Pelagic Sargassum community change over a 40-year period: temporal and spatial variability.

C L Huffard,K L Smith,S Von Thun,A D Sherman,K Sealey

doi:10.1007/s00227-014-2539-y

Abstract

Pelagic forms of the brown algae (Phaeophyceae) Sargassum spp. and their conspicuous rafts are defining characteristics of the Sargasso Sea in the western North Atlantic. Given rising temperatures and acidity in the surface ocean, we hypothesized that macrofauna associated with Sargassum in the Sargasso Sea have changed with respect to species composition, diversity, evenness, and sessile epibiota coverage since studies were conducted 40 years ago. Sargassum communities were sampled along a transect through the Sargasso Sea in 2011 and 2012 and compared to samples collected in the Sargasso Sea, Gulf Stream, and south of the subtropical convergence zone from 1966 to 1975. Mobile macrofauna communities exhibited changes in community structure and declines in diversity and evenness within a 6-month time period (August 2011–February 2012). Equivalent declines in diversity and evenness were recorded in the same region (Sargasso Sea, 25°–29°N) in 1972–1973. Recent community structures were unlike any documented historically, whether compared to sites of the same latitude range within the Sargasso Sea, or the broader historical dataset of sites ranging across the Sargasso Sea, Gulf Stream, and south of the subtropical convergence zone. Recent samples also recorded low coverage by sessile epibionts, both calcifying forms and hydroids. The diversity and species composition of macrofauna communities associated with Sargassum might be inherently unstable. While several biological and oceanographic factors might have contributed to these observations, including a decline in pH, increase in summer temperatures, and changes in the abundance and distribution of Sargassum seaweed in the area, it is not currently possible to attribute direct causal links. Electronic supplementary materialThe online version of this article (doi:10.1007/s00227-014-2539-y) contains supplementary material, which is available to authorized users.

Highlights

Pelagic forms of the brown algae (Phaeophyceae) Sargassum are a defining characteristic of the Sargasso Sea
The Sargasso Sea is bisected by the subtropical convergence zone (STCZ), a frontal region which is often associated with increased productivity (Andersen et al 2011)
We found no relationship between sea surface temperatures (SST) and Shannon H′ loge diversity index among the samples for which we had both of these values (N = 5 recent sites, 12 historical sites; r2 = 0.0049, F1,15 = 0.0745, P = 0.7887)

Summary

Introduction

Pelagic forms of the brown algae (Phaeophyceae) Sargassum are a defining characteristic of the Sargasso Sea. Ranging from approximately 20°–40°N and 30°–75°W (Butler et al 1983; Coston-Clements et al 1991; Laffoley et al 2011), the Sargasso Sea is bordered by the Gulf Stream to the west, north equatorial current to the south, and weak circulation to the east (Siegel et al 2001) This floating ecosystem is located in the center of the oligotrophic North Atlantic Gyre (Coston-Clements et al 1991), it has higher concentrations of metazooplankton (Riemann et al 2011) and phosphorous (Culliney 1970) than the surrounding waters. Sargassum floating on the surface ranges in size from fist-sized clumps to massive rafts tens of meters in diameter (Marmorino et al 2011) forming contiguous features possibly extending 100 miles or more (Carr 1986).

Methods

Results

Discussion

Conclusion