Scheibling, Robert E.

Permanent URI for this collectionhttps://hdl.handle.net/10222/22316

Browse

Now showing 1 - 20 of 38

Open Access
Variations in Sampling Procedure and Frequency Affect Estimates of Recruitment of Barnacles
(1993-09) MINCHINTON, TE; Scheibling, Robert Eric
The influence of variations in sampling procedure and frequency on estimates of recruitment of the barnacle Semibalanus balanoides (L.) on an intertidal rocky shore at Sandy Cove, Nova Scotia, Canada was determined from approximately daily monitoring of recently settled individuals. The effect of sampling procedure (removing or not removing barnacles after sampling) on estimates of recruitment varied with intertidal height. There was no effect of sampling procedure in the mid-intertidal zone, but in the low zone, where recruitment was on average 4 times- larger, the estimate of recruitment was significantly greater from quadrats where barnacles had been removed after sampling. Estimates of recruitment and post-settlement mortality decreased exponentially as the sampling frequency decreased. In both the mid- and low-intertidal zones, significantly larger estimates of recruitment and post-settlement mortality were obtained when sampling every 1.3 d compared to sampling ca every 2 d. These findings indicate that comparisons of results between studies will be difficult if estimates of recruitment or post-settlement mortality are confounded by variations in sampling frequency. Unless individuals can be tracked over time, estimates of recruitment made by sampling without removal of recruits will tend to underestimate settlement by incorporating some post-settlement mortality, the magnitude of which will be directly related to the interval between samples. Without quantitative data on the effect of sampling frequency on estimates of recruitment and post-settlement mortality, results of tests of hypotheses requiring accurate estimates of recruitment or post-settlement mortality may be compromised if sampling is not done as frequently as possible.
Open Access
Fecal production by sea urchins in native and invaded algal beds
(2009) Sauchyn, Leah K.; Scheibling, Robert Eric
To examine the role of fecal production by the sea urchin Strongylocentrotus droobachiensis in native and invaded algal assemblages in Nova Scotia, Canada, we examined the quantity (production rate by urchin feeding aggregations, or 'fronts') and quality (organic carbon and nitrogen content) of feces egested on 3 diets: (1) a native kelp Saccharina longicruris, (2) kelp encrusted with the invasive bryozoan Membranipora membranacea, and (3) an invasive green alga Codium fragile ssp. fragile. Relatively low absorption of clean and encrusted kelp resulted in high fecal output from urchins grazing native and invaded kelp beds (74 and 81 g feces m(-1) front d(-1), respectively), In contrast, high absorption of C. fragile resulted in low fecal output from urchins grazing meadows of the invasive alga (31 g feces m(-1) front d(-1)). Low C:N ratios of feces obtained from grazing C. fragile (8.5) or encrusted kelp (11.4), compared to clean kelp (30.7), suggest that feces from invaded assemblages are a higher quality food source for microbes and detritivores. To compare dispersal characteristics of feces from each diet, we measured pellet shape, size, density, and settling and critical erosion velocities. Using a wave model, we estimated the depth at which feces would be deposited under seasonally varying wave conditions. We found that pellets of C. fragile are likely deposited at the greatest depths (33 to 55 m), pellets of clean kelp at intermediate depths (28 to 47 m), and pellets of encrusted kelp at the shallowest depths (22 to 40 m). Our findings suggest a smaller amount of higher quality feces enters the detrital food web at, greater depths from Codium meadows than from native kelp beds.
