Feast or famine for intertidal grazing molluscs: a mis-match between seasonal variations in grazing intensity and the abundance of microbial resources

Distinct seasonal variations in the abundance of photosynthetic microbiota and limpet grazing intensity were recorded at Port St Mary, Isle of Man between January 1994 and June 1996. Microbial abundance was negatively correlated with insolation stress, while grazing intensity was positively correlated with sea and air temperature. These patterns result in a mis-match between the supply of and the demand for microbial resources with maximal grazing intensity during the summer and autumn, but maximal microbial standing stock during the winter and early spring. The importance of top-down control of microbial assemblages by grazing was demonstrated by experimental exclusion of limpets during autumn 1993. This resulted in a four-fold increase in the abundance of cyanobacteria within 6 days, followed by a more gradual proliferation of ephemeral algae during the next 4 weeks. The abundance of diatoms remained relatively constant and was not influenced by the removal of grazers at this time of year. The influence of microbial resource availability on the growth and mortality of limpets was examined using experimental enclosures of differing densities of either Patella vulgata or P. depressa. After 6 months, there were significant relationships between grazer density and both mortality and growth with increased mortality and reduced growth for P. vulgata at increased densities, and reduced growth for P. depressa at increased densities. Hence, the availability of microbial resources may also influence the biomass of grazers on rocky shores from the bottom upwards. A conceptual model is presented which describes seasonal and annual variations in microbial resources and grazing intensity and their potential consequences for other shore dwellers.     

The vertical distribution and seasonal abundance of intertidal microalgae on a rocky shore in New South Wales

The abundance of intertidal microalgal food resources was estimated on shores at Cape Banks (Botany Bay, New South Wales) by indirect assays of the concentration of chlorophyll on and in the surface of sandstone rocks. Examination of the relationships between chlorophyll concentration and weight and area of rock samples allowed valid assays to be made despite uncontrollable variations from replicate to replicate in the amount of rock sampled. Microalgal cells were also sampled directly by brushing them from the surface of the rock. The relationship between amount of chlorophyll in samples, and the number of microalgal cells in immediately adjacent brushed areas indicated a relatively constant amount of chlorophyll per cell at different heights and on different parts of the shore throughout different seasons of the year. These preliminaries determined that assays of chlorophyll provided reliable, repeatable estimates of the number of microalgal cells present.Replicated sampling at five heights on a transect spanning the mid-shore regions of a sheltered shore dominated by grazing gastropods, at approximately monthly intervals from 1977 to 1979, revealed a vertical gradient of increasing abundance of microalgae towards the bottom of the shore, except during summer. This trend was not correlated with decreasing abundance of microalgal grazers. During summer, the abundance of microalgae declined abruptly at all levels. The decrease was greatest at the lower levels, resulting in little or no difference among the abundances of microalgae at the five heights during summer. The seasonal changes were again unrelated to any major changes in densities of grazers. These trends confirm the general patterns of distribution of intertidal microalgae in previous studies in other parts of the world. The present data are discussed with respect to problems of interpretation of microalgal food resources for intertidal grazers.     

Analysis of inter- and intra-specific competition amongst intertidal limpets with different methods of feeding

