This blog is guest authored by Ian Miller, Ph.D., from Washington Sea Grant. Ian is also a Board member of Feiro Marine Life Center.
The winter season is marked on Hollywood Beach next to the Feiro Marine Life Center in part by the Ulva and other algae species that pile up on the beach. These huge piles have some consequences – they make the beach must less pleasant to walk or play on, and they can also stink as the organic matter breaks down. Inevitably, the green piles draw interest and questions from visitors to the FMLC.
Where do these piles come from, though? And why do they appear so rapidly? In order to try to answer those question a time-lapse camera was mounted on the roof of the FMLC on December 20,2013, and recorded a photo of Hollywood Beach every hour until June 23, 2014. Here is the raw footage:
and here is a condensed version, that averages all the photos from each day into a single frame:
In terms of trying to understand the timing of the appearance of these algae piles, this “daily-average” time-lapse is a bit more useful. As a first stab at understanding processes driving the development of these algae piles I went through and simply tried to classify each day as algae present/not present and come up with something that looks like the bar plot below, where a blue bar marks a day where an algae pile was present, and no bar marks days when there were no algae piles on the beach:
So, at least based on this very simple and incomplete analysis, it looks like whatever is driving the development of these piles of algae is a winter process…as spring turns to summer the frequency of occurrence of algae on the beach (at least in 2014), drops way off. That, in and of itself, Is interesting to me, since I may have assumed at the outset that the growth of the algae in these piles would be greatest in spring and early summer…but I digress.
I’ve often thought that these piles must be driven by wave stress on the bottom just offshore of Hollywood Beach, which would transport algae growing on the shallow bottom on to the beach. There is a fairly narrow directional window over which wind blowing into Port Angeles Harbor would be expected to generate waves on Hollywood Beach:
Waves from that direction are primarily small, short period waves associated with “local” wind. So for a first pass at this, I downloaded wind speed and direction data from the NOAA tide gauge station in Port Angeles Harbor, and plotted It as daily averages below the bar plot of pile occurrences:
One thing pops out – the daily average wind direction is really only within that narrow directional window (between 45 and 80 degrees) one time in the whole record, around late February. There are a variety of potential reasons for this, some of which might include the position of the anemometer at the tide gauge (which is at the Port of Port Angeles pier), or that I am daily averaging data that is really variable. After all, It only takes a few hours of wind to generate waves.
But for the purposes of furthering this simple analysis I will cast a wider net and pull out all of the instances in the wind dataset in which the wind averages above 5 knots over the course of the day, and also instances in which the average direction is between 45 degrees (northeast) and 180 degrees (south). The wider directional window may be physically justified – there are times during which strong south storms blow through Puget Sound in the winter, out Admiralty Inlet, and then seem to generate westward propagating waves in the Strait of Juan de Fuca which may influence P.A. Harbor. These really begs more analysis, but for this very simple starting point, let’s go with it. I’ve marked all eleven days that satisfy those conditions (daily average wind speed exceeds 5 knots and daily average wind direction is between 45 and 180 degrees) with a red star on the bar plot with the algae pile presence/absence:
So looking at those red stars tells me, perhaps, a bit…but not the whole story. There appears to be some general correspondence between the red stars and the presence of algae piles, but it is not perfect. There are times when there is a red star, but no apparent algae pile. The long period of algae-pile coverage starting on January 31 doesn’t seem to START with “red star” wind conditions, which one would expect. And then you have these three different events starting in late April that correspond to times when the wind is moving into its summer state and blowing mostly from the north and northwest. So are these piles associated with wave scour on Hollywood Beach? I still suspect yes, but need more to really convince myself.
