That is rotting kelp, or seaweed, which this time of year has been washing up heavily and with warmer weather has been rotting faster.
Sunday and Monday the area around Cowell was described as a fish market full of rotting fish, by the writer whose legal name is DNA. In Aptos, the fumes from the beach were so strong, they stung peoples' eyes all the way up to Highway 1.
Santa Cruz County's Water Quality Monitor Steve Peters says the smell of kelp and ocean bacteria can wash up in three flavors of smell: rotten fish, sewage and geraniums, with the latter being most desireable.
Luckily, his tests show that the smell isn't linked to dangerous bacteria levels, although for much of the summer advisories that levels were high were posted on beaches such as Seacliff and Cowell.
When the smell is stinging, Peters advises getting upwind quickly because it can contain hydrogen sulfide, the same material that killed two pig farmers who were pumping out a pond filled with manure.
Peters and UCSC marine biologists test the beaches regularly, particularly Cowell, which has been found to have high bacteria levels that have puzzled scientists.
Peters says he regularly gets calls from tourists from around the world checking on beach conditions to make sure they want to visit.
Cowell Beach presents a number of problems, one of which is clearing off the kelp before it rots. Peters said they have removed 400 tons of the material this summer, but it comes back so quickly that if it's clear at 10 a.m., the beach can be full again by noon.
UCSC Marine Biologist Lisa Ziccarelli regularly tests the area on both ends of the Santa Cruz Wharf and said she detected no bad smell this week. However, she added, there was a Ceratium bloom earlier this summer. The one-celled, microscopic organism creates a bad smell and can discolor the ocean in what is called a "red tide."
But she said it is mostly gone now.
You can read about her work around the wharf here.
And you can check live maps of algae blooms here.