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Naturalist’s Notebook: April 19 to April 25

In reading the reports from this week’s trips, I could not miss the continued references to the color of the waters around Cape Cod.  If you have been to the Florida Keys or the Carribean, the water there is crystal clear and allows a vast amount of visibility.  If you have been whalewatching in the more northern waters around the Cape, you will have noticed that the water here has a green tint to it and that visibility is very limited.  Before you jump to the conclusion that something is wrong with the waters here, give me a paragraph or two to explain.

The beautiful green color of the waters here is caused by the presence of billions upon billions of small plant-like organisms and algaes that we collectively refer to as the phytoplankton. The broad term refers to cyanobacteria, diatoms, dinoflagellates, and single-celled plants.  It is helpful to think of the phytoplankton as the grass of the sea.  Just like grass on land, it is the primary producer and, therefore forms the base of the food chain.  And also, just like plants on land, it is responsible for the majority of the oxygen dissolved in the waters around it that is so vital to everything else living there.  This oxygen is a byproduct of the phytoplankton’s photosynthesis in which (simply) nutrients plus carbon dioxide, fuelled by the light of the sun, and in the presence of chlorophyll makes carbohydrates needed by the plant (our phytoplankton) and oxygen.  Even more important to you and I is the fact that the phytoplankton in the world’s ocean is also responsible for more that half of the atmospheric oxygen that we, ourselves, rely on.  The phytoplankton in the world’s ocean produces more atmospheric oxygen than all of the trees, in all of the forests, on all of the continents combined.

Let’s go back briefly to the phytoplankton being the primary producer, the grass of the sea.  As such it is the base of the food chain, or the center of the food web, if you prefer.  It is fed upon by the tiny, little animals that cannot make food on their own (the zooplankton) just as grasses are fed upon by cattle.    So, any time there is a large amount of phytoplankton, there will likely be a large amount of zooplankton.  In the waters close to Cape Cod, the mixing waters of the spring trigger a bloom of phytoplankton that creates a widespread restaurant.  As summer comes on and the surface waters warm, a kind of temperature band developes that prevents the bottom waters (where the dissolved nutrients and oxygen are plentiful) from mixing with the surface waters.  When this happens, the concentration of phytoplankton drops, meaning less food for the zooplankton (and on up the food chain).

Fortunately, there are forces in the ocean greater than the thermocline.  One of these is is physical obstruction.  When the cold bottom currents meet enough of a physical obstruction, they can be forced to the surface, bringing with them the nutrients they hold (nitrates, nitrites, and phosphates).  Where this happens, it is very much like dumping fertilizer on a lawn.  The nutrients feed the photosynthetic needs of the existing phytoplankton, allowing for growth and reproduction, a mini-bloom.  But this mini-bloom can coninue as long as the nutrients are brought to the surface.

This is the case at the southern end of Stellwagen Bank.  The steep sides of the submerged mountain force the cold, nutrient rich waters to the surface with their nutrients, creating a vast oasis in the middle of what really is, in the heat of the summer, a kind of a desert.  The center of the bank is relieved by the flow of the tides over its top on a daily basis, but the edges of the bank are productive almost continually.  It is this flow of nutrients that causes the edges of the bank to be productive places to look for zooplankton, small fishes, and whales.

Portuguese fishermen knew this.  They were fishing the edges of the Bank long before there was a Provincetown.  Whalers knew it too.  Many a Right Whale (and other species too) lost its life on the southern edge of Stellwagen.  And, today, whale biologists and whalewatchers know it, as well.  That is simply why many whalewatch trips from the Cape and eastern shores of Massachussets routinely look to find whales around the edges of the Bank.