Dissolved oxygen deficits in a shallow eutrophic aquatic ecosystem (fishpond) - Sediment oxygen demand and water column respiration alternately drive the oxygen regime.

Biological processes tend to dominate the oxygen regime of productive waters. However, in shallow aquatic ecosystems, it is unclear whether the oxygen regime is driven by oxygen production and consumption in the water column or by sediment oxygen demand (SOD). In managed eutrophic ecosystems, this question is especially important in the context of extreme daily oscillations of dissolved oxygen (DO) that could breach physiological limits of heterotrophic aerobic organisms. High-frequency measurement of DO, temperature, global radiation (Gl.Rad.), and pH in a 0.6 m deep, 22 ha eutrophic fishpond Rod (Czech Republic) shows that the oxygen regime depended on the ecosystem state. Over the clearwater period in the early season, the DO level reflected ecosystem heterotrophy with relatively low daily DO oscillations. However, during the summer phytoplankton bloom, the fishpond was primarily autotrophic with extreme DO fluctuation. During late summer, a collapse of the phytoplankton bloom and an associated shift towards heterotrophy and DO deficit frequently occur. In-situ mesocosm experiments in Rod fishpond were conducted throughout 2018 and 2019 growing seasons, to address the importance of SOD to the oxygen regime. We enclosed the water column in transparent and opaque/dark plastic cylinders open or closed to the sediment. The results show that the proportional contribution of SOD to total respiration decreased from 70 to 90% at low phytoplankton biomass (expressed as Chlorophyll-a (Chl-a) concentration) to approximately 10% at phytoplankton bloom. At night, the difference between the oxygen consumption in the cylinders with or without sediment was statistically significant, when the concentration of Chl-a was <100 µg·L-1. On the contrary, the difference was not significant when the concentration of Chl-a was >100 µg·L-1. This revealed that the impact of SOD is negligible at high phytoplankton biomass.

The economics of mutualisms: optimal utilization of mycorrhizal mutualistic partners by plants.

Can choice of mutualistic partners and the degree of their utilization determine (1) mutualistic partner coexistence, (2) relative abundance of mutualistic partners, and (3) environment-dependent changes in relative abundance? We investigate these questions in the context of the plant-mycorrhizal fungal mutualism by building a biological market model potentially applicable to other mutualisms as well. We examine the situation where a single plant selectively utilizes member(s) of a group of ectomycorrhizal potential trading partners. Under biologically realistic circumstances, the plant may simultaneously utilize multiple partners, its degree of utilization determining the community structure of the fungi. If utilization of multiple partners is optimal, the marginal cost of acquiring additional nitrogen from every trading partner must be equal while the marginal cost of acquiring it from any unutilized partner must be larger. Because the plant's nitrogen demand is light dependent, the composition of the fungal species among its trading partners changes along light-availability gradients. We discuss the design of an experiment to test the key prediction of our model, the equalization of marginal cost.

Ecological persistence of the plant-mycorrhizal mutualism: a hypothesis from species coexistence theory.

In diverse mutualisms, it is common for potential partners to vary in the quality of benefits they provide. When weakly beneficial mutualists and parasites have a competitive advantage over strongly beneficial mutualists, it is not clear how strongly beneficial mutualists persist. If mutualism is destabilized by competitive superiority of weakly beneficial mutualists or cheaters, then mechanisms providing for stable coexistence among competing species may also provide for the persistence of mutualism. We analyze coexistence of species within a mutualist guild using a simple spatial model of patch occupancy to suggest hypotheses about the ecological persistence of mutualism in the interaction between plants and ectomycorrhizal fungi. We suggest that plants could facilitate the persistence of mutualistic mycorrhizal fungi by enhancing the mortality of root tips colonized by competitively superior and less mutualistic fungi. We also discuss previous empirical studies and present original data from field observations in plant-ectomycorrhizal systems to address our predictions and to suggest profitable avenues for further work.