The Seascapes

The Seascapes

Monday, June 28, 2010

A Cruise to New Jersey

As this was the first day of our June cruise, our intern, Jessica flipped the coin and it came up "New Jersey".  So we sampled ichthyplankton off New Jersey today. Our high speed wireless internet reception on the boat was consistently good so we were able use the Stevens Institute NYHOPS model output in google earth to route our sampling.  In the past we have depended only on near real time satellite (map 1, just to the left)and HF radar observations of the ocean (mat 2 below). Those data are great but the observations are at least an hour behind and cannot tell us what the ocean looks like at present or an hour in the future on our research ship.   The observations give us a view from space of what just happened in the ocean 1 to 48 hours earlier.  The assimilation model gives us a birds eye view of a good mathematical guess tuned with the real observations, of what the ocean looks like right now and in the near future (map 3 below).  With the NYHOPs we were looking at nowcasts or short term forecasts that seemed remarkable accurate and could tell us where we aught to sample an hour or more from the present.

As forecast, we had upwelling favorable conditions along the coast of New Jersey.  The wind at the ambrose bouy at 8AM (1200 GMT) was out of the southwest ( ~ 210T) at 12 knots. The early morning satellite image showed cool water near the New Jersey coast (map 1) and daily averaged HF radar current data (map 2) showed water moving offshore as a result of the southwest wind.

In the NYHOPs model (map 3) the edges of the Hudson Raritan River Plume defined the frontal boundaries in the area we would sample. In the forecast it appeared that the offshore edge of the estuarine plume would move east so quickly with the wind and ebbing tide that we wouldn't be able to keep up with our ship while sampling.  So we decided to target the inshore edge of the estuarine plume and the area of upwelled water inshore.  
We laid out a transect from the nearshore to the edge of the Hudson Shelf Valley which we ran using just a YSI temperature, salinity and oxygen probe mounted on a pole arm at the surface along with fisheries hydroacoustics and an acoustic dopler profiler.  This rapid morning transect is the line between the two white symbols with black dots on this map of NYHOPs forecast surface salinities and current flows. The transect allowed us to identify where the strongest surface features were and confirmed the position of the inshore front in the NYHOPs model forecast.

The computer screen on the left shows the output of the fisheries hydroacoustics profiles (bottom panel), acoustic doppler current profile of surface current speed and direction (top panel).  On the upper right of the screen is data from the YSI CTD probe that is measuring salinity and temperature every second at the surface as the ship moves through the water.  This real time data helps us to identify structure in the ocean water we will sample.  Strong fronts and current shears are detected by these acoustic instruments as well as by sharp changes in temperature and salinities over short distances.  The acoustic instruments work by emitting sound and measuring its properties when it returns to ship after it has bounced off particles including animals.  As a result we can use the sound to tell us where the animals are beneath the surface of the ocean. 

Here is the salinity data we measured with the surface YSI during the morning transect to confirm the model.  We have overlaid it on a side scan sonar image of the bottom sediment. The western edge of the hudson shelf valley is on the right of the image.  The colored line is the salinity which is high (red) in a thin band along the coast coast and lower (light blue) offshore.  Surface temperatures were also as much as 5C lower inshore (20 C) than offshore.  This cold salty water along the coast was brought in from deep and offshore by the effects of the southwest wind driving the surface water offshore.  The green dots in the map are the locations where we did casts with a conductivity, depth, and temperature (CTD) profiler that also measures DO, Chlorophyll-A and turbidity.  The red dots are the locations where we towed the tucker trawl for larval fish.

To the left is our live ship track (red arrow) plotted in a Geographic Information System (GIS) along with the surface YSI data we measured in the morning.  The shapes are patches of different types of sediment on the bottom. We are able to import satellite ocean color and HF radar surface current data codar data into this GIS very quickly when we have a good internet connection.  But we didn't need to do that today because we were able to stay connected to the internet and with google earth at sea.  In the upper right part of the screen is the output from our last CTD cast of the day.

This is our tucker trawl for sampling larval fish at multiple depths.  We have strapped a YSI CTD with depth. temperature, salinity, oxygen, Chlorophyll-A, and turbidity sensors to the trawl and have a wire running from the net to the computers in the bridge of the ship.  We can therefor see exactly what depth the net is fishing as well as all the ocean characteristics at all times.  These data are also saved on the computer.  Using this system we can fly the net very precisely through layers we can see acoustic instruments and CTD casts.  In the end we have continuous records of the characteristics of the environment during in the water where the larval fish were caught.

A great naturalist (Linda Stehlik) who knows all the fish (and the birds) sieving a plankton sample which we will take home and sort in the lab.  Unfortunately the sorting takes months. There did not appear to be a lot of larval fish or crabs in our samples. (An update.  I was wrong. We have begun sorting and it turns out there is a lot more in these samples, including larval fish, than we could see on the ship with the naked eye.  More to come).

And finally this is the first of the data we collected today that we had time to process.  It is a cross section made from the temperature, salinity, and density CTD profile records collected along our inshore transect to sample planton with the net.  It shows that warm freshwater was on the surface offshore (down to about 4-5M) and a upwelling front was located at approximately km 9 of our track.  We sampled plankton with our tuckertrawl on this front as well as at two stations inshore and two stations offshore of it.  Based on the CTD casts, fisheries hydroacoustics and ADCP current profiles we decided to tow nets at the surface to a depth of 4 or 5 meters and then other nets from ~ 4 meters to a safe depth above the bottom at 4 of the 5 stations. The ADCP showed us that where the direction and speed with which the water was flowing. At the offshore stations it flowed in different directions at the surface, mid depth and bottom.  Fish larvae and other plankton at different depths may have been in different lanes of divided two and sometime three way highways.

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