(Portland, OR)  --  Parks officials are warning residents about possible toxic algae at a Portland riverfront park following the sudden death of a dog who was playing in the river.  Portland Parks and Recreation officials posted "dog safety alerts" at Sellwood Riverfront Park over the weekend after being asked to do so by state health officials.  The Oregonian reports the dog named Nutmeg died September 1st just hours after playing in the river at the park.  A veterinarian who treated Nutmeg says it appears she ingested some kind of toxin, but a blood test was unable to definitively conclude that she died from cyanotoxin poisoning.