12.5 What are the main divisions of the marine environment?

oceans can be divided into two main environments:

(1) pelagic = ocean water itself, where drifters and swimmers are

(2) benthic = ocean bottom, where marine algae and animals that don’t float or swim live

Pelagic (Open Sea) Environment

can be divided into distinctive life zones (biozones) that possess unique physical characteristics.

pelagic environment divided into neritic and oceanic provinces

neritic = extends from shore seaward and includes all water less than 200m deep

oceanic = seaward of neritic, and basically rest of ocean, divided into four biozones which are defined according to depth:

(1) epipelagic = surface to 200m

(2) mesopelagic = 200-1000m deep

(3) bathypelagic = 1000-4000m deep

(4) abyssopelagic zone = deepest part of ocean deeper than 4000m

single most important factor that determines the distribution of life in the oceanic province = availability of sunlight:

  • euphotic zone = extends from surface to a depth where enough light still exists to support photosynthesis, rarely deeper than 100m

  • dysphotic zone = small but measurable quantities of light

  • aphotic zone = no light, below 1000m

Epipelagic zone

upper half of the epipelagic zone is the only place in the ocean where there’s sufficient light to support photosynthesis - boundary between epipelagic and mesopelagic zones (200m) = where level of dissolved oxygen begins to decrease significantly

  • oxygen decreases at this depth bc no photosyntheti algae live below about 150m and dead organic tissue descending from biologically productive upper waters is decomposing by bacterial oxidation, which consumes dissolved oxygen and releases nutrients back into the water

  • nutrient content increases abruptly below 200m

boundary between epipelagic and mesopelagic zones = approximate bottom of the mixed layer, seasonal thermocline, and surface water mass

Mesopelagic Zone

dissolved oxygen minimum layer (OML) occurs at a depth of about 500-1000m

intermediate water masses that move horizontally in this depth range often have the highest levels of nutrients in the ocean

organisms capable of bioluminescence are common in mesopelagic and deeper zones - in areas below sunlit surface waters, having the ability to produce ligght has many advantages so vast majority of organisms are capable of producing light i.e. shrimp, squid, deep-sea fish

Bathypelagic and Abyssopelagic Zones

aphotic bathypelagic and abyssopelagic zones = >75% of living space in oceanic province. many completely blind fish exist in the inky blackness of this region and all are small, bizarre-looking, and predaceous

  • many species of shrimp that normally feed on detritus become predators at these depths, where the food supply is greatly reduced compared to surface waters

  • animals that live in these deep zones feed mostly upon one another

  • they have evolved impressive warning devices and unusual apparatuses that make them extremely efficient predators

  • many have sharp teeth and extremely large mouths relative to body size

  • oxygen content increases with depth below the oxygen minimum layer bc it’s replenished by deep currents originating in polar regions as cold surface water high in oxygen. the abyssopelagic zone inis the realm of the bottom-water masses

Benthic (Sea Bottom) Environment

sea bottom also divided into diff provinces:

supralittoral zone = transitional region from land to sea floor above spring high tide line (spray zone), covered with water only during periods odf extremely high tides and when tsunami or large storm waves break on shore

rest of benthic is divided into 2 main units that correspond to the neritic and oceanic provinces of the pelagic environment:

(1) subneritic = extends from spring high tide to depth of 200m, approximately encompassing cont. shelf

(2) suboceanic = includes benthic enviornment below 200m

Subneritic Province

subdivided into littoral and sublittoral:

  • littoral = intertidal zone (zone between high and low tides)

  • sublittoral = low tide shoreline out to 200m, consists of inner and outer regions:

    • inner sublittoral zone extends to depth at which marine algae no longer grow attached to ocean bottom, approx. 50m

    • outer sublittoral zone = extends from inner sublittoral zone out to depth of 200m at seaward edge of cont shelf

Suboceanic province

suboceanic provine subdivided into bathyal, abyussal, and hadal zones

  • bathyal zone = extends from 200-4000m, corresponded generally to cont. slope

  • abyssal zone = 4000-6000m, more than 80% of benthic environment

    • ocean floor here covered by soft oceanic sediment, mostly abyssal clay

  • hadal zone = inhospitable high-pressure environment below 6000m, only consists of deep trenches along margins of continents, very unique adaptations

RECAP

pelagic environment includes water column and benthic environemnt includes sea bottom. du

CONCEPT CHECK 12.5

(1) construct a table listing the subdivisions of the pelagic and benthic environments and the physical factors used in assigning their boundaries.

(2) Describe the 3 zones based on the availability of sunlight. which one is where most marine life exiists?