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Consultants: sediment releases, not creek flow, drive summer phosphorus spikes at Weiser Lake
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Summary
Consultants presenting to Whatcom County said data collected from May 2023 through mid‑April 2024 indicate Weiser Lake’s summer phosphorus spikes come mainly from internal loading — phosphorus released from lake sediments — while nitrogen inputs come largely from the watershed.
Consultants presenting to Whatcom County said data collected from May 2023 through mid‑April 2024 indicate that Weiser Lake’s summer spikes in phosphorus are driven mainly by internal loading — phosphorus released from the lake’s sediments — rather than by inflow from Cougar Creek.
The finding matters because high phosphorus concentrations feed algal and cyanobacterial blooms, which can reduce water clarity, release odors and toxins, and affect recreational use. The consultant team said the lake’s summer total phosphorus is many times higher than the roughly 20 micrograms per liter target commonly used to limit nuisance algae.
Mark Rosenkrantz, a consultant with Aquatic Insight, described monitoring at two stations — WL1 (West Basin, shallow) and WL2 (East Basin, deeper) — and at the inlet stream. He said orthophosphorus (the immediately bioavailable form) and total phosphorus both rose in the lake during the summer months while phosphorus in the creek remained comparatively low. “Most of the phosphorus is coming from the sediment. Most of the nitrogen is coming from the watershed,” Rosenkrantz said.
The team presented several lines of evidence: dissolved‑oxygen profiles collected during daytime sampling showed oxygen declines near the bottom even when daytime temperatures appeared well mixed; sediment cores sliced at 5‑centimeter intervals down to 30 cm showed substantial iron‑bound and organic phosphorus in the top layers; and orthophosphate concentrations increased in the lake in summer when streamflow was low. The consultants explained that iron‑bound phosphorus can be released to the water column under anoxic (low‑oxygen) sediment conditions and that high pH from photosynthetic activity can free phosphorus bound to aluminum.
Nitrogen behaved differently: nitrate and total nitrogen concentrations in the creek exceeded lake values in winter and increased with winter flow, indicating the watershed is the dominant annual nitrogen source. Consultants noted the lake’s summertime nitrogen:phosphorus ratio often fell below the roughly 16:1 stoichiometric ratio for non‑nitrogen‑fixing algae; that imbalance likely favored nitrogen‑fixing cyanobacteria observed in the samples.
The consultants identified cyanobacteria dominated by a filamentous, nitrogen‑fixing taxon recorded in the samples (transcribed in the meeting as “Anabena…”). They also showed sensor and laboratory measures (chlorophyll and phycocyanin) that tracked cyanobacterial activity, including a notable October onset of cyanobacteria in one basin.
The presenters cautioned that daytime profiles can mask nocturnal oxygen depletion: daytime wind mixing can temporarily reoxygenate deeper water, while calm nights allow bottom oxygen to fall and release phosphorus. They recommended continued, targeted monitoring (including overnight dissolved‑oxygen profiles and more spatial coverage) to quantify how frequently and where sediments become anoxic and to refine management choices.
No formal decisions or funding approvals were made during the meeting; the presentation and Q&A were informational and followed by a public comment period on the draft Lake and Catchment Management Plan.
