Marine Vegetation

plants that inhabit the seas and oceans. They include algae, grasses, mangrove trees and shrubs, bacteria, and, to a lesser degree, fungi. The most common form of marine vegetation found in the seas is algae, which populate the pelagic zone to the lowest point of light penetration (100–400 m, where photosynthesis is still possible). Some types of vegetation, usually microscopic forms, are found in upper zones of the water (plankton); others, mostly large forms, inhabit the bottom (benthos). The vertical and horizontal distribution of marine vegetation depends on the amount of solar radiation that penetrates the water and the ability of algae to capture rays of a definite wavelength at various depths.

The development of the coastal belt of marine vegetation is a result of climatic and local factors, most important of which are the quantity of biogenic elements; the temperature, salinity, and transparency of the water; the type of sea floor; the degree of coast protection; and the depth. The largest of the marine algae (green, brown, and red), or macrophytes, are found in the comparatively narrow coastal zone of the sea known as the sublittoral (at depths to 50–75 m, usually 15–20 m), in the littoral (tidal zone), and in the supralittoral (zone of surf and spray). Macrophytes form underwater forests, particularly in the temperate zone, consisting of laminariaceous and fucalean algae (Alaria, Laminaria, Macrocystis, Nereocystis, Ecklonia, Lessonia, Sargassum). Numerous small algae, usually arranged in layers, settle on the macrophytes as epiphytes; the more photophilic ones grow on the upper part of the thallus of the host algae, while the shade-tolerant ones grow on the lower. Variously colored small algae usually grow among the large algae.

The marine algae are represented by both unicellular and multicellular forms that externally resemble terrestrial higher plants. They range in size from microscopic to giant (several dozen meters in length). Many algae have nonsegmented thalluses of various shapes—for example, threadlike, ribbonlike, platelike, or leaflike. The algae secure themselves to the bottom (usually to a rock substratum) by means of rhizoids; less frequently they are found on sandy and silty bottoms, to which they are usually unattached.

The algae are the principal producers of organic matter in the seas and oceans. Each year they produce approximately 10¹¹ tons of organic matter (in carbon), or 48–64 tons per hectare of converted dry matter. The large algae serve as shelter and food for many invertebrates and fish, particularly for their young. The existence of all life in the water depends directly or indirectly on algae. The nutritional value of marine algae, particularly plankton, is equal to that of the best meadow hay. Large marine algae contain 6–29 percent protein, 17–60 percent carbohydrates, 4 percent lipids, and many vitamins and mineral salts. They are able to accumulate certain trace elements that are contained in minute quantities in seawater.

The higher plant life of seas and oceans consists predominantly of grasses of the families Potamogetonaceae and Hydrocharitaceae. The grasses, comprising approximately 50 species, form true underwater fields, which are found primarily at depths to 100 m. Eelgrass, Poseidonia, and Thalassia are particularly common.

The total quantity of marine vegetation products in the world’s oceans has been estimated at 550 billion tons. According to the geographic distribution of benthic algae and grasses, the world’s oceans are divided into five biogeographic regions: two polar zones (arctic and antarctic) with fairly sparse vegetation in the sublittoral, two temperate zones (Boreal and Austral) with rich growths of brown algae and grasses (distributed in both the littoral and sublittoral), and one tropical zone with numerous species of algae (particularly green and red) and predominantly sublittoral grasses. Mangrove forests are found in the tropics, growing in bays on soft bottoms. In the subpolar and temperate latitudes a regular seasonal change in marine vegetation and cycles in the life of marine plants are observed. Among the algae and grasses there are perennial, annual, and ephemeral forms (primarily in the polar and tropical regions).

Like the other resources of the world’s oceans, marine plant resources are not inexhaustible. The seas of the USSR are rich in marine vegetation: the potential yield of macrophytes are 22 million tons (the total quantity, about 70 million tons). The northern seas of the USSR contain 277 species of plant organisms. Marine vegetation in the arctic zone is quite sparse. Macrophytes are abundant in the tidal and sublittoral zones of the White and Barents seas, and the total reserves of marine vegetation in these seas are more than 2 million tons. Marine vegetation in the southern seas of the USSR consists of 291 species of algae and ten species of grasses; of the southern seas the Black Sea is the richest in plant life (278 species). Filamentous algae and grasses are abundant in the Azov and Caspian seas, where the total reserves of marine vegetation are more than 10 million tons. The seas of the Soviet Far East are the richest in marine vegetation, with 550 species of algae and 15 species of grasses.

Marine vegetation has many uses. Algae yield a number of valuable food and industrial products. They are used in medicine and by the food, textile, leather, and petroleum industries. About ten species of algae are collected and processed in the USSR; they are obtained from the White Sea (Ahnfeltia, Laminaria, and, to a lesser degree, Fucales), the Baltic Sea (Furcellaria), Black Sea (Phillophora, Cystoseira), and the Sea of Japan (Laminaria and Ahnfeltia).