Evaluating stress in rocky shore and shallow reef habitats using the macrofauna of kelp holdfasts

Kelp often dominates hard rocky substrata in the coastal watersof temperate, boreal and subantarctic regions of the world's oceans. The habitat providedby kelp holdfasts supports high species richness and high abundance of a range of invertebratetaxa. Studies conducted on the effects of stress on these communities have indicated that theyare sensitive to stressors and that the response is often predictable. With the growing body ofliterature on natural variation in holdfast community structure, studies of the impacts of stressare becoming more readily interpretable making the macrofauna of this habitat a potentiallyimportant tool for evaluating anthropogenic effects on hard substrata in coastal environments. Limitations of the use of holdfast fauna in studies of stress include the fact that brownseaweeds may disappear from polluted areas of reef, and where they persist, holdfaststructure may be altered directly by the stressor. An additional problem is that manipulative,experimental studies are difficult. Artificial holdfasts may provide a means by which thedemonstrated sensitivity of the holdfast community can be combined with a manipulative approach to testspecific hypotheses related to effects of stress in shallow coastal environments.     

Colonization of gastropods on subtidal reefs depends on density in adjacent habitats, not on disturbance regime

Habitats dominated by algal canopies are often altered by physicaldisturbances of varying severity, changing environmental conditionsand biological processes. We used Artificial Seaweed Units (ASUs)to test whether severity of physical disturbances on algal canopiesaffects the post-disturbance colonization of gastropods on subtidalreefs. Specifically, we examined patterns of assemblage structureof gastropods to test the hypothesis that the extent and intensityof canopy removal affects the post-disturbance colonizationof ASUs, testing the consistency of these effects among fourregions encompassing a 6° latitudinal gradient in southwesternAustralia. Because adjacent habitats can act as a source ofnew colonists (either as drifting migrants or as a source ofpropagules) from the perimeter surrounding perturbed areas,we also predicted that patterns of colonization (types and totalabundances of colonizers) were influenced by the available poolof individuals at the scale of reefs. Three reefs were selectedwithin each region. On each reef, ASUs were placed in the centreof circular canopy clearings of different size (0, 3, 13 and50 m²) and intensity (50% vs 100%), and retrieved after3 months. Resulting assemblages occupying the ASUs were quantitativelyrepresentative of the adjacent (undisturbed), algal-associated,assemblages at the scale of reef. Within reefs, recruited assemblageslargely mimicked those associated with erect red algae. However,neither disturbance size nor intensity affected the colonizationpatterns across reefs and regions. These results suggest thatalgal-associated gastropods, regardless of the prevalent modeof dispersion, are resilient to physical disturbances to canopiesacross broad geographical regions as long as the pool of potentialcolonists is maintained. A high dispersal ability of gastropodslikely ensures a quick colonization of recovering algal habitats. (Received 14 April 2008; accepted 18 August 2008)     

The effects of macroalgal cover on the spatial distribution of macrobenthic invertebrates: the effect of macroalgal morphology

The growth of green macro-algae in response to nutrient inputs is a common phenomenon in marine estuaries and sheltered bays. While the ecological effects of the growth of the most commonly occurring macroalgal taxa (Enteromorpha, Chaetomorpha, Ulva, Cladophora) have been well studied, the effects of a morphologically very different species, Vaucheria subsimplex, have not been investigated. This study investigated the ecological effects of the establishment of V. subsimplex on a relatively exposed intertidal sandflat, Drum Sands, Firth of Forth, Scotland. Because of the spatially heterogeneous development of the weed, the short term (4 weeks) and long term (20 weeks) effects of the weed could be studied using a survey approach in which the weed-affected and weed-free plots were interspersed. After 4 weeks, V. subsimplex significantly increased the mean number of individuals and diversity of the macrofauna, eight of the ten most abundant species showed significant increases in abundance compared to weed-free areas. After 20 weeks, mean number of species and individuals were significantly higher under weed patches, while species diversity was reduced due to the numerical dominance of Pygospio elegans (Claparède). The weed, therefore, had an enriching effect on the macrofaunal communities on Drum Sands. The increased numbers of P. elegans, the numerical dominant infaunal species on Drum Sands, resulted mainly from enhanced larval recruitment to weed-affected areas. The effects of V. subsimplex on sediment characteristics were similar to those reported for other macroalgal taxa, i.e., increased water, organic and silt/clay contents, medium particle size and sorting coefficients, and reduced redox potentials. The results from this study are compared to those for other, morphologically different macroalgal species, with particular reference to an Enteromorpha-implanted experiment on the same sandflat. Since the general effect of such macroalgal taxa on macrofaunal communities is a detrimental one, the present study supports the contention that macroalgal morphology is an important feature in algal–faunal interactions.     

