A Tale of Two Tidepools

Every once in a while I like to remember that summer's just a few months away.  Sitting by the wood stove with a dog at my feet, I've been thinking about tidepools in the rocky intertidal zone, remembering sunny warm days by the seaside.  My poor students were forced to spend a perfect fall day at Giant Steps researching tidepools, and it was a fascinating lab.  It turns out tidepools vary greatly depending on their tidal height (or elevation), and it's easy to see the differences.

The tidal height of a tidepool affects how long it's separated from the ocean.  Pools near the top of the tidal range are rarely submerged by the tides, whereas those at the bottom are submerged most of the time.  This affects the water chemistry in the pool.  Some of this is obvious, like temperature.  On a hot summer day, a tidepool at the top of the rocky intertidal warms up rapidly, and is downright hot if you're an animal who's used to being in the cold ocean of Maine.  This works the same way in winter -- tidepools commonly get very cold in winter compared to surrounding oceans.  So tidepools at the top of the intertidal zone get hotter and colder than those at the bottom.  Other things are different too, though we can't see them.  Oxygen and carbon dioxide will differ, and pH will too.  As seaweeds photosynthesize during the day, oxygen increases and CO2 decreases; this increases pH (making it less acidic).  On the other hand, don't forget plants have to respire as well as photosynthesize, so at night this is all reversed.

A tidepool at the top of Pemaquid Point, frozen over.  This is rare lower in the intertidal.

Although we can't see differences in water chemistry, it's easy to see their effects.  Visit any rocky intertidal ecosystem with tidepools and here's what you're likely to see:

Tidepools at Giant Steps.  The one at the top is full of Enteromorpha intestinalis.

Pools near the top of the rocky intertidal will be chock-full of a brilliant green seaweed called Enteromorpha intestinalis -- and little else.  Move down the intertidal zone and tidepools will look very different.  There won't be a speck of Enteromorpha to be seen.  Instead the pool is likely to have a greater diversity of seaweeds, and those seaweeds are likely to be dominated by Chondrus crispus -- Irish moss.  You'll probably see a lot more animals in those lower tidepools too -- common periwinkles, slipper shells, and mussels.

Common periwinkle; Littorina littorea.  Always hungry.

Much of the difference in tidepools can be attributed to the innocent-looking common periwinkle.  Periwinkles are ravenous, and it turns out they have very particular tastes.  Enteromorpha?  Delicious.  Chondrus?  Not so much.  Wherever common periwinkles are found, Enteromorpha is not -- it's just too darn tasty to stick around long.  Those lower tidepools turn out to be very comfortable homes for periwinkles, but the ones at the top have water chemistry that excludes snails.  Enteromorpha has a grand time up there -- so much so that it excludes other seaweeds, and animals for that matter.  It's competitively dominant.  Down lower, Enteromorpha gets gobbled up, allowing many other species to grow without competition.  Hence the high diversity.

An Enteromorpha-dominated tidepool at a high tidal height

A high-diversity tidepool at a low tidal height

Jane Lubchenco laid much of the ground work for our understanding the relationship between snails, seaweed, and diversity.  Looking out on intertidal ecosystems, it's hard for me to imagine common periwinkles are an invasive species -- they are ubiquitous.  It's literally impossible to walk around without stepping on them.  Given their major influence on just these tidepools, I have to wonder what rocky intertidal ecosystems looked like 300 years ago.  Would every tidepool have looked like those up at the top of the ecosystem?  Were organisms like slipper shells rare?  It's doubtful I'll be out in the tidepools until it warms up.  Until then, the Enteromorpha filled pools will be frozen, and the periwinkles will be munching away at whatever they can find in lower pools.

A very very low tidepool; dominated by coralline algae (Corallina officinalis).  What's up with that?