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ABSTRACT: In the Northwest Atlantic, overgrowth of the competitively dominant, native kelps by an invasive bryozoan Membranipora membranacea increases frond erosion, which has facilitated the establishment and spread of the invasive macroalga Codium fragile ssp fragile. To document the spread of both introduced species along the Atlantic coast of Nova Scotia from initial introduction points
(the ‘epicentre’) southwest of Halifax, we conducted video-surveys of shallow rocky habitats along the southwestern shore
of Nova Scotia (100km linear distance, encompassing the range of M. membranacea) in 2000, and then along the entire Atlantic coast in 2007 (650km). Membranipora membranacea was observed continuously throughout the surveyed ranges in 2000 and 2007, wherever kelps were present, suggesting natural
dispersal via planktonic larvae. Codium fragile was observed along 95km of the surveyed range in 2000 and along 445km in 2007, with a relatively patchy distribution beyond
the epicentre, suggesting a combination of natural and anthropogenic dispersal mechanisms. Rockweed-dominated (Fucus spp.) or mixed algal assemblages common outside the epicentre may alter the interaction between M. membranacea and C. fragile, since seaweeds other than kelp are not subject to defoliation by the bryozoan. Percent cover of kelp at the epicentre generally increased from 2000 to 2007, while that of C. fragile generally decreased. Codium fragile was the dominant canopy alga at 54% of sites in 2000 and at only 15% of sites in 2007. These findings indicate that, at near
decadal timescales, C. fragile does not prevent re-colonization by native kelps.
KeywordsInvasive species-Range expansion-Spread-Facilitation-Seaweed-Bryozoan
Biological Invasions 04/2012; 12(7):2329-2342. · 2.90 Impact Factor
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ABSTRACT: Distribution, abundance and demographic composition of populations of Oreaster reticulatus (Linnaeus, 1758) were sampled in mangrove, fringing reef, seagrass, and sand habitats at sand cayes off the southern coast of Belize. Density and biomass were 1.7-18.3 ind 100 m−2 and 0.9-5.5 kg 100 m−2 among six sites. Dispersion was random, except at a fringing reef where sea stars were aggregated at our sampling scale (15 m2). Mean individual size (radius, R) ranged from 9.7 to 13.7 cm among sites. Populations in mangrove and reef habitats were mainly juveniles (R < 12 cm: 83% and 91% respectively), with characteristic cryptic coloration. Juveniles occurred primarily along mangrove banks and were closely associated with coral colonies in fringing reefs. The proportion of juveniles also was high on a sand flat (54%), although juvenile color morphs were not observed there. Most sea stars had their stomach everted on sediments or detrital matter, but sponge and ascidian prey also were consumed in a mangrove habitat. Mangrove and fringing reefs, like dense seagrass beds, may provide a protective environment for recruits that undergo an ontogenic shift to adult habitats, such as sand flats, as they mature reproductively. Our findings are comparable with previous studies of O. reticulatus in Panama and suggest that levels of recruitment in Central America may be higher than in other parts of the Caribbean.
Bulletin of Marine Science -Miami- 12/2009; 86(1):133-148. · 1.15 Impact Factor
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ABSTRACT: Protoreaster nodosus (Linnaeus, 1758) is a common sea star in shallow sand and seagrass habitats in the tropical Indo-pacific region. It has been extensively harvested as an ornament, but little is known of its biology and ecology. We quantitatively sampled populations at five sites in palau to obtain baseline information on abundance, spatial distribution, and population structure of this species in areas unaffected by human depredation. Density on sand bottoms ranged from 4 to 32 individuals 100 m-2 and biomass from 3.1 to 20.7 kg 100 m-2. Population size estimates at two locations were 100 and 630 individuals. Mean size (radius) of individuals ranged from 14 to 16 cm among populations, and was negatively related to density. Density was greatest in a seagrass bed (51 individuals 100 m-2, 12.3 kg 100 m-2) where the population was composed of juveniles (8-12 cm). Spatial distribution of sea stars in each population was random, as indicated by nearest-neighbor distance. Manipulations of density and spatial distribution, performed by removing or by adding and centrally aggregating individuals in circular plots (78.5 or 314 m2), showed that sea stars reestablish pre-manipulation densities and nearest-neighbor spacing within 2-5 d. Protoreaster nodosus feeds on meiobenthos and microbial/microalgal films by extra-oral ingestion of sediments and seagrass. The incidence of daytime feeding is generally high (> 60% of individuals), suggesting these sea stars are important consumers and bioturbators in sedimentary habitats. Synchronous spawning occurred at full moon (in May) in the laboratory, yielding planktotrophic larvae.
