Community Dynamics Following a Mussel Mortality
in Winter 2004 in the Mid-Intertidal of the Maine Coast:
An Annual Photographic Record at the Giant Stairs Natural Area, Harpswell, Maine

 

Photos taken in late February/ early March each year.
 

In the winter of 2004, we went to the Giant Stairs to do our routine sampling as part of the ecology class (Bio 270) and
discovered that the vast majority of the mussels (Mytilus edulis) in the lower mid-intertidal zone (+1.8 m and below) were gone,
although many patches of empty shells or decomposing brssal threads were observed. The remaining mussels were located
primarily in refuge habitats, around the holdfasts of the large macroalgae (mostly Ascophyllum), and in tidepools. Similar
mortality was observed on the rocky headland off Popham Beach State Park, suggesting that it was not an isolated event.
The exceptionally cold air temperature in winter 2004 is a likely source of the mortality - the mussels probably froze to death
and then were dislodged by winter storm waves. The photos below document the loss of mussels in 2004, and then the
subsequent successional dynamics of one area that we are monitoring with annual sampling to measure percent cover of
macroscopic primary space consumers. This particular area had been largely a monoculture of mussels for many
years prior to 2004 (our course has been working here annually since the early 90's).

 
Figure 1. Mussels killed by freezing.(Feb 2004)

  

Figure 2. The white material is decomposing byssal threads of mussels.(Feb 2004)


 
Figure 3. White areas are where mussels had formerly been
dominant space consumer.(Feb 2004)
  
Figure 4. In 2004, this area was reduced to < 5% cover of mussels.
This is one year later in Feb 2005, and mussel % cover was approximately 40-50% in this area. This is part of the area we now monitor annually for % cover of primary space consumers. Tide elevation ~ +0.5m above MLLW.
 
Figure 5. Ditto figure 4. (Feb 2005) Both the mussels and Irish moss (Chondrus crispus) were present in high abundance.
  
Figure 6. Ditto previous. (Feb 2005)
 
Figure 7. Ditto previous. (Feb 2005)
 

Figure 8. Same area as in Figures 4-7, in Feb 2006. Mussel density has dropped off again to approximately 10% cover or less. Percent cover of Irish moss has dropped way off, as well.
  
Figure 9. Same area as in Figures 4-7, in Feb 2006. Mussels that remain are located primarly in the refuge sites.

 Figure 10. Same area as in Figures 4-9, in March 2007. Percent cover of mussels was way down again this year, including along the margin with barnacles (Fig 12). A lush growth of Fucus sp. occurs here, and throughout the zone in the whole Giant Stairs area and persisted until at least September 2007.

  

Figure 11. Same area as in Figures 4-10, in March 2007.

Figure 12. Loss of mussels along lower edge of barnacle zone in March 2007. Subsequent recruitment of barnacles has filled in almost all of the gaps in the barnacle zone seen in the picture as of March 2008.

 

Figure 13. View l(ooking north) of barnacle zone showing increase in cover since March 2007. The relative location of the site we are monitoring is indicated in the photo.

Figure 14. Sampling area in March 2008, showing loss of Fucus spp. cover since March 2007.

Figure 15. Sampling area on March 14, 2009.

Figure 16. Sampling area on March 19, 2010 - no mussels!! Mussels were largely absent from the entire mid-tidal except for in refugia and in small cluster around the holdfasts of a few Ascophyllum. More mussels were found among the barnacles higher on the rocks.

Figure 17. Photos of the mid-tidal ("brown algae-mussel zone") on March 19, 2010. The area is largely devoid of blue mussels (Mytilus edulis).

Figure 18. Sampling area on March 17, 2011. No mussels present in this tide level anywhere along the Giant Stairs
area at this time. Looks very similar to the condition in 2010.

Figure 19. Sampling area on March 15, 2012. More mussels than in the previous year, particularly around the holdfasts of Ascophyllum, and note the reestablishment of barnacles along the upper edge that had been absent in 2011.

   

© 2012 Bates College.
All Rights Reserved.
Greg Anderson