Open Access
Free-Space Availability and Larval Substratum Selection as Determinants of Barnacle Population-Structure in a Developing Rocky Intertidal Community
(1993-05) MINCHINTON, TE; Scheibling, Robert Eric
Variation in settlement and recruitment of the barnacle Semibalanus balanoides (L.) was experimentally investigated in the high, mid and low intertidal zones at 2 adjacent sites (A and B) on a rocky shore in Nova Scotia, Canada, following a rare occurrence of ice-scouring. Manipulations in 100 cm2 quadrats at each intertidal height involved removal of various components of the sessile macrofauna and macroflora, removal of the total community and a control treatment. The availability of free space on the substratum in each quadrat was measured prior to the onset of settlement to determine whether differences in the density of barnacles among treatments were due to differences in the availability of free space or to manipulation. Where the density of settlers was high, it was positively related to the availability of free space suggesting that settlement was a simple function of the availability of free space. However, the pattern of settlement varied from the onset to the end of settlement: quadrats from which only barnacles had been removed were occupied early in the settlement period and later-arriving larvae were restricted to less favourable sites where free space was available. Thus, settlement preferences may be masked when the supply of larvae is saturating or the duration of the selction experiment is too long. Where the density of settlers was low and free space was non-limiting, there was no relationship between the density of settlers and the availability of free space. In the high intertidal zone at Site A, the density of settlers was greater in treatments with ephemeral algae (wetter quadrats) than in those without (drier quadrats). In general, early post-settlement mortality in treatments where algae had been removed increased with intertidal height, whereas in treatments where algae were present it remained relatively constant among heights. Post-recruitment mortality did not differ significantly among treatments in the high intertidal zone, suggesting that factors which influence selection of the substratum by cyprid larvae and promote early post-settlement survival may be particularly important in determining subsequent population structure of barnacles in this zone. Post-recruitment mortality, mainly due to predation by whelks, was highest in the low intertidal zone at both sites and did not differ significantly between treatments. In the mid intertidal zone at Site B, whelk foraging appeared to be constrained by desiccation stress and post-recruitment mortality was highest in treatments with Fucus spp. Where predation is intense, initial selection of the substratum by cyprid larvae may have little effect on the subsequent population structure of barnacles.
Open Access
Spatial and Temporal Variability of Tidepool Hyperbenthos on a Rocky Shore in Nova-Scotia, Canada
(1994-05) METAXAS, A.; Scheibling, Robert Eric
We examined the distribution and abundance of the hyperbenthos in tidepools on a rocky shore, near Halifax, Nova Scotia, Canada, at approximately monthly intervals between March and November 1991, and between April and June 1992. We sampled 4 pools at each of 3 intertidal heights: mid and high intertidal and splash zones. Additional samples were taken at the sea surface for comparison. Calanoid and harpacticoid copepods, cladocerans, foraminiferans, nematodes and rotifers were the most abundant groups within our samples. Significant differences in the abundance of these groups among zones were detected on only 5 out of 12 sampling dates. Total hyperbenthos, as well as calanoid and harpacticoid copepods, nematodes and rotifers, generally were more abundant in the high and/or the splash pools. The abundance of total hyperbenthos varied significantly among pools within zones from May to October 1991 and in June 1992. Significant differences in abundance among pools within zones were also detected for harpacticoid copepods and nematodes on 3 sampling dates, for calanoid copepods and rotifers on 5 sampling dates, and for cladocerans and foraminiferans on 2 sampling dates. In most cases, the significant variability in abundance among pools arose within the high intertidal or the splash zones. Over the entire sampling period, the diversity of faunal groups was lower in the splash pools, possibly as a result of harsh conditions in these pools. The few groups that survived in these high pools, such as rotifers and the calanoid copepod Eurytemora affinis, reached densities of up to 106 ind. m-3. Dominant populations may be established in particular pools by founder effects and persist due to low flushing rates. Thus, variability among pools in the high intertidal and splash zones may exceed that observed among pools in lower intertidal zones.
Open Access
Spatial heterogeneity of phytoplankton assemblages in tidepools: Effects of abiotic and biotic factors
(1996-01) Metaxas, A.; Scheibling, Robert Eric
In any ecological system, the factors that regulate the abundance of species vary with spatial 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. planktonic and benthic micrograzers, mussels) and abiotic (e.g. nutrients, temperature, salinity, pH) factors that may regulate these assemblages in 4 tidepools at 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 phytoplankton assemblages along the intertidal gradient, and differences among zones rarely explained more than 30% of the spatial variability 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 phytoplankton varied significantly among pools within intertidal zones on most sampling dates, and differences 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 abundance 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 at the scale of the individual pool rather than the intertidal zone.
