Rocky Subtidal Community Research Articles

The interplay of natural variability, productivity and management of the benthic ecosystem in the Humboldt Current System: Twenty years of assessment of Concholepas concholepas fishery under a TURF management system

Areas with controlled access, as Marine Protected Areas (MPAs) and Territorial Use Rights for Fisheries (TURF), are conceived as tools to contribute to the ecosystem conservation and fisheries sustainability. Management and Exploitation Areas for Benthic Resources (MEABR), a TURF system implemented in Chile, offers the opportunity to test this hypothesis. The management in these areas is based mainly on the assessment of the resources population status, with low or no consideration of the ecological - environmental effects that condition their productivity. In this study, we evaluate ecological trophic dynamics of the rocky subtidal community configuration in four periods along 20 years of the highly productive Choros Island MEABR – MPA, including one pre-implementation period. In this TURF, the carnivore gastropod Concholepas concholepas (locally known as “loco”) is the main targeted resource. The performance of ecosystem indexes of this TURF increased during the MEABR regime as compared with the pre MEABR implementation. Nevertheless, the most conspicuous increase occurred in 2013, primarily in response to changes in environmental conditions and their direct and indirect effects on community interactions. The analysis of interactions between guilds indicated that the main drivers were the interaction between the primary productivity, prey abundance and the top predator loco. This produces a long-term dynamic of the system modulated by the interplay of environmental variability, which at benthic community level increases prey populations thus sustaining increased loco productivity. In turn, intensified predation of loco reduces its prey. This produces natural productivity fluctuations, which counteract the expected outcome of management actions, impeding to achieve stable landings in the long term.

Biodiversity of an Antarctic rocky subtidal community and its relationship with glacier meltdown processes

Glacier meltdown is a major environmental response to climate change in the West Antarctic Peninsula. Yet, the consequences of this process for local biodiversity are still not well understood. Here, we analyse the diversity and structure of a species-rich marine subtidal macrobenthic community (consumers and primary producers) across two abiotic environmental gradients defined by the distance from a glacier (several km) and depth (between 5 and 20 m depth) in Fildes Bay, King George Island. The analysis of spatially extensive records of seawater turbidity, high-frequency temperature and salinity data, and suction dredge samples of macrobenthic organisms revealed non-linear and functional group-dependent associations between biodiversity, glacier influence, and depth. Turbidity peaked in shallow waters and in the nearby of the glacier. Temperature and salinity, on the other hand, slightly decreased in the proximity of the glacier relative to reference sites. According to the spatial pattern in turbidity, species richness of consumers was lowest in shallow waters and near to the glacier. Also, Shannon's diversity of consumers significantly decreased in the nearby of glacier across depths. Moreover, the spatial variation in community structure of consumers and primary producers depended on both glacier distance and depth. These results suggest that glacier melting can have significant effects on diversity and community structure. Therefore, the accelerated glacier meltdown may have major consequences for the biodiversity in this ecosystem.

Role of predators in the recruitment of invertebrates in a rocky subtidal community in the southwest Bay of Fundy, NB

ABSTRACTPredation is potentially an influential source of early post-settlement mortality of benthic marine invertebrates, but previous studies demonstrate conflicting results. We investigated the effect of large predators on developing subtidal invertebrate communities in cobble-filled collectors in the southwest Bay of Fundy, Canada. Two predation treatments (exclusion of predators ≥ 7 mm body width or full access for predators < 50 mm) were used to test the effect of large predators, and a partial-cage control tested for caging artefacts. Despite a reduction in the abundance and biomass of large predators, multivariate analyses indicated no effect of predator exclusion on the composition of the prey and micropredator communities. Results indicated that the largest differences were between the predator-access treatment and the other two treatments, which was potentially influenced by the caging material. There were significant but weak positive correlations between the micropredator and the other two communities. Previous studies indicate that at low predation intensity, as seemed to be the case here, other factors may play a stronger role in controlling recruit abundance. Predation control probably varies spatially and temporally, and the influence of large predators was not likely the driving force for early post-settlement mortality in the invertebrate communities measured in this study.

