Tide Pools in the Rocky Intertidal   1 comment

Tide Pools in the Rocky Intertidal

When the tide recedes, water often remains trapped in rock depressions. These pools are colonized to varying degrees by intertidal animals and plants, many of which have been previously described. Tide pools  may be classified either according to their position within the intertidal zone—Low middle or high—or according to the dominant plant form within the pool. On this basis, tide pools can be distinguished as dominated, respectively by green algae, calcareous red algae, mixed brown and red algae, and mixed green, brown and red algae.

Pools located highest in the intertidal, because of the extended time they are exposed to the elements, undergo considerable fluctuations in temperature and salinity. Rainwater dilutes the more saline pool water , whereas evaporation increases the salinity. In either case, eventual contact with the ocean restores the original pool salinity. During winter, water of low salinity may freeze while in summer pool water temperature may rise above that of the air. On the other hand, tide pools in the lower intertidal provide a relatively constant chemical and physical environment since they are not exposed to the elements as much as upper pools.

Since pools seldom reached by seawater frequently contain water of low salinity as a result of freshwater runoff  and rainwater accumulation, one would expect to find freshwater organisms that are adapted to low salinity or marine organisms that can tolerate exposure to freshwater. In fact, insects and insect larvae, typical freshwater forms are common in upper intertidal pools. The positions of high, mid and low intertidal pools are shown below:


Pools Above The High Tide Line

The highest pools are essentially filled with freshwater, however they receive salt water from time to time. The first pool shown below has a rich growth of the filamentous green alga  About two weeks after this picture was taken, on December 19th 2011, the pool was completely frozen. The second pool has a layer of diatoms floating on the surface while the third and fourth have a layer of detritus on the bottom along with filter feeding mosquito larvae and pupae.




Often the water boatman and backswimmer (true bugs), adult predatory hemipterans, and dragonfly nymphs can be found in some of the larger pools. Other essentially freshwater organisms may also be found in the highest pools.




Moving downward in the intertidal towards the low tide line, below the pools described above, one may encounter pools with growths of single cell green algae, Clamydomonas  for example. The pools are situated in areas where Gulls hangout and deposit fecal matter that  washes into pools and accelerates algal growth. Can you find any evidence in the three photographs below that supports this statement?


Pools At Or Below The High Tide Line

A. Upper Intertidal Pools

Green algae (Chlorophyta) are often the dominant algae in upper pool ecosystems.  Upper pools are often dominated by the tubular green Enteromorpha  as shown below.



Can you explain why Enteromorpha on the left hand side of the pool are longer than specimens in the pool center?

The water in the pool above was at the high tide line and therefore received more seawater than the frozen pool above it. Explain why the upper pool froze and the other pool did not.

Enteromorpha has a holdfast connected to an elongate tube. Oxygen bubbles  produced photosynthetically, are released into the water from the alga.



Other green algae often found in upper tide pools include the green, branched, filamentous algae Cladophora , Rhizoclonium  (not shown) Spongomorpha
and Ulothrix.





Spongomorpha .


Cladophora, Enteromorpha, Rhizoclonium and Spongomorpha have the following adaptations that allow them to colonize upper tide pools:

  1. High reproductive potential and a short life span allowing them to quickly colonize free space.

  2. A holdfast that secures the plant to rock surfaces.

  3. The entire plant is flexible and therefore able to conform to the incoming wave reducing the amount of drag thereby minimizing the chance of being torn loose.


During winter months pool water often freezes from top to bottom encasing algae in ice.This causes heavy mortality, as evidenced by bleaching in Enteromorpha populations seen below.



In some of the upper pools, the brown alga Fucus filliformis attaches to the pool bottom with its holdfast and extends towards the surface. Fucoid anatomy and adaptations are discussed in the Brown Algal Zone.



Surprisingly, the red alga Chondrus crispus (Irish Moss), normally occurring in the Red Algal Zone can be found in shaded portions of some upper pools. Can you think of a reason why C. crispus thrives in this situation?



