Trevor A. Norton

Dispersal and colonization in Sargassum muticum (Yendo) Fensholt

Abstract The ecology of the propagules of the invasive brown seaweed Sargassum muticum (Yendo) Fensholt was studied both in culture and in the sea. In stationary water the propagules sink at an average rate of >0.5 mm·s −1 and stick on contact. Germlings retained on the surface of the reproductive receptacles beyond the normal incubation time seem unable to liberate themselves. When released forcibly or by the eventual decay of the underlying receptacle, they seem unable to re-attach to the substratum. In the sea the vast majority of the germlings arise within 2–3 m of the parent plants although a few settle up to 30 m away. Seasonal recruitment follows closely the onset and degree of fertility of the adult populations. Successful colonization of the substratum requires not only the close proximity of a fertile plant, but also the availability of cleared rock. The presence of an existing canopy or turf of any one of at least eight species of algae can totally inhibit colonization by interposing a physical barrier between the settling germlings and the substratum. In southern California the barrier can be effectively re-established on denuded surfaces within 1–3 months. Germlings cultured in suspension rapidly lose their ability to attach on settlement but in culture unattached germlings remain healthy and seem able to grow well indefinitely. In nature drifting germlings can be found in the plankton up to at least 1.3 km from the nearest fertile plant. Drifting germlings seem to represent a mechanism for short-range dispersal, yet the species has almost certainly migrated 1100 km down the entire coast of California in a single step. Neither drifting germlings nor abscissed fertile branches could have made this migration, for they are produced too late in the year to take advantage of the inshore currents which are only seasonally favourable for southward dispersal. Floating vegetative branches on the other hand can depart at any time of the year and their southward migration would be greatly accelerated by the southerly winds that augment the currents. The branches could become fertile en route and as the plant is both monoecious and self-fertile, a single migrant could give rise to an extensive population. The possibility that man may have introduced S. muticum into southern California is discussed, but considered unlikely. The facility with which S. muticum not only migrates long distances, but also produces dense populations on arrival, reflects the efficiency of the varied dispersal mechanisms that it possesses.

Recruitment and mortality of early post-settlement stages of benthic algae

Four transitional life history stages are generally recognized for benthic marine algae. On the basis of differences in size, we propose two more: young germlings and young juveniles. Three of these (spores or zygotes, young germlings, and germlings) are considered early post-settlement (EPS) stages. Many of the available data on recruitment and mortality were not collected with EPS stages specifically in mind, and considerable extrapolation and inference are required to interpret effects on early phases. Data on EPS stages, and grazing in particular, are based on the disappearance of early stages (laboratory or field experiments, including outplantings), on indirect information on mortality from manipulations of grazers or juvenile stages, and combinations of observation and manipulation. We also provide original data on the effects of water motion and canopy sweeping on zygote mortality. The most notable finding is the variability in, and multiplicity of, factors preventing successful recruitment of ear...

Dispersal by macroalgae

Seaweed dispersal is discussed in relation to the properties of the propagules, the parent plant and the water motion. The magnitude of water flow velocities is overwhelmingly important. Even motile propagules have little control over their destiny. The fate of both liberated propagules and drifting plants is discussed in relation to dispersal distance and the likelihood of successful colonization at journeys end. Analogies are drawn between the dispersion and drift of algal propagules and that of smoke particles and fungal spores. The fecundity of the parent plant, the point and mode of propagule release are also influential. Many larger plants have release mechanisms that limit rather than enhance dissemination. Moreover, contrary to accepted dogma, invertebrates that consume or transport spores also tend to limit rather than extend dispersal range. The relative merits of local consolidation vs. widespread dispersal are discussed.

