Sargassum

PI

• Dennis McGillicuddy

Overview

Occupation of GO-SHIP lines A20 by R/V Thomas G. Thompson offered an exceptional opportunity to sample the Great Atlantic Sargassum Belt [Wang2019]. Satellite imagery indicates another significant bloom began just before the cruise, with the abundance of Sargassum in February of 2021 near the top of that observed in Februaries of the last five years, second only to February 2018. Given the seasonality of the phenomenon, Sargassum abundance was expected to increase during the course of the cruise.

Recent evidence suggests a long-term shift in the elemental stoichiometry of the seaweed (particularly N:P), which may reflect changes in nutrient supply fueling these blooms [Lapointe_submitted]. Sargassum tissue samples in the high-abundance region of the tropical and southern subtropical Atlantic are very few in number, with opportunistic sampling by the R/V Thomas G. Thompson in August 2019 providing most of the measurements of which we are aware.

Clearly more observations are needed to test the hypothesis of a long-term shift in N:P and its implications for nutrient supply and Sargassum bloom dynamics. A20 extended into the high-abundance region, and the core hydrographic and inorganic nutrient measurements will be extremely valuable for interpreting satellite-based Sargassum abundance. The critical need for opportunistic sampling is Sargassum tissue.

Procedure

Seaweed sampling was conducted by dipnet affixed to a standard recovery pole. A standard sample is 30-40g, an amount that fits easily into a quart-sized Ziploc bag. When sufficient biomass was available, 12 samples per station were collected, 6 dried and 6 frozen, each comprised of triplicates for the two species S. fluitans and S. natans which are easily distinguishable by their pods and leaves. In the event that sufficient biomass was not available, dried samples were prioritized.

Samples to be dried were rinsed with DI water, shaken dry, and placed in drying oven on parchment paper with name of designated species and station. Drying oven temperature were set between 55 and 65 C and checked periodically with a thermometer inserted into top dryer vent. Once sample was “bone dry” or crispy (typically 24-48 hours), sample were placed in Ziploc bag and labeled with species, station location collected, and date of collection.

Samples to be frozen were separated by species and placed in Ziploc bags, labeled with a code referencing date, location, type. Excess water was removed (with paper towel) prior to sealing bags and bags were stored in a freezer and covered with a black blanket to keep samples dark. Additional sample details were recorded on log sheets, including date, time, location, GPS, etc.

Lapointe_submitted

Lapointe, B. E., R. A. Brewton, L. W. Herren, M. Wang, C. Hu, D. J. McGillicuddy, S. Lindell, F. J. Hernandez, and P. L. Morton, submitted: Nutrient content and stoichiometry of pelagic Sargassum reflects increasing nitrogen availability in the Atlantic Basin. Nature Communications.

Wang2019

Wang, M., C. Hu, B. B. Barnes, G. Mitchum, B. Lapointe, and J. P. Montoya, 2019: The great Atlantic Sargassum belt. Science, 365, 83-87.