Summary The prosobranch limpet Cellana tramoserica grazes on microalgae, including the spores of macroalgae, and coexists with the pulmonate limpets Siphonaria denticulata and S. virgulata at mid-tidal levels on sandstone shores in New South Wales. These siphonarians graze on macroalgae, leaving the basal parts of the thalli intact. Where Cellana graze, they are capable of removing all algae; where Siphonaria graze, they leave at least a thin film of alga on the rocks, which is available to Cellana. S. denticulata normally show invariant homing behaviour, whereas S. virgulata tend to move around at random when Cellana are present, but apparently home when Cellana are absent. Both siphonarians have been observed to show opportunistic behavioural responses by moving towards patches of macroalgal foods when they become available.Experimental ecclosures of limpets at different densities and in different combinations revealed that Cellana tramoserica suffered increased mortality and reduced growth due to intraspecific competition when at increased densities. There was no effect on Cellana of increased densities of either species of Siphonaria. Nor was there any interspecific interaction between the siphonarians. Both species of Siphonaria showed some reduction of growth at increased intraspecific density. More importantly, both showed increased mortality when enclosed with low densities of Cellana. Larger densities of Cellana had no effect; the numbers of Cellana could not be maintained because of the reductions caused by intraspecific competition. Even after 27 weeks in enclosures with Cellana, the numbers of Siphonaria never declined to zero in any experimental enclosure. Thus, Cellana has a competitive effect on the survival of siphonarian limpets, but is unable to exclude them from an area of the shore. Siphonaria spp., in contrast, have no effect on Cellana.The nature of the competitive interactions between these types of limpets is explaied in terms of their methods of feeding; Cellana can exploit the food-resource before it reaches a suitable size for Siphonaria. The coexistence of Siphonaria spp. with Cellana is discussed with respect to the behaviour of the pulmonates. Intraspecific competition leading to reduced densitities of Cellana, however, will ensure that Cellana cannot exploit all the food resources, and some will be available to Siphonaria. The consequences of inter- and intra-specific competition among grazing gastropods are discussed with reference to the structure of intertidal communities, and it appears that competition for food is fundamentally different from competitive interactions for space in the organization of such communities.     

Herbivorous caddisflies,macroalgae, and epilithic microalgae: dynamic interactions in a stream grazing system

Summary 1. During the low-flow period (April–October) in sunlit pools of Big Sulphur Creek (northern coastal California), the attached algal community predictably changes from an assemblage dominated by lush, upright Cladophora glomerata filaments in spring and early summer to one dominated by epilithic diatoms and blue-green algae (together=microalgae) in late summer through early autumn. Previous studies in this stream indicated that grazing by the caddisflies Helicopsyche borealis and Gumaga nigricula maintain low algal biomass during the latter part of this period. We used a combination of in situ exclusion/enclosure experiments to examine (1) the separate and combined effects of these grazers on Cladophora and microalgal assemblages, and (2) food preferences, growth, and microdistribution patterns of grazers when offered these different algal foods. 2. Grazers exerted strong but divergent effects on algal assemblages. Selective grazing on Cladophora by G. nigricula greatly accelerated the transition from upright Cladophora to epilithic microalgae, whereas selective grazing on microalgae by H. borealis dramatically reduced biomass of these forms. Grazers were largely ineffective at reducing the non-preferred algal food source (i.e. Cladophora by H. borealis, microalgae by G. nigricula). In the case of each grazer, growth was highest on the preferred algal food. Together, the activity of these grazers produced a low-biomass assemblage dominated by microalgal cells. 3. Removal of the Cladophora overstory by G. nigricula resulted in a three-fold increase in the abundance of epilithic microalgae, the preferred food of H. borealis. Elimination of Cladophora by G. nigricula can increase food availability for H. borealis and, in so doing, can indirectly facilitate the growth of this grazer during food-limited conditions. However, microdistribution of G. nigricula shifts from high overlap with H. borealis in spring and early summer when Cladophora is abundant to low overlap in late summer after Cladophora has been eliminated. This may indicate intense competition between these species for limited epilithic algae, and a concomitant movement by G. nigricula to areas in the stream where food resources are more available.     

Effects of interactions between algae and grazing gastropods on the structure of a low-shore intertidal algal community

