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Recent observations document increased abundance of algae in the headwater streams of Hubbard Brook Experimental Forest (HBEF). It is possible that this “greening up” of HBEF streams may be due to climate change, with rising temperatures, altering terrestrial phenology, and shifting hydrologic regimes. Alternatively, stream “greening” could be from the slow recovery of stream chemistry after decades of acid rain, which has led to rising pH, declining concentrations of toxic Al3+, and low solute concentrations. Four years of weekly algal measurements on artificial moss and ceramic tiles, along with six nutrient enrichment experiments, revealed new insights about the interactions between these two autotrophs. We found that in protected weir ponds and in stream channels, algal biomass was higher on artificial moss substrates than on tiles—with this effect amplified in the stream channels. These results suggest that bryophytes can provide physical protection from flood scour or may trap nutrients to support algal growth. In stream channels, algal biomass was higher in well‐lit habitats and time periods indicating strong light limitation. We only measured nitrogen and phosphorus limitation of algal biomass in nutrient enrichment experiments conducted within weir ponds, with higher light availability and lower flow. By comparison, results from the remaining four instream experiments provided little evidence for nutrient limitation, with only one trial showing increased algal growth in response to nutrient addition. The most striking implication of our study is the role of bryophytes in providing refugia, and potentially nutrients, to algae in shaded and oligotrophic headwater streams. 

Solar wind provides an example of a weakly collisional plasma expanding from a thermal source in the presence of spatially diverging magnetic-field lines. Observations show that in the inner heliosphere, the electron temperature declines with the distance approximately as $T_e\left(r\right)\sim r^{-0.3}\dots r^{-0.7}$, which is significantly slower than the adiabatic expansion law $\sim r^{-4/3}$. Motivated by such observations, we propose a kinetic theory that addresses the nonadiabatic evolution of a nearly collisionless plasma expanding from a central thermal source. We concentrate on the dynamics of energetic electrons propagating along a radially diverging magnetic-flux tube. Due to conservation of their magnetic moments, the electrons form a beam collimated along the magnetic-field lines. Due to weak energy exchange with the background plasma, the beam population slowly loses its energy and heats the background plasma. We propose that no matter how weak the collisions are, at large enough distances from the source a universal regime of expansion is established where the electron temperature declines as $T_e\left(r\right)\propto r^{-2/5}$. This is close to the observed scaling of the electron temperature in the inner heliosphere. Our first-principle kinetic derivation may thus provide an explanation for the slower-than-adiabatic temperature decline in the solar wind. More broadly, it may be useful for describing magnetized collisionless winds from G-type stars. 

Quasi-periodic plasmoid formation at the tip of magnetic streamer structures is observed to occur in experiments on the Big Red Ball as well as in simulations of these experiments performed with the extended magnetohydrodynamics code, NIMROD. This plasmoid formation is found to occur on a characteristic time scale dependent on pressure gradients and magnetic curvature in both experiment and simulation. Single mode, or laminar, plasmoids exist when the pressure gradient is modest, but give way to turbulent plasmoid ejection when the system drive is higher, which produces plasmoids of many sizes. However, a critical pressure gradient is also observed, below which plasmoids are never formed. A simple heuristic model of this plasmoid formation process is presented and suggested to be a consequence of a dynamic loss of equilibrium in the high- $$\beta$$ region of the helmet streamer. This model is capable of explaining the periodicity of plasmoids observed in the experiment and simulations, and produces plasmoid periods of 90 minutes when applied to two-dimensional models of solar streamers with a height of $$3R_\odot$$ . This is consistent with the location and frequency at which periodic plasma blobs have been observed to form by Large Angle and Spectrometric Coronograph and Sun Earth Connection Coronal and Heliospheric Investigation instruments.