Small depressions  in the rock surface above the Bluegreen Bacterial Zone  qualify as upper level tide pools, and are inhabited by a variety of organisms such as bluegreen bacteria, blue mussels, barnacles and snails. In pools with  abundant bluegreen bacteria, the microscopic flora and fauna are generally the same as those described for the Bluegreen Bacterial Zone.


In some of the lower upper tide level and upper mid tide level tide pools, the following organisms may also be found:


Encrusting, Calcareous red algae characterized by a thin layer of calcium carbonate, secreted by the algae, are attached directly to the pool bottom.




Hildenbrandia spp. . This species is an encrusting red alga that often covers the bottom of the pool. It will often smother other attached organisms as it grows.



Ralfsia spp.  is an encrusting brown alga that either will attach to bare rock or grow over other attached algae.



The periwinkle snail Littorina littorea  is common in upper pools dominated by green algae. For the most part they graze on the abundant algae with their toothed radula. Small blue mussels, nematodes, oligochaete annelids, copepods and attached small barnacles may also be abundant. Periwinkle adaptations are discussed in the rocky intertidal brown algal section.



Mytilus edulis (Blue Mussel) , discussed in the Barnacle Zone Page, is found in all of the rocky intertidal zones and in tide pools at all levels.



Mytilus edulis


Semibalanus balanoides (Barnacle) is found in all but the highest intertidal zones. Refer to the Barnacle Zone section of the rocky intertidal for more information.



As we descend to mid level tide pools the tidal pool flora and fauna become more diverse. The following organisms are generally common here.


Enteromorpha  and Ulva (sea lettuce) may seasonally colonize large areas within the Brown and Red Algal zones as well as Tide Pools at all but the highest intertidal levels.




Littorina littorea (Periwinkle) , discussed in the Brown Algal Zone page, is common here.



The dogwhelk Nucella lapillus, a predatory snail, is a common inhabitant of mid level tide pools. The predatory snail feeds almost exclusively on barnacles and mussels. It is responsible for preventing barnacles and mussels from colonizing areas in the lower brown and red algal zones.Refer to the Barnacle Zone page  for more information on this species.




Limpets (Tectaria testudinalis) are gastropods that have a conical rather than spiral shell. Limpets move about and feed on algal films when covered with water or while the rocks are still wet. This species is discussed in more depth in the Brown Algal Zone.



Blue mussels (Mytilus edulis) discussed above, are also common here.



The primitive flightless insect Anurida martitima (Springtail) is often found floating on the pool surface here and in upper tide pools. They basically feed on organic matter that accumulates at the air water interface.



Most of the invertebrates pictured at the end of the Red Algal Zone page are also found in Mid-level Tide pools from time to time.


Additional algae common to mid level tide pools include Ceramium,Cladophora (see above); Hildenbrandia  (See above) ; Petalonia   and Scytosiphon 






Scytosiphon spp.


Mid level tide pools are often completely frozen during the coldest part of the winter. Barnacles can be seen encased in ice. Pool organisms are often killed as a result of being frozen.


As one  moves downward into lower mid tide and upper low tide pools, kelps, discussed in the Sub-tidal Kelp Zone, are more abundant. Laminaria , Laminaria digitata, Alaria and Agarum are shown in the photographs below.




Laminaria spp.


Laminaria digitata


Alaria spp.


Agarum spp.


Porphyra, a red alga,  is often found here. It is described in the Red Algal Zone page.



Palmaria palmata (Dulse)


Palmaria palmata (Dulse), a red alga, is described in the Red Algal Zone page.

Chondrus crispus


Chondrus crispus (Irish Moss), a red alga, often covers most of the pool bottom. Irish moss is discussed in more depth in the Red Algal Zone page.


Corallina officinalis





 The following photographs of Tidal Pools were taken near or slightly below the position of mean low tide.

The  animals described at the end of the Red Algal Zone Page are often found in mid to low intertidal pools. (Refer also to the subtidal animal portion of the rocky intertidal).


Posted December 16, 2011 by zottoli

One response to “Tide Pools in the Rocky Intertidal

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  1. This site is a fabulous resource – thank you!

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