Absence Of Grazers Research Articles

Turbulence effects on cold ocean microbial communities: an enclosure study

There have been numerous studies on turbulence effects on macro- and zooplankton during the last decade, but studies on microplankton have been fewer, sometimes contradictory, and conducted largely under temperate conditions. In the present study, an enclosure experiment was carried out with natural seawater of Logy Bay, Newfoundland to determine the effects of turbulence on heterotrophic and autotrophic growth in the presence and absence of micrograzers. The experiment was conducted in February with water temperatures at 0 °C. We found no difference in bacterial growth between the static and turbulence enclosures in either the presence or absence of micrograzers. Heterotrophic nanoflagellate growth, however, was found to be significantly higher in the presence of turbulence but only when micrograzers were absent. A similar pattern was found with the small autotrophic community, determined as chlorophyll a passing through a 5-μm filter, in which growth was significantly higher with turbulence but only when micrograzers were absent. Turbulence significantly enhanced growth rates of the large autotrophic community (chlorophyll a retained by a 5-μm filter), which was largely composed of diatoms, both in the presence and absence of grazers. These results suggest that turbulence effects are small under cold ocean conditions, but increase with the size of the organism. It thus appears that turbulence could affect patterns of biogenic carbon export differentially, being dependent upon the conditions of the marine environment.

Effects of benthic grazers on microalgal communities of morphologically different encrusting corallines: implications for abalone recruits

Recruits of the South African abalone Haliotis midae are more abundant on irregularly textured than on smooth encrusting corallines, and experimental removal of the urchin Parechinus angulosus leads to the virtual disappearance of H. midae recruits. Explanations include the following; (1) In the absence of larger grazers, prostrate diatoms that are preferentially consumed by abalone recruits become displaced by less suitable overstorey diatoms, filamentous algae or blue-green algae; (2) textured corallines protect pockets of suitable diatoms, whereas large grazers eliminate these from smooth corallines. We tested these possibilities in aquarium experiments in which microalgal communities were grown on artificial coralline mimics with different textures and then exposed to different grazers at densities simulating those in the field. Microalgal standing stocks (measured as chlorophyll a) increased in the absence of grazers, but this was reflected only in the amounts of fila- mentous and blue-green algae. Diatoms remained unchanged in quantity and community composi- tion between different grazing treatments and substratum textures. Changes in diatom communities following urchin removal are thus unlikely to explain the ensuing disappearance of abalone recruits, although changes in filamentous and blue-green algae may do so. Textured surfaces did, however, consistently support higher stocks of microalgae than smooth surfaces (in either the presence or the absence of grazers), which may account for the preference of abalone recruits for irregular encrust- ing corallines in the field.

Interactions between substratum rugosity, colonization density and periwinkle grazing efficiency

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 225:239-249 (2002) - doi:10.3354/meps225239 Interactions between substratum rugosity, colonization density and periwinkle grazing efficiency M. Wahl1,*, K. Hoppe2 1University of Namibia, Windhoek, Namibia 2Zoologisches Insitut, Universität Kiel, Olshausenstr. 40, 24098 Kiel, Germany *E-mail: mwahl@zoologie.uni-kiel.de ABSTRACT: Both surface texture and littorinid grazing are known to influence the establishment of many shallow-water benthic hard-bottom communities. However, the effects of these factors and their interactions have not yet been investigated in a quantifiable manner. This investigation aims to assess the interactive effect of both factors in a strictly standardized manner. Natural recruitment by diatoms, the barnacle Balanus improvisus and the tube-building polychaete Polydora sp. was monitored under a 2-factorial treatment: grazing by the periwinkle Littorina littorea (Factor 1; 2 levels: 1 or no snails per plate) on artificial recruitment plates of different initial surface rugosities (Factor 2; 5 levels: smooth, 0.1, 0.5, 1 and 5 mm rugosity elements). In the absence of grazers, barnacle recruitment decreased with increasing initial rugosity, polychaete recruitment peaked at intermediate rugosities, and diatoms exhibited contrasting recruitment patterns in an in vitro and an in situ experiment. When preferred recruitment sites coincided for Polydora and B. improvisus, a competition for space could be inferred from a negative correlation between the 2 species. However, when the overlap of requirements weakened on the 5 rugosities, the relationship was positive, but was not statistically significant. Grazing efficiency by L. littorea depended on initial rugosity, generally showing minimum values on intermediate rugosities which is attributable to a mismatch between radula dimensions and surface structures in these rugosity classes. Additionally, grazing effects tended to increase with higher prey densities. As all factors‹initial rugosity, grazing, colonizer species‹interact with each other, the outcome of recruitment under combined factors is difficult to predict from single factor effects. KEY WORDS: Grazing efficiency · Surface rugosity · Recruitment · Multiple interactions Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 225. Online publication date: January 11, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