Summary At low levels on shores in New South Wales, foliose algae are abundant and often occupy all substrata; microalgal grazing gastropods are rare or absent. At higher levels, foliose algae are sparse or absent and grazing gastropods are abundant. Hypotheses for the causes of the lower vertical limits of distribution of these grazers include the effects of increased predation or the deleterious physiological effects of increased period of submergence at lower levels on the shore. Alternatively, the presence of the algae, because they occupy space and deprive the grazers of substratum for feeding, may prevent the downward movement, or survival of the grazers at low levels. Under the first two of these hypotheses, algae are able to colonize and grow in low-shore areas as an indirect result of factors which remove grazers. Under the third hypothesis, the algae are directly responsible for the lack of grazers.Experimental clearings of the low-shore algae and introductions of the mid-shore limpets Cellana tramoserica and Siphonaria denticulata were used to test these hypotheses. C. tramoserica grazes microalgae and removes them from the substratum, preventing colonization. S. denticulata, in contrust, crops the algae, leaving a visible cover of algae on the substratum, which can grow rapidly. Because of its method of feeding, S. denticulata had no measurable impact on the rates of colonization, nor on the dry weights of algae, compared with those of ungrazed areas. C. tramoserica could keep cleared areas tree from foliose algae, but only when the limpets were mainfained in great density (10 per 900 cm²). They were less effective where wave-action was greater.Neither species of limpets could survive when placed onto beds of mature algae, because they had no substratum on which to cling and were swept away by the waves. C. tramoserica did not invade clearings below their lower limit of distribution where they had to move over a bed of foliose algae. Few C. tramoserica moved directly downshore into cleared areas. When placed on bare rock within low-shore beds of algae of different ages, S. denticulata remained amongst the algae and maintained their tissue-weights. Few C. tramoserica remained in areas with well-developed algae, compared with areas having sparse algal growth. Those Cellana which remained amongst well-developed algae lost weight, whereas limpets in areas with less algal growth mammtained their weights. In experimental cages in low-shore beds of algae, where the limpets were inaccessible to potential predators, C. tramoserica lost weight and died. On cleared areas they survived for many weeks, but lost weight and died as algae grew and covered the substratum. In the absence of predation, the micro-algal grazer C. tramoserica could not survive in lowshore areas because algae grew too fast and occupied the substratum, making it inaccessible for the limpets to graze; the algae, once grown beyond small sporelings, are not a suitable food-source for C. tramoserica, and the loss of weight and death of these limpets is attributable to starvation.The lower limit of distribution of C. tramoserica is not due to the direct effects of physical factors associated with prolonged submersion, nor to the impact of predators, but is apparently determined by the presence of rapidly growing, extensive beds of foliose algae at low levels on the shore. The cause of the limit of distribution of S. denticulata is not yet known and predation may prove to be important. Removal of S. denticulata from low-shore algal beds would not, however, affect the domination of substrata by algae. Grazing by S. denticulata at very great density had no effect on algal cover nor weight. In the intertidal community studied, the persistence of a low-shore algal zone, bounded above by abundant grazers is not influenced by the activities of predators, but is a direct result of interactions between the grazers and the algae.     

Positive abundance and negative distribution effects of a gastropod on an intertidal hermit crab

Summary Field experiments were used to determine the effect of a common intertidal snail (Nerita funiculata) on the use of space for foraging by the hermit crab Clibanarius digueti. Removals of Nerita resulted in an increased density of foraging Clibanarius, while additions of the gastropod had the opposite effect. The observed negative effect of the gastropod on individual hermit crabs appears to be food-related. Field surveys, however, suggested that the hermit crab population is limited by shell number, rather than food. Because Nerita contributes to the shell resource, its effect on the hermit crab population is positive. Nerita, therefore, has a negative effect on the distribution of foraging hermit crabs, but a positive effect on their abundance. Such decouplings of distribution and abundance effects are rare.     

Experimental studies on homing in the intertidal patellid limpetCellana tramoserica (Sowerby)

Summary A model of two-dimensional random walk was developed to allow statistical tests for the presence of homing behaviour in intertidal populations of the limpetCellana tramoserica. Not all limpets return to a home-site after feeding excursions. Some move around at random. The latter pattern of movement was tested to justify the assumptions of the model. Limpets can stop homing and begin to move at random; moving limpets can become homers. Thus, random movement and consistent homing behaviour are not discrete patterns of behaviour in a population.Controlled field experiments showed that the proportion of animals which home is not affected by the height on the shore, force of wave action, the cover of macroalga, nor the irregularity of the substratum. Small limpets, however, home more than large ones.Close proximity to other homing limpets causes individuals to stop homing and move away. Alterations of density demonstrated thatCellana shows density-dependent dispersal. More limpets emigrate from areas of increased density and more immigrate into areas of decreased density, when compared with control areas. Less animals immigrate into, and remain in, areas from which microalgal food was experimentally removed. More animals emigrate from these areas.These experiments support the hypothesis that homing behaviour is an adaptation which regulates local density and dispersion to maximize utilization of food resources and, thus, to reduce intraspecific competition for food at high densities of limpets.     

