Prevalence of chytrid parasitism among diatom populations in the lower Columbia River (2009–2013)

– increased with infection prevalence, but the population average never exceed two parasites per host.
1. Net growth rates of chytrid populations exceeded those of A. formosa host diatoms in the spring under low‐flow conditions. In spring 2013, the prevalence of infection increased from 35 to 78% and the parasite load increased from 1.5 to 2.2 along a downstream transect, suggesting that zoospores were actively infecting hosts in situ and not simply transported allochthonously.
2. The estimated loss of organic carbon associated with diatoms via the parasitic pathway was greatest in spring (averaging c. 10%), with a peak of c. 85 μg carbon L^?1 (c. 45% of total large‐diatom carbon) lost from the diatom carbon pool to parasitism in May 2011. Parasites of microalgae may actively shape aquatic food webs within systems like the Columbia River by increasing the availability of large‐diatom carbon to higher trophic levels, thus decreasing downstream export losses to the estuary and coastal ocean.