Application of Oxygen Microelectrode for Measuring the Primary Production of Microphytobenthos on Daebu Mudflats, Korea

Abstract

This study aimed to establish and improve a measurement method for the primary production of microphytobenthos using oxygen microelectrode and apply it to the measurement process of the primary production of microphytobenthos. The methodogical improvements were as follows: the fabrication of an oxygen microelectrode, standardization of the microelectrode-enabled measurement process, setting a production measurement system and its outcomes were emphasized. As fabrication of oxygen microelectrode involves several technical challenges in itself, it was highlighted how such challenges were overcome in the fabrication process. In addition, following its fabrication, it was also important how microelectrode would be applied to the measurement of primary production. To that end, a standardized water bath was made, and the primary production was measured through exposure of microphytobenthos in the water bath and standardization of the conditions of experiment. This process also required creation of the several conditions of experiment.

The microelectrode was largely fabricated in four steps: step one - fabrication of working cathode; step two – fabrication of outer casing; step three – fabrication of reference electrode/guard cathode, and; step four - assemblage. In the working electrode fabrication step, the method of processing its edge was improved. Outer casing is an important component in determining the specification of microelectrode, and its edge needs to be made to be thin simultaneously with acquiring space to house working cathode and guard cathode. Step three was designed to resolve the two contradicting problems. In addition, the tip of outer casing was also fabricated hard as well as thin to prevent the tip of microelectrode from being damaged when it was used in tidal flat sediments. Stability of microelectrode signal was improved while external interference was reduced by inserting a guard cathode.

To measure the primary production of microphytobenthos, first, experimental water bath was fabricated, second, constant temperature conditions were controlled in the water bath, and lastly, a system that could measure up to eight deposit specimens simultaneously was implemented. Small experimental water bath was used in preceding studies. However, large water bath to hold 20 liter of seawater was fabricated in this study to keep seawater saturated and maintain constant rate of flow. It was possible to stably maintain water temperature while measuring primary production using a lot of experimental seawater in a temperature-adjustable laboratory.

To resolve the issue of non-homogeneity of deposit specimen, a weakness of oxygen microelectrode method, and to measure deposit specimens containing differing amount of living organism content, the experiment core and the experimental water bath were improved to measure primary production of up to eight different core samples under the same conditions simultaneously with arrangement and use of measuring instruments. This study is the first attempt to improve the heterogeneous problem inherent to in the measurement of microphytobenthos. Primary production rate was calculated by summing up the ratios of oxygen produced from deposits and diffused into water layer and diffused under deposits respectively using Ficks first law of diffusion in depth-specific oxygen concentration distribution curve.

After the usefulness of the microelectrode method to the measurement of the measurement of primary production of microphytobenthos was determined, p.p. measurements using in situ core samples were conducted 13 times from March 2009 to April 2010. All measurements were done under the temperature conditions of four to five at 5 oC intervals in consideration of prevailing field temperature in each month.

Primary production measured in each month revealed significant seasonal change. It was 25.4 mmol O2 m-2 h-1 in May and fell to 14.1-22.3 mmol O2 m-2 h-1 in June to August before declining to 12.4-13.9 mmol O2 m-2 h-1 in October-November and sliding to the lowest level of 9.9-11.9 mmol O2 m-2 h-1 in December-January. It rose to 18.7 mmol O2 m-2 h-1 in February and reached the highest annual level of 35.3 mmol O2 m-2 h-1 at the end of March. Such outcomes confirmed that the blooming happened in early spring in the Daebu mud flat in the west coast of Korea. When compared with tidal flats in other countries, primary production rate of microphytobenthos measured in this study was comparable to those recorded in the tidal flats of Europe and America.

The microphytobenthos primary production value measured by oxygen microelectrode was comparable the value measured by 14C incubation method. Additionally, the hourly production rate and normalized maximal production rate were comparable with other tidal flats around the world. So it was found that this method may be replaced with the commonly used 14C method.