Experiments were done on sheltered shores of Swan’s Island in the Gulf of Maine, USA (44°10’ N, 68°25’ W). The shores are dominated by stands of fucoid rockweeds (Ascophyllum nodosum and Fucus vesiculosus) and until recently, interspersed with patches of mussels (Mytilus edulis). Winter ice scour events periodically open up areas for colonization by algae and invertebrates, although winter ice scour events since 2003 have been increasingly rare because of warming in the Gulf of Maine.
Experimental clearings were established in 1996 – 1997 to mimic the effects of ice scour. Experimental clearings along with uncleared control plots were created at three sites within each of four bays for a total of 12 replicate sites. At each site, four circular clearings (1, 2, 4 and 8 m in diameter) were made. Plots within a site are separated by at least 10 m, and sites within bays by at least 750 m. Center of plots were marked with two stainless steel reference bolts which were anchored in drilled holes. GPS coordinates for each plot can are available online (Petraitis and Dudgeon 2023). We are currently in the process of setting bronze survey markers in the center of each plot (in Petraitis and Dudgeon 2023, see Marker_close_up.jpeg in Other Entities for photograph of and details about the markers).
The size range of the experimental clearings is within the normal range of major ice scour events. For example, ice removed an average of 13% of the A. nodosum cover from the 12 experimental sites during the winter of 2002-2003 (range: 0 – 39%, P. S. Petraitis, unpublished data). At the three sites with the greatest damage, the average area of naturally created clearings was 25.4 sq. m. (range: 2.4 – 78.5 sq. m., n = 5 clearings per site). The experimental clearings are 0.8 - 50.3 sq. m. in area.
While clearings were made to mimic the effects of ice scour, it was not possible to make the clearings during the winter because of limited access and dangerous conditions due to snow and ice. Thus, plots were originally scraped between 22 June and 17 August 1996 and then re-scraped between 6 and 10 February 1997 to mimic winter ice events. Scraping was done using paint scrapers and removed all A. nodosum.
As part of our larger project, we re-scraped about half of the plots in the winter of 2010 – 2011 to examine if succession in these plots followed similar trajectories as observed from 1997 to 2008. We used three criteria to select which plots to re-scrape. First, we attempted to re-scrape no more than half the plots in each bay to control for variation among bays. Likewise, we re-scraped half of the plots at each level of clearing size. Plots were initially re-scraped in August 2010 and then scraped again in early March 2011 to mimic winter ice scour. Plots that were re-scraped are identified in a separate data table (List of plots that were re-scraped).
Counts of barnacle, fucoid and mussel recruits were carried out by various people over the years. While all counts was overseen by P. S. Petraitis, E. C. Rhile or S. R. Dudgeon, there is danger of observer bias among years. Barnacle counts were the easiest to do and least likely to have a bias.
The plates used for barnacle recruitment were resin castings made from latex molds of natural granite rock (Fig. 1 in Other Entities ). Plates were disk-shaped; approximately 0.67 cm thick and 7.1 cm in diameter (39.6 sq. cm. in area). Barnacle recruitment in Maine begins in March and continues through May, and so plates for barnacle recruitment were usually placed into the plots in late March (median date: 28 March; range: 4 March – 17 May) and usually collected in late May (median date: 22 May; range: 7 April – 24 June) for a median duration of 56 days (range: 27–84 days). A complete count of cyprid larvae, newly metamorphosed spat and dead individuals (i.e., empty tests and dead cyprids) each plate was done within a day or two of collection using a dissecting scope at 25×. Data file provides counts per plate of each. Barnacle recruitment was not sampled in 2009, 2010 and 2020.
White ceramic tiles (approx. 5.0 × 5.0 × 0.6 cm) were used for collect recruits of the brown algae Fucus vesiculosus and Ascophyllum nodosum (Fig. 1). Surface of tiles had a grid of grooves that divided the tile into 0.5 × 0.5 cm squares forming a 9 × 9 grid. Grooves were approximately 0.5 mm deep and 0.5 mm wide, and tended to trap developing embryos. A. nodosum recruitment in eastern Maine near Swans Island occurs over 3–7 weeks in spring (Bacon and Vadas 1991). F. vesiculosus releases gametes in late April – early May and recruitment peaks in May – June; however, individuals remain reproductive until October. For most years, recruits of the two species were lumped together because it is impossible to distinguish embryos and extremely difficult to distinguish young germlings. However, in 1997, we are confident the counts are nearly all A. nodosum for three reasons (Dudgeon and Petraitis 2001). First, F. vesiculosus adults were extremely rare prior to 2002 and so the source for zygotes was very limited. Background percentage cover in 1999 averaged 4.8% excluding the July 4th site (JL), which had 52% F. vesiculosus cover. Second, tiles in 1997 were taken in prior to the peak of F. vesiculosus recruitment. And finally, F. vesciulosus germlings have apical hairs and A. nodosum germlings do not; nearly of the germlings in 1997 lacked apical hairs By 2002, F. vesiculosus had invaded most clearings and formed large beds of mature reproductive individuals (P. S. Petraitis, personal observation), and it is likely most of the recruits in large clearings after 2002 were F. vesiculosus.