Vertical distribution of rocky subtidal assemblages along the exposed coast of north-central Chile

Through a systematic study of a stretch of coast in north central Chile, the variety and vertical zonation patterns of the rocky subtidal communities are described, thereby revising apparent uniformity and lack of vertical zonation of the rocky subtidal of southeastern Pacific shores previously reported in the literature. Over the 600km of coast studied, the following pattern of depth-zonation is described: an upper fringe (lower part of the sublittoral fringe) characterized by barren grounds dominated by calcareous encrusting algae and the sea urchin Tetrapygus niger; an intermediate fringe (upper Infralittoral sub-zone) characterized either by deep barren grounds similar to the former, or kelp beds of Lessonia trabeculata, or an assemblage of suspension feeding organisms, as the big barnacle Austromegabalanus psittacus and/or the tunicate Pyura chilensis; and a deep fringe (Circalittoral sub-zone) dominated by small encrusting or mobile invertebrates. This basic vertical zonation pattern of the shallow rocky subtidal communities seems to be common to most of the temperate coasts of the world. The analysis, first of the occurrence of the general zonation pattern and second of the species composition within the assemblages corresponding to each sub-zone, offers a useful framework for the assessment of the eventual impacts and changes within the shallow rocky subtidal habitat, for example within environmental monitoring programs.

Effects of the Discharge of Iron Ore Tailings on Subtidal Rocky-Bottom Communities in Northern Chile

ABSTRACT Gonzalez, S.A.; Stotz, W., and Lancellotti, D., 2014. Effects of the discharge of iron ore tailings on subtidal rocky-bottom communities in northern Chile. Some of the many mining operations that exist in northern Chile dispose their mine tailings directly into the sea. Little is known about the effects of these tailings on shallow rocky subtidal communities. This study reports these effects for an iron ore pelletization plant that has discharged for ca. 23 years in the intertidal of a cove on the exposed rocky coast of northern Chile, generating high turbidity in the water column and deposit of sediments on the bottom. A nonparametric multidimensional scaling (nMDS) ordination analysis for the species of the rocky subtidal communities, based on the Bray-Curtis dissimilarity index, showed great differences, the affected communities being dominated by incrusting invertebrates, whereas the unaffected areas at the same depth had kelp beds. The effects were observed only close to the discharge and we...

Effects of ocean acidification and diet on thickness and carbonate elemental composition of the test of juvenile sea urchins

Continuous anthropogenic CO2 emissions to the atmosphere and uptake by the oceans will cause a reduction of seawater pH and saturation state (Ω) of CaCO3 minerals from which marine calcifiers build their shells and skeletons. Sea urchins use the most soluble form of calcium carbonate, high-magnesium calcite, to build their skeleton, spines and grazing apparatus. In order to highlight the effects of increased pCO2 on the test thickness and carbonate elemental composition of juvenile sea urchins and potential differences in their responses linked to the diet, we performed a laboratory experiment on juvenile Paracentrotus lividus, grazing on calcifying (Corallina elongata) and non-calcifying (Cystoseira amentacea, Dictyota dichotoma) macroalgae, under different pH (corresponding to pCO2 values of 390, 550, 750 and 1000 μatm). Results highlighted the importance of the diet in determining sea urchin size irrespectively of the pCO2 level, and the relevance of macroalgal diet in modulating urchin Mg/Ca ratio. The present study provides relevant clues both in terms of the mechanism of mineral incorporation and in terms of bottom-up processes (algal diet) affecting top-down ones (fish predation) in rocky subtidal communities.

