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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 L. (Le Jolis) and Fucus vesiculosus L.) and until recently, interspersed with patches of mussels (Mytilus edulis L.). Winter ice scour events periodically open up areas for colonization by algae and invertebrates.

In 1996-97, experimental clearings were established 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. 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. Center of plots were marked with two stainless steel reference bolts which were anchored in drilled holes. We are currently in the process of setting bronze survey markers in the center of each plot; 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, which occur infrequently on Swan’s Island. 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of the A. nodosum cover from the 12 experimental sites during the winter of 2002-2003 (range: 0 – 39%, P. S. Petraitis, unpublished data).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 m2 (range: 2.4 - 78.5 m2, n = 5 clearings per site). The experimental clearings are 0.8 - 50.3 m2 in area.

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. Finally, Petraitis et al. (2009) classified plots having become mussel beds or rockweed stands, and so we attempted to re-scrape half the plots in each group. 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 column labelled Re_scraped (Yes = re-scraped; No = left intact).

An additional small clearing was made in August 2012 in large plots that were dominated by F. vesiculosus stands in 2012 to examine the resilience of F. vesiculosus to small perturbations. The small clearings were circular and 1 m diameter, and placed near the outside edge of the plot in six plots at five sites. All plots were 4 or 8 m in diameter and had fully established F. vesiculosus stands. Five had been re-cleared only 18 months previous in winter 2010-11, but the remaining plot had not been cleared since 1996-97. These small clearings are identified in column labelled F_scraped (Yes = small clearing was created; No = left intact).

Abundance and percentage cover of the most common species have been sampled at least once a year since 1996. Sampling was usually done in the summer (mid-June to mid-August) and the year code for these data (column label: Yr_code) ends with a "b" (e.g., Y1996b). In 1997, 1998, 2000, spring sampling also done and the year code ends with an "a" (e.g., Y1997a). For eleven years from 2004 to 2022, high school students from the Cheverus School in Portland, ME sampled five to six sites in the last week of May and the year code ends with "c" (e.g., Y2004c). From 1996 to 2003, five 50 x 50 cm quadrats per plot were sampled. For 2004 onwards, three quadrats per plot were sampled. Students from the Cheverus School usually sampled three quadrats per plot (range: 1-6). Quadrats were haphazardly placed near the center of each plot. For uncleared controls, quadrats were placed within 2 m of the stainless steel reference bolts. Uncleared control plots were not sampled in 1996. Data were not collected in 2009.

Abundance data were collected for mussels (Mytilus edulis), barnacles (Semibalanus balanoides (L.)), gastropods (Tectura testudinalis = Testudinalia testudinalis (Müller), Littorina littorea L., Littorina obtusata (L.) and Nucella lapillus (L.)), and fucoid algae (Ascophyllum nodosum and Fucus vesiculosus). Mussels and gastropods were counted in 50 x 50 cm quadrats. Fucoid algae were counted in the five 10 x 10 cm squares on the diagonal of each 50 x 50 cm quadrat because of the large number of recruits. Counts for fucoids were re-scaled to the number per 0.25 m2. The barnacle S. balanoides was counted in two 2 x 2 cm squares per 50 x 50 cm quadrat except in 1998 when one square per quadrat was used. Barnacle counts were divided into the current year’s recruits and older individuals, which could be distinguished by differences in size and color. For data collected after 2004, fucoids and mussels were also divided into young and old individuals (young fucoids < 1 cm, young mussels < 0.5 cm in length).

Cover was estimated by counting the most common space occupier in the twenty-five 10 x 10 cm squares per 50 x 50 cm quadrat. Percent cover was thus calculated on the basis of 125 (through 2003) or 75 (from 2004) counts of presence/absence (25 squares per quadrat x 5 [or 3] quadrats per plot). Initially, Ascophyllum nodosum, considered the normal canopy species, was counted first (range 0-100%). The canopy cover was then pulled back to reveal the common understory species (Fucus vesiculosus, Semibalanus balanoides and Mytilus edulis), which were then counted. From 1996 to 2000, A. nodosum and F. vesiculosus were rare because we completely removed these species during initial scraping done in 1996-1997. Thus, percentage cover by these species was not usually distinguished as being canopy or the surface. From 1996 to 2004, percentage cover by all other species, which were usually rare, or common in only a few quadrats, were lumped with the cover by unoccupied space. Starting in 2005, other species besides Ascophyllum nodosum were counted as both canopy and understory. Cover by other species and unoccupied surface were also split into separate entries.

Earlier archives of some of these data contain additional information of rare species (Petraitis and Vidargas 2006, Petraitis et al. 2008). The species tallied were: the red algae Chondrus crispus Stackhouse and Mastocarpus stellatus Stackhouse(Guiry), other algal species, the horse mussel Modiolus modiolus (L.), and other sessile invertebrate species.

Petraitis, P. S., H. Liu, and E. C. Rhile. 2008. Densities and cover data for intertidal organisms in the Gulf of Maine, USA from 2003 to 2007. Ecology 89:588, (http://esapubs.org/archive/ecol/E089/032/)

Petraitis, P. S., and N. Vidargas. 2006. Marine intertidal organisms found in experimental clearings on sheltered shores in the Gulf of Maine, USA. Ecology 87:796, (http://esapubs.org/archive/ecol/E087/047/).