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

Michelle A. Maier, Tawnya D. Peterson

Research output: Contribution to journalArticlepeer-review

6 Scopus citations


We documented temporal (2009–2013) and spatial [river kilometre (rkm) 8–rkm 196] patterns of parasitic fungal infections on diatoms in the lower Columbia River, as evidenced by the presence of sporangia attached to diatom frustules. Multiple diatom species were infected by zoosporic fungi (‘chytrids’) throughout the year, with a maximum infection prevalence (Ip) among large diatoms of 36% in spring 2011. At a fixed site at rkm 85, up to 30–45% of the population of Asterionella formosa, the dominant spring diatom, was infected each year during the spring (2010–2013). During this time, high Ip was associated with high host density, low precipitation and low orthophosphate concentrations. Parasite load – the number of sporangia per infected diatom cell, also referred to as the mean intensity of infection, ī – increased with infection prevalence, but the population average never exceed two parasites per host. 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. 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.

Original languageEnglish (US)
Pages (from-to)414-428
Number of pages15
JournalFreshwater Biology
Issue number2
StatePublished - Feb 1 2017


  • Asterionella formosa
  • Columbia River
  • chytrid
  • diatom
  • parasite

ASJC Scopus subject areas

  • Aquatic Science


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