INFECTION, GROWTH, AND COMMUNITY-LEVEL CONSEQUENCES OF A DIATOM PATHOGEN IN A SONORAN DESERT STREAM

We describe effects of a pathogen that spread through a dense, rapidly growing, benthic diatom community during two infection periods (Febru ary and mid-April 1991) in Sycamore Creek, Arizona. Infected areas app eared as gray rings within a matrix of healthy diatom growth and sprea d rapidly, eventually covering all benthic substrata and causing algal sloughing (within 2 wk in February and 1 wk in April). Examination of algal material with transmission electron microscopy revealed the pre sence of invasive bacteria within diatom cells from infected areas, su ggesting a pathogenic bacterium as the most probable cause of this phe nomenon. Infected areas supported lower chlorophyll a concentrations a nd contained higher percentages of diatom cells with fragmented or red uced chloroplasts than uninfected areas. Spread of the pathogen appear ed to be linked most strongly with diatom densities. The infection spr ead most rapidly in April, when cell densities were highest, and decim ated all diatom species populations. The February infection was more s pecies-specific in its action, affecting large motile and rosette-form ing taxa more strongly than small, adnate diatoms. This latter group l ikely resided at the base of communities and may have been buffered fr om pathogen transfer by mucilage and/or detrital particles. Consequent ly, relative abundance of small, adnate diatom taxa increased in algal communities as a result of the February infection. Pathogen-induced a lteration of diatom species composition and abundance should influence primary production in this ecosystem and affect the dynamics of organ isms that exploit algae as a resource.