Influences of wave fetch, tidal flow and ocean colour on subtidal rocky communities

Community assemblages on subtidal rock change markedly along gradients of wave energy, tidal flow, and turbidity. The importance of these assemblages for rare and delicate spe- cies, for shellfish, as nursery areas for fish, and for their contribution to ecosystem functioning in coastal areas has prompted much conservation effort in many countries. I applied a rapid method of calculating a large high-resolution (200 m scale) map of wave exposure 40% cover had a median of 22 species. Laminaria hyper- borea, and the most diverse communities, is found in areas of estimated low chlorophyll concen- trations and in the most wave-exposed environments, which are often but not always in areas of high tidal flow.

Stability, resilience, and phase shifts in rocky subtidal communities along the west coast of Vancouver Island, Canada

We used the extirpation, reintroduction, and spread of sea otters (Enhydra lutris) along the west coast of Vancouver Island, Canada, to evaluate how the otter–urchin–algae trophic cascade creates variation in rocky reef community structure over space and time. By repeatedly sampling both randomly selected and permanently marked sites in areas where sea otters were continuously present, continuously absent, or became reestablished during a 23-year study period, we found a highly predictable association between community phase states (algae abundant or urchins abundant) and the population status of sea otters. In areas where sea otters were continuously present, urchins were rare and algae dominated, whereas in areas where otters were continuously absent, urchins were abundant and algae were rare. Despite this predictability, the species composition and abundance of algae within otter-dominated sites and the abundance of urchins in otter-free sites were spatially and temporally variable. The transition from...

Effect of the degree of autotomy on feeding, growth, and reproductive capacity in the multi-armed sea star Heliaster helianthus

The term autotomy refers to the process by which some species lose limbs or parts of limbs in response to adverse biotic or abiotic conditions, as for example, predation or abnormally high temperatures. The multi-armed sea star Heliaster helianthus is a key predator of the intertidal and the shallow rocky subtidal communities of north-central Chile. Natural populations of this sea star have been found with up to 60% of the individuals showing some degree of autotomy. The present study evaluated the effects of autotomy on feeding rate and growth of juvenile and adult H. helianthus after experimentally induced autotomy of 17% and 33% of their arms, as well as on the energy content of the pyloric caeca and gonads of adults during the reproductive period. Experimental juvenile sea stars were maintained and fed in the laboratory over a period of five months and adult sea stars for one month, Intact individuals were maintained as parallel controls. The results showed that juveniles undergoing 33% autotomy decreased their feeding rates, and as a consequence showed lowered net individual growth. In contrast, adults with 17% and 33% autotomy showed marked reductions in feeding. The results showed that autotomized adults had between five and seven times lower contents of carbohydrates, lipids, and proteins (and thus energetic content) in their pyloric caeca and gonads. The loss of the arms not only decreased the capacity for feeding in sea stars, but also allocated energy away from growth and reproduction into the process of regeneration of arms. This suggests that autotomy reduces the fitness of H. helianthus. Growth was reduced in the juveniles, while adults became limited in their ability to store energy which then limited their reproductive potential. Finally, based on the important effect of autotomy on reducing the feeding capacity of H. helianthus, the role of this sea star as a predator in the environment may be strongly affected.

Invertebrate fauna associated with kelp enhances reproductive output of the green sea urchin Strongylocentrotus droebachiensis

Kelp beds are species rich communities providing food and shelter for a wide range of invertebrate and fish species. As the dominant subtidal grazer in the northwest Atlantic, the green sea urchin Strongylocentrotus droebachiensis can severely limit the distribution and abundance of kelps and influence the structure of the rocky subtidal community. Urchins form large mobile feeding aggregations (termed fronts) along the margins of kelp beds that destructively graze kelps and, incidentally, the associated invertebrate fauna. Urchins along the leading edge of fronts climb upon and consume kelp fronds and trailing urchins graze the remaining stipes and holdfasts. The fronds are often encrusted with an epiphytic bryozoan Membranipora membranacea, while the holdfasts harbour many sessile and cryptic invertebrates, such as mussels and brittle stars. The effects on urchin growth (as test diameter and body weight) and reproduction (as gonad index) of feeding on kelp fronds or holdfasts, both with or without the natural associated fauna, were examined in a 95-day experiment in the laboratory. Female urchins had a greater gonad index than males in all diet treatments at the end of the experiment. Females fed holdfasts with associated fauna had a greater gonad index that those fed cleaned holdfasts, but there was no difference between these treatments for males. For urchins fed fronds, there was no significant difference in gonad index of either sex between treatments with and without M. membranacea, although urchins fed at a lower rate on bryozoan-encrusted fronds. Test diameter increased by 1–7 mm (based on tank means) during the experiment, but there was no effect of diet or sex on growth rate. The finding that a diet of kelp with naturally associated fauna can significantly enhance gonad production in S. droebachiensis has important implications for the development of optimal aquaculture and harvesting practices.