Physical stress and biological control regulate the producer-consumer balance in intertidal biofilms

Epilithic biofilms play a key role in marine ecosystems. They also provide a tractable system to investigate the relative roles of environmental stressors, bottom-up physicochemical factors and top-down biological control in regulating communities. Patterns of photosynthetic microbial biomass were recorded over a four-year period at several tidal levels on shores in the Isle of Man. Photosynthetic biomass and the abundance of diatoms were consistently greater during winter than summer. Biomass was negatively correlated with insolation stress and air temperature, but was not correlated with grazing intensity, dissolved nutrients, sea temperature, or planktonic chlorophyll. Field experiments confirmed that reducing insolation stress led to substantial increases in photosynthetic biomass, predominantly of diatoms and macroalgal germlings. Reducing grazing intensity also led to considerable increases in photosynthetic biomass, but reducing desiccation stress or increasing nutrient availability had no effect. Although grazing can regulate microalgal biomass, seasonal patterns of grazing activity were driven by temperature and were decoupled from photosynthetic biomass. Our study demonstrates the importance of physiological stresses for the direct and indirect regulation of the balance between primary producers and consumers. Based on these findings, we present a model that combines the roles of stressors together with bottom-up forcing and top-down regulation in controlling communities on wave-exposed shores.

Scraping a living: a review of littorinid grazing

Littorinid snails are predominantly herbivorous and the versatility of their radulae enables them to feed on a variety of macroscopic and microscopic plants in a diversity of habitats. Some are selective feeders preferring some species of algae to others, and rejecting some even after a prolonged period of starvation. Different species of snail exhibit different preferences. The factors affecting the attractiveness and edibility of food plants are discussed and food value considered.Foraging behaviour of littorinids is briefly reviewed in relation to the influence of chemical cues from the algae. Littorinids appear to be able to select or reject algae without having ingested them, having perceived the plants from a distance, moving towards favoured foods (or habitat-providing plants) and away from those that it rejects. The nature of the chemical cues emitted by the algae is discussed. Temporal patterns of foraging activity show some evidence of an endogenous component which can be overridden by responses to environmental conditions. These patterns place restraints on energy intake.The structural and chemical defences used by algae against littorinid grazing are considered. The importance of polyphenolic compounds is evaluated. The effects of grazing as a selective agency and a factor influencing algal populations are discussed. There is some evidence that life history patterns are a response to grazing. The influence of external physical factors, such as salinity on grazing pressure is demonstrated.Finally, the impact of littorinid snails on intertidal communities is assessed in relation to their abundance and biogeographical distribution. The relative importance of littorinids is contrasted on shores possessing or lacking limpets.

Dietary preferences of the common periwinkle, Littorinalittorea (L.)

In gustatory choice experiments with mature macroalgae, grazing by the common periwinkle, Littorina littorea (L.), was highly selective in favour of the foliose ephemeral green algae, Ulva lactuca L. and Enteromorpha intestinalis (L.) Link. The fucoid, Ascophyllum nodosum (L.) Le Jol., and the calcareous red alga, Corallina officinalis L., were rejected even under conditions of considerable nutritional duress. Results of attractiveness trials were similar, emphasizing the important selective role of gustatory considerations. Macroalgal edibility was not influenced by a prolonged period of starvation or by enforced habituation to a non-preferred species. There was, however, some evidence to suggest that previous feeding experience altered algal attractiveness significantly, prompting less discriminate feeding. A high degree of selectivity was also apparent for algal germlings. Lawns of germling Ulva lactuca and Enteromorpha intestinalis were strongly preferred to juvenile Mastocarpus stellatus (Stackh.) Guiry (= Gigartina stellata (Stackh.) Batt.), Fucus serratus L. and especially Ascophyllum nodosum. Consistent preferences were displayed for adult over juvenile Ulva lactuca and for juvenile rather than adult Fucus serratus. Experimental examination Littorina littorea microdistribution on the mid-and lower shore reveals that, at sheltered sites, individuals forage predominantly on the rock surface and amongst the understorey of ephemeral algae. Higher up the shore and at exposed sites cast-up “drift” seaweeds assume an important dietary role. In controlled population experiments on the shore, Ascophyllum nodosum was not grazed. Consequently, this species cannot be considered a potential food source for Littorina littorea, even on those regions of the shore where it may constitute the only macroalgal cover.