Intertidal zonation of prosobranch gastropods: Analysis of densities of four co-existing species

The densities of populations of Nerita atramentosa Reeve, Austrocochlea constricta Lamarck, Bembiciuin nanum (Lamarck), and Cellana tramoserica (Sowerby) were recorded in 0.25 m² quadrats on landward and seaward halves of a sheltered and a moderately exposed rock platform from June 1972 to June 1973. The data have been subjected to variance and multiple regression analysis. Differences between densities of each species on the two shores and in the two areas of each shore are discussed with respect to the regressions on littoral height. Densities of Nerita on the sheltered shore, Austrocochlea and Bembicium on both shores, and Cellana, on the exposed shore, increased with height. The density of Cellana on the sheltered shore decreased as height increased. The density differences in the landward and seaward areas on the two shores were attributable to the effect of height for Nerita and Austrocochlea. Height was the major variable affecting the density of these two species on the sheltered shore. Bembicium and Cellana densities on both shores, and Austrocochlea density on the exposed shore, are not primarily determined by height.On the sheltered shore, Nerita and Cellana increased in density with increasing cover of water in rock pools. On the exposed shore, Bembicium and Cellana became sparser with increasing water cover. Bembicium on both shores increased in density with increasing cover of the alga Peyssonelia, as did Cellana on the exposed shore. Cellana and Austrocochlea tended to increase in density with Nerita on the sheltered shore. Cellana tended to be sparse where Bembicium was dense on the exposed shore. The biological effects of substrata and other species on the densities of each species remain unexplained.Significant time effects were found for variations in the densities of Nerita and Cellana on the sheltered shore and of Austrocochlea and Cellana on the exposed shore. The seasonal change in density of Nerita was parabolic with a minimum in summer, and is consistent with the known period of recruitment of this species. The lack of significant time effects on density of Austrocochlea on the sheltered shore is consistent with the known continuous recruitment of this species. The declining density of Austrocochlea on the moderately exposed shore was found, by size-frequency analysis, to be due to reduced recruitment compared with surrounding shores. The density of Cellana on both shores showed seasonal changes, with a maximum in summer, which is consistent with the known period of recruitment in Cellana. The lack of seasonal changes in density of Bembicium is briefly discussed.This type of multifactor analysis identifies biological interactions affecting variation in density of these species. It also corroborates the results of other ecological investigations and determines the relative importance of a variety of physical and biological factors in the distribution of density of intertidal gastropods.     

Distribution and abundance of the acmaeid limpet,Patelloida latistrigata,and its interaction with barnacles

Summary At Cape Banks, New South Wales, adults of the small intertidal limpet, Patelloida latistrigata occur exclusively in the barnacle zone, and are primarily associated with the barnacle, Tesseropora rosea. Limpet density increases with barnacle density. Juvenile limpets can be found throughout the barnacle zone, and on patches of bare rock that may be temporarily available at lower levels on the shore. The failure of juveniles to survive and grow in places other than among Tesseropora is due to a combination of factors. These include desication at high levels on the shore, smothering by rapidly growing algae low on the shore, and the grazing activities of the larger limpet, Cellana tramoserica. These latter two factors also reduce the survival of experimentally transplanted adult Patelloida: algae by covering the substratum and smothering the limpets, and Cellana by outcompeting them for food. The density of Cellana is greater on patches of bare rock than among barnacles, and these large limpets may be unable to move and feed effectively over the irregular surface created by Tesseropora. Patelloida, however, is small enough to feed over and among these barnacles, and hence has a refuge from competition with Cellana. Barnacles may also provide shelter from the effects of desiccation and strong wave action, and thus increase the survival of juvenile Patelloida. By being associated with barnacles, however, Patelloida becomes vulnerable to intermittent predation by the whelk, Morula marginalba. This effect may be serious enough to eliminate small local populations of limpets, either by direct predation or by removing the refuge-providing barnacles. The association with barnacles may also limit the maximum size to which Patelloida can grow. Patelloida is not always found with Tesseropora, and adults in different localities can be found in association with other sessile organisms. It may be argued that small species of limpets require a spatial refuge from physical and/or biological pressures. To examine this hypothesis, the relationship between Patelloida and Tesseropora is compared to other published accounts of limpets with specialised modes of life.     