Mechanisms of recovery and resilience of different components of mosaics of habitats on shallow rocky reefs

Understanding the mechanisms producing and maintaining discontinuities between patches in mosaics of habitats is necessary to predict changes in patterns of abundance and distribution of species. On temperate rocky reefs, physical and biological disturbance can result in a mosaic of patches of encrusting coralline, turf- or canopy-forming algae. We experimentally investigated the effects of disturbance within and at the boundary between these patches, in order to identify mechanisms accounting for re-colonisation of space and to assess whether the response of boundaries can alter the relative size of contrasting habitats. Also, we tested whether the resilience of the different types of assemblages depends on species richness (i.e. number of taxa present) of habitats or, alternatively, on other properties of systems like differences in life-history traits of dominating species. Although the nature of the mechanisms generating differences among habitats changed among different stages of the colonisation, local processes (within patches) prevailed over larger scale processes (among patches) in determining early patterns of colonisation of space in mosaics. By the end of the experiment, assemblages in clearings at boundaries had recovered to the undisturbed reference condition, in contrast to clearings within barren patches or algal turfs. Boundaries represent, therefore, relatively more stable components of the mosaic, with greater resilience than adjacent habitats. Although sea urchins are often indicated as the main force regulating the proportion of contrasting habitats on shallow rocky reefs, determining the nature of variation in the interaction between species dominating each type of habitat is crucial to understand dynamics of mosaics. Finally, our study provides evidence that resilience could not be entirely controlled by initial species diversity, suggesting that different proportions of dominant taxa could influence the stability of natural systems.     

Relationships between benthic macrofauna and biogeochemical properties of sediments at different spatial scales and among different habitats in mangrove forests

In muddy intertidal sediments, there are reported complex interactions between the biological, physical and chemical properties of the sediment and the benthos that live in and on it. These are expected to be reflected in strong relationships between benthic animals and particular properties of the sediment, although some research has shown these relationships to be relatively weak. This study investigates the relationships between benthic macrofauna and biogeochemical properties of sediments within and among different habitats in multiple mangrove forests in a temperate estuary, in order to address the generality of any such relationships. Matched samples of benthos and sediment were collected from three habitats, which differed with respect to shading, the amounts of algae and leaf litter and the presence of pneumatophores. The sediment was sampled for water content, grain size, organic matter, chlorophylls a and b and colloidal and total carbohydrate.Spatial variation in sediment and benthos were significantly correlated across habitats in two of the three bays, but the sediment properties that contributed to differences between habitats and those that best correlated with the benthos varied among bays. In all bays, the single taxon that best correlated with the sediment was spionid polychaetes, but correlations were generally weak. There was no generality in the patterns of variation among the benthos or sedimentary properties among habitats. The benthos differed significantly among bays in all habitats, with large variation within and among sites. The sediment varied significantly at small scales in all habitats, but significant differences among bays were only found in two habitats. All spatial scales contributed to the total amount of variability in the sediment and there was little predictability from the patterns shown in one habitat to those in other habitats, or from one component of sediment to other components. Such variability suggests that there may be structural redundancy in this fauna, with different components of the benthos contributing to similar functions in different places. Such variability must be considered in experiments designed to understand relationships between ecological structure and function in these complex habitats and in any sampling studies to identify environmental impacts in these habitats.     

The effect of litter type and macrofauna community on litter decomposition and organic matter accumulation in post-mining sites

Field microcosms consisting of mineral soil (spoil substrate) and two types of litter taken either from an unreclaimed site with spontaneously developed vegetation (mostly Salix caprea) or from an alder plantation (a mixture of Alnus glutinosa and A. incana) were exposed in spontaneously developed or reclaimed sites at a post-mining heap near Sokolov (Czech Republic) for one year. The litter types differed remarkably in C:N ratio which was 29 for spontaneous litter and 14 for alder litter. The two microcosm types were either accessible or not accessible to soil macrofauna. The effect of macrofauna exclusion on soil mixing was complex and depended on litter quality and the site that determined soil fauna composition. In reclaimed sites where macrofauna was dominated by saprophags, mainly earthworms, the macrofauna access increased soil mixing. In sites where predators dominated, the macrofauna exclusion probably suppressed fragmentation and mixing activity of the mesofauna.     

