“My toes are cold,” shouted my dad who had just jumped off our dock to swim with his grand daughters. “Why is that?” he asked me with a delightful confidence in my knowledge of all things ocean-related. “It’s the thermocline,” I replied, “You know, layers of hot and cold water.” I hoped this would satisfy his curiosity, but was not surprised when he followed up with, “Well, how does that work?” 

I turns out that there is much more to it than just layers of hot and cold water and that I had to dive in a bit deeper (into some good books and the internet) to more fully explain the reason for cold toes. Think of the water column as a kind of layer cake composed of three different layers. The surface layer is the warmest because of warming by the sun. The bottom layer is the coolest. The word thermocline describes the transition layer between the two. Thermo means heat and cline means slope, so it’s like a heat slide between the warm and cool layers. Sometimes this slide can be shallow and other times it can be steep depending on the difference in temperature. 

Cold water is denser than warm water, so it likes to stay down at the bottom, whereas warm water is lighter and stays near the top. This seems to explain the cold toes my dad felt. But, the principles of density are the same for both air and water, and the interesting part comes when warm and cool mix. This may sound familiar if you read my column a couple of weeks ago about how onshore and offshore breezes occur when warm air from the land rises and pushes cool air back towards the shore at the end of the day. When the water is calm, the cold water stays at the bottom and the warm water stays at the top. And, even when there is a light breeze, it only stirs up the surface layer – often also called the “mixed” layer. But, if you get enough of a breeze, the mixing starts to reach deeper down and can pull the cool water up closer to the surface.  

In the case of the cold toes, it seems that there wasn’t a lot of mixing happening and that the warm water was at the surface that afternoon. The other factor to consider, however, is the tides. To refer to the column I wrote on ocean breezes again, there I discovered how the wind direction often shifted with the tides as a result of the warming of the mudflats during low tide. When looking at layers of warm and cool water, tides have a big impact. The movement of the water as it comes in and moves further up the shore causes more mixing deeper into the water column. This happens at the times of peak water movement when the tide is changing. When the water isn’t moving as much right during the periods right around high and low tides, the water settles out and the layers are more distinct. The day that we were swimming, it happened to be right around high tide, so toes were cold and shoulders warm. 

This is all interesting for swimming purposes, but what it is really important for is productivity. Last week, I wrote about how nutrient levels that are too high can cause nuisance algal blooms in the ocean. But, nutrients are necessary at the right levels for marine plants and algae to grow and for all the ocean creatures that consume them.  

The nutrients often settle to the bottom, but the plants need to be up at the top where there is light. So, the mixing of the water is necessary to bring those nutrients back up to the surface. The biggest mixing happens in the spring during the biggest flip-flop of the thermocline. In the winter when the air is cold, the temperature of the surface water drops and often freezes while the bottom layer stays warmer. In the spring, when the ice melts, the cold dense water sinks to the bottom and pushes up all the good stuff that has accumulated on the bottom.  

That brings it to the surface where it can provide nutrition to all the newly blooming marine plants and algae. This cycle is incredibly important for refueling the marine ecosystem. 

At this time of year, the mixing that occurs is on a smaller scale, but is equally important for replenishing the surface layer. So, next time you feel the cold water on your toes, be grateful for all of the nutrition that it holds in that deep layer just waiting to be brought back up to the surface.