Pelagic Sargassum community change over a 40-year period: temporal and spatial variability - PubMed (original) (raw)

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

C L Huffard et al. Mar Biol. 2014.

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.

PubMed Disclaimer

Figures

Fig. 1

Fig. 1

Study area map: stations of recent sample stations (small yellow boxes, stations numbered 1, 3, 5, and 6) and historical sample stations (Weis ; Fine ; Butler et al. 1983) including Hydrostation S in green. Samples used for direct comparison across time series in Sargasso Sea, 25°–29°N (“restricted block”) depicted by white triangles inside larger white box. Additional historical stations included in broader analysis of all samples shown as orange triangles. Long time-series records of sea surface temperature and pCO2 recorded in vicinity of Hydrostation S. Map prepared in ArcMap 10.1

Fig. 2

Fig. 2

Sea surface temperature and pCO2, a Sea surface temperature at recent survey stations at time of sampling, b time series sea surface temperature (black line) and long-term trend (red line) collected at Hydrostation S (32°N, 64°W, near recent Station 1), c Summer sea surface temperature (black dots) and long-term trend (red line) at Hydrostation S, d pCO2 (black line) and long-term trend (red line) at Hydrostation S. Smoothing in b and d performed using negative exponential model and 0.7 sampling proportion. Data for panels b–d available through the Bermuda Atlantic Time Series program (Bates et al. ; Bates et al. 2014)

Fig. 3

Fig. 3

Mobile macrofauna community composition (MMCS) similarity, diversity, and evenness for Sargasso Sea stations examined, Aug 2011 and Feb 2012. a Cluster diagram showing percent similarity between samples. MMCS >48 % similar across sampling periods. Red lines denote statistically significant clustering according to the Simprof test, b Shannon diversity by recent cruise date. Station 1 (black dot) lies north of Sargasso Sea, 25°–29°N restricted block c) Pielou’s evenness index by recent cruise date. Station 1 (black dot) lies north of Sargasso Sea, 25°–29°N restricted block

Fig. 4

Fig. 4

Recent and historical (Butler et al. 1983) samples of mobile fauna associated with Sargassum in Sargasso Sea, between 25° and 29°N (“restricted block” identified by white triangles in Fig. 1). a Non-metric multi-dimensional scaling plot of mobile fauna communities, with similarity cluster overlays. All diversity (b) and evenness (c) groups significantly different from each other based on pairwise Mann–Whitney U tests

Fig. 5

Fig. 5

Multi-dimensional scaling plots of _Sargassum_-associated mobile macrofauna community structures documented in August 2011 and February 2012 (open shapes, legends designated with an “R”) compared with communities documented in historical samples collected between 1966 and 1975 (Weis ; Fine ; Butler et al. 1983), all geographic and latitude groups (both orange and white triangles in Fig. 1), labeled by a geographic group, b latitude group, c season, and d year

Fig. 6

Fig. 6

Trophic groups of mobile macrofauna associated with Sargassum in recent and historical samples. Relative fraction of all individuals in each sample represented by each trophic group, an average of all samples in each latitude block ± standard deviation error bars. Groups specified by an R represent recent surveys. The other/unknown category includes parasites and taxa for which diets were unknown. Trophic category designations and citations in Online Resource 3

References

    1. Abé H, Komatsu T, Kokubu Y, Natheer A, Rothäusler EA, Shishido H, Yoshizawa S, Ajisaka T. Invertebrate fauna associated with floating Sargassum horneri (Fucales: Sargassaceae) in the East China Sea. Spec Divers. 2013;18:75–85. doi: 10.12782/sd.18.1.075. -DOI
    1. Andersen NG, Nielsen TG, Jakobsen HH, Munk P, Riemann L. Distribution and production of plankton communities in the subtropical convergence zone of the Sargasso Sea. II. Protozooplankton and copepods. Mar Ecol Prog Ser. 2011;426:71–86. doi: 10.3354/meps09047. -DOI
    1. Banta WC, Perez FM, Santagata S. A setigerous collar in Membranipora chesapeakensis n. sp. (Bryozoa): implications for the evolution of Cheilostomes from Ctenostomes. Invertebr Biol. 1995;114:83–88. doi: 10.2307/3226957. -DOI
    1. Bates NR, Takahashi T, Chipman DW, Knap AH (1998) Variability of pCO2 on diel to seasonal timescales in the Sargasso Sea near Bermuda. J Geophys Res C (1978–2012) 103:15567–15585
    1. Bates NR, Best MHP, Neely K, Garley R, Dickson AG, Johnson RJ. Detecting anthropogenic carbon dioxide uptake and ocean acidification in the North Atlantic Ocean. Biogeosci Discuss. 2012;9:2509–2522. doi: 10.5194/bg-9-2509-2012. -DOI

LinkOut - more resources