Optical control of fish and zooplankton populations

Aquatic food webs are affected from the bottom up by light through its effect on photosynthesis and productivity. But light also has a top-down effect, because it is crucial for the visual foraging efficiency in many fish. Here we present data suggesting that marine pelagic food webs are primarily structured top-down by light through its effect on vision in fish. For light-limited fjord ecosystems, we show that the abundance of zooplanktivorous fish is proportional to the vertical extension of a visual feeding habitat, represented by the inverse of the light absorbance coefficient of the water column. We also show that both zooplankton abundance and body size are proportional to the size of a vision-protected habitat that can be defined as the dimensionless product of the light absorbance coefficient and the depth of the water column. Natural and human-driven environmental change may involve alterations in the amount of surface radiation as well as in the optical properties of the water column. Our results imply that such changes are likely to affect aquatic food webs top-down through vision as well as bottom-up through photosynthesis.