Fig 1.
Map of the Gulf of Maine showing the location of the coastal and offshore (Cashes Ledge) sites.
Cashes Ledge is located 145 km east of the New Hampshire coast with Ammen Rock at the center of the 29 km-long ridge. The site names are abbreviated as: AR1, AR2, AR3 designating Ammen Rock Sites 1–3 on Cashes Ledge and the coastal sites as: Spout Shoal (SS), Duck Island (DI), Mingo Rock (MR), Star Island (SI) and Lunging Island (LI). Sea Surface Temperature (SST) and wave height data were obtained from oceanographic buoy stations indicated by a diamond symbol with CL for the Cashes Ledge Buoy and WMS for the Western Maine Shelf Buoy. The scale bar is 10 km. Specific GPS coordinates for centrally located sites in the coastal and offshore region are: DI 43 00. 667 N x 70 60. 517 W, and AR1 42 53.4348 N x 068 56.6106 W. This map was produced by L. Carlson using Esri software licensed to Brown University.
Table 1.
Information on the sampling for kelp and fish.
Number of replicate quadrats for urchin counts reported in S7 Table.
Fig 2.
Sea surface temperature (SST) anomalies for the Gulf of Maine between 1980 and 2016.
Data were compiled from the Met Office Hadley Centre data set (HadISST). Orange shaded bars correspond to the years that kelp forest communities were sampled on Cashes Ledge. Note that SST anomalies were consistently positive from late 2006 to 2016, and that a record 2.75° C warm anomaly occurred in 2012. A trend line (blue) was fit across the time series using a rolling function based on the median anomaly using a 24-month window with the R program stat_rollapplyr(). Grey dots represent individual SST anomaly data points.
Fig 3.
Boxplots display the median value of the data as a horizontal bar, the first and third quartiles of the data (25th and 75th percentiles) as a shaded box around the mean, 1.5x the interquartile range (IQR) as a vertical line, and all outliers beyond +/- 1.5 IQR as individual dots. Ammen Rock sites on Cashes Ledge are colored orange while data from coastal sites are purple. These graphical representations are repeated in all subsequent plots. Note that the y-axis ranges are different for each species. Grey shaded areas indicate sites that were not sampled in that year.
Fig 4.
Data shown are number of individual kelp stipes per 1.0 m2. Boxplots as in Fig 3.
Fig 5.
Temporal variation in average S. latissima density at Ammen Rock Site 1 during four sampling periods from 1987 to 2015.
A significant 36.2% reduction of average kelp density occurred between 1987 and 2015 (S4 Table). The low kelp densities in 2012 occurred during anomalously warm sea surface temperatures (Fig 2, S1 and S2 Figs) when the kelp plants were extensively covered by the invasive bryozoan Membranipora membranacea (Fig 6E, S5 Table). The boxplots display the median value of the data as a horizontal bar, the first and third quartiles of the data (25th and 75th percentiles) as a shaded box around the mean, 1.5x the interquartile range (IQR) as a vertical line, and all outliers beyond +/- 1.5 IQR as individual dots. The diamond symbol represents the mean value.
Fig 6.
Kelp communities in the coastal zone and offshore on Cashes Ledge.
A. view of common macroalgae at a coastal site, Mingo Rock. Blades of the dominant shotgun kelp Agarum clathratum are visible in addition to the red algae Euthora sp. and the invasive Dasysiphonia japonica. B. Side view of Saccharina latisssima forest at AR site 2 on Cashes Ledge, showing the tall stature and complexity of the habitat created by this foundation species. C. View of S. latissima forest at AR site 1 showing understory red algae and small colonies of the ascidian Aplidium constellatum. D. Patch of Saccharina digitata kelp at AR site 2 in 2015. S. digitata density is inversely related to that of S. latissima, suggesting interspecific competition. Small fish in the background are cunner (Tautogolabrus adspersus). E. Photo of a deteriorating assemblage of S. latissima kelp at AR1 site during the anomalously warm summer of 2012. Note the high incidence of kelp covered with white colonies of the invasive bryozoan Membranipora membranacea, kelp stipes lacking blades and abundant copper colored fish (cunner). Photo credits: Fig 6A, B and D by Brett Seymour, Fig 6C and E by Brian Skerry.
Table 2.
Percent cover of macroalgae at Cashes Ledge sites (AR1, AR2) in 2014 and 2015.
Data are means and standard deviations (in parentheses) of the kelp canopy assessed by “freeze frame” images sampled at random locations along 30 m long video transects at 12–15 m depth. D. aculeata is the fucoid alga Desmarestia aculeata. Each image sampled measured 2.6 to 3.0 m2 in 2014, and 3.0 to 3.4 m2 in 2015.
Table 3.
Percent cover of the invasive red alga Dasysiphonia japonica at the study sites.
Data represent the mean +/- one Standard Error (SE) percent cover. Sample sizes are numbers of 0.25 m2 area photo quadrats analyzed. Data are from 2015 unless noted otherwise.
Fig 7.
nMDS plot indicating differences in benthic community structure (sessile invertebrates, understory algae) between coastal and offshore sites on Cashes Ledge (Ammen Rock 1, 2, Ammen Rock 1 1987).
Note the spatial segregation between coastal sites (colored purple) and offshore sites (colored orange), and the shift in the composition of the benthic community at the Ammen Rock sites between 1987 and 2015, when Ammen Rock 1 and 2 sites were sampled. Each individual data point represents the benthic community sampled in a 0.25 m2 quadrat. Replication consisted of the following number of photo quadrats per site: LI 31, SS 24, SI 24, MR 38, AR1 43, AR2 24 and AR1 1987 24.
Fig 8.
Fish biomass standardized to 10.0 x 5.0 m census areas for the three most abundant species: cod, pollock, cunner, comprising > 99.7% of total fish biomass.
Grey bars indicate sites that were not visited in a specific year. Boxplots as in Fig 3. Note different scales of y-axes.
Fig 9.
Abundance (maximum number of individuals observed in a single frame–MaxN–during a 10-minute segment) of the three fish species observed in stationary video deployments.
Data are compiled for all years (2014–2016) to facilitate coastal to Cashes Ledge comparisons. Boxplots as in Fig 3. Note different scales on y-axes.
Fig 10.
Temporal variation in fish abundance on Cashes Ledge 1987–2016.
Data shown are MaxN per 10-minute video segment [54]. Coastal sites are not shown since data are only available for 2015. Boxplots as in Fig 3. Note different scales on y-axes.