Open Access
Top down and bottom-up regulation of phytoplankton assemblages in tidepools
(1996-12) Metaxas, A.; Scheibling, Robert Eric
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 high intertidal and splash zones in 3 repeated, 1 to 2 wk experiments in November 1992 and June and August 1993. For each experiment, 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 abundance over 1 wk intervals of 5 taxonomic groups of phytoplankton: centric diatoms, pennate diatoms, cryptomonads, prasinophytes, and chlorophytes. We examined the effects of nutrient concentration, grazer density, and pool on the phytoplankton assemblage using 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 positive effect on pennate diatoms in the second week. In another pool in the second week, grazer reduction had a positive effect on prasinophytes but a negative effect on chlorophytes. In 1 pool in August, nutrient enrichment had a positive effect on prasinophytes 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 chlorophytes. 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.
Open Access
Estimating fertilization success in marine benthic invertebrates: A case study with the tropical sea star Oreaster reticulatus
(2002-01) Metaxas, Anna; Scheibling, Robert Eric; Young, Craig M.
Several factors can influence fertilization success, and for marine broadcast spawners, the main constraint is rapid dilution of gametes. Because the measurement of fertilization success in the field is logistically difficult, theoretical models have been used as an alternative method of estimation. We tested the predictive ability of the existing models (time-averaged diffusion model in conjunction with a fertilization kinetics model) by making direct comparisons between empirical and corresponding predicted rates of fertilization in the tropical sea star Oreaster reticulatus. Using induced spawnings, we measured in situ fertilization in field experiments at 4 sites on sand bottoms and seagrass beds in the Bahamas. Rate of fertilization decreased exponentially with increasing distance downstream (x) from a spawning male for all experimental runs at all sites, and when averaged across all sites, it ranged from 74% at x = 1 m to 31% at x = 32 m. For each experimental run, we parameterized the 2 models by quantifying the flow field, and thus obtained predicted estimates of fertilization success. The shape of the fitted exponentially-decaying curves was similar between the observed and predicted data for sites on sand bottoms, but not in seagrass beds. There was a highly significant correlation between the observed and predicted data at each distance directly downstream for each run, but the predicted values were 1 to 10 orders of magnitude lower than the observed values for distances off the main axis of advection. We also used dye dilution runs to test the validity of the diffusion model and, in agreement with the observed values, the predicted concentrations of dye decreased as a power function of distance downstream from the point of release. Turbulent diffusivity was quantified in 2 ways: by measuring the standard deviation of dye concentration across-stream and vertically (sigmay and sigmaz), and by calculating coefficients of diffusivity (alphay and alphaz). Using alphay and alphaz, the values predicted with the model of diffusion were up to 24 orders of magnitude lower than the observed values at heights >0.2 m above the bottom, but this inconsistency was alleviated when sigmay and sigmaz were used. Thus, the combination of the 2 models currently used can predict fertilization rate reliably for a particular parameter space, which can be increased by quantifying turbulent diffusivity more accurately. These modified models can substitute field experiments to estimate fertilization success in species of marine benthic invertebrates that are fragile, such as O. reticulatus, or are relatively inaccessible, such as inhabitants of the deep sea.