Interacting effects of leaf litter species and macrofauna on decomposition in different litter environments

The leaf litter environment (single species versus mixed species), and interactions between litter diversity and macrofauna are thought to be important in influencing decomposition rates. However, the role of soil macrofauna in the breakdown of different species of leaf litter is poorly understood. In this study we examine the multiple biotic controls of decomposition – litter quality, soil macrofauna and litter environment and their interactions. The influence of soil macrofauna and litter environment on the decomposition of six deciduous tree species (Fraxinus excelsior L., Acer pseudoplatanus L., Acer campestre L., Corylus avellana L., Quercus robur L., Fagus sylvatica L.) was investigated in a temperate forest, Wytham Woods, Southern England. We used litterbags that selectively excluded macrofauna to assess the relative importance of macrofauna versus microbial, micro and mesofauna decomposition, and placed single species bags in either conspecific single species or mixed species litter environments. The study was designed to separate plant species composition effects on litter decomposition rates, allowing us to evaluate whether mixed species litter environments affect decomposition rates compared to single species litter environments, and if so whether the effects vary among litter species, over time, and with regard to the presence of soil macrofauna. All species had faster rates of decomposition when macrofauna were present, with 22–41% of the total mass loss attributed to macrofauna. Macrofauna were most important for easily decomposable species as soon as the leaves were placed on the ground, but were most important for recalcitrant species after nine months in the field. The mass loss rates did not differ between mixed and single species litter environments, indicating that observed differences between single species and mixed species litterbags in previous field studies are due to the direct contact of neighbouring species inside the litterbag rather than the litter environment in which they are placed.     

Differential effects of habitat complexity,predators and competitors on abundance of juvenile and adult coral reef fishes

Greater structural complexity is often associated with greater abundance and diversity, perhaps because high complexity habitats reduce predation and competition. Using 16 spatially isolated live-coral reefs in the Bahamas, I examined how abundance of juvenile (recruit) and adult (non-recruit) fishes was affected by two factors: (1) structural habitat complexity and (2) the presence of predators and interference competitors. Manipulating the abundance of low and high complexity corals created two levels of habitat complexity, which was cross-factored with the presence or absence of resident predators (sea basses and moray eels) plus interference competitors (territorial damselfishes). Over 60 days, predators and competitors greatly reduced recruit abundance regardless of habitat complexity, but did not affect adult abundance. In contrast, increased habitat complexity had a strong positive effect on adult abundance and a weak positive effect on recruit abundance. Differential responses of recruits and adults may be related to the differential effects of habitat complexity on their primary predators. Sedentary recruits are likely most preyed upon by small resident predators that ambush prey, while larger adult fishes that forage widely and use reefs primarily for shelter are likely most preyed upon by large transient predators that chase prey. Increased habitat complexity may have inhibited foraging by transient predators but not resident predators. Results demonstrate the importance of habitat complexity to community dynamics, which is of concern given the accelerated degradation of habitats worldwide.     

Seedling growth strategies and seed size effects in fourteen oak species native to different soil moisture habitats

 Seedling growth and morphology are thought to reflect evolutionary responses to habitat or influences of seed size. To test these hypotheses, we selected fourteen species of North American oaks differing in soil moisture habitat preference and seed size. Seedlings were grown for 1 – 2 years with abundant soil water and moderate soil nutrition in pots placed outdoors and in a common garden. Oak species native to xeric environments produced the smallest seedlings. Oaks from hydric soils had more shoot weight per unit of root weight and more height per unit of total plant weight than did mesic or xeric oaks. Essentially no differences in leaf area per unit of total plant weight were detected. Species with thinner and larger individual leaves tended to produce larger seedlings. Within species, seed size was generally unrelated to seedling growth, although results may have been complicated by uncontrolled genotypic variability. However, when species were compared, those with larger mean seed size produced larger seedlings. Root/shoot allometry, height growth and leaf thickness in the tested species may reflect evolutionary responses to soil moisture and flooding. Although seed size influenced seedling growth, no clear relationship between seed size and soil moisture habitat was found. Received: 26 March 1995 / Accepted: 30 November 1995     

