Heat in the lower intertidal: The role of tidal propagation

 

9 July 2021 photo of dead cockles from the intertidal off of Marrowstone Island 

By this point the intertidal ramifications of the late June Pacific Northwest heat wave have been well reported on, with some pretty dire accounts coming in from throughout the Salish Sea and Pacific coast region of Washington and British Columbia.  I've been seeing the influence as I visit various shorelines, including dead cockles on Marrowstone Island (photo above), and the same off of the Dungeness River delta:

7 July 2021 photo of more dead cockles, taken about here on the Dungeness River delta.  

Yesterday I also hunted around Kalaloch Beach on the coast of Washington and for the most part was impressed by how little mortality there seemed to be on the intertidal rocks, though some impacts were visible:

11 July 2021, dead mussels still attached to the rock on the rocks in the lower intertidal at Kalaloch about here.

There has been one interesting wrinkle to the interaction between that June heat wave and the very low tides that weekend that I wanted to focus on a bit, which has to do with the pattern of propagation of tides from the Washington Coast into and through the Salish Sea.  Low tides in the summer tend to occur during the daytime in coastal Washington, but not at the same time during the day.  The low tide propagates as a wave, with the trough (i.e. the lowest part of the tide) generally hitting the outer coast in the morning, generally reaching the eastern Strait of Juan de Fuca about two hours later (generally late morning), reaching Seattle in the after-lunch hours in general, and then reaching Olympia about an hour after that.  So the long and the short of it is that a low tide that is lowest at La Push on the Washington coast at 8am in the morning won't reach Olympia until 5.5 hours later, at 1:30 pm.  

Measured water level along a west-to-east gradient - at La Push, Port Angeles, and Friday Harbor - on Monday June 28th 2021, illustrating the pace of propagation of the low tide from the coast into the Strait of Juan de Fuca. 

So what this means, of course, is that the heating of the intertidal will be different along that spatial gradient as well, since temperature varies so much throughout the day, and is generally highest in the afternoon.  Its much more likely that uncomfortable temperatures will be reached in the lower intertidal in south Puget Sound, for example, as compared to the coast, just because the summer low tides occur later in the day when its warmer.  

So was this evident during the heat wave?  It appears so, at least based on a quick look at air temperature data collected at some tide gauges, that I was able to couple with water level data.  I ended up working with data from La Push, Port Angeles and Friday Harbor simply because I'm well set up to download and manipulate data served from the NOAA tides and currents website, but found that air temperature were not available for any of the Puget Sound stations (i.e. Seattle or Tacoma), so wasn't able to incorporate those locations into my analysis.  But La Push, Port Angeles and Friday Harbor provided me with plenty to work with.  

Air temperature recorded at tide gauges in La Push, Port Angeles and Friday Harbor during the late June heat wave

So I focused in on the three hottest days - Saturday the 27th, Sunday the 28th and Monday the 29th...and the air temperature recorded at three tide gauges are shown above.  The first thing that pops out to me is that La Push was considerably cooler than Port Angeles and Friday Harbor, especially on Saturday and Sunday.  Not a huge surprise.  Generally temperatures were a bit warmer in Friday Harbor as compared to Port Angeles, though not by a lot.  Interesting, on both Sunday and Monday Friday Harbor cooled an hour or so sooner than Port Angeles at the end of the day...something I tentatively attribute to shading of the stations as the sun drops (Friday Harbor's station would be blocked in the late afternoon by the mass of San Juan Island, whereas the Port Angeles tide gauge likely doesn't have that same later afternoon shading).  

My next step was to couple those air temperature data with water level data to calculate how much time various elevations in the intertidal exceeded a critical threshold temperature...I chose 25 degrees celsius as a relevant ecological threshold based on conversations with a few marine ecologists.  Perhaps not surprisingly given the air temperature time-series above, when I calculate the hours above 25 degrees celsius and plot it as an elevation profile I get this for the three locations:

Amount of time (in hours) that various intertidal elevations exceeded 25 degrees celsius between 27 and 29 June 2021 at three locations in coastal Washington

In the profiles, La Push stands out here as having way less exposure to temperatures over 25 celsius...simply because it was cooler there, especially on Saturday and Sunday.  Before I move on though, its worth noting that the data from La Push do suggest heating exposure over 25C really low in the intertidal (i.e. below MLLW), and even though its nothing compared to what the intertidal to the east likely experienced, it may be unprecedented.  The NOAA buoy that sits off the Washington Coast off of Taholah, for example, has only measured hourly average air temperature over 25 degrees celsius 34 times since 1988 (all during a series of warm days in September 2017 that did not correspond with low tides).  

Average number of hours exceeding 25C for the lowest part of the intertidal (below MLLW) in La Push, Port Angeles and Friday Harbor (top panel), and the same but expressed as a percentage of the total number of hours between 27 and 29 June 2021 that were above 25C.  This is where we see the influence of the later low tides inside the Salish Sea.

Okay, but can we see an influence of tidal propagation in heating?  Yes we do.  In the bar plot above I averaged the number of hours over 25C experienced by the lowest parts of the intertidal (below MLLW) at each of the three location, and we see that west to east gradient.  Again, that COULD just be due to it being a bit warmer in Friday Harbor than it was in Port Angeles and La Push, but the difference between Port Angeles and Friday Harbor is telling - the tide gauge in Port Angeles actually experienced more time above 25C (31.7 hours in Port Angeles over those three days, versus 30.7 hours in Friday Harbor)...but there were more hours at the Friday Harbor tide gauge that the lowest part of the intertidal (again below MLLW) experienced those elevated temperatures (4.8 hours in Friday Harbor versus 3.8 in Port Angeles).  To pull that out a bit I've expressed it as a percentage of the total time exceeding 25C at those three locations (lower panel in the figure above)...and we still see that gradient...that is the tidal influence at work.

I hadn't really thought of this interaction between tidal propagation and daily heating before, but since heating and dessication are critical stressors for intertidal organisms it may exert an important influence on what can live in the intertidal in various parts of coastal Washington.  The low intertidal of south Puget Sound is likely a very different place to live than the lower intertidal of the coast of Washington, and not only because of the wave protection differences and the different geology and geography.