Weedlines: A Sargassum Research Blog

PS-18-05 Daily Log

May 31, 2018 (Day 2)*

*Note: these will be posted on a one-day delay, i.e., this report is for yesterday’s activities.

Another fine day on the water, though we still have not found the motherlode of Sargassum we are looking for out in the Gulf.  Here is a short synopsis of some of the day’s activities.

CTD_2018.05.30

Mengqiu Wang (left) and Zabe Premo (right) collect water samples from the rosette bottles. Photo: Frank Hernandez

Overnight the vessel steamed west along the 28°N latitude, and by morning we were in offshore waters south of Barataria Bay, Louisiana. The winds had picked up a bit during the evening, so the seas were choppy with whitecaps. We did find a convergence line, with small clumps of Sargassum, though no large features. We followed this line for several miles, but still no large mats or weedlines. It was evident that the winds were having an impact; we could see clumps of Sargassum below the surface, likely a result of surface mixing. Also, we observed many detached Sargassum air bladders floating along the convergence zone, again likely due to surface mixing and agitation. The loss of air bladders, which keeps Sargassum buoyant, likely contributed to the subsurface algal biomass we observed.

Having relatively little luck in this area, we changed course and headed south-southeast towards the northwest edge of the Loop Current. Recent analyses of remote sensing observations indicated high Sargassum biomass in this region. It is a relatively long transit to reach this area (12+ hours), so we kept watch during the morning and afternoon in hopes of finding Sargassum along the way. We did find thin, scattered lines of Sargassum throughout our transit, but ultimately decided the habitat was too sparse to sample. The reason we prefer to sample the larger mats and weedlines is that these are the Sargassum features detectable through remote sensing (satellite) observations; smaller clumps and thin, scattered lines of Sargassum are too small to be detected. So our goal at sea is to sample Sargassum habitat at the same scale of the habitats that are detectable from space.

Plankton_2018.05.31

Verena Wang (left), Courtney Stachowiak (center), and Carla Culpepper (right) set up the plankton sample processing station prior to deploying the neuston net. Photo: Frank Hernandez

Continuing along our heading, we decided to stop during mid-afternoon to sample an “open water” station. Here we took reflectance measures and water samples. Our neuston net sample collected a few clumps of Sargassum, and along with those, a juvenile mahi mahi and amberjack. The mahi was set aside for additional processing of muscle tissue for stable isotope and condition analysis. We also towed the large frame trawl again, and although the tow went smoothly, we did not catch any juvenile fishes, which is our target for this gear. We’re not exactly sure why: perhaps they are just not present in our chosen sampling region, or they are large enough still to avoid our net. We are determined not to give up on the frame trawl yet, so we will continue experimenting with it over the next few days.

Seriola_2018.05.31

A juvenile Amberjack (Seriola sp.) collected in a neuston net tow today. Photo: Frank Hernandez

Having wrapped up our sampling around 1730, we once again set our course for a region near the Loop Current, where we hope to (finally) find suitable Sargassum habitat to sample. The team is ready for it!

 

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