There are several hypotheses as to why DVM occurs. Perhaps the simplest explanation is that it reflects a trade-off for zooplankton between the safety of the dark deep sea and the bounty of food, such as phytoplankton, which tend to remain near the surface (see the figure here which illustrates this). Being near the surface where there is more light makes it easier for a predator to spot you, so zooplankton have been hypothesized to feed at the surface when darkness falls and move to deeper, darker water during the day to avoid predators. This means they can make the most of the available food supply without making themselves an easy meal. Another hypothesis is that it is bio-energetically more efficient for zooplankton and small fish to perform DVM than to just stay consistently at the surface, because metabolic rates will be lower in the cooler deep waters. Of course, any potential energy savings here must be weighed against the energy costs of migrating several hundred meters vertically in the water column twice a day. It should also be noted that there is a growing body of evidence for diel vertical migration in deep sea plankton. Like plankton at the surface, those at depth have been found to migrate up and down in the water column to the rhythm of the Sun despite the absence of light. In addition, the vertical movements of at least some organisms follow the variation in day length with latitude, which raises questions about what the trigger for DVM is if not the availability of light. One suggestion is that a very precise biochemical clock may be involved, and efforts are underway to test this. - In cooler temperate waters, the changes that accompany the seasons are much more dramatic. Overturn and mixing during the winter when the thermocline breaks down brings large quantities of nutrients to the surface. At the same time, markedly lower light levels prevent phytoplankton from utilizing the available nutrients, leading to low, light-limited productivity in the winter. As spring arrives, however, the days get longer, the light in temperate latitudes becomes more intense, and the surface waters contain abundant nutrients because of overturn and mixing during the preceding winter. This leads to ideal conditions for phytoplankton growth and produces a period of rapid population expansion known as the spring bloom (peaking in April in the figure you should still be looking at).
- As this bloom develops, two things happen that negatively impact phytoplankton growth. First, the increase in sunlight warms the surface water, making it less dense and the water column more stable. This eliminates the overturning and mixing of nutrient-rich deep water to the surface. Second, the phytoplankton rapidly use up the available nutrients in the photic zone as they photosynthesize and grow, which occurs at a time when the deep nutrient supply is increasingly blocked. (causes them to be nutrient limited)
- fall bloom in temperate latitudes: autumn cooling of the surface water breaks down the stratification and allows mixing of deep nutrients back to the surface again. However, a fall bloom will only occur if light levels remain high enough to stimulate photosynthesis as the nutrients again become abundant. Once light levels drop down to full winter values, the system switches from being nutrient-limited to light limited.