Aquatic Biodiversity

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Aquatic species richness and rarity measurements depend on techniques similar to those used on land. Fish are mobile animals that go through several life-cycle stages. Monitoring methods must take these factors into consideration; usually fish counts are done on a specific age. Small pools and clear streams lend themselves to visual surveys, temporary catches, or traps. Migrating fish in larger, fast-flowing streams may require artificial structures that funnel them through a specific area. Biologists then visually count them or use an electronic device.

In deep, still waters, marine biologists monitor species using sonar readings or a variety of netting techniques, especially gill nets, trawl nets, and seine nets. Gill nets consist of mesh that allows in fish of only a certain size. Smaller fish swim back out of gill nets and larger fish do not fit into the mesh openings. Collections may also be done with trawl nets that a
boat (trawler) drags by a long cable through the water or along the water’s bottom. Seine fishing nets have weights that hold the net’s middle area on the water’s bottom and allow the open mouth to reach the surface. Fishermen bring in their catch by drawing on ropes until the net closes, like closing a cloth sac. Trawling and seine fishing are not usually selective; they catch fish of various sizes.

Aquatic invertebrate monitoring requires specialized techniques because these species contain a wide range of size, life cycles, and microenvironments. Some of the most common monitoring methods are the following: aquatic vegetation sampling to count the species attached to aquatic plants; net sampling for species suspended in the open water; kick sampling to dislodge species living among the gravel, sand, and pebbles at the water’s bottom; cylinder sampling to collect a set volume of water and bottom matter; and artificial substrates onto which invertebrates attach and grow over time.

Aquatic algae and other simple organisms usually require microbiology methods. One collects and dilutes a water sample, then applies it to a mixture containing nutrients. After an incubation period, a microbiologist counts the number of cells per unit of sample volume.

Marine biologists often resort to diving or snorkeling to capture representative aquatic samples. In general, aquatic biodiversity monitoring uses specialties that require extra training. The case study “Ocean Warming and Phytoplankton” describes technologies adopted for global studies of aquatic conditions. Though all biodiversity measurement depends on estimates rather than actual counts, aquatic methods present the most difficult
challenges of all monitoring methods.