A ?microbialite carbonate crust at the Permian–Triassic boundary in South China, and its palaeoenvironmental significance

A 1 m thick carbonate crust, layered and commonly domal, caps crinoidal limestones on reef complexes of the top Permian Changxing Formation in the Huaying Mountains, eastern Sichuan, China. The crust's stratigraphic level lies at a sharp change in facies, and is overlain by poorly fossiliferous laminated micrites and shales of the Lower Triassic Feixianguan Formation. The crust therefore appears to coincide with the end-Permian extinction event, although the dating of the strata is currently imprecise. The crust is composed mostly of digitate carbonate, locally thrombolitic, with remnant lobate fabric, and resembles microbialites, but is mostly recrystallised and a microbial origin is unconfirmed. It is enclosed in micrite with pyrite crystals, ostracode and other shell debris. The crust is absent from interreef areas. Previous interpretations of karstification and calcrete formation are not upheld, and the facies were deposited under water. Overlying sediments are low energy, with abundant ferroan calcite and pyrite, reflecting anoxia associated with rising sea level. Various interpretations of the crust are possible: (1) it was organic, and microbia grew preferentially on topographic highs of reef tops; (2) microbia were a disaster biota in the absence of grazers; (3) microbia took advantage of favourable conditions for calcification, associated with a rapidly rising environmental-CO2 content during the Late Permian; (4) the crust was an inorganic precipitate associated with CO2-rich water. Microbial crusts are rare after the Cambrian, but whether this crust is organic or not, its presence in these strata reinforces the view that oceanic-atmospheric conditions in the Permian–Triassic boundary interval were unusual.

Carriage of dwarf males by adult female puelche oysters: the role of chitons

Ostrea puelchana (d'Orbigny) is the only ostreid for which carriage of epibiotic males by adult females (>55 mm) has been reported. On the natural grounds of San Matı́as Gulf (Argentina), larvae settle on the shells of adult female oysters. Spat concentrate on an expansion of the anterior margin of the concave shell (the platform), where they mature as males at about 2 mm of shell size, their growth being severely retarded as a result of interaction with the carrying female oyster. High concentration of spat on the platform relative to their density on the surface of the shell has suggested that a chemical released by the female might trigger a localized settlement response by the larvae. In this study I explore a different hypothesis, namely that the pattern results from higher survival of the spat settled on the platform, which provides protection from foraging activities of chitons. Most recruits die during their first month of life, chitons being an important cause of mortality. The chiton populations on two oyster grounds were evaluated. Results showed that in one of the oyster grounds, where chiton densities are high, the density of oyster recruits on oyster shells is low. The inverse pattern was observed in the second oyster ground, where chiton density is low. The effect of grazing by chitons on oyster recruits was experimentally assessed. The density of oyster spat on oyster shells was three times higher in the absence of grazers than when grazers were experimentally added. Results thus support the hypothesis that differential survival may contribute to the carriage phenomenon.

Behavioural response in plants: adjustment in algal recruitment induced by herbivores

Behavioural responses to chemical cues are common among animals, whereas there is still intense debate as to whether plants are able to detect environmental information and adjust their behaviour accordingly. Here, I report that several freshwater algal species respond to the presence or absence of grazers in the water column by regulating their recruitment rate from sediment to water. In lakes, as well as in controlled laboratory experiments with low abundances of grazers, recruitment of certain flagellated algal species was high, whereas recruitment was negligible when grazer abundances were high. Hence, certain plants equipped with locomotion organelles can use cues released from herbivores to adjust the timing of recruitment from the 'seed-bank', thereby reducing the exposure to grazing. Such an adaptation may have important consequences for dominance and temporal succession in algal communities. In an evolutionary context, this study shows that consumer-avoidance behaviour is not an adaptation exclusive to animals.

