Photic zone:

♤ It is the upper layer of the aquatic ecosystems, up to which light penetrates and within which photosynthetic activity is confined.

♤ The depth of this zone depends on the transparency of water.

♤ Both photosynthesis and respiration activity takes place.

♤ photic (or “euphotic”) zone is the lighted and usually well-mixed portion that extends from the lake surface down to where the light level is 1% of that at the surface.

Aphotic zone:

♤ The lower layers of the aquatic ecosystems, where light penetration and plant growth are restricted forms the aphotic zone.

♤ Only respiration activity takes place.

♤ aphotic zone is positioned below the littoral and photic zones to bottom of the lake where light levels are too low for photosynthesis. Respiration occurs at all depths so the aphotic zone is a region of oxygen consumption. This deep, unlit region is also known as the profundal zone.

Winterkill

Snow cover of ice on water body can effectively cut off light, plunging the waters into darkness. Hence photosynthesis stops but respiration continues. Thus in shallow lakes, the oxygen get depleted. Fish die, but we won’t know it until the ice melts and we find floating fish. This condition is known as winterkill.