Seasonal succession of ciliates in lake constance

We found a recurrent seasonal pattern in abundance and composition of planktonic ciliates in Lake Constance, FRG, over a three-year period. Abundance peaks occurred in early spring and summer/autumn, while ciliate numbers were low in late spring (clear-water phase) and winter. Prostomatida and Oligotrichida dominated in early spring. They responded immediately to the phytoplankton spring bloom, while Haptorida, Peritrichida, and large Scuticociliatida (Histiobalantium) were delayed by 1 to 2 weeks. The spring community broke down at the onset of the clear-water phase.Pelagohalteria viridis containing symbiontic algae appeared shortly after this event. A highly diverse community was recorded in summer/autumn. Peritrichida, small Oligotrichida, and large Scuticociliatida reached their maxima during this season. Small Scuticociliatida were rare throughout the year and contributed moderately to total ciliate numbers only during the cold season. The observed seasonal sequence of pelagic ciliates in Lake Constance is discussed in relation to simultaneously collected data on potential food organisms and grazers.     

Seasonal variation in epifaunal communities associated with giant kelp (Macrocystis pyrifera) at an upwelling‐dominated site

Kelp forests are highly productive and species‐rich benthic ecosystems in temperate regions that provide biogenic habitat for numerous associated species. Diverse epifaunal communities inhabit kelp sporophytes and are subject to variations in the physical environment and to changes experienced by the kelp habitat itself. We assessed seasonal variations in epifaunal invertebrate communities inhabiting giant kelps, Macrocystis pyrifera, and their effects on this seaweed. Six seasonal samplings were conducted over a year at an upwelling‐dominated site in northern‐central Chile where physical conditions are known to fluctuate temporally. More than 30 taxa were identified, among which peracarid crustaceans stood out in both diversity and abundance. Species richness and abundance differed among sporophyte sections (holdfast and fronds) and throughout the year. The frond community was dominated by two grazers (the amphipod Peramphithoe femorata and the isopod Amphoroidea typa), while suspension feeders, grazers, and omnivores (the amphipod Aora typica, the isopod Limnoria quadripunctata, and polychaetes) dominated the holdfasts. Abundances of the dominant species fluctuated throughout the year but patterns of variation differed among species. The most abundant grazer (P. femorata) had highest densities in summer, while the less abundant grazer (A. typa) reached its peak densities in winter. Interestingly, the area of kelp damaged by grazers was highest in autumn and early winter, suggesting that grazing impacts accumulate during periods of low kelp growth, which can thus be considered as ‘vestiges of herbivory past.’ Among the factors determining the observed seasonal patterns, strong variability of environmental conditions, reproductive cycles of associated fauna, and predation by fishes vary in importance. Our results suggest that during spring and early summer, bottom‐up processes shape the community structure of organisms inhabiting large perennial seaweeds, whereas during late summer and autumn, top‐down processes are more important.     

Persistent spatial and temporal abundance patterns for the late-stage copepodites of Centropages typicus (Copepoda: Calanoida) in the US Northeast continental shelf ecosystem

The annual cycle of abundance and the monthly distributions of the copepod Centropages typicus are described for US Northeast Continental Shelf waters from samples collected on broadscale plankton surveys during 1977-87. High numbers of the copepod were captured throughout the region during the autumn months in weak south-north and onshore-offshore abundance gradients. The highest individual station densities were found near the mouth of the major estuaries and the heaviest broad-scale concentrations were usually located where bottom depth ranged from 20 to 39 m. Numbers declined throughout the ecosystem after winter arrived, less so in the southern half of the region where C.typicus abundance remained high year round in nearshore and midshelf waters between New York City and Chesapeake Bay. The copepod's abundance fell to much lower levels further north in the Georges Bank and Gulf of Maine subareas, and disappeared entirely from shelf waters in the northernmost offshore region until summer. Interannual abundance variability was substantial, but no long-term trend was detected. Analyses of samples collected from 1988 to 1996 on Georges Bank during early autumn indicate that abundance levels of C.typicus have been high here in the 1990s, completely recovered from low density values measured there in 1986 and 1987. Temperature and food availability were found to be the key factors that determine the copepod's distribution and annual abundance cycle. Mean abundance was high throughout the ecosystem where surface temperature was >9C and in regions where annual mean chlorophyll levels exceeded 1 mg m^-3. The copepod's abundance appeared to be independent from variation in water column salinity.      

