A Cruise to New York (continued)

Once we arrived at the inshore point on the Google Earth map pictured in the 2nd to last post, we turned toward the southeast to try to slice into the warm fresher water we saw in the model and ocean observations. The map to the left shows a rapid morning transect and histograms of the density (sigma_theta), salinity, and temperatures we measured at the surface along the way. We don't fish on this first transect; just collect physical and acoustic data to pinpoint where our samples should be when we turn around and retrace our steps later in the day.  Early and inshore where the water was cold and salty, we saw Mother Carey's chickens (Wilsons Storm Petrel ) pattering the water, fishing for plankton with their feet. The clam dredge boat below was working about half way along the transect in an offshore anchorage where cargo ship wait to be permitted entry to New York Harbor.

When we finished this exploratory transect we quickly plotted up the data and found that it confirmed the model, satellite and HF radar observations of inshore to offshore gradient of cold salty water to warm fresher water.

A Mother Carey's Chicken
(Wilson's storm petrel)

The clam dredge











This is the track we took toward Long Island on return trip when we fished the tucker trawl.  It is colored by the density of the surface water with less dense warm and fresher offshore water in light blue and more dense cold and salty water inshore in red.  (A plot of surface water PH looks very similar but with low PH inshore; higher PH offshore). The green dots along our track indicate the 5 locations where we towed the tucker trawl for plankton. The red dots are the locations where we  did casts with our conductivity, temperature, depth profiler (CTD) to map out the vertical structure of the water column. (We also do CTD's at the plankton sampling stations.)  Below are cross sections of temperature, salinity and density along our track that we made with our  CTD cast data.  The sharpest change in temperature and density occurred at about kilometer 12 or 13 where we made our third net tow (The third green dot in the map above)

The cross sections show a two layered ocean more or less. 

But is it really? 

The two acoustic images below taken during our first trawl tow offshore and last tow inshore suggest the vertical structure can be more complex.

This image shows at least 3 scattering layers in the area of our first net tow offshore.  And this is a simple structure compared to what we sometimes see in the vacinity of the Hudson Raritan River plume in our New Jersey Seascape.

Our basic sampling strategy is to use the ocean observations and models to map out our sampling in the horizontal dimension across the ocean surface.  (This would be impossible without an exceptional operational ocean observatory in the region run by friends at Rutgers University (http://rucool.marine.rutgers.edu/) and their collaborators in MARCOOS (http://www.marcoos.us/)).  We then look at our shipboard data; individual CTD casts, the fisheries hydro-acoustic images, our acoustic doppler current profiler, and make decisions on the fly as to how to partition the water column and determine the depths at which we will fish our net for larval fish.

 

This acoustic image was taken during the final tucker trawl tow of the day at the sampling station nearest to the Long Island shoreline. We are in shallow water and there was a fairly big ocean swell toward the shore (Big long waves).  The waviness of the bottom is from the heave of our "ship" in the swell.  The structure of the water column visible in this acoustic image was less organized than the structure in deeper water because of the waves in shallow water make more turbulence.