Open Access
Variation in egg spawning among subpopulations of sea urchins Strongylocentrotus droebachiensis: a theoretical approach
(2001) Meidel, SK; Scheibling, Robert Eric
Variation in zygote production, by determining the initial size of the larvalpool, can aff ect the population dynamics of marine invertebrates with a planktonic larval stage. In this paper, we model temporal and spatial patterns in egg spawning of the sea urchin Strongylocentrotus droebachiensis in the shallow subtidal zone along the Atlantic coast of Nova Scotia. In this region, population outbreaks of sea urchins have caused major transitions from kelp beds to barrens (areas devoid of fleshy macroalgae). We define 3 community states and associated subpopulations of sea urchins: (1) the established kelp bed state, where sea urchins are in low density and small; (2) the transition state, with sea urchins in kelp beds, newly created (transitional) barrens, and grazing fronts (dense aggregations of large sea urchins along the edges of kelp beds); and (3) the barrens state after fronts have dispersed and intermediate densities of moderately sized sea urchins remain in post-transitional barrens. Using data from the literature and unpublished sources, we parameterize mathematical models to predict egg spawning for each subpopulation, both on an areal basis and for the entire coast. The predicted number of eggs spawned per m(2) is 1 order of magnitude higher in grazing fronts (7.1 x 10(7)) than in transitional and post-transitional barrens (5.8 x 10(6) and 4.4 x 10(6), respectively), and 4 to 6 times higher in barrens than in established kelp beds (1.0 x 10(6)). Differences among subpopulations in the number of eggs spawned are directly related to differences in adult density, female gonad output, and body size. The total number of eggs spawned (all subpopulations combined) on a coastal scale increases similar to6-fold from the established kelp bed state (5.6 x 10(14)) to the late transition state (3.1 x 10(15)) and then drops slightly in the barrens state (2.4 x 10(15)). During most of the transition state, sea urchins in barrens spawn the greatest number of eggs. Based on published values, we estimate that fertilization rates are highest in grazing fronts (62 %), intermediate in transitional and post-transitional barrens (36 and 43 %,respectively) and lowest in kelp beds (15 %), resulting in up to 2 orders of magnitude differences in the number of zygotes produced in these subpopulations 14.4 x 10(7), 2.1 x 10(6), 1.9 x 10(6), and 1.5 x 10(5)). Total zygote production tall subpopulations combined) on a coastal scale increases similar to 16-fold from the established kelp bed state (8.1 x 10(13)) to the late transition state (1.3 x 10(15)) and then drops slightly in the barrens state (1.1 x 10(15)). During most of the transition state, sea urchins in barrens contribute the greatest number of zygotes to the total zygote pool, with those in grazing fronts contributing up to 44 % of all zygotes. Sensitivity analysis and evaluation of model assumptions indicate that our predictions should be correct in a relative sense (i.e. comparing among subpopulations), although absolute numbers may be overestimated. A disparity between temporal patterns of estimated zygote production and observed settlement rates of S, droebachiensis in the shallow subtidal zone suggests that zygote production interacts with other factors to determine settlement rates of this species.
Open Access
Spatial and Temporal Variability of Macrobenthic Communities in Tidepools on a Rocky Shore in Nova-Scotia, Canada
(1994-02) METAXAS, A.; HUNT, HL; Scheibling, Robert Eric
Spatial and temporal patterns in the distribution and abundance of macroalgae and macrofauna were measured in 4 pools in each of 3 intertidal zones (mid, high and splash) on a rocky shore near Halifax, Nova Scotia, Canada. Sheets, filamentous, coarsely branched, thick leathery, jointed calcareous, and crustose forms were the most abundant macroalgal functional groups, and mussels, littorinid snails and whelks were the most abundant macroinvertebrates in the pools. Percentage cover of the late-successional macroalgal forms, such as thick leathery and crustose forms, was greatest in the mid pools where environmental fluctuations are smaller and more predictable. Percentage cover of the more opportunistic forms, such as sheets and filamentous algae, was variable among pools in all zones, whereas percentage cover of the other functional forms was most variable among mid pools on most sampling dates. Mussels and whelks were more abundant in mid pools, whereas littorines were more abundant in the high pools. Macrofaunal abundance also was variable among pools within most zones on most sampling dates. Multiple regressions showed that grazer density, nutrient regime and physical characteristics of the tidepools can consistently explain the variability in percentage cover of late-successional macroalgal functional forms, but not of the more opportunistic forms. The horizontal spatial variability in percentage cover of macroalgae may result from differential recruitment of opportunistic forms, and/or differences in the physical characteristics of the pools which affect slow-growing, late-successional forms. The Shannon Diversity Index for the macroalgal and the macrofaunal assemblages was greatest in mid pools, suggesting that a smaller number of macroalgal and macroinvertebrate species can survive in the higher pools where conditions are harsher. Cluster analyses showed that the macroalgal assemblages were mostly similar among mid pools, and that macrofaunal assemblages of mid pools were similar to high pools, but not splash pools. Our study shows that macrobenthic community structure in tidepools does not vary consistently with the period of tidal isolation and there is large variability among pools within intertidal zones.
