Describe the division of the ocean into pelagic and benthic realms and into depth zones, and explain how light, temperature, pressure and food availability change across them

What this dot point is asking

QCAA wants you to divide the ocean into its main environments and depth zones and explain how the abiotic factors change across them. This organises the abiotic-factors material into a map of where different communities live, and supports reading depth profiles in IA1.

Pelagic and benthic realms

The first division is between the water and the bottom.

• The pelagic realm is the open water column, everything between the surface and the seabed. It is the home of plankton, swimming fish (nekton) and marine mammals. It is further divided by distance from shore into the neritic zone (the shallow water over the continental shelf) and the oceanic zone (the deep open ocean beyond the shelf).
• The benthic realm is the seabed itself and the organisms living on or in it, the benthos, from intertidal shores down to the deep abyssal plains. Coral reefs, seagrass and rocky shores are benthic communities.

Depth zones and light

Light is the master variable that defines the vertical zones.

• The photic (euphotic) zone is the sunlit surface layer, usually the top tens of metres, where there is enough light for net photosynthesis. Almost all primary production happens here.
• The twilight (dysphotic) zone below has some light, enough to see by but not enough for net photosynthesis.
• The aphotic zone is the permanently dark deep ocean. With no light there are no producers, so its food webs depend on material sinking from above (or, at vents, on chemosynthesis).

The depth that light reaches depends on water clarity, which is why clear tropical water has a deeper photic zone than turbid coastal water.

How abiotic factors change with depth

Reading a depth profile, four factors change in predictable directions.

• Light falls rapidly and is gone within a few hundred metres at most.
• Temperature is warm and fairly stable in the surface mixed layer, drops sharply through the thermocline, then stays cold (near 2 to 4 degrees C) in the deep sea.
• Pressure rises steadily with depth, by about one atmosphere every ten metres, so deep-sea organisms face crushing pressure.
• Food (and oxygen) become scarce with depth because most production is at the surface; the deep sea relies on a slow rain of sinking organic matter.

As a result, biomass and diversity are generally highest in the sunlit, food-rich surface and shallow benthic zones and decline into the deep sea, with notable exceptions such as hydrothermal vent communities.

Australian context

Australian waters span the full range. The Great Barrier Reef lagoon is shallow neritic and benthic, bathed in light. Just offshore, the continental shelf drops into the deep oceanic Coral Sea, where the seabed lies in the dark aphotic zone under enormous pressure. The East Australian Current flows through the pelagic realm, carrying warm surface water and larvae south. Mapping a named habitat onto these zones is a common short-response task.

Why this matters

The zones organise everything else in Unit 3: producers and the highest productivity sit in the photic neritic zone, reefs and seagrass are shallow benthic communities, and the deep sea is a low-food, high-pressure world with very different adaptations. The same depth profiles reappear in IA1 stimulus, so being fluent in reading light, temperature, pressure and oxygen against depth is directly assessable.