About the Cover.

Next article FreeAbout the CoverPDFPDF PLUSFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinked InRedditEmailQR Code SectionsMoreCoverThe green sea urchin Strongylocentrotus droebachiensis has a circumboreal distribution in cold-temperate and arctic waters. This herbivore plays a pivotal role in determining the structure of rocky subtidal communities throughout its range. Dense aggregations of urchins (or feeding fronts) form along the margins of kelp beds and destructively graze all upright seaweeds, creating urchin barrens of much lower productivity, habitat complexity, and biodiversity. In this photograph, urchins in a front are grazing laminarian kelps (Laminaria digitata and L. longicruris) at 10-m depth near Halifax, Nova Scotia, Canada. The formation of these fronts results in the emergence of three distinct zones on the rocky seabed, each with its characteristic subpopulation of sea urchins. Fronts are bands 2- to 3-m wide composed of large adult urchins with a high reproductive effort (gonadic index) at maximal densities (hundreds per square meter). Kelp beds in advance of these fronts harbor low densities of urchins that are generally smaller and have an intermediate reproductive effort. Barrens in the wake of feeding fronts have intermediate densities of relatively small urchins with a low reproductive effort. The differences in density and subpopulation characteristics influence the fertilization success of these free-spawning invertebrates (releasing gametes into the water column for external fertilization) in each zone.On pages 195-205 of this issue, Lauzon-Guay and Scheibling present a spatial fertilization model for Strongylocentrotus droebachiensis. By incorporating population characteristics such as urchin density, size, and gonadic index into their model, they show that fertilization success is much greater than estimated from previous field experiments and theoretical studies. Their results suggest that sea urchins can achieve high fertilization rates, even at relatively low density (as in kelp beds) or with low reproductive effort (as in barrens), when eggs are allowed to disperse over realistic times and distances to encounter sperm from multiple males located upstream and downstream. These results challenge the conventional hypothesis that sperm is limiting, and consequently fertilization rates are low, except in very dense populations of free spawners.Credits: Photo, Robert E. Scheibling (Dalhousie University); cover layout, Beth Liles (Marine Biological Laboratory). Next article DetailsFiguresReferencesCited by The Biological Bulletin Volume 212, Number 3June 2007 Published in association with the Marine Biological Laboratory Article DOIhttps://doi.org/10.1086/BBLv212n3cover Views: 186 © 2007 by Marine Biological Laboratory. All rights reserved.PDF download Crossref reports no articles citing this article.

Differences in somatic and gonadic growth of sea urchins (Stronglyocentrotus droebachiensis) fed kelp (Laminaria longicruris) or the invasive alga Codium fragile ssp. tomentosoides are related to energy acquisition