Bulletin of Marine Science -Miami- 02/2008; 82(2):221-235. · 1.15 Impact Factor
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ABSTRACT: Repopulation by green sea urchins Strongylocentrotus
droebachiensis of a steeply sloping rock bottom was monitored at a wave-exposed headland (Chebucto Head) following a disease outbreak that caused mass mortality in September 1999. Density and size of urchins were sampled in four depth strata: at 8–10 m in an urchin grazing aggregation (front) along the lower margin of a kelp bed, at 12 m and 16 m on a bedrock ramp, and at 24 m on a cobble and boulder field where urchins were unaffected by the disease. Shoreward migration of adults along the ramp from the surviving population at 24 m was the primary means of repopulation, which was augmented by recruitment via planktonic larvae. At 16 m, urchin density stabilized (at [similar]50 urchins m−2) within six months of the die-off while repopulation at 12 m took more than eight months. A grazing front of large urchins (40–60 mm, test diameter) had formed along the lower edge of a kelp bed by January 2002, which reached densities of up to 284 urchins m−2. Video surveys at Chebucto Head and two adjacent locations of similar bathymetry revealed an extensive urchin population between 25 and 55 m depth, with a mean density on rocky substrata of 73 urchins m−2.
Journal of the Marine Biological Association of the UK 11/2005; 85(06):1511 - 1517. · 1.00 Impact Factor
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ABSTRACT: We examined the relative importance of bottom-up (nutrient availability) and top-down (grazing) factors in regulating phytoplankton assemblages in tidepools on a rocky shore near Halifax, Nova Scotia, Canada. We manipulated the concentration of nutrients and density of micrograzers in pools in the h ~ g h intertidal and splash zones in 3 repeated, 1 to 2 wk experiments in November 1992 and June and August 1993 For each expenment, we set up 4 orthogonal treatments in enclosures in each of 3 or 4 pools: (1) micrograzers removed and nutrients enriched, (2) micrograzers removed and nutrients at natural levels, (3) micrograzers at natural densities and nutrients enriched, and (4) both micrograzers and nutrients at natural levels. For each treatment, we measured the change in abun-dance over 1 wk intervals of 5 taxonomic groups of phytoplankton: centric diatoms, pennate diatoms, cryptomonads, prasinophytes, and chlorophytes. We examlned the effects of nutrient concentration, grazer denslty, and pool on the phytoplankton assenthlage uslng multivariate analysis of variance. There were significant effects of grazer density in June and August, and of nutrient concentration in August, which varied among phytoplankton groups and tidepools. In 1 pool in June, reduction in grazer density had a negative effect on pennate diatoms, cryptomonads and chlorophytes in the first week of the experiment, but a posihve effect on pennate diatoms In the second week. In another pool in the second week, grazer reduction had a positive effect on prasinoph.ytes but a negative effect on chloro-phytes. In 1 pool in August, nutrient enrichment had a posltlve effect on pras~nophytes in the first week of the experiment, and grazer reduction had a negative effect on cryptomonads and chlorophytes in the second week. In another pool in the first week, nutrient enrichment had a negative effect on chloro-phytes. Based on the frequency of significant effects, we concluded that grazing is more important than the nutrient regime in regulating phytoplankton assemblages in tidepools. The large variability among tidepools in the response of phytoplankton to our manipulations supports our previous suggestion that regulation of these assemblages occurs at the scale of the individual pool rather than the intertidal zone.
