During World War II, underwater sonar scans of the ocean showed relatively shallow bottoms at some areas that also changed in depth depending on the time of day. This isn’t a supernatural activity moving the ocean floor up and down every day. What the navy recorded was actually a densely packed layer of plankton in the water column. The layer was so dense that it looked like the ocean floor on the sonar.
As part of our weekend trip at Bamfield Marine Science Centre, we wanted to see the different types of plankton that can be found in our surrounding waters. We collected samples from both shallow and deeper waters with different sized mesh nets. Although planktons are microscopic, they do still differ in sizes. Phytoplankton are smaller in size and they can photosynthesize using sunlight similar to plants. Zooplanktons are larger in size and they need to consume other organism to acquire energy instead of using sunlight. We used nets with large meshes for deep water collection aimed to collected zooplanktons and finer mesh nets for shallow water collection aimed to collect phytoplankton. We then examined the samples under the microscope. In the deeper water samples, we saw a diverse range of zooplankton such as copepods, crab larvae, and jellies while in the shallow water we saw many phytoplankton such as diatoms and dinoflagellate.
But why did we only see zooplankton in deeper water while phytoplankton in shallower waters? This, like the strange plankton layer the navy of WWII saw, are both due to a process called Diel Vertical Migration (DVM).
During this migration zooplankton move up to the water surface during night time and to deeper waters during the day. This is a huge distance to travel for tiny creatures like plankton so why do they do it? It all comes down to food. Phytoplankton which require sunlight to photosynthesize is found only in the surface water where sunlight is available. Zooplankton must migrate up to the water column to eat these phytoplankton. However, being at the top of the water also make zooplankton easy targets as food themselves by small fish and filter feeders such as whales which feed near the water surface. Thus to better protect themselves, zooplankton only feed in the surface during night time when there are less predators while moving down to deeper, darker waters during the day where there are less predators. You can learn more about this fascinating process and some of the mechanisms driving this movement from this Nature Education blog.
The DVM along with other oceanic processes help remove excess carbon from the atmosphere to be stored in deep ocean waters. With increasing stress from global climate change, plankton diversity and distribution are also predicted to change and potentially affecting the vertical migrations of certain areas. You can learn about the links between climate change and diel vertical migration and the potential impacts of climate change here.