Imagine that you are on a boat in the ocean and you decide to take a bucket and collect some water samples to look for aquatic life. With the naked eye, you will more than likely be disappointed, as you will not see anything. However, you still have hope and decide to take the samples back to a nearby lab, pour a couple drops of the water on a slide and place it under a microscope. The following image is what you would see.
The organisms in this microscopic world are called plankton, which are those that drift in water. Plankton are broken down into two classes: phytoplankton and zooplankton. Phytoplankton are those that photosynthesize (use light to produce energy and O2) and in fact produce more than half of the atmosphere’s oxygen. Zooplankton on the other hand are plankton that do not photosynthesize. Just to name a few, this diverse class consists of crab, shrimp and fish larvae, sea star and sea urchin juveniles, comb jellies and jellyfish.
But is there a difference in plankton diversity and abundance between different environments, such as between deep and shallow waters? To find out, we went to the Bamfield Marine Science Center (BMSC) in Vancouver Island, where we collected water samples in both deep and shallow water during a sunny winter day. When taking the samples back to a lab in BMSC and looking at them under microscopes, we saw differences in the plankton populations between the shallow and deep aquatic environments. First, phytoplankton was nearly absent in the deep-water samples, while being abundant in the shallow samples. This makes sense because in deep bodies of water, light does not readily enter and therefore photosynthesis is extremely rare. Because photosynthesis is what gives phytoplankton their energy to survive, it would be very difficult for them to live in a lightless habitat. In contrast, light readily reaches shallow waters and therefore is a suitable environment for phytoplankton.
Regarding zooplankton, we noticed a higher diversity based on phyla (a taxonomic rank) in the deep-water samples compared to the shallow water samples. A possible explanation for this is that during the day, zooplankton tend to drift towards deeper waters to avoid their predators that live in the upper regions of the water column during the day. Therefore, while we were on boats at the deep water at daytime, we saw higher plankton diversity. That being said, the population of zooplankton was much higher in the shallow waters than in the deeper waters. This is understandable due to the fact that shallow waters have higher primary productivity as can be seen by the prevalent sunlight and therefore photosynthetic organisms including phytoplankton. Therefore, shallow waters can sustain more zooplankton.
If you are interested in learning about the microscopic world around you (different plankton species), the following website is a good place to start: http://www.encyclopedia.com/plants-and-animals/microbes-algae-and-fungi/moneran-and-protistan/plankton.