Long-term effects of local disturbance history on mobile stream invertebrates

The patchy distribution of benthic invertebrates in streams and rivers is an important and widely researched phenomenon. Previous studies on reasons for this patchiness have neglected the potential role of local disturbance history, probably because most lotic invertebrates are mobile and any effect of disturbance history was thought to be short-lived. Here we demonstrate for a New Zealand gravel-bed stream that local disturbance history can have long-term effects on the distribution of highly mobile stream invertebrates. Buried scour chains (100 at each of three 20-m sites within a 350-m reach) indicated that a spate with a return period of 5 months caused a mosaic of bed patches with different stabilities. More than 2 months after the spate, we took random, quantitative samples at each site from five patches that had experienced 4 cm or more of scour during the spate, from five patches with 4 cm or more of fill, and from five stable patches. Density of the dominant invertebrate taxon, the highly mobile mayfly Deleatidium spp., and densities of another three of the seven most common taxa differed significantly between patch stability categories. Larvae of Deleatidium, the black fly Austrosimulium spp. and the dipteran Eriopterini were most abundant in fill patches, whereas Isopoda were most abundant in scour patches. Total invertebrate densities and densities of six common taxa also differed between sites, although these were only 95–120 m apart. These results show that local disturbance history can have long-term effects on lotic invertebrates and be an important cause of invertebrate patchiness. The observed effects might have been even stronger had we sampled sooner after the spate or after a large flood. Disturbance history may influence invertebrates both directly (through dislodgement or mortality) and indirectly, through effects on the spatial distribution of their resources. Our results suggest that the role of disturbance in structuring animal communities dominated by mobile species may be more important than previously thought. Received: 25 January 2000 / Accepted: 14 April 2000     

Density and environmental effects on shell size in some sand dune snail populations

Populations ol three species of land snail, Helicella itala. Candidula intersecta and Cochlicella acuta are studied on sand dunes on Coll in the Inner Hebrides.
Population density and mean shell size were estimated on 18 sample sites. The sites were ranked for six environmental factors. Environmental factors which are related to the dune vegetational succession account for much of the variation in the densities of C. intersecta and C. acuta . The density of H. itala , however, shows no strong association with these factors. Both snail density and shell size are reactively independent of total soil calcium levels. For each of the three species, mean shell size is negatively associated with population density; the environmental factors account for little of the variation in shell size.
Four possible mechanisms are suggested to account for the variation of shell size with density. It is argued that a direct influence of density on shell size, possibly mediated by mucus conditioning of the environment is the most probable mechanism. There is some evidence to suggest an inter-specific effect whereby C. intersecta density affects H. itala shell size.     

Density and environmental effects on shell size in some sand dune snail populations

Populations ol three species of land snail, Helicella itala. Candidula intersecta and Cochlicella acuta are studied on sand dunes on Coll in the Inner Hebrides. Population density and mean shell size were estimated on 18 sample sites. The sites were ranked for six environmental factors. Environmental factors which are related to the dune vegetational succession account for much of the variation in the densities of C. intersecta and C. acuta. The density of H. itala, however, shows no strong association with these factors. Both snail density and shell size are reactively independent of total soil calcium levels. For each of the three species, mean shell size is negatively associated with population density; the environmental factors account for little of the variation in shell size. Four possible mechanisms are suggested to account for the variation of shell size with density. It is argued that a direct influence of density on shell size, possibly mediated by mucus conditioning of the environment is the most probable mechanism. There is some evidence to suggest an inter-specific effect whereby C. intersecta density affects H. itala shell size.     

Effects of chronic bottom trawling disturbance on benthic biomass, production and size spectra in different habitats

Bottom trawling has widespread impacts on benthic communities and habitats. It is argued that the impact of chronic bottom trawling on benthic infauna depends on the natural disturbance levels to which benthic communities are adapted. We analysed biomass, production and size structure of two communities from a muddy and a sandy habitat, in relation to quantified gradients of trawling disturbance on real fishing grounds. We used an allometric relationship between body mass and individual production to biomass ratio to estimate community production. Chronic trawling had a negative impact on the biomass and production of benthic communities in the muddy habitat, while no impact was identified on benthic communities from the sandy habitat. These differences are the result of differences in size structure within the two communities that occur in response to increasing trawling disturbance.     