Open Access
Community structure and organization of tidepools
(1993) Metaxas, Anna; Scheibling, Robert Eric
Although tidepools are conspicuous components of rocky intertidal shores, their biotic communities have not been studied as extensively as those on emergent substrata. We examine processes regulating the structure of tidepool communities for comparison with emergent substrata. The physical environment of tidepools is highly regulated by the tidal cycle, although fluctuations in physical factors are smaller in tidepools, and the organisms remain submerged for the entire tidal cycle. As a result, the upper limits of the distribution of organisms are extended in tidepools and some species either tend to aggregate in pools or avoid them. The vertical zonation of organisms is not as pronounced in tidepools as on emergent substrata. Herbivory has been shown to influence the distribution and abundance of algal species in tidepools, but the effect of predation in regulating community structure has been less well documented. The importance of interspecific competition has been consistently shown in tidepools, particularly among algal species, which usually are the dominant space occupiers. Although the introduction of most species into tidepools depends upon recruitment from the surrounding water, the effects of variation in the supply of new individuals has not been examined. Aspects of the physical regime such as habitat complexity and wave exposure affect the community structure of tidepools, as they do communities on emergent substrata. However, the specific characteristics of tidepools such as pool depth, volume, orientation, shading and flushing rate make individual pools unique, resulting in large spatial variability in tidepool community structure. For this reason, replication in tidepool studies should be carefully selected. Because of their variable characteristics, well-defined boundaries and tidepools of manageable size can serve as experimental mesocosms to test general ecological theories about community organization.
Open Access
Survival analysis of tethered juvenile sea scallops Placopecten magellanicus in field experiments: Effects of predators, scallop size and density, site and season
(1994) Barbeau, M. A.; Scheibling, Robert Eric; Hatcher, B. G.; Taylor, L. H.; Hennigar, A. W.
The impact of predation on survival of juvenile sea scallops Placopecten magellanicus has important implications for the population dynamics and aquaculture of this species. In field experiments using tethered juvenile scallops, we examined the effect of biological and physical factors, such as prey size, predator and prey density, water temperature (season) and site, on mortality rates of scallops in Lunenburg Bay, Nova Scotia, Canada. The major predators of juvenile scallops were crabs Cancer irroratus and sea stars Asterias spp. In an experiment manipulating scallop density, crab predation rate increased significantly with scallop density, but crab density did not, suggesting a functional response. Sea star predation rate on scallops was not affected by scallop density. In a series of experiments with different size classes of scallops tethered at different sites and seasons, and in different densities of surrounding scallops and predators, the effects of covariates on scallop survival times were assessed using the statistical procedure of survival analysis. Since the tethering technique affected crab and sea star predation differently, the competing causes of scallop mortality were analysed separately. Water temperature, site, and scallop density were the important variables affecting crab predation. There was a significant interaction between temperature and site, such that crab predation on scallops increased with temperature at one site and was independent of temperature at the other site. Predation by crabs also increased with scallop density and, to a lesser extent, with crab density. Water temperature and scallop size were the primary variables affecting sea star predation. Sea star predation on scallops increased with temperature and decreased with scallop size. The results of this study suggest options for minimizing predation of scallops by crabs and sea stars in bottom culture operations.
Open Access
Detrital production in Nova Scotian kelp beds: patterns and processes
(2011) Krumhansl, Kira A.; Scheibling, Robert Eric
Connectivity via the transport of detrital material from areas of high to low productivity may be an important determinant of secondary productivity and biodiversity in receiving communities. On the Atlantic coast of Nova Scotia, detritus exported from subtidal kelp beds contributes to food webs in communities inhabiting deeper waters offshore. To estimate the amount of energy available via this pathway, we measured rates of detrital production via erosion of kelp blades in kelp beds at 5 sites varying in wave exposure. Specifically, we measured productivity and erosion of the 2 dominant species of kelp, Laminaria digitata and Saccharina longicruris, over 16 mo. We also examined the effect of environmental and biological factors on erosion rates, including temperature, wave exposure, grazing by snails and cover by the invasive bryozoan Membranipora membranacea. We observed clear seasonal and spatial patterns in productivity, with the highest production in spring. Erosion rate was highly variable across sites and seasons, and was strongly related to the cover of M. membranacea and the intensity of snail grazing at the distal end of kelp blades. Both factors cause tissue degradation, abrasions and perforations that weaken blades. Erosion rate was also positively related to water temperature and site exposure. Annual detrital production from the erosion of kelp blades increased linearly with kelp bed biomass, and ranged from 0.5 to 1.71 kg dry weight m(-2) (150-513 g C m(-2)) across sites. These rates equal or exceed annual phytoplankton production off the Atlantic coast of Nova Scotia and estimates of detrital production in seagrass beds in North America.