Tiles were usually placed out in late March (median date: 29 March; range: 4 March – 16 April) and usually collected in late May (median date: 22 May; range: 3 May – 24 June) for a median duration of 55 days (range: 32–84 days). Collected tiles were kept moist, and the number of embryos and germlings were counted in five 0.5 × 0.5 cm squares (i.e., the flats) and five 0.5 cm grooves (i.e., the grooves) per tile using a dissecting scope. Squares to be counted were randomly chosen; for each square, one of the grooves that bordered the square was then chosen. The total number of recruits per five squares (1.25 cm2 in total area) and per five grooves (1.25 cm in total length) are reported. Fucoid recruitment was not sampled in 1999, 2009, 2010 and 2020.
Settling mussel larvae tend to attach initially to filamentous algae and so fibrous pads were used as substrates for recruitment. Pads were approximately 5 × 5 cm and cut from furnace filters (range of fiber diameter = 0.3 – 0.5 mm). Pads were wrapped in a 5 cm × 13 cm piece of plastic mesh (width of mesh opening: 4.6–5.0 mm; diameter 1.5 mm), which was closed with two cable ties (Fig. 1). Pads both erode and trap sediment during emersion and so in some years, they were weighed after collection. On Swans Island, mussels begin to recruit in June, peak in July and early August, and continue through September (Bethel 1973, Petraitis 1991). Pads were usually placed out in late May (median date: 23 May; range: 5 May – 11 July) and usually collected in late August (median date: 25 August; range: 8 August – 22 August) for a duration of 90 days (range: 66–115 days). Upon collection, pads were preserved in 70% isopropyl alcohol.
For counting, pads were torn open and rinsed in water over 425 µm and 300 µm sieves. Pads were then dried and weighed. In 1997, counts were collected as 10 circular fields, 1 cm in diameter for a total area of 7.85 sq. cm. (Dudgeon and Petraitis 2001). In most years, mussels trapped on each sieve were rinsed onto a tray with a 2 × 2 cm grid of 32 squares (4 × 8), and mussels were counted in five (2000 and 2001) or ten (2003 – 2014) randomly chosen squares. In 1999 and 2015 – 2021, mussels in the entire pad were counted. In 2004, mussels in both the entire pad and ten 2 x 2 cm squares were sampled; these data are provided as a separate file (see Recruit_sub_vs_all_v4.csv). Mussel recruitment was not sampled in 1998, 2002, 2009 and 2010.
The column labeled "M_area" gives the area sampled, which can be used to standardize the counts per area or as an offset in a Poisson or quasiPoisson model. For years in which the pads were subsampled, the area and the entry for M_area is either 7.85, 20 (five 2 x 2 cm squares sampled) or 40 (ten 2 x 2 cm squares sampled). For years in which mussels in the entire pad were counted, effective area sampled can be set by the user, and so the entry for M_area is coded as -99. The 2004 data in data table "Mussel recruitment: subsamples and entire counts" allows one to estimate the effective area sampled in the entire tray by regressing the count for the entire pad on the count for 40 sq. cm. Using ranged major axis regressions (Legendre 2024, Warton et al. 2012), the estimates and 95% confidence limits of effective sampling area in sq. cm. for mussels trapped on the 300 µm sieve, the 425 µm sieve and the sum of the two counts are: 107 (99 – 116), 106 (98 – 115) and 107 (104 – 110), respectively. These estimates can be used to replace the -99 entries in the M_offset column. R script for calculating these estimates is provided in Other Entries. If desired, users can use the 2004 data and other approaches to calculate the effective area sampled.
References
Bacon, LC, and RL Vadas. 1991. A model for gamete release in Ascophyllum nodosum (Phaeophyta). Journal of Phycology 27:166–173.
Dudgeon, SR, and PS Petraitis. 2001. Scale-dependent recruitment and divergence of intertidal communities. Ecology 82:991–1006.
Legendre, P. 2024. lmodel2: Model II Regression. https://CRAN.R-project.org/package=lmodel2.
Petraitis, PS, and SR Dudgeon. 2023. Densities and cover data for intertidal organisms from an LTREB project in the Gulf of Maine, USA, from 1996 to 2023. ver 2. Environmental Data Initiative. https://doi.org/10.6073/pasta/04bf8647c5bb39025aeeecae2e31d404 (Accessed 2024-12-09).
Warton, DI, Duursma, RA, Falster, DS, and S Taskinen. 2012. smatr 3 - an R package for estimation and inference about allometric lines Methods in Ecology and Evolution, 3(2), 257-259
