Rising Above It All, Life Above The Stream Bottom
by Andy McAllister, Watershed Coordinator
Since the “Life in Our Streams” series began, our journey has taken us through a myriad of life forms inhabiting our underwater world. From the bacteria, smallest of the small, to the bottom dwelling creatures known as benthic invertebrates, all form an important part of the aquatic food web. As we leave the relative protection of the stream bottom and journey into the expanse of the water column travelling toward the surface, other life forms begin to become apparent.
Aquatic organisms are often categorized by where they live in the water. As we’ve already seen, benthic organisms live in or on the stream bottom and feed on plant and animal material that collects on the bottom. Crayfish, mussels, and stonefly and mayfly larvae are examples of benthic organisms.
Pelagic organisms are those that live within the water column. These organisms may float or swim and include everything from tiny plants and animals known as plankton, to fishes, frogs, turtles, and a wide variety of insects.
Fishes are probably the best-known pelagic inhabitants of our streams and rivers. The fish’s ability to live in the water column depends upon the speed at which it can swim and how long it can sustain that speed. Of course, this ability often varies greatly between species. Continuous swimming expends a tremendous amount of energy, so fishes tend to spend only short periods in full current. Instead, individuals remain close to the bottom or the banks, behind obstacles, sheltered from the current, often swimming in the current only to feed or change locations. Some fish species have adapted to living primarily on the stream bottom, rarely venturing into the open water column. These fishes are flattened top to bottom to reduce resistance to the current and often have eyes on top of their heads to see what is happening above them. These types of fishes are known as “demersal” fishes. An example of a demersal fish is the sculpin.
Other vertebrates that inhabit streams include amphibians, such as salamanders, reptiles (e.g. snakes and turtles) various bird species, and mammals (e.g. otters and beavers). With the exception of a few species, these vertebrates are not tied to water as fishes are and spend part of their time in terrestrial habitats.
Abandoned Mine Drainage (AMD) is just as dangerous for inhabitants of the water column as it is for benthic organisms. Fish in particular are susceptible in several ways. The primary causes of fish death in acid waters are through loss of sodium ions from the blood and loss of oxygen in their tissues. Sodium loss interferes with the process known as osmoregulation, the way in which fish maintain the concentration of their body fluids. As a result of a sodium loss, there is a constant influx of water into the body and loss of salts and ions from the blood outwards. The end product is that the fish would rapidly accumulate water and die. Acid water also attacks the fish’s gills, increasing the gills’ permeability to water, adversely affecting gill function and reducing the amount of oxygen they can absorb.
| Brook Trout. Illustration from PA Fish and Boat Commission |
Low pH levels that are not directly lethal adversely affect fish growth rates and reproduction . Some fish, such as brook trout, are tolerant of low pH. However, it’s the presence of dissolved metals found in many AMD discharges, that decreases the fish’s ability to tolerate low pH. Finally, metals precipitating out in the water column, collects on the gill structures, further restricting oxygen absorption into the blood.
AMD impacts on Pennsylvania’s native brook trout populations for example, are localized. But where it happens, it’s generally severe. In fact, if we were to total up the entire area of brook trout habitat in Pennsylvania that’s impacted by AMD, it would be larger than the state of Connecticut.
For more information:
Conserving the Eastern Brook Trout by the Eastern Brook Trout Joint Venture
Gallery of PA Fishes by the PA Fish and Boat Commission