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ABSTRACT: In any ecological system, the factors that regulate the abundance of species vary with spa-tial scale; therefore, the sources of spatial variability should be described. We examined different sources of variability in the spatial distribution of phytoplankton assemblages and biotic (e.g. plank-tonic and benthic micrograzers, mussels) and abiotic (e.g. nutrients, temperature, salinity, pH) factors that may regulate these assemblages in 4 tidepools a t each of 3 intertidal zones (mid, high and splash) on a rocky shore in Nova Scotia, Canada, over a period of 15 mo. Stratum (defined as the depth within a pool) was a significant source of variability, particularly for pennate diatoms which were consistently more abundant near the bottom of pools. There was no indication of vertical zonation of the phyto-plankton assemblages along the intertidal gradient, and differences among zones rarely explained more than 30':: of the spatial vanabillty in phytoplankton abundance. Also, among-zone variation was not apparent for the biotic and abiotic factors. We suggest that among-zone variability in these factors does not adequately explain vertical variability in phytoplankton assemblages. All groups of phyto-plankton vaned significantly among pools within intertidal zones on most sampling dates, and differ-ences among pools explained up to 96% of the variability In phytoplankton abundance. Furthermore, there was significant variability among pools within zones for all biotic and abiotic characteristics of the pools on most sampling dates. We detected significant relationships between the density of benthic micrograzers and small mussels, and the concentration of nutrients in individual pools with the abun-dance of pennate diatoms, cryptomonads and chlorophytes Among the abiotic characteristics of the tidepools, there was a significant relationship between flushing rate and temperature of individual pools, with the abundance of cryptomonads and chlorophytes. We suggest that the factors that regulate phytoplankton assemblages in tidepools probably operate more a t the scale of the individual pool rather than the intertidal zone.
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ABSTRACT: Since its introduction in 1989, the invasive green alga Codium fragile ssp. fragile (formerly Codium fragile ssp. tomentosoides) has spread rapidly in Atlantic Canada. Although its spread has likely been facilitated by human transport, C. fragile possesses diverse modes of natural dispersal. In addition to parthenogenetically developing swarmer cells, it can propagate through the release of vegetative buds, thallus fragments, and entire, dislodged thalli. We examined the natural dispersal potential of these propagules using a combination of field and laboratory experiments. Vegetative buds were most abundant on thalli in late August, coinciding with increasing late summer wave activity. At two locations within two field sites, we examined the effects of tidal state, wave action and topography on the dispersal of fragments and intact thalli. Over a 4-hour period, retention rate was generally lower and dispersal distance of retained propagules higher: during flood than ebb tide; at the more wave-exposed locations; and for fragments than thalli. Dispersal direction corresponded with topography or flood tide currents at certain locations, but was often bi-directional or random, suggesting an added role of wave action. Over periods of weeks, the retention rate of marked detrital thalli was negatively related to the magnitude of wave activity. In the laboratory, settling rate was negatively related to propagule length, and large buds had higher critical shear erosional velocities than either small buds or fragments. Our results indicate that large propagules of C. fragile, such as intact adult thalli, have a generally low dispersal potential (meters), but may be transported longer distances (kilometers) during storms or when positively buoyant. Fragments and buds are regularly transported 10's of meters at a time at average flow velocities. However, they also become resuspended in average flows, and probably disperse longer distances over multiple resuspension events, or when positively buoyant. Once settled, the smallest propagules may be less easily resuspended or transported along the bottom as they become entrapped by small-scale topographic features or turf algae. The wide variety of propagules produced by C. fragile and the variable distances over which different propagule types may be transported give C. fragile the advantage of both short- and long-distance dispersal, and have likely played a role in the invasive success of this alga.
