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.

Free-swimming fauna associated with influxes of pelagic sargassum: Implications for management and harvesting

“…As raft size decreased, individual abundance and species richness also decreased, as expected by ecological theory (Simberloff, 1976;Lomolino, 2000). The observed positive relationships between sargassum raft size and species richness and abundance is consistent with the earlier studies (Settle, 1993;Moser et al, 1998;Casazza and Ross, 2008;Martin et al, 2021;Goodwin et al, 2022) and explains why Barbadian fishers target larger rafts.…”

“…Cameras were deployed under two types of pelagic sargassum aggregation states: mats and windrows (Figure 2). Mats were classified as densely packed aggregations of sargassum with an irregular to round shape and measured 5 to 100s of meters in diameter (distance across) (Marmorino et al, 2011;Goodwin et al, 2022). Windrows were defined as aggregations of sargassum generally arranged in a line and ranged from 0.5 meter to several meters in diameter (Marmorino et al, 2011;Goodwin et al, 2022).…”

“…Mats were classified as densely packed aggregations of sargassum with an irregular to round shape and measured 5 to 100s of meters in diameter (distance across) (Marmorino et al, 2011;Goodwin et al, 2022). Windrows were defined as aggregations of sargassum generally arranged in a line and ranged from 0.5 meter to several meters in diameter (Marmorino et al, 2011;Goodwin et al, 2022). The term 'raft' is hereinafter used to refer to aggregations of mats and/or windrows.…”

Holopelagic Sargassum aggregations provide warmer microhabitats for associated fauna

“…Sampling was restricted to these conditions to minimize variation in incoming solar radiation and water movement and control for temporal variation in environmental conditions that affect seawater temperature 32 and Sargassum aggregation state 33 . Drifting Sargassum occurs as dispersed fragments and individual thalli or “clumps” 17 , 21 , 33 , 34 typically 0.1–0.5 m across 3 ; lines or “windrows” of Sargassum aggregated parallel with the wind direction and stabilized by Langmuir circulation 2 , 3 varying in width from < 0.5 m to several m 22 , 33 , 34 ; and “mats” or “rafts” of aggregated Sargassum several m in extent up to tens or hundreds of m across 3 , 17 , 22 , 33 , 34 , which may be associated with windrows 33 , 34 or occur as discrete aggregations more likely to form in calm conditions 2 , 3 . Sargassum distribution in the Sargasso Sea is irregular and patchy 3 , large aggregations are uncommon, and clumps and windrows are most frequently observed in groups of 1–5 33 .…”

“…Drifting Sargassum occurs as dispersed fragments and individual thalli or “clumps” 17 , 21 , 33 , 34 typically 0.1–0.5 m across 3 ; lines or “windrows” of Sargassum aggregated parallel with the wind direction and stabilized by Langmuir circulation 2 , 3 varying in width from < 0.5 m to several m 22 , 33 , 34 ; and “mats” or “rafts” of aggregated Sargassum several m in extent up to tens or hundreds of m across 3 , 17 , 22 , 33 , 34 , which may be associated with windrows 33 , 34 or occur as discrete aggregations more likely to form in calm conditions 2 , 3 . Sargassum distribution in the Sargasso Sea is irregular and patchy 3 , large aggregations are uncommon, and clumps and windrows are most frequently observed in groups of 1–5 33 . Because patterns of Sargassum aggregation and distribution observed during the study period were consistent with previous reports, we sampled Sargassum aggregations as they were encountered unless aggregations of multiple size strata were visible from the RIB.…”