Biochemical and energy dynamics throughout the reproductive cycle of the striped venus Chamelea gallina (Mollusca,Bivalvia)

The striped venus Chamelea gallina is an important commercial bivalve species in Europe. However, large inter-annual fluctuations in stock abundance and periodic recruitment failure threaten the biological and economic sustainability of this fishery. This study aimed to improve the knowledge of the reproductive cycle and reproductive strategies of this species from the Algarve coast (southern Portugal) in order to contribute to the establishment of management measures and to assess its potential for aquaculture. The reproductive cycle of C. gallina followed a seasonal cycle, significantly influenced by sea surface temperature and food availability. Gametogenesis took place in winter, coinciding with the phytoplankton bloom. Spawning occurred during summer, followed by a short period of sexual inactivity in autumn. Condition index did not reflect the reproductive cycle of C. gallina and generally, followed the same trend of chlorophyll a. Glycogen was positively correlated with gonadal index and chlorophyll a. High total lipid values were recorded throughout gonad ripeness and spawning, but decreased at the end of the spawning and in the rest period. The extended spawning period of C. gallina will allow larvae to be obtained for much of the year by artificial spawning of wild broodstock.     

Molluscan grazing and macroalgal zonation on a rocky intertidal platform at Perth,Western Australia

Abstract Intertidal limestone platforms off Perth show a characteristic pattern of algal zonation, with dense macroalgal beds nearshore bounded by a ‘barren zone’ along the seaward edge. Abalone (Haliotis roei) and several species of limpets and chitons are abundant in the barren zone, which is generally devoid of non-coralline macroalgae. The relative importance of abalone versus limpets and chitons in limiting macroalgal abundance in the barren zone was evaluated by manipulating the presence of each group in a factorial experiment. Percentage algal cover was measured photographically in 0.25m² plots at 1–2 month intervals for 9 months. Mean algal cover (mainly the foliose green alga, Ulva rigida) was highest in plots where all grazers were excluded (77–99%), intermediate where only limpets and chitons were excluded (37–85%), and lowest where only abalone were excluded (4–30%) or where no grazers were excluded (2–19%). The effect of limpets and chitons accounted for 55–89% of the variance in total algal cover, whereas the effect of abalone generally accounted for <10% of the variance. Similar results were obtained in terms of the biomass of Ulva rigida at the end of the experiment. Haliotis roei are relatively large and sedentary herbivores, feeding mainly on drift algae. Their effect on algal abundance was mediated both through pre-emption of space, which might otherwise be colonized by algae, and by grazing around their home scar. Limpets and chitons are smaller than abalone, but were much more abundant. Intensive grazing of the reef surface by limpets and chitons precluded the establishment of non-coralline macroalgae, even where abalone were absent.     

The effects of grazing by gastropods and physical factors on the upper limits of distribution of intertidal macroalgae

Summary The cover of foliose algae is sparse to non-existent above a low-level algal zone on many shores in N.S.W., except in rock-pools. Above this algal zone, encrusting algae, mostly Hildenbrandia prototypus, occupy most of the primary substratum on sheltered shores. Experimental manipulations at midtidal levels were used to test hypotheses about the effects of grazing by molluses and of physical factors during low tide on this pattern of algal community structure.Fences and cages were used to exclude grazers: molluscs grazed under roofs and in open areas. Cages and roofs provided shade, and decreased the harshness of the environment during low tide: fences and open areas had the normal environmental regime.In the absence of grazers, rapid colonization of Ulva and slower colonization by other foliose algae occurred in all experimental areas. The rate of colonization by Ulva sporelings was initially retarded on existing encrusting algae, but after a few months, cover of Ulva equalled that on cleared rock.Most species of algae only grew to maturity inside cages, and remained as a turf of sporelings inside fences. No foliose algae grew to a visible size in open, grazed areas. Grazing thus prevents the establishment of foliose algae above their normal upper limit on the shore, but the effects of physical factors during low tide prevent the growth of algae which become established when grazers are removed. Physical factors thus limit the abundance of foliose algae at mid-tidal levels.The recolonization of cleared areas by Hildenbrandia was not affected by the presence of a turf of sporelings, nor by the shade cast by roofs, but was retarded in cages where mature algae formed a canopy. Even under such a canopy, Hildenbrandia eventually covered as much primary substratum as in open, grazed areas. This encrusting alga is able to escape from the effects of grazing by having a tough thallus, and by its vegetative growth which allows individual plants to cover a lot of substratum, and by the tendency for new individuals to start growing from small cracks and pits in the rock, which are apparently inaccessible to the grazers.Mature foliose algae are removed from the substratum by waves, and many individual plants died during periods of hot weather. Sporelings in a turf were eliminated, after experimental fences were removed, by the combined effects of macroalgal grazers, which invaded the areas, and microalgal grarers which ate the turt from the edges inwards.The results obtained here are discussed with respect to other studies on limits to distribution of intertidal macroalgae, and the role of grazing in the diversity and structure of intertidal algal communities. Some problems of these experimental treatments are also discussed.     

