We were chomping at the bit to get on the boat, but not before we had a friendly discourse between our resident geologist and ecologist. There's no doubt that the area we were exploring was an estuary. The question was: at what point does an estuary become a freshwater river? Geologically, an estuary becomes a river when there is no longer tidal influence. Ecologically, an estuary becomes a river when the salinity is below a certain level. (There's a name and a number for it, but I can only remember so much! :) ) As trivial as this may seem, it's something that I kept in my head throughout the day as I was collecting data.
We were finally turned loose to get on the boat, where we met Captain Mark and his English Bulldog, Beamer. Both of these guys are awesome! When we pulled out and headed downriver, the first thing we noticed was Osprey nests atop the buoys. Captain Mark is so knowledgeable about the wildlife in the area and informed us that the birds in the nests at that point are not adults but young Ospreys and aren't able to fly yet. They're pretty big for being babies-- about the size of a crow!
We soon moved downriver and came to a stop in the harbor. At this point we performed our first test, a turbidity test. We used the Secchi disk to measure the turbidity of the water:
As you can see in the first picture, you put the disk into the water and you measure how far down you can see the disk by the tick marks on the pole (every 2 cm). Next, you bring up the disk until you can see it again, like in the second picture. It took me about 3 tries to figure out how to get accurate measurements, since I was looking down over the tick marks instead of straight at them. I had to make use of the difference in coloration on the pole and the number of tick marks between the coloration the water level where I could see the disk again.
Once I found the two numbers, I calculated the median and discovered that the visibility was about 59 cm at that point in the harbor. It didn't surprise me considering the morning was overcast and rainy.
Our next stop was at the mouth of the Occoquan, where it met the Potomac. There was a dramatic shift in the width of the river, by at least 500 or 600 yards. This allowed for greater "fetch". (I love new words! Until today, I thought fetch was just a "fun" game people play with their dogs) Fetch is a distance of water that a given wind has blown over. That being said, I was interested to see what the changes were from the harbor to the mouth of the river and I wasted no time in finding out!
Dr. McBride asked me if I wanted to do an Ekman drop, and once I found out what it was I was more than happy to oblige! An Ekman Grab looks like a trap of sorts:
The bottom "doors" are hinged and are hooked onto the crosspiece (where the rope is) so they are open. The entire unit is then dropped into the water. Once it hits the bottom, a weight that looks like a bullet is slid forcefully down the rope and this releases the spring that the hooks are attached to, slamming the doors shut and capturing whatever sediment and organisms are on the riverbed.
Dr. McBride and I brought the Ekman grab up and emptied it and sifted the clay away and I was left with this:
There were TONS of bivalves (2-shelled) organisms, like freshwater clams in this sample. In the previous sample a group had pulled up an invasive species called an Asian Clam. They were first spotted in the Potomac in 1968 and came from the southern Asian to Eastern Mediterranean regions and is thought to have entered U.S. waterways via Chinese immigrants who used the clam as a food source. (http://nas.er.usgs.gov/queries/factsheet.aspx?speciesid=92)
Dr. Sklarew pointed out that although the Asian Clam is considered an invasive species, it's effects on the ecosystem are not completely negative. The Asian Clam is a filter feeder and removes particles from the water and could, perhaps, assist in clearing the Potomac.
The bivalves that I found happened to be native to the Potomac. Being the curious person that I am, I was a little disappointed that the clams weren't open so I could see the inside. But never fear! Captain Mark saved the day! Apparently, he is a Jack of all trades and shucked the clam in a matter of seconds. Karla, the co-director of the VISTA program, brought up a phenomenal diagram of the clam and I was able to find the foot, the ligament, the stomach, and yes, even the anus.
After I finished exploring my clam, we were headed further downstream, near an on-the-water seafood restaurant called Tim's Rivershore. We took our third sample here and our group tested water quality. We tested dissolved oxygen, nitrates, and phosphates. Dissolved Oxygen is an indicator of whether or not the water being tested can support life and/0r how much life is available and respiring. Using our water monitoring kit, we took a sample and put the Dissolved Oxygen indicators in the test tube:
The reddish-pink color indicates that the dissolved oxygen measures about 8 parts per million which is more than enough to sustain life. Good stuff!
We then tested the nitrates and phosphates, as both are indicators of pollution by fertilizers or other chemicals. Thankfully, there was no evidence of nitrates in our sample and there were only trace amounts of phosphate. Less than 1 part per million.
Now it was time for lunch at Tim's. Ordinarily, I would have gone for some steamed clams, especially since I didn't have to pay; but after having examined a clam's anus earlier, I really wasn't in the mood for clams! I decided on a fish sandwich instead!
After lunch we were challenged to create hypotheses and test them using one area that we had studied that day and our group chose water quality. We hypothesized that the pH would go down from morning to afternoon, because the sun was now out. We also hypothesized that salinity would go up because the high tide had gone out and less water with more salt would mean higher salinity. We also hypothesized that the amount of dissolved oxygen would go up because we thought that more plants would be respiring due to the increased sunlight.
We repeated each test, at each previous site on the way back and compared the two data points. pH did not go down, it went up instead. Salinity went up in one region and down in the other. However, the region where it went down is not a region where salinity is typically measured and is considered freshwater. The amount of dissolved oxygen did go up and did so at a remarkably fast rate at each incremental depth marker.
Each group reported out their data and before we knew it we were docked! Time just flew by! It was such a great day and Dr. Sklarew made it a point to thank us for our contributions. I didn't realize that this was the first time a group that he had helped direct had systematically collected data at prescribed points in the river(s). He said what we did today is a precursor to research on the river.
If that's not enough to have you leaving for the day on a high note, I don't know what is! What an awesome day!
Tomorrow- Discourse and Misconceptions :)
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