In Situ Observations of Pelagic Sargassum Total and Morphotype-specific Density and Distribution Across the Western North Atlantic from 1993 to 2023.
Description
Between 1993 and 2023, 5,587 neuston net tows were conducted onboard Sea Education Association’s (www.sea.edu) SSVs Westward and Corwith Cramer during oceanographic research cruises across the North Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. Cruise tracks varied seasonally and annually. The neuston net (1.0m × 0.5m frame, 333 um mesh) was deployed from a boom extended 5 m off the ship’s port side while sailing on a port tack. Any windrows of Sargassum were thus crossed perpendicularly and any bow wake effect was minimized. Tows were conducted at two knots for 30 minutes for a typical distance of ~1.0 nm; tow distance (m) was calculated as the difference between start/end GPS locations (1993–2003) and from minute-by-minute GPS locations (2004–2023). Standard net processing included identifying pelagic Sargassum specimens to morphotype, patting dry, and weighing using a spring scale to yield mass (g) for each morphotype. Three common holopelagic morphotypes are reported: Sargassum natans var. natans (Sn_n), S. natans var. wingei (Sn_w), and S. fluitans (Sf). Weighted arithmetic mean Sargassum density (g/km2) was calculated from the sum of Sargassum mass divided by the sum of tow area for all tows in a given year (season, time frame) for a given region. Morphotypes reported as percentage of total collected biomass in a given year (season, time frame) for a given region. The date and GPS position were captured with each sample. All net tows were assigned to one of five regions defined on the basis of physical oceanographic boundaries. Analysis of data was organized by ecological year (March–February, corresponding with the start of the annual spring bloom period in the Sargasso Sea) and season (spring/summer March-August, fall/winter September-February). In most years, neuston tows occurred during both the spring/summer and fall/winter seasons in a region; sporadic changes in cruise track may have caused occasional missed seasons. Examined time frames (pre-MODIS 1993-1999, pre-GASB 2000-2010, early GASB 2011-2014, and recent GASB 2015-2023) were selected for comparison with satellite data and Great Atlantic Sargassum Belt (GASB) dynamics. Quality control confirmed accurate, consistent data entry and GPS positions, and filtered out incomplete records. Blank cell = no data.