How do marine habitats differ from freshwater habitat physically, chemically, an

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Chemically: Marine environments are characterized by salinity of approximately 3.5‰ while freshwater environments have an average of 0.5‰. Physically: Oceans are large, deep (up to 11,000 m), and very active bodies of water with considerable mixing occurring especially in surface water layers. Freshwater environments are either fairly static (lakes) or running (rivers and streams). Lakes vary in depth from a few to >1000 m and similarly vary considerably in size. Microbially: Both freshwater and marine environments have a wide diversity of microorganisms. The physical and chemical differences in these environments dictate differences in the number and distribution of microorganisms. In general, lakes contain extensive primary and secondary producers that interact dynamically. Primary producers (photosynthetic microorganisms) are found at the surface to the depth of light penetration. The amount of secondary production by heterotrophs is usually 20 to 30% of primary production. Heterotroph concentrations vary with depth, but there are three areas that generally have elevated numbers of heterotrophs, the neuston, the thermocline, and the upper layer of the benthos where the populations are primarily anaerobic. In ocean environments, especially in deep waters, microbial concentrations are highest at the neuston layer. Total bacterial numbers are on average one order of magnitude higher in coastal water than in the open ocean. As in lake environments, the vertical distribution of the heterotrophs shows an increase at the thermocline. At greater depths, the numbers of heterotrophs quickly diminish until, at a depth of 200 m, concentrations are very low. Heterotrophs increase again immediately above the ocean floor. Thermocline-induced stratification is less important in coastal waters because of the mixing of water by winds, currents, and temperature. For this reason, bacterial numbers are uniform at all depths except when the weather is very calm for long periods of time. In addition, seasonal fluctuations occur in coastal bacterial numbers, which are not observed in the open ocean. In general, there are two times of the year when there is an increase in bacterial populations in coastal waters, late spring–early summer and late summer–early fall, times when the phytoplankton are most active.

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