The rocky subtidal community off the Atlantic coast of Nova Scotia has historically undergone a cyclical transition between Laminaria-dominated kelp beds and sea urchin-dominated barrens. Since the introduction of the invasive alga Codium fragile ssp. tomentosoides, a third community state has emerged: Codium-dominated algal beds. We conducted a 42-week feeding experiment in the laboratory, which mimicked the quantity and quality of food available to urchins (Strongylocentrotus droebachiensis) in each of these community states. Feeding rate, growth, reproduction, and survival of urchins fed either Laminaria longicruris or C. fragile ad libidum, or L. longicruris 2 days per month, were measured. Although the ad libidum feeding rate on C. fragile was higher than that on kelp, energy intake was lower. Urchins in the ad libidum kelp treatment were larger and had larger gonads than those in the C. fragile treatment. Urchins fed kelp infrequently exhibited little somatic and gonadic growth over the course of the experiment. Regression analysis revealed that urchin performance on these diets was strongly related to energy intake. Diet treatment had no effect on survival or gonad maturation. Although urchins can consume substantial amounts of C. fragile, it appears that they cannot, or do not, feed quickly enough to compensate for its lower nutritional value. Our results suggest that, although urchins feeding on C. fragile are capable of surviving, growing, and reproducing, the replacement of kelp by C. fragile in some areas might negatively affect urchin populations as they continue to repopulate the shallow subtidal zone.

Using Markov chain successional models backwards

Summary Markov chains are commonly used to model succession in plant and animal communities. Once fitted to data, these models are typically used to address ecological issues concerning future successional states. In some situations it may be of interest to use the present successional state to reconstruct past conditions. The properties of a time‐reversed Markov chain are reviewed and used to provide an expression for the conditional probability distribution of the most recent time that the chain was in a particular successional state given its present state. The estimation of this conditional probability is discussed and a parametric bootstrap is described for constructing a confidence interval. The calculations are illustrated using a published Markov chain model of succession in a rocky subtidal community. Synthesis and applications. The term succession refers to the progressive changes over time in the state of an ecological community. Although most analyses of succession focus on characterizing the future, in some situations interest centres on reconstructing the past. For example, forensic entomologists are commonly interested in estimating time of death from the insect community present on a corpse. When succession can be modelled as a Markov chain, the results obtained here can be used for this kind of reconstruction.

Markov chain analysis of succession in a rocky subtidal community.

We present a Markov chain model of succession in a rocky subtidal community based on a long-term (1986-1994) study of subtidal invertebrates (14 species) at Ammen Rock Pinnacle in the Gulf of Maine. The model describes successional processes (disturbance, colonization, species persistence, and replacement), the equilibrium (stationary) community, and the rate of convergence. We described successional dynamics by species turnover rates, recurrence times, and the entropy of the transition matrix. We used perturbation analysis to quantify the response of diversity to successional rates and species removals. The equilibrium community was dominated by an encrusting sponge (Hymedesmia) and a bryozoan (Crisia eburnea). The equilibrium structure explained 98% of the variance in observed species frequencies. Dominant species have low probabilities of disturbance and high rates of colonization and persistence. On average, species turn over every 3.4 years. Recurrence times varied among species (7-268 years); rare species had the longest recurrence times. The community converged to equilibrium quickly (9.5 years), as measured by Dobrushin's coefficient of ergodicity. The largest changes in evenness would result from removal of the dominant sponge Hymedesmia. Subdominant species appear to increase evenness by slowing the dominance of Hymedesmia. Comparison of the subtidal community with intertidal and coral reef communities revealed that disturbance rates are an order of magnitude higher in coral reef than in rocky intertidal and subtidal communities. Colonization rates and turnover times, however, are lowest and longest in coral reefs, highest and shortest in intertidal communities, and intermediate in subtidal communities.

Spatio‐temporal variation in Markov chain models of subtidal community succession

AbstractIn this paper we ask whether succession in a rocky subtidal community varies in space and time, and if so how much affect that variation has on predictions of community dynamics and structure. We describe succession by Markov chain models based on observed frequencies of species replacements. We use loglinear analysis to detect and quantify spatio‐temporal variation in the transition matrices describing succession. The analysis shows that space and time, but not their interaction, have highly significant effects on transition probabilities. To explore the ecological importance of the spatio‐temporal variability detected in this analysis, we compare the equilibria and the transient dynamics among three Markov chain models: a time‐averaged model that includes the effects of space on succession, a spatially averaged model that include the effects of time, and a constant matrix that averages over the effects of space and time. All three models predicted similar equilibrium composition and similar rates of convergence to equilibrium, as measured by the damping ratio or the subdominant Lyapunov exponent. The predicted equilibria from all three models were very similar to the observed community structure. Thus, although spatial and temporal variation is statistically significant, at least in this system this variation does not prevent homogeneous models from predicting community structure.

