ensure that no air bubbles remain trapped inside the incubation bottles once they are closed after adding the samples. Using this procedure, it is possible to quantify oxygen exchange between the water body and any organism or community of organisms of a suitable size. This method allows studying the year-round primary productivity of selected organism in situ by making the natural condition as much as possible, thus giving a better idea of its relative importance for whole-life carbon metabolism.