Additional USGS surveys were conducted on May 20 with plankton net tow samples collected from the Willamette River at Ross Island Lagoon, and plankton net tow samples from the Willamette River downstream from the Holgate Channel at the OMSI dock, on July 8, 18, and 24^th, 2024. 

Plankton were dominated by diatoms during the spring and early summer period, but there was a persistent low-level presence of potentially toxigenic cyanobacteria (Dolichospermum and Aphanizomenon identified microscopically from samples taken with a plankton tow net.)  

On July 8, Microcystis (Figure 1) joined Dolichospermum and Aphanizomenon in the plankton, but their overall abundance continued to be low, and diatoms were numerically more abundant.  

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After a prolonged heat wave in mid-July, signs that the bloom was starting to exit the lagoon started to appear, with two periods of elevated phycocyanin fluorescence units at the OMSI dock indicating the occurrence of blue-green algae (cyanobacteria) (Figure 2). 

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July 24, 2024, Water Quality Mapping Survey:

On July 24, USGS ORWSC conducted its first high-frequency water-quality mapping survey in the lower Willamette River, covering the reach from Milwaukie through the Ross Island Lagoon down to the Columbia River. This system was modeled with help from the USGS California Water Science Center Biogeochemistry Group who use a similar system to monitor water-quality within a broad area within the Bay Delta.  

The novel system collects water-quality measurements by boat traveling 20-25 miles per hour, covering much of the river within a few short hours. The on-board system uses a pumped flow-through system, bringing smooth water to a YSI multi-parameter sonde logging 8 parameters twice per second.  

The surveys characterize the extent of algae and cyanobacteria in the Willamette River and provide a spatially dense snapshot of surface water-quality. In addition to providing scientists with a ‘read’ of water-quality to complement observations on these surveys, detailed maps provide never-before seen details of the relative water-quality throughout the lower Willamette River. Maps from this and other planned surveys can be used by the public, researchers, the Oregon Department of Environmental Quality (ODEQ) and the Oregon Health Authority (OHA).  

Anecdotally, measured fluorescence of pigments -- total chlorophyll and phycocyanin (produced by blue-green algae) -- showed where the bloom was located and its relative intensity, as well as the photosynthetic activity based on measurements of surface pH and percent dissolved oxygen saturation. The blue-green pigment phycocyanin (PC) values were not, however, overly elevated in these surface measurements on this first trip. There were some areas of higher PC values where pH and %DO were also elevated, including inside the Ross Island Lagoon (though no surface scums observed), and some of the backwater areas in the Swan Island and other industrial areas within the lateral ports. These shallower locations with exposed shores were a bit more turbid, apparently from the observed boat induced wave action both in the Holgate Channel and at some of the downstream areas.

The phytoplankton assemblage was again dominated by colonial diatoms (Fragilaria crotonensis, Aulacoseira) in the upper site at OMSI, although the three potentially toxigenic cyanobacteria previously reported for the river (Microcystis, Dolichospermum, Aphanizomenon) were also present at the OMSI dock, and at the farthest downstream sampling site near the mouth of the Columbia River  At all sites, small fully green balls of Microcystis were observed, and verified microscopically (see images, Figures 1-4), along with several types of planktonic diatoms. 

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