Links between Rain, Salinity, and Predation in a Rocky Subtidal Community

Physical control of predation appeared likely in subtidal habitats of fjords in southwestern New Zealand because they intersect a shallow low-salinity layer (LSL) where marine predators might be excluded by low-salinity stress. We investigated links between predator distributions, predation intensity, rainfall, the depth of the LSL, and sessile invertebrate community structure at two sites in Doubtful Sound, New Zealand. Sessile invertebrate communities living on the rock walls of the fjord showed striking patterns of vertical zonation corresponding to the depth of the LSL. Barnacles and mussels, Mytilus edulis galloprovincialis, were most abundant at the 1–3 m depth range commonly bathed by the LSL, while sponges, bryozoans, and ascidians dominated space below it (6–18 m depth). Mussel abundance declined sharply with depth at both sites, from maxima of 52–80% cover in the LSL (3 m) to <2% cover just below the LSL (6 m). To test the hypothesis that the LSL influences the lower limit of the mussel zone by excluding predators and reducing predation from mobile invertebrates living below it, we measured predator densities and tracked predation on transplanted mussels, M. e. galloprovincialis, in the LSL (2.5 m depth) and at 3–4 depths below. Predators were most abundant just below the lower boundary of the LSL during February and April 1993. No transplanted mussels were consumed in the LSL at 2.5 m, but 20–80% of them were consumed in experiments conducted below it, indicating that the LSL represents a spatial refuge from predation. The shallowing of the LSL to 1.5 m depth during low rainfall in November 1993 provided an additional opportunity to evaluate the influence of the LSL on predation. Repeated predator surveys showed that sea stars and urchins moved up with the ascending LSL to create a pattern of high predator abundance at 2.5 m depth in November, suggesting that the shallow zone previously buffered from predation by the LSL was not a predation refuge in November. Rainfall and salinity at 2.5 m depth were inversely related. A significant regression of rainfall one day prior to salinity profiling and the depth of the LSL boundary (30 mg/g salinity) explained 61.5% of the variation in the depth of the boundary, which represented the maximum depth of the predation refuge. The regression enabled us to predict the depth of the predation refuge over a 1.5-yr period beginning in October 1992. The average depth of the refuge was 2.35 m, ranging from 1.5 to 5.6 m as a function of rainfall. Daily tidal variation increased the vertical excursion of the refuge by 0.9–1.7 m. Distinguishing between a partial predation refuge where the LSL occurred at a given depth for a part of a day, and a complete refuge where a depth occurred in the refuge for the entire day, we estimated that the 2.5-m habitat was a complete refuge during 10% of the 18-mo period and a partial predation refuge 90% of the time. The close correspondence between the average depth of the predation refuge (2.35 m) and the depth of maximum mussel cover (3.0 m) suggests that the mussel zone is restricted to the LSL by high predation at the lower boundary of the layer. The alternative hypothesis, that the lower limit of the mussel zone (6 m depth) was determined by a failure of mussel recruitment to this depth, was tested by a 1-yr recruitment experiment. Mussels recruited to the lower edge of the mussel zone at 6 m, although overall levels of mussel recruitment during May 1993–April 1994 were too low to maintain the observed zonation pattern. By increasing the spatial extent of the low-salinity layer, we speculate that changes in climatic conditions leading to increased freshwater runoff may alter salinity gradients, bathymetric distributions of predators, and the structure of shallow subtidal communities, especially in fjords.

Small-scale association measures in epibenthic communities as a clue for allelochemical interactions.

