By: Miram Gleiber
June 18, 2015
This morning we woke up off the coast of Bimini Island, Bahamas, with the sun shining and surrounded by sapphire and turquoise water. We anchored here for an hour to clear customs, since the next week of sampling will take us in and out of Bahamian waters. We have a jam-packed week of sampling ahead, so time is of the essence and we won’t see much more of the Bahamas than the sandy coastline on the horizon while sampling offshore.
During Leg 2 of OSTRICH 2015 we are conducting a “spatial” study. This is because we will sample stations on either side the cross strait (East, West) and North-South regions of the Straits of Florida. The schematic (left) shows an example of stations we may sample during the spatial study. Each station has a general regional location, but is not an actual physical location with a set latitude and longitude. Instead, we deploy a “drifter” each morning, and follow this drifter for the day’s sampling. The drifter is a cylindrical cloth tube with holes, suspended about 30 feet below the surface, and marked at the surface by a red buoy and GPS tracker (see photo). The drifter marks a parcel of water that we are hoping to sample with different pieces of equipment throughout the day.
An overarching aim of the OSTRICH project is to understand how larval fish food web dynamics (i.e. predator-prey interactions) vary with physical features in the ocean that can create hot spots, or “patches” in larval fish distribution. The eddy we sampled in the first week of the cruise is a big physical feature, creating different water masses that can entrain the larval fish, their predators, and prey. During the spatial study this week, we are sampling broadly throughout the Straits of Florida, hoping to find smaller physical features in either the vertical or horizontal (called fronts; see example drawing). We expect to see regional differences in frequency and magnitude of patchiness correlated with different fronts.
Similar to the eddy sampling (see previous blog post) we are sampling each spatial station with DPI-2 (formerly ISIIS), MOC4, MOC1, and the CTD. The spatial stations differ slightly in the order, depth and duration of sampling, as well as generally only sampling during daytime hours. This is because many zooplankton vertically migrate from depth to the surface at night, thus increasing biomass. In order to equally compare stations, all tows need to be conducted in similar light conditions. Since we’d prefer to not be vampires for the next week, we do all tows during daylight. After the round-the-clock sampling during the Eddy Study (Leg 1), it’s nice to eat at regular meal times and get a real night’s sleep… because who knows how crazy things can get when we are sleep deprived (see photo at left).