Since its first observation in the Gulf of Maine (Northwest Atlantic) in 1987, the epiphytic bryozoan Membranipora membranacea has become the dominant epiphyte on laminarian kelps. This note describes changes in the structure of a kelp bed at Cape Neddick ( M a n e , USA) after the coincident increase of M. membranacea, evaluates the potential causes of the observed changes, and documents the shortterm recovery of the kelp bed. Percent cover, length and density of kelps decreased significantly during 1989 through 1991 The dispersion of Laminaria spp. within the kelp bed was clumped on each sampling date at a large spatial scale (meters), while the distribution of Laminana spp. changed from a random pattern to a clumped d~st r ibut~on on a smaller spatial scale (0.25 m'). There were no consistent differences in storm intensity between years; densities of herbivores within the kelp bed were low and also have not changed between years. The coverage of M. membranacea on laminarian kelps increased 3-fold from 1989 to 1990, and the total coverage of other epiphytes decreased. It appears that the presence of M. membranacea on kelps has contributed to the defoliation of the kelp bed at Cape Neddick. This phenomenon may have important consequences to organisms that utilize kelps as habitat and shelter In fall 1990 we noticed changes in kelp beds in the southern Gulf of Maine (Northwest Atlantic) at the Isles of Shoals, and off the coasts of New Hampshire and southern Maine, USA, that consisted of increased abundances of the epiphytic bryozoan Membranipora n~embranacea and the decrease in both density and size of kelp plants within these beds. The physical structure of these beds appeared to change dramatically from being dominated by large laminarian kelps to containing many broken stipes with few intact laminae. We decided to focus our studies on a previously persistent kelp bed at Cape Neddick, Maine, since this kelp bed had been under monthly observaContribution No. 263 from the Center for Marine Biology " Present address: School of Biological and Medical Sciences, Gatty Marine Lab, University of St. Andrews, St. Andrews KY16 8LB, Scotland. UK tion since 1985. After a series of fall storms passed through the area, kelps within the kelp bed retaining intact laminae appeared to be smaller and relatively free of epiphytes, while the majority of drift kelps stranded on nearby beaches were covered by the introduced bryozoan M. membranacea. We hypothesized 3 potential causes to account for the changes observed within the kelp bed: (1) grazing by herbivores (Strongylocentrotus droebachiensis and Lacuna vincta), (2) differences in the severity of fall storms between 1989 and 1990, and (3) increased epiphyte abundances, particularly M. membranacea since 1989. The bryozoan Mernbranlpora membranacea was first observed in the Gulf of Maine in 1987 (Lambert 1990), and has since become the dominant epibiont on laminarian kelps (Berman et al. 1992). In the Gulf of Maine M. membranacea primarily recruits during the summer and once established on kelps, it often grows to cover entire surfaces of blades (Berman unpubl.). This note documents the changes in the structure of a kelp bed after the invasion by Membranipora membranacea, presents an evaluation of the potential causes of the change, and documents the short-term recovery of the kelp bed. Our data indicate that the defoliation of the kelp bed was not consistent with differences in the intensity of storms between years, but did coincide with a dramatic increase in M. membranacea. Therefore, we suggest that the heavy encrustations of M. rnembranacea may have enhanced the susceptibility of kelp fronds to storm damage and infuenced the abundances and identities of organisms that utilize kelps as habitat and shelter. Materials and methods. Field work was conducted from June to October, 1989 and 1990, and in May and July 1991 in a kelp bed at Cape Neddick, York, Maine, USA (43"101N, 70°36' W). The site has large granitic outcrops covered with algae to a depth of 10 m MLW, and below 10 m the substratum is sand. Several age O Inter-Research 1992 304 Mar. Ecol. Prog. Ser. 88: 303-307, 1992 classes of laminarian kelps (Laminaria saccharina and L, digitata) dominate the kelp canopy and these species have been abundant and persistent at this site for at least 27 yr (Mathieson unpubl.). Percent cover, length of intact lamina, and density of kelps were enumerated in randomly placed 0.25 m2 quadrats. Randomization was accomplished by pairing random compass headings and distances from a haphazard starting point. Randomized quadrats only contained Laminaria saccharina. Percent cover of the kelp canopy was determined by recording the presence of kelps under each of 40 points within quadrats. Length was determined as the total distance between holdfast and tip of lamina, and length measurements were taken of all kelps within each quadrat. The dispersion of kelps within the kelp bed was determined at 2 spatial scales. T-square sampling was used to test for random spatial patterns at a relatively large scale (meters) using Hines' test statistic for randomness (hT) (Krebs 1989). Smaller scale spatial patterns were examined by enumerating