Separating the grazing effects of periwinkles and amphipods on a seaweed community dominated byFucus Distichus

Abstract In situ caging experiments showed that both littorinid snails and gammarid amphipods exerted minimal effects on the suite of algae comprising the canopy of littoral fringe tide pools dominated by Fucus distichus. In the absence of grazers, the Fucus canopy was dramatically reduced by fatal endophyte infections. Among the 10dependent variables making up the algal species and forms at the substratum level below the canopy, 3 were highly affected by grazing. The germlings of F. distiehus were greatly reduced by littorinids. Gamrnarids also reduced germling cover, but were less effective. In addition, the two grazer guilds interactively reduced the cover of algae belonging to the Uniseriate functional form at the substratum level. Sheet/Tube form algae were reduced in cover only by littorinids. Other forms, including calcareous crusts, thalli of Chondrus crispus and 3 Cladophora species were unaffected by grazing. There was no indication of food resource partitioning between littorinids and gammarids.

Factors controlling phytoplankton ice-edge blooms in the marginal ice-zone of the northwestern Weddell Sea during sea ice retreat 1988: Field observations and mathematical modelling

The factors controlling phytoplankton bloom development in the marginal ice zone of the northwestern Weddell Sea were investigated during the EPOS (Leg 2) expedition (1988). Measurements were made of physical and chemical processes and biological activities associated with the process of ice-melting and their controlling variables particularly light limitation mediated by vertical stability and ice-cover, trace metal deficiency and grazing pressure. The combined observations and process studies show that the initiation of the phytoplankton bloom, dominated by nanoplanktonic species, was determined by the physical processes operating in the marginal ice zone at the time of ice melting. The additional effects of grazing pressure by protozoa and deep mixing appeared responsible for a rather moderate phytoplankton biomass (4 mg Chla m−3) with a relatively narrow geographical extent (100–150 km). The rôle of trace constituents, in particular iron, was minor. The importance of each factor during the seasonal development of the ice-edge phytoplankton bloom was studied through modelling of reasonable scenarios of meteorological and biological forcing, making use of a one-dimensional coupled physicalbiological model. The analysis of simulations clearly shows that wind mixing events — their duration, strength and frequency — determines both the distance from the iceedge of the sea ice associated phytoplankton bloom and the occurrence in the ice-free area of secondary phytoplankton blooms during the summer period. The magnitude and extent of the ice-edge bloom is determined by the combined action of meteorological conditions and grazing pressure. In the absence of grazers, a maximum ice-edge bloom of 7.5 mg Chla m−3 is predicted under averaged wind conditions of 8 m s−1. Extreme constant wind scenarios (4–14 m s−1) combined with realistic grazing pressure predict maximum ice-edge phytoplankton concentrations varying from 11.5 to 2 mg Chla m−3. Persistent violent wind conditions (≥ 14 m s−1) are shown to prevent blooms from developing even during the brightest period of the year.

The zooplankton of the forth estuary

Regular samples at nine stations ranging from fresh water to fully marine conditions in the Forth Estuary and Firth, on the East coast of Scotland, allow a description of the community structure, composition and distribution of the zooplankton over an eighteen month period. A transition gradient in the occurrence of species along the length of the estuary was clearly identified that could be linked to salinity and hydrographic features using both traditional and nonparametric techniques. The fauna included both resident estuarine and incursive marine populations, having strong seasonal components of variation. Calanoid copepods dominated the estuary, withEurytemora and severalAcartia species present, independent from analogous populations to seaward. The data reflected distinct seasonal episodes of predatory dominance, resulting in the temporary but complete absence of grazers in the lower estuary. In the middle and upper estuaries there was an apparent complete dislinkage between the phytoplankton levels and the zooplankton. It is hypothesised that the zooplankton of the upper and middle estuaries were dependant not upon phytoplankton but upon either anthropogenic inputs of distillery waste and sewage, or the microphytobenthos.

Temporal variability in chlorophyll a and phaeopigment concentrations during incubations in the absence of grazers