Marten and fisher responses to fluctuations in prey populations and mast crops in the northern hardwood forest

Many forest tree species produce seed (mast) crops that are consumed by a variety of wildlife species and these pulsed resources may mediate interactions among predator and prey populations. In the northern hardwood forests of New York, we investigated interactions among mast production, prey abundance, and harvests of American martens (Martes americana) and fishers (Martes pennanti) during 1988–2009. Mast production for beech (Fagus grandifolia), sugar maple (Acer saccharum), and mountain ash (Sorbus americana) was synchronous and an alternate-year pattern in production was evident for most of the time series. We documented considerable temporal variation in summer small mammal relative abundance and our numerical response models received substantial support for 5 of the 8 species, indicating lagged responses to autumn mast crops. Trap response of martens to the autumn production of beech mast and mountain ash berries was immediate and numerical responses to the relative abundance of small mammal prey occurred during the preceding summer. The age structure of the marten harvest differed based on the dominant alternate-year pattern of summer prey relative abundance and autumn mast production (χ²₄ = 33.06, P < 0.001). The proportion of juvenile marten in the autumn harvest was 52% and 34% following summers when small mammal relative abundance was high and low, respectively and these differences resulted in a persistent cohort effect that was apparent until age 3.5. Trap response of fishers to the autumn production of beech mast was immediate and numerical responses to the relative abundance of Sciurid prey occurred during the preceding summer. Marten and fisher harvests fluctuated similarly among New York, Maine, and New Brunswick, which may indicate regional synchronization of mast crops and responses of martens and fishers to similar prey dynamics. A better understanding of how food availability influences demographic responses and trapping vulnerability of martens and fishers would aid our ability to manage harvests of these species on a sustained yield basis. © 2011 The Wildlife Society.     

The feeding of trout during the filling phase of Rutland Water

Fingerling Salmo trutta L. and S. gairdneri R. were stocked in Rutland Water in early 1975. Initial rapid growth was associated with feeding on littoral and planktonic Crustacea during this year. Rapid water level changes occurred over the winter months, interspersed with longer periods of stability during the summer. These changes caused changes in trout diet. Terrestrial food (e.g. earthworms), released from flooded land was important during winter months; aquatic items which increased in abundance during periods of stable water levels (e.g. Gammarus pulex, Limnaea pereger, Chironomidae larvae and pupae) were important during spring, summer and autumn.It is suggested that the rapid growth rates recorded in the first two years of the reservoir are the result of the ready availability of different food sources.Present address: Department of Adult Education, University of Leicester, Leicester, EnglandPresent address: Department of Adult Education, University of Leicester, Leicester, England     

Predation by small mobile aquatic predators regulates populations of the intertidal limpet Patella vulgata (L.)