The effect of deposition of organic matter on phosphorus dynamics in experimental marine sediment systems

The effect of deposition of organic matter on phosphorus dynamics in sandy marine sediments was evaluated using an experimental system (boxcosms) and three different strategies: (1) no supply (2) one single addition (3) weekly additions of a suspension of algal cells (Phaeocystis spec.). Macrofauna (3 species, 6 individuals of each) were added to half of the boxes. Both in the case of the single and weekly additions a clear effect of increased organic matter loading on phosphorus dynamics was found. Following the organic matter addition, porewater phosphate concentrations in the upper sediment layer increased, phosphate release rates from the sediment increased by a factor 3–5 and in the boxes to which a single addition was applied NaOH-extractable phosphorus increased substantially. The increase in phosphate release rates from the sediment was attributed to mineralization of the added material and to direct release from the algal cells. No clear effect of the presence of macrofauna on sediment-water exchange of phosphate could be discovered. The macrofauna were very effective at reworking the sediment, however, as illustrated by the organic carbon profiles. It is hypothesized that the sediment-water exchange rates of phosphate were regulated by the layer of algal material which was present on the sediment surface in the fed boxes. In the boxes to which the single addition was applied porewater phosphate concentrations were lower and NaOH-extractable phosphorus was higher in the presence of macrofauna, suggesting that macrofauna can stimulate phosphate binding in the sediment.Publication no. 40 of the project Applied Scientific Research Netherlands Institute for Sea Research (BEWON)     

Evaluating the importance of predation on subtidal benthic assemblages in sandy habitats around rocky reefs

It was proposed that predation could explain differences in the structure of the benthic macrofaunal assemblages in subtidal sandy sediments close to compared with those far from rocky reefs. This hypothesis was tested using experimental exclusion cages and partial cages at two sites at two distances at two different rocky reefs. Undisturbed uncaged assemblages of macrofauna close to the rocky reefs were generally different from those in the partial cages and full cages. However, caging artifacts could not be detected and there were no strong correlations between the macrofauna and the proportions of different grain size and organic content. The structure of the macrofaunal assemblages close to the rocky reefs was, nevertheless, different from those far from the reefs and the sediments were finer far from than close to the rocky reefs. The results indicated that factors other than predation or grain size caused the differences in the macrofauna. For the spatial and temporal scales used in this study, it was clear that, although predation maybe intense, on its own it cannot explain the differences in the structure of the assemblages close and far from rocky reefs. The importance of adequate replication on caging experiments is discussed and it is suggested that alternative ways need to be found to test predictions about the influence of predation on soft sediment benthic assemblages.     

Pollen-pistil size correlation and pollen size-number trade-off in species of Argentinian Nyctaginaceae with different pollen reserves

Data on pollen and pistil traits from 14 Argentinean Nyctaginaceae species with starch or lipids as pollen reserves are presented. We expect differences in the traits between these two groups of species, but the same pattern within each group for (a) the relationship between pollen size and pistil length assuming that pollen tube length is predetermined by provisions in the pollen independently of the pollen reserve type, and for (b) a trade-off between size and number because available resources for male function are not unlimited. In particular we expect that (a) species with longer pistils will have larger pollen grains, (b) pollen grain size and the number of pollen grains per flower will be negatively correlated. Significant differences in the mean pollen number per flower and mean pollen size were observed between species with different pollen reserve type but not for pistil length. Then, correlation analyses were performed for species with starchy pollen or with pollen with lipids separately. Pollen size - pistil length correlation was positive and significant for species with starchy pollen but not for species with pollen with lipids. On the other hand, pollen size - number correlation was not significant for both starchy and oil-rich species. Results suggest that pollen reserve type would be a relevant factor that constraint pollen size in species of Nyctaginaceae, and that this pollen trait should be considered when studying pollen-pistil relationships, mainly between species of those families with mixed pollen reserves.     

Biodiversity of testate amoebae (Arcellinida and Euglyphida) in different habitats of a lake in the Upper Paraná River floodplain

This study aimed to investigate the testate amoebae (Arcellinida and Eugliphida) species diversity in plankton, macrophytes and aquatic sediment samples from a shallow lake of the Upper Paraná River floodplain. Samples were carried out from April 2007 to March 2008. We recorded 89 taxa, belonging to 10 families. Eighty-two taxa were found in the aquatic sediment, 71 in the macrophytes and 53 in the plankton. Highest values of alpha diversity were observed in the aquatic sediment. Although the plankton had the highest number of accidental species, accessory and constant species were also observed in this habitat. Most of the species classified as constant for the plankton belonged to the genus Arcella. Most of the constant species in the macrophytes and aquatic sediment belonged to the genus Difflugia. This study supports the idea that the presence of these protists in the plankton should not be attributed only to stochastic processes because (i) the species diversity recorded in this habitat was remarkably high in relation to the total biodiversity of the lake, and (ii) we also recorded frequent and constant species in the plankton.