Combination of factors

  

One morning last week when we were headed out to do field work, we noticed an odd smell in the area and later we found the source — some small patches of California Mussels (Mytilus californianus) that had died recently.

Although it's been warm inland, I didn't recall noticeably high air temperatures at the coast, so at first I was a bit puzzled.  But then I thought about how calm the ocean has been (hardly any large swell) and that this site is quite high on the rocks.  It's likely that the mussels growing here are used to getting splashed by waves, and if they didn't experience any wave action during a warm day, it could be a problem for them.

So we did a little sleuthing regarding the conditions that might have caused this mussel die-off.  Questions we wondered about included: 

 - Were there any higher air temperatures in Bodega Bay recently?  We looked back at some temperature records and noted a couple of days in early August when the air temperatures reached ~67°F (~19°C).  That was the warmest we found, so the air didn't get that hot.

- But we know that mussels and the rocks they grow on reach warmer temperatures than the air.  So how hot was the rock at the time?  Luckily, Eric had some data loggers on the rocks near mussels and so he looked back and found that rock temperatures reached ~98-100°F (~37-38°C) on those warmer afternoons.  That's warm enough to impact mussels, and if exposed long enough, it might kill them, but it would depend on other conditions at the time. So...

- What was the timing and height of the tides on those days?  Interestingly, this was during a neap tide series. Compared to tide heights during a spring tide series (around full moons and new moons)  the high tide heights during neap tides are relatively low.  For example, the morning high tide on August 1st was only 3.7 feet (higher high tides in our area are often over 6 feet).  And the low tide on August 1st was 2.7 feet at ~12 p.m.  This combination of a very low high tide in the morning followed by a low tide in the middle of the day is potentially dangerous for a marine organism living in the high intertidal zone, especially if the wave conditions are extremely calm and the air is relatively warm.  It basically means that mussels growing at that site (at about 5 feet elevation) were exposed to warm air/rock temperatures and no wave splash for the entire day — especially from late morning through late afternoon (the hottest part of the day).  On 1-3 August 2021, these mussels might have reached temperatures >86°F (>30°C) for 4.5-6 hours each day and they might not have experienced any wave splash for that entire time.  [I think it's even possible they didn't get splashed during the night-time high tide (which on August 1st was 5.6 feet at 7 p.m.)]

So questions remain about this die-off, but rather than being caused by extreme warm air temperatures alone, in this case I'm guessing it was a convergence of factors, including (1) these mussels lived very high in the intertidal zone at a very splashy site; (2) there was a low high tide in the morning (the mussels weren't submerged during high tide); (3) there was a low tide in the middle of the day (the mussels were exposed to warm air for many daylight hours); (4) there were warm air temperatures (this might not have happened if air temperatures had been cooler); and (5) this occurred during very calm sea conditions (the mussels weren't cooled down by wave splash).

This mussel die-off was restricted to a very high intertidal splash zone site.  Most of the mussels in the rest of the intertidal zone were fine.  But seeing this event was insightful for considering combinations of factors that could be lethal for mussels.

One more photo for the record: