Density
When my father would tell me that I was dense, I did not take it as a compliment, but I did not quite understand the insult either. It did not occur to me that what he meant to tell me was that I was incapable of learning, stupid, even, because I knew that for my father, to be dense was to be a jackass: stubborn. But density is oh so much more. There is a certain power in density, mass divided by volume, in certain circumstances that my father may have, in some askew manner, been actually keying me in on.
In the world of our river, density comes mostly in to play in the interplay between salt water and fresh. If you just consider the comparison between a glass of fresh water, and a glass of water of the same volume into which a table spoon of salt has been added, it is plain that the added salt would make the water weigh more and, therefore, be more dense than the glass of fresh water. But water is water right? So when salt water meets fresh, they should mix and find some medium between the two, right? This is true, but the mixing is not instantaneous. The variables of density, temperature and flow affect the time that it takes for the solution to find this medium point.
There is a fun way to test this effect. You will need four glasses, two of which are filled with water, blue and yellow food coloring and ½ cup of salt. One glass of water will remain as is, but into the other you should add the ½ cup of salt. Should the salt not dissolve entirely upon stirring, you can heat the water slightly in a microwave, stirring occasionally until dissolved. Add 4 drops of yellow food coloring to the fresh water, and 4 drops of blue to the salty water. Pour half of each color into one of the empty glasses so that you know have a two half glasses of yellow (fresh) water and two half glasses of blue (salty) water at approximately the same temperature. Slowly add the salty water to the fresh and ved versa and compare how the two mix and become another color or find where you can see where the two remain separate. If you have tries the experiment, or perhaps if you have not, you will note that the salty water tends to stay towards the bottom of the glass. The same happens in a tidal estuary such as in the Soundview area and farther upstream, only instead of water being added from above, you have two walls of water moving towards one another. The salt water is carried upstream by the tides and the fresh water is carried downstream by gravity. There is a point where these two meet and it is called the salt wedge. The Denser salt water pushes under the fresh water, settling towards the bottom, as the lighter fresh water floats over the top.
This wedge can stretch for miles as it does in the Hudson, or it can be much more vertical if the flow of the river is equally matched by the rise of the tide. In the case of the Hudson, the salt wedge is carefully watched since Poughkeepsie gets much of its drinking water directly from the Hudson. If the salt wedge runs too far north, the flow is increased to push it back down south. Sampling the water at different depths will show the differing salinities.
Fresh water means less than .5 parts per thousand (ppt) of salt, while brackish water is between .5 ppt and 17ppt, and oceans average 35 ppt. Most estuaries, like the Bronx River Estuary, are brackish. At our last monitoring of Drew Gardens miles above the mouth, Sally found2-3 ppt salinity from a surface sample. As a I walked upstream to get a few pictures of The Phipps Youth Employment Program students that were being introduced to water quality monitoring, I saw a nice size blue crab crawling across the bottom of the river in an area were the flow was quite slow.
I thought that the water would be dangerously low in salt for this creature which thrives 10ppt-25ppt but there could be mitigating factors. One is that the test we had done was surface which means that towards the bottom it could have been a little more salty. There is also an interesting study from 2005(POSEY Martin H. (1) ; ALPHIN Troy D. (1) ; HARWELL Heather (1) ; ALLEN Bryan (1) ) that discusses the tendency for juvenile blue crabs to seek areas of lower salinity(3ppt) in the summer and the fall. The theory put forward by the authors is that they are actually looking for waters safer from predators since salinity and predators were positively related. In any case, it shows the river’s improvement and, should this crab not be atypical, demonstrates the importance of this part of the estuary to all waters down river.
Dad may have been right about my being dense, but now I know that density can also have its advantages.
DG
Add comment July 21st, 2008