Journal of Experimental Marine Biology and Ecology 381(2):114-125. · 1.88 Impact Factor
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ABSTRACT: We examined the relative importance of parental nutritional condition and larval food ration on the rates of development, growth and metamorphosis of larvae of Strongylocentrotus droebachiensis (Müller) in a laboratory experiment. Parents were reared for 22 months on either a high ration of kelp (Laminaria spp., 6 days week−1) supplemented with mussel flesh (Mytilus spp., 1 day week−1) (KM), or a low ration of kelp (1 day week−1) (KL). Larvae were fed either a high ration (5000 cells ml−1) or a low ration (500 cells ml−1) of microalgae (Dunaliella tertiolecta). Larval food ration had a strong effect on the rates of development, growth, and metamorphosis, which were all significantly greater in larvae fed the high ration. Test diameter of settlers also was significantly greater in the high than the low ration. Parental nutritional condition had little or no effect on the rates of development and growth, and no effect on settler size. The rate of metamorphosis was significantly higher in larvae from the KM than the KL treatment in the high but not the low ration (where rates of metamorphosis were similar). Although parental condition generally had a small effect on larval development, our results suggest that when planktonic food is abundant, larvae of adults from nutritionally rich habitats (such as kelp beds) may metamorphose sooner than those of adults from nutritionally poor habitats (such as barrens).
Journal of Experimental Marine Biology and Ecology.
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ABSTRACT: Following a disease outbreak that caused mass mortality of green sea urchins, Strongylocentrotus droebachiensis, along the Atlantic coast of Nova Scotia in September 1999, changes in growth and reproduction were monitored over 3.75 years as surviving individuals migrated from deep water to repopulate the shallow subtidal zone at a wave-exposed site. Urchins were sampled at 4 depth strata: at 24 m on a boulder field where the population was unaffected by the disease, at 12 and 16 m on a steeply sloping bedrock ramp, and at 8–10 m along the lower margin of a kelp bed (Laminaria digitata) where urchins formed a grazing front by January 2002. Urchins migrating shoreward from the deep-water refuge responded rapidly to increased algal productivity in the shallows through increased growth and reproduction. Measures of annual increments of skeletal elements (rotules) from urchins across the depth gradient indicated that the fastest growing individuals from the source population formed the grazing front. Urchins in the front reached a larger asymptotic size and produced larger gonads than urchins lower on the ramp. The annual cycle in gonad index showed a pronounced spring spawning period across all depths; a secondary fall spawning was evident at the front and 12 m. The presence of mature, fertilizable ova and short response time to spawning induction in both spring and fall supported the occurrence of two spawning periods.
Journal of Experimental Marine Biology and Ecology.
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ABSTRACT: We examined the effects of native kelps, Laminaria longicruris de la Pylaie and L. digitata (Hudson) Lamouroux, and of the invasive alga, Codium fragile ssp. tomentosoides (Van Goor) Silva, on the composition and abundance of mobile benthic macrofauna and of turf algae by measuring the response of these assemblages to experimental removal of the respective macroalgal canopy. From June 2003 to November 2004, we censused macrofauna and measured canopy cover within 4 × 10 m strips of alternating Canopy Intact (control) and Canopy Removed treatments in both a Codium- and a Laminaria-dominated habitat in the rocky subtidal zone of a semi-protected embayment on the Atlantic coast of Nova Scotia, Canada. Macroalgal canopy cover fluctuated seasonally, peaking in September/October (69% cover in 2003; 55% cover in 2004) for Codium and in May 2004 (70% cover) for kelps, and with both canopy types reaching a winter minimum in January 2004 (22 and 28% cover, respectively). In both Codium and Laminaria habitats, significant effects of canopy removal on the overall macrofaunal assemblage were evident only during periods in which canopy cover in the Canopy Intact treatment was ≥ 50%. In the Codium habitat, 4 out of 11 characteristic taxa were more abundant in the Canopy Intact treatment, where taxonomic diversity also was higher. In contrast, 4 out of 11 characteristic taxa in the Laminaria habitat were more abundant in the Canopy Removed treatment and diversity was similar between treatments. Turf algae were sampled in November 2004 and, despite between-treatment differences in the light regime, there was no significant effect of canopy removal. Our results indicate that selection of algal habitats by mobile macrofauna is likely determined by the different shelter and foraging opportunities offered by these morphologically dissimilar ecosystem engineers.
Journal of Experimental Marine Biology and Ecology.