A Review of Some Aspects of Form and Function in Seaweeds

The relationship between form and function in seaweeds is emphasized by the similarity of form in unrelated species. Morphologically similar types such äs thin blades, branched tufts, and crusts are found in several algal groups. The functional significance of a particular form is suggested when unrelated plants exhibit a similar appearance in the same habitat. For example, Norton (1970) reported algal meadows dominated by t wo strikingjy similar plants, Ciadostephus verticHiatus (a brown alga) and Halurus equisetifolius (a red alga). Similarly, the subtidal surge zone in which regulär back and forth water motion prevails (Neushul 1972) is usually dominated by laminarian algae. Representatives of other groups of seaweeds may occupy this zone, but they often bear a striking resemblance to laminarian algae — e. g. Durvillea spp. and Marginariella (Fucales) or Desmarestia ligulata var. ftrma and Phyttogigas (Desmarestiales). In deep waters, stalked, peltate thalli, which may have extensive starch reserves, occur in several species — e. g. Caulerpa spp. (Chlorophyta), Fauchea peltata, Maripelta rotata, Humbrella hydra, Constantinea simplex, and Sciadophycus stellata (Rhodophyta). With the flrst three species the deepwater, peltate plants contrast with non-peltate thalli of other shallow-water members of these genera. Presumably this attests to the light-gathering efficiency of such a morphology and its functional suitability for species growing in dirnly lit deep water. The same generalizations may also apply to the crustose seaweed architecture, which is a characteristic and common morphology in deep water habitats (Cheney and Dyer 1974, Sears and Wilce 1975).

Problems in extraction and spectrophotometric determination of chlorophyll from epilithic microbial biofilms: towards a standard method

A variety of methods are available to extract chlorophyll from epilithic biofilms using solvents. The relative efficiency of these has not been determined simultaneously and there is no recognized standard procedure. In this paper techniques for sample collection, storage, preparation and extraction are reviewed and compared experimentally. Extraction of chlorophyll was incomplete unless biofilms were fully hydrated. This factor was highly significant for all the solvents tested, with at least three times more pigment being extracted from hydrated samples than from dry ones. Methanol was the most efficient solvent, releasing over 96% of the total chlorophyll during a single extraction; hot ethanol extracted 86%, while acetone extracted less than 50%. Sonicating samples during extraction did not release any additional pigment. Centrifuging to remove suspended material did not alter estimates and was not advantageous. Rugose rock surfaces released more chlorophyll than smooth ones. However, a simple method to quantify surface rugosity at an appropriate scale was not available. Based on these observations, a standard method for chlorophyll extractions from epilithic biofilms using 100% methanol at room temperature (20°C) is proposed. This technique requires considerably less supervision than previously preferred methods and gave a chlorophyll extract which was stable for 15 h.

Ecological experiments with Sargassum muticum

In spite of the controversy surrounding the accidental introduction of the Japanese seaweed Sargassutn muticum (Yendo) Fensholt onto two continents, the startling south-ward expansion of its range on the Pacific coast of North America (Norton, 1977 a), and the failure of an eradication campaign on the south coast of England, surprisingly little is known about the ecology of this species. Work has recently begun in both southern California and Britain (Nicholson et al. 1977; Lewey & Farnham, 1977). Unfortunately the ecological studies in Britain have been hampered by the conflicting aims of trying to study the plant and at the same time trying to eradicate it.

Settlement and post-settlement interactions between Semibalanus balanoides (L.) (Crustacea: Cirripedia) and three species of fucoid canopy algae

Manipulative field experiments carried out on sheltered rocky shores investigated settlement and post-settlement interactions between Semibalanus balanoides and three species of fucoid canopy algae, Fucus spiralis L., Ascophyllum nodosum (L.) Le Jolis and Fucus serratus L. All three species had a negative effect on settlement and early recruitment of S. balanoides, densities being significantly lower on settlement tiles beneath the canopy compared to cleared areas. Investigation of the mechanism by which settlement and recruitment were inhibited showed the importance of sweeping of algal fronds at all three shore heights. In addition, we found that the F. serratus canopy inhibited settlement not only by its sweeping action but also by limiting the access of cyprids to the substratum. Detailed examination of the sweeping effect in F. serratus showed that newly settled cyprids transplanted beneath a canopy suffered extremely high mortality (between 82 and 97%) over just one high water period. Mortality was significantly reduced for individuals that had settled within a matrix of adults. Metamorphosis of cyprids to spat conferred no additional resistance to sweeping-induced mortality but resistance increased in 6 day old spat. Mortality in S. balanoides after the end of the settlement season was monitored for three months in experimental treatments and controls. Mortality rates were significantly lower under the canopy than in cleared areas in both the F. spiralis and Ascophyllum zones. The overall influence of each canopy species on the development of barnacle populations on sheltered shores is discussed.