Changes in chlorophyll a (chl a) and phaeopigment concentrations during 24 h incubations in water prefiltered through 2.0 pm Nuclepore filters were determined on a weekly basis over a period of 13 mo using water from Kaneohe Bay, a subtropical inlet in the Hawaiian Islands, USA. In bottles illuminated at a constant irradiance of 4.0 E m-' h-', both chl a and phaeopigment concentrations were consistently lower than initial values at the end of the incubations. Chl a concentrations declined at a lower rate in dark bottles than in light bottles. There was no evidence of a change in phaeopigment concentrations in dark bottles. There was no temporal pattern in the exponential decay rates of phaeopigments in light bottles over the course of the 13 mo study, the median value being 0.016 m2 E-' There was, however, evidence of a nonrandom temporal pattern in the chl a decay constants. Winter values were about twice as large as summer values, a result presumably reflecting changes in the physiology and/or species composition of the phytoplankton community. In about 30% of the incubations phaeopigment concentrations were higher than initial values at intermediate time points, in some cases by as much as a factor of 2 to 3 during the first 4 to 8 h of the incubations. These results are beheved to have been caused by stress associated with the effort to remove grazers by filtration. Inclusion of nanoplankton ( 2 to 10 pm) in the incubation bottles consistently resulted in a higher concentration of phaeopigments in the picoplankton fraction after 24 h than was the case in control bottles containing only picoplankton. In this study as in other work, prescreening through 10 pm filters appeared to be insufficient to eliminate grazing artifacts from phaeopigment photodegradation experiments. Dilution rather than filtration may be a more practical way to account for grazing effects in such studies.

Nutrients, algae and grazers in some British streams of contrasting pH

SUMMARY1. The relationship between algal biomass accumulation, invertebrate colonization, and stream‐water pH was investigated in seven streams in three regions of England and Wales. Possible nutrient limitation of algal production at all sites was examined with diffusion substrata.2. Periphyton assemblages on experimental substrata after 30 days were dominated by diatoms, notably Eunotia spp., at all sites. Algal pigment concentration (chlorophyll a and phaeopigments) was not correlated with stream‐water pH, and mean concentrations on control (unenriched) substrata ranged from 0.08 to 1.94 μg cm−2.3. The growth response of periphyton to nutrient additions was site specific. Algal production was stimulated by nutrient additions at sites in the English Lake District and Llyn Brianne (south‐west Wales), but not in the Ashdown Forest (southern England).4. Larval Chironomidae were the main invertebrates retrieved from substrata at all sites. Within all three regions, larval abundance was positively related to algal pigment concentration (biomass). Abundance of the stonefly Nemurella pictetii was also positively correlated with algal biomass at the one site where it occurred.5. Our results indicate that epilithic algal production in small, oligotrophic streams is unlikely to be determined primarily by pH. Neither do they support the view that an absence of grazers from acid streams is necessarily due to an inadequate food supply.

Dietary adaptations and paleoecology of the Late Miocene ruminants from Pikermi and Samos in Greece

The late Miocene (Turolian) localities of Pikermi and Samos in Greece present numerous ruminant species which have been interpreted to represent savanna-adapted taxa. The masticatory morphology of these extinct ruminants is compared qualitatively to twenty-seven extant ruminant species from Asia and Africa using thirteen masticatory features. Statistical analysis of these features on the modern species shows significant differences between the morphology of browsers, intermediates and grazers and suggests that intermediate and grazing adaptations evolved independently more than once. In addition, certain extant ruminant browsers and intermediates may have assumed such diets from ancestral grazing species recently. Application of this method to the extinct species from Pikermi and Samos shows that, with the exception of one species, the bovids were either browsers or intermediates. Parurmiatherium rugosifrons from Samos was probably the only grazer. Pachytragus laticeps, a presumably advanced species, has masticatory morphology very similar to Miotragocerus species, commonly interpreted as primitive species. Samotherium boissieri and Helladotherium duvernoyi were intermediate feeders, although giraffids. We propose that the absence of grazers at these localities suggests that savanna-like dietary adaptations for ruminants were not as widespread as now. The presence of savannas similar to those of East Africa at Pikermi, Samos and other Miocene localities is unlikely. Such savannas were originally proposed because of modernization of fossil taxa, the presence of numerous ungulate and hyaenid species, the misconception that hipparions represent zebra-like species, and the abundance of fossil specimens. Ruminant faunal comparisons using masticatory morphology point out a greater similarity of Pikermi and Samos to extant forest and woodland ruminants; the extinct ruminant faunas were not similar to extant savanna-adapted species. A good physiognomic model for the Pikermi-Samos ruminants is the forest-woodland ruminants of Sichuan, China, Southeast Asia, and Kanha, India. The hypothesis that the Pikermi ruminants evolved in forested environments is supported by analysis of the local paleofloras. The Pikermi is similar to modern warm mixed riparian evergreen and broadleaf deciduous temperate woodland-forest. The Turolian mountains near Pikermi had mixed forests, although evergreens dominated.