Open Access
Modelling phase shifts in a rocky subtidal ecosystem
(Inter-Research, Nordbuente 23 Oldendorf/Luhe 21385 Germany, [mailto:ir@int-res.com], [URL:http://www.int-res.com/], 2008-03) Lauzon-Guay, Jean-Sebastien; Scheibling, Robert Eric; Barbeau, Myriam A.
The rocky subtidal ecosystem of the Atlantic coast of Nova Scotia is characterised by 2 community states, kelp beds and urchin barrens that alternate on a decadal time scale. While the shift from barrens to a kelp bed requires a large perturbation, namely a disease outbreak causing urchin mass mortality, the reverse shift is more gradual and mediated by the formation and propagation of destructive grazing aggregations (or fronts) of sea urchins Strongylocentrotus droebachiensis. We have developed a spatial model for the re-establishment of kelp beds and the formation of a feeding front of urchins in order to describe transitions (phase shifts) between kelp bed and barrens states following an urchin mass mortality event. Our model includes size-specific movement, growth, and survival of urchins, as well as growth of kelp and grazing by urchins. The position of the kelp bed-barrens interface (or grazing front) predicted by our model is in general agreement with field observations at 2 sites and for up to 7 yr after a mass mortality event. Elasticity analysis showed that daily foraging movements by individual urchins have the greatest effect on the position of the feeding front and the density of the urchins there. Survival and recruitment rates of urchins also have large effects on urchin density at the front. The density of urchin populations in deep-water thermal refuges (from a disease agent) and the distance between these source populations and remnant or emergent kelp beds have large effects on the time required for the re-formation of a feeding front after an event of urchin mass mortality. These findings suggest that site-specific characteristics should be taken into account in the development of sustainable harvesting strategies and habitat-based management of the urchin fishery.
Open Access
Spatial and temporal variation in DMSP content in the invasive seaweed Codium fragile ssp fragile: effects of temperature, light and grazing
(2010) Lyons, D. A.; Scheibling, Robert Eric; Van Alstyne, K. L.
The methionine-derived secondary metabolite dimethylsulfoniopropionate (DMSP) is believed to function in cryoprotection, defense against herbivory, and reduction of oxidative stress in some seaweeds. The ability to produce DMSP, and modify its concentration according to environmental conditions, may confer adaptive advantages to these algae. We studied the spatial and temporal distribution of DMSP content in Codium fragile off the Atlantic coast of Nova Scotia. Levels of DMSP in C. fragile were highest in boreal spring (similar to 2% of total dry wt) and lowest in fall (similar to 1%), and negatively related to seawater temperature. DMSP content differed among sites and tended to decrease or remain constant with depth. In tide pools, partially bleached algae contained less DMSP than undamaged individuals, consistent with the putative function of DMSP in response to oxidative stress. Results of field-and laboratory-based experiments, designed to examine plasticity in DMSP production by C. fragile, indicate that it increases with light intensity, exposure to sea urchin grazing, and decreasing temperature.
Open Access
Modelling phase shifts in a rocky subtidal ecosystem
(2009) Lauzon-Guay, Jean-Sebastien; Scheibling, Robert Eric; Barbeau, Myriam A.