the number of kelp plants in 0.25 m2 quadrats, and then using the standardized Morisita index (Id) to test for random distributions (Krebs 1989). Weather data for the southern Gulf of Maine were obtained from the U.S. National Climatological Data Center. The site is moderately exposed to waves and wind from the Northwest to East, thus we examined variation in wind speed and number of days the wind blew from these directions between seasons and years to compare the intensity of storms in 1989 when the kelp bed persisted and in 1990 when it was defoliated. The percent cover of epiphytes (the bryozoans Membranipora membranacea, Electra pilosa and the hydroid Obelia geniculata) on large kelps (> 800 cm2) was quantified by randomly collecting 10 blades on 3 occasions during September and October 1989 and on 4 occasions during September to November 1990. We documented the coverage of all epiphytes on kelps to determine whether their abundances were affected by the invasion of M. membranacea. The outlines of each kelp blade and attached epifauna were traced onto paper and their areas were determined by digitlzlng them with a graphics tablet. Analyses of variance were used to assess differences among seasons in percent cover, blade length and density of kelps, percent cover of epiphytes, and wind velocities. Quadrat counts were log(x+ l ) transformed and proportions were arcsine transformed to correct unequal variances (Bartlett's test, p < 0.05) (Zar 1984). Post-hoc multiple comparisons on percent cover of epiphytes were performed using Tukey's HSD test and unequal sample sizes were corrected with a TukeyKramer adjustment (Wilkinson 1990). Multiple MannWhitney U-tests with a Bonferonni adjustment to protect the experiment-wise alpha (Wilkinson 1990) were used to analyze data on epiphytes that did not conform to assumptions of least squares analysis. Results. The structure of the kelp bed at Cape Neddick changed dramatically from 1989 to 1991. In summer 1990 the percent cover of kelps on the rocky substrata, and the length of kelps was greater than on other sampling dates (Fig. lA,B) . In addition, the density of intact kelps declined from summer 1990 to spring 1991. However, unlike the pattern exhibited for percent cover and length of kelps, the density of kelps in summer 1991 returned to a value similar to that observed in summer 1990 (Fig. 1C). The dominant organisms on primary substrata immediately after the observed changes were filamentous red algae and crustose coralline algae, which comprised the previous understory species within the kelp bed. The dispersion of kelps did not change and was clumped at each sample date at the large spatial scale (summer 1990: hT = 1.72, p < 0.005; fall 1990: hT = 1.98, p < 0.001; spring 1991: hT = 1.62, p < 0.005). However, at the smaller scale, the spatial pattern changed over time; kelp plants were randomly dispersed (h = 0.417) in summer 1990, but clumped in fall 1990 ( I , = 0.518) and spring 1991 (h = 0.558). Grazers of kelps were scarce within the kelp bed during the study period. Sea urchins averaged 0.47 ind. m-2 shortly after the defoliation of the kelp bed and Lacuna vincta averaged 0.16 snails (SE = 0.08) per kelp blade (n = 25) during June to August 1989. Weather patterns did not differ significantly in summer and fall between 1989 and 1990. Wind speed was significantly greater in the fall than in the summer of each year, and this difference was greater in 1990 than in 1989 (Table 1). There was no significant difference in the number of days the wind blew from the NW to E in the summer and fall of 1989 and 1990 ( X 2 = 0.092, p = 0.762). The abundance of Mem branipora rnem branacea on kelps increased 3-fold from 1989 to 1990. In September 1990 a significantly greater amount of M. membranacea (52.5 O/O + 8.5) covered the surfaces of kelps at Cape Neddick than at any other time (Tukey's p = 0.002). In October and November 1990 after a series of fall storms passed through the area, only 22.7 % f 6.5 of surfaces of kelps were covered by M. membranacea; this was not significantly different than the amount of M. membranacea (21.2 O/O ? 3.9) on kelps in fall 1989 (Tukey's p = 0.973) (Flg. 2). The combined coverage of Electra pilosa and Obelia geniculata decreased (Fig. 3), but only the abundance of 0. geniculata showed a significant decline between years (p = 0.013). Discussion. Since its first observation in the Gulf of Maine in 1987 the epiphytic bryozoan Membranipora membranipora has become the dominant epiphyte on Lambert et al.: Introduced species alters kelp bed 305 Fig. 1. Laminaria spp. Mean percent cover (f SE), length ( f SE) and density (2 SE) of kelp on rocky substrata in randomly sampled 0.25 m2 quadrats in 1990 an
Changes in the structure of a New England (USA) kelp bed: the effects of an introduced species?
Published 1993 in Oceanographic Literature Review
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1993
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Oceanographic Literature Review
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1993-07-01
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Biology, Environmental Science
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