Highly mobile aquatic predators are known to forage in the intertidal during periods of immersion. There is limited quantitative information, however, on the extent to which these predators influence the abundance of grazing molluscs which are known to have a key role in structuring intertidal assemblages. Our preliminary video observations revealed that crabs and small fish were abundant on shores in southwest England during high-tide. We then used manipulative experiments to quantify the effect of small mobile aquatic predators on the abundance of limpets (Patella vulgata L.). On the lower shore at two moderately sheltered rocky shores three treatments were established: complete cage, partial cage (cage control) and uncaged (natural condition). The complete cages excluded all predators. The partial cage treatment allowed full access to small predators and the uncaged treatment allowed access to all predators. After two months, limpet abundance in uncaged and partial cage treatments had declined by around 50% compared to the complete cage treatment. Population structure also changed with survival of larger individuals being greater than smaller individuals in the open and partial cage treatments compared to the complete cage treatment. The effects of excluding predators were consistent at small (meters) and large spatial scales (kilometres) and hence, it would appear that the outcomes of our research are generally applicable to similar shores in the region.To explore the mechanism behind the differential effects of predators according to prey size, we compared the detachment force required to remove limpets of differing sizes from the shore. This was around four times greater for larger individuals than for smaller ones indicating that smaller limpets were more vulnerable to predation. These effects were also consistent between locations. Subsequent laboratory observations showed that the crabs Carcinus maenas (L.), Necora puber (L.) and Cancer pagurus (L.) which are locally abundant predators of limpets, had differing handling behaviour but were all highly efficient at removing limpets from substratum. Hence, shell width and attachment force appeared to be critical factors influencing the vulnerability of limpets to predation by these crabs. Limpets are known to control the abundance of macroalgae on shores in the North-east Atlantic and so our conclusions about the role of mobile predators in regulating the abundance of these grazers are important to our broader understanding of the ecology of these shores.     

Seasonal Depth-Related Gradients in Virioplankton: Standing Stock and Relationships with Microbial Communities in Lake Pavin (France)

This study presents a depth-related survey of virioplankton abundance in Lake Pavin (Massif Central, France), in relation to the abundances of heterotrophic prokaryotes, picocyanobacteria (Pcy), autotrophic picoeukaryotes (Peu), and of autotrophic (ANF) and heterotrophic (HNF) nanoflagellates. The sampling strategy was designed to be representative of the physico-chemical gradients of the whole water column of the lake, and the seasonal variability as well. In mixolimnic surface waters, all communities were present and viral abundance peaked in summer and autumn. Viral abundance was significantly correlated (p < 0.001) with Pcy, Peu, and ANF, indicating that cyanophages and perhaps other phytoplankton viruses represent a significant pool of viral standing stocks in the mixolimnion of Lake Pavin. Microautotrophs were absent in the deep monimolimnic water masses, where viruses and heterotrophic prokaryotes exhibited highest seasonal abundances in summer and/or autumn and were significantly correlated (p < 0.001) to each other. This indicates that the anoxic monimolimnion of Lake Pavin is an exclusive habitat for viruses and heterotrophic prokaryotes. We conclude that in this habitat, host availability is prevalent over other factors (temperature, oxygen, nutrients, grazers) in favoring viral proliferation.     

Seascape structure and complexity influence temperate seagrass fish assemblage composition

Understanding how spatial patterning relates to ecological processes is fundamental to define important species–environment associations at broader scales. Analyses targeting habitat structure (i.e. composition and configuration) in terrestrial landscapes are increasing, but similar studies in marine landscapes are still relatively uncommon. In this study, we explored how seascape structure and complexity (determined from significant spatial pattern metrics) influenced summer and autumn fish assemblage composition in 30 seagrass (Zostera marina) meadows along the west coast of Sweden. Species density was not influenced by seascape structure in any season. In contrast, the majority of significant fish assemblage variables were influenced by seascape structure during the summer (i.e. abundance and proportion of juveniles, abundance of Labridae and abundance of occasional shallow‐water visitors) whilst fewer in the autumn (i.e. abundance of occasional shallow‐water visitors and Synganthidae). For instance, less complex seascapes were more suitable for juvenile assemblages in summer, as these seascapes exhibit larger patch sizes of appropriate habitat (e.g. Z. marina) and less edge boundaries providing refuges from predators and food resources. Abundances of migrating fish, such as the sea trout Salmo trutta, also responded positively to a less complex seascape in the summer though perhaps ecological processes, such as prey availability, were additional contributing factors driving this relationship. High complexity seascapes only had a positive influence on the abundance of taxa using multiple habitats (Labridae during the summer). Our study shows that fish assemblages in temperate marine environments are significantly linked to spatial habitat patterning and seascape complexity. This offers valuable insights into species–habitat–seascape linkages, information important for coastal conservation and marine spatial planning.