The Living Ocean

The worlds oceans are a very diverse place. Many different types of flora and fauna call the ocean home. Species can be found throughout the entire ocean, from the top, sunlit areas, to deep down where no light can penetrate and some species even live on or within the seafloor. The ocean can be divided into two general zones; the pelagic zone, also known as the water environment, and the benthic zone, also know as the seafloor environment. The pelagic zone can be divided into sub-categories. They are the coastal, or neritic zone, the area above the continental shelf, and the oceanic zone, the area of open ocean away from the direct influence of land. Depending on the depth, the benthic and pelagic zones can be further divided.
The most abundant organisms found in the ocean are called microbes or microorganisms. They are single celled organisms and they carry out a variety of processes. They are very small in size compared to other marine species, averaging about a few microns. Invertebrates, fish and whale species can be seen really easy compared to microorganisms. Most of the other species found in the ocean, like invertebrates, fish and whales, are considered multi-cellular organisms. This means they have several cells that interact and form tissues, organs and limbs. There are many other types of organisms that make up the food chain withint the oceans. The next species up from microorganisms would be plankton, then zooplankton, nekton, benthos and larger species.
There are many complex interactions that occur in the ocean. Species. The marine environment is made up of a variety of different ecosystems, with diverse communities. Species are grouped together based on either their relatedness or their function. Species can either be prokaryotes, unicellular organisms, with no internal membrane structures, or eukaryotes, unicellular or multi cellular organisms containing internal membrane-bound cell structures. After determining if the species belongs to the group of prokaryotes or eukaryotes, they are then further categorized by kingdoms, families, classes, orders, etc. This is a process known as taxonomy. Taxonomy is based on species who share common morphologies.
Species can also be categorized in two general types, based on how they obtain energy. First there are autotrphs, which generate organic matter from inorganic matter. Then there are heterotrophs consume the organic matter by consuming autotrophs.
Life in the ocean does no only stop at the species and what groups they are in, but there are other factors to be considered when looking at life in the oceans. Things like carbon, salinity, temperature, etc. Since the oceans are so diverse and complex, it is what makes them interesting and fun to learn about.
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~Chapter 14 in "An Introduction to the World's Oceans", by Keith A. Sverdrup and E. Virginia Armbrust.