The rocky subtidal ecosystem of the Atlantic coast of Nova Scotia is characterised by 2 community states, kelp beds and urchin barrens that alternate on a decadal time scale. While the shift from barrens to a kelp bed requires a large perturbation, namely a disease outbreak causing urchin mass mortality, the reverse shift is more gradual and mediated by the formation and propagation of destructive grazing aggregations (or fronts) of sea urchins Strongylocentrotus droebachiensis. We have developed a spatial model for the re-establishment of kelp beds and the formation of a feeding front of urchins in order to describe transitions (phase shifts) between kelp bed and barrens states following an urchin mass mortality event. Our model includes size-specific movement, growth, and survival of urchins, as well as growth of kelp and grazing by urchins. The position of the kelp bed-barrens interface (or grazing front) predicted by our model is in general agreement with field observations at 2 sites and for up to 7 yr after a mass mortality event. Elasticity analysis showed that daily foraging movements by individual urchins have the greatest effect on the position of the feeding front and the density of the urchins there. Survival and recruitment rates of urchins also have large effects on urchin density at the front. The density of urchin populations in deep-water thermal refuges (from a disease agent) and the distance between these source populations and remnant or emergent kelp beds have large effects on the time required for the re-formation of a feeding front after an event of urchin mass mortality. These findings suggest that site-specific characteristics should be taken into account in the development of sustainable harvesting strategies and habitat-based management of the urchin fishery.
Open Access
Behaviour of sea urchin Strongylocentrotus droebachiensis grazing fronts: food-mediated aggregation and density-dependent facilitation
(2007) Lauzon-Guay, Jean-Sebastien; Scheibling, Robert Eric
The occurrence of destructive grazing fronts is a common phenomenon in sea urchins, but mechanisms governing front formation and dynamics remain poorly understood. We experimentally examined the effect of kelp biomass on the aggregative behavior and movement of a front of green sea urchins Strongylocentrotus droebachiensis at a wave-exposed site on the Atlantic coast of Nova Scotia. We varied kelp (Laminaria digitata and L. longicruris) abundance in 2 x 2 m plots adjacent to the front in 3 treatments: 50 % plant removal, and 100 % frond removal and unmanipulated control. In each treatment, we monitored the position of the front and urchin density at the leading edge over 24 d. The mean advance of the front in 24 d (2.27 m) did not differ between treatments, but urchin density was greatest in the control (74.9 urchins 0.25 m(-2)) and lower in whole plant (54.3) and frond (39.4) removal treatments. When urchin density was used as a covariate, front advance was inversely related to kelp biomass and greater in frond and plant removal treatments than in the control. Together, urchin density and kelp biomass explained 75 % of variation in front advance. These findings provide the first direct evidence that urchins redistribute themselves along a front to concentrate in patches of greatest food availability. Temporal variation in urchin density at the front was inversely correlated with wave height, and individual grazing rates increased with urchin density, which may explain seasonal variation in front dynamics observed in previous studies.
Open Access
Spatial dynamics, ecological thresholds and phase shifts: modelling grazer aggregation and gap formation in kelp beds
(2010) Lauzon-Guay, Jean-Sebastien; Scheibling, Robert E.
On the Atlantic coast of Nova Scotia, transitions between alternative states of the subtidal ecosystem, kelp beds and sea urchin barrens, occur on a decadal scale. To explore the process of urchin aggregation within kelp beds that leads to the shift to barrens, we developed a coupled map lattice model to simulate the spatial dynamics of kelp and green sea urchin Strongylocentrotus droebachiensis abundance over time. Our simulations show that the following factors can cause sea urchins to form grazing aggregations that create gaps in a kelp bed: (1) random movement: by > 60% of sea urchins residing in the bed, (2) moderate to high levels of spatial variability in sea urchin recruitment (30 to 90 [urchins m(-2)](2)), (3) localized aggregation of sea urchins (150 urchins m(-2)) amid a moderate to high background density of sea urchins within the kelp bed (> 10 urchins m(-2)), with or without a chemotactic response of sea urchins to kelp, and (4) removal of kelp from areas > 20 m(2) (to simulate physical or biological disturbance, or harvesting). Gaps formed at random locations within the spatial domain and expanded and coalesced to form barrens in which sea urchins were randomly distributed, Sea urchins formed circular fronts around gaps in the kelp bed. The rate of advance of fronts (and increase in gap size) was linearly related to the density of sea urchins at the front. The duration of the transition to the barrens state decreased with increases in (1) the proportion (1),) of sea urchins moving (from > 6 yr for P-m = 0.8 to 5 yr for 30 [urchins m(-2)](2) to < 3 yr for 90 [urchins m(-2)](2)). Our findings support observations of gap formation within kelp beds that resulted in widespread destructive grazing on this coast in the late 1960s. Our model provides a predictive tool for the design of monitoring programs and field experiments to explore the underlying mechanisms of an ecosystem phase shift that has major ecological consequences.
