Effects of Increasing Water Temperature and Nutrient Concentration on Fungi Activity and Subsequent Litter Decomposition.

Woodland streams are characterized as small forest streams located in moderate to high latitude and altitude magnitudes. The ecosystem of these streams is dependent on the decomposition of organic matter which is primarily decomposed by aquatic hyphomycetes. The streams do not obtain energy through photosynthesis because riparian vegetation, aside from supplying organic matter, provides a lot of shade to the streams. This shade, in addition to the streams’ location, causes the streams to have low water temperatures, and be, therefore, more susceptible to temperature increases from global warming. Two factors associated with global warming and which are expected to have a great impact on litter decomposition in streams are water temperature and nutrient concentrations (Ferreira and Chauvet 2010). These two factors are expected to increase and to affect litter decomposition, specifically alder leaves; aquatic hyphomycetes will also be affected as they are the primary decomposers. The hypothesis states that increases in water temperature and nutrient concentration will increase hyphomycete activity and decomposition rates.  —Daniella Barraza

Ferreira, V., Chauvet, E., 2010. Synergistic effects of water temperature and dissolved nutrients on litter decomposition and associated fungi. Global Change Biology 17, 551–564.

Ferreira and Chauvet simulated stream-like conditions at the lab by creating fungal microcosms.  A microcosm consisted of a glass chamber with an opening at the bottom for air to enter and create turbulence for the leaf discs.  The leaf discs represent the organic matter to be decomposed. Inside the discs are samples of Alnus glutinosa (alder leaves). Also, at the bottom, was a valve to allow the glass chamber to drain and to obtain the conidial suspension for analysis. A conidium is an asexual spore of the hyphomycete. The strains were acquired from a conidium found in three streams of different biomes. A total of six species were collected to form an assemblage for experimentation to represent the fungal diversity of a decomposing leaf in a stream. This number was indicated as adequate by another study. Articulospora tetracladia was gotten from a lowland stream in Portugal. Clavariopsis aquatica, Flagellospora curvula, and Tetracladium marchalianum were gotten from a Mediterranean stream in the French Pyrenees. Heliscus lugdunensis and Tumularia aquatica were gotten from a temperate mountain stream in the southwest of France. These strains were grown in petri dishes until they produced conidia. These conidia were then placed in a solution to be used in the microcosm. There were six microcosms and they were replicated twelve times. The treatment each microcosm received was a variation of the pair of factors being studied: water temperature and nutrient concentration. Water temperature was three levels: 5°C, 10°C, and 15°C. Nutrient concentration (NP) was two levels: low and high. The information procured from the microcosms was rate of oxygen consumption of the leaf discs, biomass of hyphomycetes by converting from the mass loss of the leaves, and fungal carbon budget which is the percentage of carbon dioxide the hyphomycetes produced. The data was statistically analyzed through three-way ANOVA and Tukey HSD.

The results were consistent across the whole spectrum and fell in line with the stated hypothesis. High NP levels and high water temperatures (10°C and 15°C) resulted in higher hyphomycete activity and higher decomposition rates. This means that if global warming does not occur and temperatures stay low, despite high NP levels, hyphomycete activity will remain low. Placing these results into the scenarios of eutrophic waters versus oligotrophic waters yields different interpretations of the results. In eutrophic waters, carbon mineralization might occur due to stimulation of decomposition rates and oxygen consumption rates but this might not occur in oligotrophic waters. It might not occur in oligotrophic waters because the maximum predicted global temperature increase is 6.4°C for this century. Oligotrophic waters need at least a 10°C increase in temperature to stimulate decomposition and oxygen consumption rates before carbon mineralization can take place. Carbon mineralization is the process by which fungi (and other organisms) obtain carbon from the decomposing litter which they then respire and release into the atmosphere as carbon dioxide. Therefore, litter decomposition rates in oligotrophic waters will remain relatively the same in spite of the global warming.

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