Importance of physical and biological processes in structuring tropical intertidal populations of Lobophora variegata (Phaeophyceae)

On the moderately exposed tropical SW coast of Curaqao (Netherlands Antilles) the encrusting brown alga Lobophora variegata (Lamouroux) Womersley occurs in a narrow belt in the intertidal zone just above the mean sea level. We studied seasonal fluctuations in cover, the influence of grazers, recovery from storm damage and competitive interactions with other algae. Near the upper Limit of distribution, fluctuations in cover were observed whlch paralleled seasonal fluctuations in tidal levels, but lower down, cover remained more or less constant. Grazing did not affect cover or vertical distribution, nor were the L. variegata crusts being overgrown by upright algae in the absence of grazers. L. variegata recovered quickly from denudation after a storm, and competition with other algae was slight. Dominance of this species over upright algae is attributed to successful adaptation to physical stress (dessication, wave force).

Interactions of bacteria, protozoa and plants leading to mineralization of soil nitrogen

The capacity of bacteria and protozoa to mineralize soil nitrogen was studied in microcosms with sterilized soil with or without wheat plants. The effect of small additions of glucose or ammonium nitrate or both, twice a week was also tested. Plant dry weight and N-content, number of microorganisms and biomass plus inorganic N were determined after 6 weeks. The introduction of plants profoundly influenced the N transformations. In the presence of root-derived carbon, much more N was mineralized from the organic matter and immobilized mainly in plant biomass. “Total observable change in biomass N plus inorganic N” was negative in the unvegetated soils without additions, while a mineralization of 1.7 mg N microcosm −1 was observed in microcosms with wheat plants grown with bacteria only. When protozoa were included, the N taken up by plants increased by 75%. Sugar additions resulted in an 18% increase of total N in the shoots when protozoa were present, but had no significant effect in the absence of grazers. Plants with the same root weight were more efficient in their uptake of inorganic N when protozoa were present. Plants grown with protozoa also had a lower R/S ratio, indicating a less stressed N availability situation. The lowest ratio was found with N additions in the presence of protozoa. The results indicate that, with energy supplied by plant roots or with external glucose additions, soil bacteria can mineralize N from the soil organic matter to support their own growth. Grazing of the bacteria is necessary to make bacterial biomass N available for plant uptake.

Measuring microzooplankton grazing on planktonic marine bacteria by its impact on bacterial production

Grazing on planktonic bacteria by microzooplankton was estimated by separating bacteria from the larger plankton with 1μm pore Nuclepore filtration and measuring changes in bacteria in filtered and unfiltered samples over 24 hours. In the absence of grazers, bacteria increased linearly. The regression coefficient of linear increase was used to estimatein situ bacterial production. When grazers were present, the changes in bacteria concentration usually took the form of a linear decline, and grazing was estimated by subtracting the regression coefficient of the unfiltered sample from that of the 1μm filtrate. Results from the Essex estuary-coastal system of northern Massachusetts show grazing and production at rates that indicate a daily turnover of the standing crop of bacteria, with highest values in mid-estuarine waters. Experiments on the size distribution of grazing showed that microzooplankton from 1-3μm were responsible for most of the observed decrease in bacteria. It was suggested that the basic pattern of linear increase of the bacteria in the absence of grazing reflects density-dependent limitation by substrate present at the outset of the incubation and is indicative of a population that has been maintained around the mid-point of the logistic growth curve by grazing.

Epiphyte-seagrass relationships with an emphasis on the role of micrograzing: A review

Despite the recent advances in seagrass ecology over the last ten years, there are still numerous aspects of the ecological and biological interactions that occur in seagrass ecosystems that remain poorly understood. We have attempted, here, to place into perspective one interrelationship that could have important implications in the production and vigor of seagrasses. This is the relationship between epiphytic fouling by macroalgae and periphyton and the grazers which consume them as a food source while leaving the leaves intact. Our approach to this review was first to describe the relationships between macroalgae, periphyton and the seagrass host in terms of physical benefits, biochemical interactions, factors which reduce fouling on the host, and the effects of epiphytism on seagrass photosynthesis. We then examined the importance of epiphytes as a food source for those herbivores found in seagrass beds, and looked at the consequences of this grazing and removal of epiphytes for the seagrass host. Based on the potential impact of epiphytes on seagrass and grazers on epiphytes, we developed a hypothetical model that describes the effect of increasing epiphytic fouling on seagrass production in the presence and absence of grazers. From this model, we have made predictions on the direction of seagrass decline with diminishing light along depth and estuarine gradients. Lastly, we touched briefly on the problem of eutrophication and how it affects the balance of these interrelationships, and the management options to insure the health and survival of seagrass habitats in the face of increasing stress by man on these critically important ecosystems.