Open Access
Structure and Dynamics of Epifaunal Assemblages on Intertidal Macroalgae Ascophyllum-Nodosum and Fucus-Vesiculosus in Nova-Scotia, Canada
(1987-05) JOHNSON, SC; Scheibling, Robert Eric
No abstract available.
Open Access
Fatty acids as dietary tracers in benthic food webs
(Inter-Research, Nordbuente 23 Oldendorf/Luhe 21385 Germany, 2011) Kelly, Jennifer R.; Scheibling, Robert Eric
Fatty acid (FA) analysis is a well-established tool for studying trophic interactions in marine habitats. However, its application to benthic food webs poses 2 particular challenges. First, unlike pelagic zooplankton, benthic consumers have access to different sizes and functional groups of primary producers and may consume a highly mixed diet. Classes of benthic primary producers are distinct in their overall FA composition, but most do not possess unique marker FAs that can be used to identify their contribution to higher trophic levels. Second, unlike mammalian predators, benthic invertebrates have the capacity to significantly modify their dietary FAs and thereby obscure markers for food sources. Controlled feeding studies have been used to distinguish dietary tracer FAs from those that are modified by the consumer in several benthic invertebrates, but more such studies are needed. Despite these challenges, FAs have been used to study trophic structure in a variety of benthic habitats including the deep sea, polar regions, estuaries, and the rocky subtidal zone. However, the complexity of benthic food webs and lack of unique markers impose uncertainties in the interpretation of FA data from field studies. Multivariate analyses are necessary for analyzing FA datasets, although univariate tests can be useful for comparing levels of informative FAs among food sources and consumers. Combining FA analysis with other lines of evidence, such as stable isotope analysis, offers a more reliable approach to examining trophic interactions in benthic systems.
Open Access
Fatty acid profiles in the gonads of the sea urchin Strongylocentrotus droebachiensis on natural algal diets
(2008) Kelly, Jennifer R.; Scheibling, Robert Eric; Iverson, Sara J.; Gagnon, Patrick
We examined fatty acid (FA) compositions of gonads of the green sea urchin Strongylocentrotus droebachiensis collected from a grazing aggregation (front) at the edge of a kelp bed and from barrens dominated by coralline algae, and those of urchins fed single algal diets in the laboratory. We compared these gonad FAs with those of the algal diets, which represented known urchin food sources in rocky subtidal habitats. Gonads of urchins collected from both wild habitats, and of urchins fed kelp Saccharina longicruris in the laboratory, contained more lipid than did urchins fed single diets of barrens macroalgae (Agarum clathratum, coralline red algae or Desmarestia viridis). Substantial biosynthesis of non-methylene interrupted dienes and other FAs by urchins markedly affected their overall FA signatures. Although the FA compositions of gonads of laboratory-fed urchins did not clearly correspond with those of their diets, 3 clusters of urchins were distinguished in multivariate space using multidimensional scaling (PADS): (1) urchins fed single diets of barrens macroalgae in the laboratory; (2) urchins fed S. longicruris in the laboratory or collected from the grazing front; and (3) urchins from the barrens. Characteristics of FA signatures found in urchins from the barrens suggested the occurrence of benthic diatoms in their diet. Our results indicate that, while the FA signatures of urchin gonads are affected by diet and can be used to differentiate feeding groups of urchins in the laboratory and field, significant de novo biosynthesis and/or modification of FAs precludes correspondence of urchin FAs to those of their algal diets.