In situ observation of holopelagic Sargassum distribution and aggregation state across the entire North Atlantic from 2011 to 2020

Abstract

Between 2011 and 2020, 6,790 visual observations of holopelagic Sargassum were recorded across the North Atlantic Ocean to describe regional distribution, presence, and aggregation state at hourly and 10 km scales. Influences of oceanographic region and wind/sea conditions as well as temporal trends were considered; marine megafauna associates documented the ecological value of aggregations. Holopelagic Sargassum was present in 64% of observations from the western North Atlantic. Dispersed holopelagic Sargassum fragments and clumps were found in 97% of positive observations whereas aggregated windrows (37%) and mats (1%) were less common. Most field observations noted holopelagic Sargassum in quantities below the AFAI algorithm detection limit for the MODIS sensor. Aggregation state patterns were similar across regions; windrow proportion increased with higher wind speeds. In 8 of 10 years in the Sargasso Sea holopelagic Sargassum was found in over 65% of observations. In contrast, the Tropical Atlantic and Caribbean Sea exhibited greater inter-annual variability (1-88% and 11-78% presence, respectively) that did not align with extremes in central Atlantic holopelagic Sargassum areal coverage determined from satellite observations. Megafauna association patterns varied by taxonomic group. While some study regions were impacted by holopelagic Sargassum dynamics in the equatorial Atlantic, the Sargasso Sea had consistently high presence and operated independently. Field observations capture important dynamics occurring at fine spatiotemporal scales, including transient aggregation processes and ecological value for megafauna associates, and therefore remain essential to future studies of holopelagic Sargassum.

Keywords: Aggregation; Holopelagic Sargassum; In situ observations; Windrow.

Figure 1. Holopelagic Sargassum offshore aggregation states.

(A) Dispersed clumps and fragments (0.2 m diameter clumps; Caribbean, winter 2021, credit J. Schell). (B) Narrow windrow (0.5–1.5 m across; Tropical Atlantic, fall 2015, credit A. Siuda). (C) Wide windrow (3–4 m across; Caribbean, winter 2015, credit J. Schell). (D) Small mats (3 × 9 m mat near small boat; Sargasso Sea, fall 2014, credit C. Morrall). (E) Large mats (50 × 100 m; near Guadeloupe, Caribbean, spring 2015, credit J. Schell).

Figure 2. Hourly observation locations within eight sampling regions.

Light blue circles indicate where holopelagic Sargassum was absent, navy circles where it was present. Dotted lines are borders between oceanographic regions. Sampling months for each region noted at lower right. GoM, Gulf of Mexico; FS, Florida Straits; Shelf, New England Shelf.

Figure 3. Holopelagic Sargassum counts.

Number of observations within each count range for dispersed (A) clumps and (B) windrows. Observations without pelagic holopelagic Sargassum not shown.

Figure 4. Holopelagic Sargassum aggregation by region and Beaufort Force.

Proportion of highest ranked aggregation states by (A) region and (B) BF. As only one positive holopelagic Sargassum observation each was made at BF 0 and 8, these levels not shown; holopelagic Sargassum was absent from the observation at BF 9. GoM, Gulf of Mexico; FL, Florida.

Figure 5. Annual patterns of holopelagic Sargassum presence and aggregation type.

(A–C) Annual (October through May) proportional presence and (D–F) ranked aggregation states in the Tropical Atlantic, merged Eastern and Western Caribbean regions, and merged North and South Sargasso Sea regions. Low annual mean satellite-derived areal coverage (<500 km²) of holopelagic Sargassum in the central Atlantic Ocean indicated by underscores; all other years were high coverage. Remote sensing data as monthly means, derived using the same approach as in Wang & Hu (2016) and Wang et al. (2019), were provided by the Optical Oceanography Laboratory at the University of South Florida (https://optics.marine.usf.edu).

Figure 6. Holopelagic Sargassum at different growth stages.

(A) Windrow of healthy holopelagic Sargassum clumps (~35 cm diameter clumps; Tropical Atlantic, fall 2014, credit J. Schell). (B) Healthy clump of Sargassum natans VIII (30 cm ruler shown; Tropical Atlantic, fall 2014, credit J. Schell). (C) Windrow of decaying fragments and pneumatocysts (~0.7 m wide; Tropical Atlantic, fall 2015, credit A. Siuda). (D) Decaying clump of Sargassum natans VIII (1 cm grid shown; Tropical Atlantic, fall 2015, credit A. Siuda).