Spatial and temporal patterns in herbivory on a Caribbean fringing reef: the effects on plant distribution.

Blades of Thalassia testudinum Banks ex König were used in a field bioassay to assess the impact of herbivorous fishes and urchins in different habitats, at different depths, and at different times of day on a fringing reef at Galeta Point, Panama. Algal transplants were then performed to determine the effect of herbivory on algal distribution and on potential plant-plant interactions in the absence of grazers. Thalassia loss on the reef flat (0%/h) and sand plain (0%/h) was very low relative to that on the shallow reef slope (14%/h). Within the reef slope habitat, herbivore activity decreased linearly with depth. On the shallow reef slope, herbivory was highest in habitats that were most topographically complex. Herbivory was reduced in microhabitats where algal turfs had decreased structural heterogeneity by overgrowing small branching corals and filling in the cavities that these corals normally provide. On the shallow reef slope, grazing of Thalassia was lowest at night (0.3%-1%/h), rose rapidly in the morning to peak between 0900-1300 h (27%-45%/h), then dropped gradually throughout the afternoon and returned to nocturnal rates at 1900-2000 h.Herbivorous fishes did 97% of their fedding during the daytime and were responsible for more than 90% of the Thalassia removed from the reef. Urchins accounted for only 9% of the Thalassia removal, and day and night grazing rates were not markedly different.Algal species from the reef flat and sand plain are excluded from the reef slope by herbivores, not physical parameters, and should outcompete reef-slope species in the absence of herbivory. On this tropical reef, plants in the more physically benign environment appear to be selected primarily for herbivore resistance. Selection for competitive ability among seaweeds becomes increasingly important in habitats where herbivores are more exposed to their own predators or where herbivore populations are low due to periodic physical disturbances.

Ultraplankton growth rates in a subtropical ecosystem

The ultraplankton (cell diameters >3 μm), which compromises about 70% of the biomass of phytoplankton in subtropical surface waters near Oahu, Hawaii, was isolated for growth rate studies. The specific growth rate (μ) was estimated from the rate of increase of the chlorophyll biomass during incubations in the absence of grazers. This growth rate of the ultraplankton ranged from 0.037 to 0.071 h−1 (=1.3 to 2.5 doublings d−1) during a period when P:B ratios of 5 to 14.5 μg C μg−1 chl a h−1 prevailed. The co-occurrence of atypically high P:B ratios and nonlimiting ambient nutrient concentrations suggests that the calculated values are higher than those characteristic of such subtropical ecosystems in general. Rates of ammonium uptake and photosynthesis by the >3 μm fraction were also compared to those of larger fractions. Organisms in the >3 μm fraction assimilated NH4+at a rate which was about 75% greater than that of the 3 to 20 μm size fraction. Comparison of μ and P:B data collected over a 2 mo period (November–December, 1980) shows that the correlation between these two rate indices is nonlinear. The predominance of small-celled phytoplankton in oligotrophic waters is explained, in part, by its higher μ, its higher nutrient assimilation rates, and the absence of its loss through sedimentation.

Differential susceptibility to grazers in two phases of an intertidal alga: Advantages of heteromorphic generations

The intertidal red alga Gigartina papillata (C. Ag.) J. Ag. alternates between two major forms in its life cycle, an upright blade and a fleshy crust. The evolution and functional significance of such alternation of heteromorphic generations is little understood. This study, done on the outer coast of Washington state (U.S.A.), considers the rôle of physical factors, measured by intertidal position, and a biological factor, interactions with herbivores, as selective pressures potentially responsible for this pattern. Observations along transects and a herbivore exclusion experiment showed that the two phases of this alga simultaneously occupy similar intertidal positions but differ significantly and oppositely in their response to grazing. The number of crust plants declined in the absence of grazers, probably due to being overgrown and shaded by a thick algal film. It is, therefore, suggested that the crust form is grazer-dependent. In contrast, the number of blade phase plants increased in the absence of grazers. “Bet-hedging” is suggested in explanation of the maintenance of two phases in one cycle; the crust is more persistent but susceptible to overgrowth, while the more productive blade is more susceptible to grazing.

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

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.