Suppression of cyanobacterial growth of Aphanizomenon flos-aquae by vascular plant decomposition products in Upper Klamath Lake, Oregon

Abstract

Upper Klamath Lake in southwestern Oregon has experienced harmful algal blooms of the nitrogen-fixing cyanobaterium Aphanizomenon flos-aquae for 5 decades. The appearance of A. flos-aquae coincided with loss of wetlands to agricultural use starting at the end of the 19th century. Plant degradation produces colored dissolved organic matter (CDOM) or humic substances that affect algal and cyanobacterial growth. We conducted a series of controlled laboratory assays and an in situ limnocorral experiment to assess the effectiveness of plant materials that contribute high levels of CDOM upon decomposition (i.e., barley straw, barley straw extracts, marsh water, and wetland plants) to suppress growth of the cyanobacterium A. flos-aquae. Our results indicate that the application of barley straw or dried wetland plants were most effective at suppressing growth or causing mortality of A. flos-aquae. Mortality was dependent on the amount of CDOM released and incident ultraviolet (UV) light. We hypothesize that mortality is due to the well-known production of reactive oxygen species from the absorption of UV light by CDOM. In general, direct application of plant material under high UV exposure resulted in near complete A. flos-aquae mortality within 24 hours. Because the mortality was rapid, these results are consistent with toxicosis and not the binding of essential nutrients. We suggest that fringing wetlands may play an important role in regulating harmful cyanobacterial blooms by providing CDOM and that this role should be considered when determining wetland value.