Ecophysiology of mutualism in a unique stream symbiosis between a polymorphic cyanobacterium, Nostoc parmelioides, and the larvae of the midge Cricotopus nostocicola

DOCTORATE DISSERTATION: In mountain streams, larvae of Cricotopus nostocicola develop symbiotically with colonies of Nostoc parmelioides. They cause the spherical colonies to become disk shaped and lie horizontally on the substrate (pies), or to protrude vertically into the flow (ears). I used field observations and manipulations, laboratory experiments and mathematical models to investigate the population ecology of this symbiosis, and its interactions with its physical environment. Field work was done in Sagehen Creek, California. In Chapter 1, I describe the seasonal changes in population dynamics of the two Nostoc morphologies. Using SOLRAD, a light interception model, I found that the induction of a seasonally appropriate morphology enables Nostoc colonies to best utilize available light in seasonally different population densities. Morphological changes are important in aquatic environments, because the magnitude of the mechanical forces exerted by the current depends on colonies size, shape and orientation to water flow. By manipulating macro-flow regime in situ, and using scaled models in a wind tunnel, I found that unoccupied spheres are susceptible to dislocation; drag forces do not significantly affect survival of ears or pies in Sagehen Creek, yet colonization seems to be flow related. In streams, thermal regime is important. Temperature can change the balance between Nostoc production and consumption by the midge, thereby changing the nature of the interaction. The results of a simple theoretical model suggest that to maintain a mutualistic interaction, the joined respiration of the symbiosis and Nostoc s photosynthetic response should change seasonally with changes in water temperature. This can be brought about by the seasonal morphological changes that the larvae induce in Nostoc colonies. In an experimental test in situ, I found that within the temperature range tested, the presence of the midge always contributed positively to growth in Nostoc colonies. I suggest that the morphological change that Cricotopus nostocicola larvae induce in Nostoc parmelioides colonies helps in maintaining the mutualistic nature of the symbiosis by optimizing the interactions between the symbiosis and its physical environment.