Dynamics of chytrid infections on diatoms in a turbulent river system

Background/Question/Methods

Often overlooked, zoosporic fungal parasites of phytoplankton (‘chytrids’) are present in lakes and rivers worldwide. Epidemics of chytrid infections can result in the decline of spring phytoplankton blooms and contribute to species succession patterns. Parasitic chytrids play a potentially important role in aquatic food webs by efficiently re-packaging organic material from the large, often inedible, colonial diatoms they infect into fungal zoospores, which are easier for zooplankton to consume. The ecological significance of chytrid infections of phytoplankton is poorly understood, particularly in lotic systems. We investigated the seasonal dynamics of diatom chytrid infections in the Columbia River. Our objectives were to: (1) document seasonal patterns in the infectivity of diatoms by chytrids in the Columbia River; (2) determine how variations in turbulence and light influence infection rates and infectivity constants in laboratory cultures isolated from the Columbia River; and (3) estimate the amount of organic carbon diverted away from diatoms due to infection. Field samples were collected from June 2009-June 2012 to analyze phytoplankton species composition and the prevalence of chytrid infections under variable turbidity and river discharge conditions.

Results/Conclusions

Results from laboratory experiments indicate that the higher growth rates of diatoms observed under high turbulence may assist populations to out-compete fungal parasites and prevent epidemic events from occurring in rivers. During the 3-year study, chytrids were prevalent in the spring on the dominant diatom species, Asterionella formosa, with a maximum infection prevalence of 40% of the population. Intermittent spring storm events decreased cell abundance and infection prevalence. During the spring bloom, the loss of diatom carbon biomass due to chytrid parasitism averaged 3.5% of the phytoplankton standing stock with a maximum of 18% during peak infection. We hypothesize that alterations in river flow due to hydroelectric dams may have opened a niche for chytrid parasites in the Columbia by increasing water retention time, which allows diatoms to bloom and provides a prolonged interaction period for chytrid parasites to infect hosts. Although epidemic proportions have not been recorded in the Columbia River in contrast to European lakes, the previously unrecognized presence of chytrids may be responsible for preventing spring blooms from reaching maximum potential and may provide an important route for the transfer of organic matter into local food webs, reducing losses due to downstream export.