The small-scale associations in a rocky subtidal community in the northwestern Mediterranean were studied by a development of the continuous line transect method. This method allowed the overall measurement of non-randomness in interspecific contacts and the assignment of an association index to each species-pair, whose, significance was tested by Monte Carlo procedures. At the same time, the continuous recording allowed the study of the weakening of the interactions with increasing distances. Our purpose was to uncover evidence for allelochemical mechanisms of space occupation and maintenance. A strong non-randomness was found in the interspecific associations. This was mostly due to the interactions of the poecilosclerid sponge Crambe crambe (Schmidt) with its neighbours, especially its negative associations with other sponge species. The strength of the relationships fell drastically over the first few centimeters from the contact borders of the different species. The results pointed strongly to an allelochemical mechanism. The extracts of this sponge featured high bioactivity in laboratory assays, and field experiments demonstrated that the sponge can inhibit the growth of species in the community studied. Standard sampling techniques would have overlooked the spatial structure present in the data. The study emphasizes the need for both contact data and distance data in order to identify the underlying processes reliably. The line transect method provides both types of information easily and allows testing of models and identification of organisms likely to use chemical defenses in space competition. Its use as a preliminary step in studies of chemical ecology might help to detect presumptive allelochemical processes prior to experimental work on the potentially active species.

Influence of physical gradients on the structure of a northern rocky subtidal community

We have examined the effect of the estuarine and depth gradients on community structure (diversity, richness, density, biomass) and biomass of trophic guilds in the rocky bottom subtidal community ...

Feeding ecology of benthic mobile predators: experimental analyses of their influence in rocky subtidal communities of the Gulf of Maine

The feeding ecology of mobile predators and their impact on benthic organisms inhabiting the rocky shallow subtidal zone off the coast of Maine, USA, was investigated. To determine diets and degree of prey selection we analyzed gut contents of lobsters ( Homarus americanus Milne-Edwards), two species of crabs ( Cancer irroratus Say and C. borealis Stimpson) and 13 fish species of which pollock ( Pollachius virens (Linnaeus)), cunners ( Tautogolabrus adspersus (Walbaum)), and sculpins (several species) were the most conspicuous species of this assemblage. Dietary data were compared with those on benthic prey availability. In the laboratory, we conducted experiments to test whether there is prey selection in relation to their energetic value. In the field, we conducted two experiments: (1) to test whether fish predators move into the intertidal zone during high tide for feeding purposes; and (2) to test the impact of increased predator abundance in these communities using artificial reefs made of concrete blocks. Large fish and crustacean predators foraged both benthic (mainly invertebrates) and pelagic (mostly fishes) prey. Juveniles or small individuals of crustaceans, mollusks and fish were the most heavily preyed, crabs ( Cancer spp.), mussels ( Mytilus edulis Linnaeus), and herring ( Clupea harengus Linnaeus) being the most common items. Juvenile sea urchins ( Stronglocentrotus droebachiensis (Muller)) were important prey of cunners. In the laboratory, lobsters, cunners and sculpins exhibited strong preference for small crabs and mussels but not for echinoderms. These results were consistent with those obtained with a prey selection index, and were closely related to the energy value of prey, suggesting that these predators behave as energy maximizers. 13 pollock and four cunners were caught in the mid-intertidal zone. These specimens contained a variety of undigested benthic intertidal organisms indicating that they utilize intertidal zones as feeding grounds. These results suggest that fishes may play an important role on the structure and organization of intertidal sessile communities. A significant number of predators colonized the artificial reefs, indicating that the availability of living space is an important factor affecting the distribution and abundance of large mobile predators. These predators reduced the abundance of juvenile or small benthic species, particularly of green sea urchins, in the area surrounding the reefs, suggesting that size-selective predation is presumably the mechanism by which predators influence the structure and organization of benthic communities in these environments.

Long-term trends in algal-dominated rocky subtidal communities on the Swedish west coast — a transitional system?

Rocky subtidal communities on the Swedish west coast were monitored over approximately 16 years (1970–1985) by means of stereophotographic recordings, at intervals, of fixed sites.