The last entry in my dive logbook was on August first, 1997. This August, seventeen years later, I signed up for a Scuba refresher class and I am looking forward to getting my face wet again. Diving is the only way to truly appreciate marine life in its natural habitat. Besides, I really missed the feel of suspension in space while floating weightless through the underwater scenery with all sound blocked or muffled except for the intermittent bubbling of my own and my partners regulators. A famous quote by dive pioneer Jaques Coustau goes something like this: “From birth man carries the weight of gravity on his shoulders, …but one only has to dive beneath the surface to be free”.
While most people dive in tropical waters, our northern coastlines have a lot to offer those who don’t shy away from frigid temperatures and lower visibility. While clear water and good visibility means that dive conditions are great, biologically it means that there is little or no plankton in the water. When the production in the surface water is high, nutrients are abundant, and the tiny plantlike organisms called phytoplankton flourish. They either absorb or scatter the incoming light. This leads to low visibility and less light penetrating through this layer of plankton.
As we all know, phytoplankton production is good news for the small animals in the zooplankton that feed on the phytoplankton and are in turn food for numerous small fish, which may either grow into big fish or are consumed by bigger fish. My dive instructor said that all that sunshine earlier this summer caused low visibility under water. This is because as plantlike organisms that get their energy from photosynthesis the phytoplankton had no shortage of its energy supply, the light. Because phytoplankton consists of either one celled or chain forming forms, growth means that more cells are produced and numbers increase quickly. This growth can occur very quickly and continue endlessly, until any one thing needed for the construction of the cells is lacking. This is called a limiting factor.
If you have ever looked at the contents in plant fertilizer, you know that the two main components are nitrogen and phosphate. Phytoplankton in the ocean usually first run out of nitrogen, so that nitrogen becomes the limiting factor. In lakes, this is different and the limiting factor is usually phosphate. When the phytoplankton cells run out of readily available nutrients in the water their growth slows and in extreme shortages the cells will die off and sink to the bottom. When this happens, the visibility at the surface gets better.
While in your garden the process of turning old potatoe plants into compost takes several months to over a year, in the ocean this process starts immediately as bacteria attach to the dying phytoplankton cells even as they begin to sink. The digestive products of these bacteria are the nutrients needed for phytoplankton growth and can sometimes be used again before they leave the light flooded surface water. This recycling process is referred to as the microbial loop and can make a difference in prolonging the growing period. Not everything gets recycled and what is left sinks to the bottom.
On the bottom a multitude of animals live off the organic materials that come raining down from above. Eventually, bacteria will turn all biological matter back into its inorganic nutrient form.
After a heavy storm, visibility is low because not only the nutrients, but a lot of silt and small particles will be stirred up from the bottom. Usually, this flurry settles after a couple of days. If the sun comes out again, the new nutrients at the surface may trigger a fall bloom of phytoplankton. The fall bloom is usually of short duration and consists of different species of phytoplankton. Just like in your gardens the tulips bloom before the pansies and those are earlier than the nasturtiums, phytoplankton species have different growing seasons as well.
At the surface most phytoplankton are neutrally buoyant. If the water mixes violently, individual cells get drawn to greater depth. Without the ability to swim actively, there is a depth at wich the cell is no longer neutrally buoyant and starts to sink, first slowly and then more rapidly. The same is true for a diver, however technology comes to our aid: with the help of the buoyancy control device, air can be transferred from the pressurized air tank to a bladder in the dive vest to maintain neutral buoyancy and shake the weight of gravity off ones shoulders for that amazing feeling of freedom in the blue realm.