We know plankton are the food for whales that use baleen to filter water. Okay, so we know plankton are super tiny, but are plankton completely invisible or not?
In fact, you can see some with your naked eyes, but they are just tiny dots like the green tea powder. I guess, there might be a ton of them on your hair once you soaked your body in the seawater. I don’t mind, but I can see that the itchy feeling is not just due to the salt maybe? However, don’t hate them so much because they have a seriously important function. Phytoplankton carry out photosynthesis in the ocean living in the surface water, the photic zone, providing a big portion of oxygen on the earth. Phytoplankton also serve as the important food source for the secondary producers in the food pyramid. Who would eat these? Many filter-feeding animals like baleen whales and sponges! However, there is another type of plankton called zooplankton. These zooplankton do not carry out photosynthesis and instead move around in the water to feed on phytoplankton. A cool fact about zooplankton is that they move up the water column during the night to feed on phytoplankton and go back to the deeper water during daytime to avoid a predation.
Thus, we assumed that we would find phytoplankton in the shallow and zooplankton in the deep seawater for our experiment at the head of Grappler inlet where Bamfield Marine Sciences Center is located nearby in BC. The picture above is the net we used in the shallow water. Other groups used the net with larger holes to capture zooplankton in the deep water. When we brought them into the lab, we had an unexpected result. We had a considerable abundance of zooplankton in the shallow water. Although we found more zooplankton in the deep water, we still found 7 different species of zooplankton in the shallow water, which belong to 5 different phyla. On the other hand, we had 10 different species of zooplankton in the deep water, which belong to 9 different phyla. The shallow water did have much more phytoplankton individuals such as diatoms (algae), and the deep water did not have phytoplankton. Altogether, we can conclude that a substantial variety of zooplankton are still found in the shallow water even during the daytime, but the abundance of phytoplankton outcompetes the abundance of the zooplankton in the shallow water.
Regarding the difference of a phyla diversity we see between the shallow and the deep water, the species that belong to certain phyla might not mind too much staying at the both depths in the sea during the daytime with risking the predation. We need further experiments to find out the reason. This might be due to phyla-specific temperature preference between the shallow and deep water or the less sensitivity to temperature, salinity, and hydrostatic pressure in measuring the water depth. For the latter, here is an article explaining how zooplankton orient themselves in the seawater. http://www.jstor.org.ezproxy.library.ubc.ca/stable/41061768?pq-origsite=summon&seq=1#page_scan_tab_contents