David M. John

Ecology of Subtidal Algae on Seasonally Devastated Cobble Substrates off Ghana

The growth of subtidal algae on calcareous cobbles is restricted to a short (2—4 mo) growing season. Most algae are destroyed during rainy season storms due to surge and cobble tumbling; algal density is low throughout the year. Significant interspecific competition does not occur on cobbles, and no species/area relationship could be demonstrated. The number of species per cobble is correlated with cobble moment of inertia (resistance to tumbling) during the rainy season. Species diversity on cobbles is significantly higher than on neighboring rocky reefs. Available evidence suggests an annual floristic cycle, although biomass and diversity are closely related to short—term weather conditions. Ephemerals, annuals, pseudoannuals, and prennials appear sequentially during the growing season. Species with greater potential size reach peak abundance late in the growing season, although many never reach maximum size or reproduce on cobbles. These longevity and size trends cannot be said to constitute adaptive strategies on cobbles. The high species diversity observed on cobbles is adequately explained by seasonal disturbance.

The fish grazing of rocky shore algae in the Gulf of Guinea

Abstract There is a conspicuous absence of large algae on five breakwater systems surveyed along the northern shore of the Gulf of Guinea. Such algae are a distinctive feature of most natural, rocky areas in the region and their absence on these artificial structures may be accounted for by heavy sedimentation and pollution inside the harbours, and as a result of fish grazing on the outside. Herbivorous pomacentrids (Abudefduf, Pomacentrus), parrot fish (Pseudoscarus) and surgeon fish (Acanthurus) are common along parts of the breakwaters sheltered from direct wave action. Preliminary experiments and observations show the importance of fish grazing and indicate diurnal differences in its intensity. The ‘broken-up’ nature of the breakwaters with moderate wave action is the probable reason for the occurrence of large fish populations and hence the presence largely of grazing-tolerant algae.

Effects of Pollution from the Torrey Canyon on Littoral and Sublittoral Ecosystems

Pollution from oil and detergent deposited on the beaches of Devon and Cornwall after the grounding of the Torrey Canyon on March 18, 1967, has altered the balance of littoral and sublittoral ecosystems at two sites. This effect is most marked on the littoral zone, and falls off below the low water mark.

Ascension Island, south Atlantic: A survey of inshore benthic macroorganisms, communities and interactions

Abstract Ascension, an isolated volcanic island 07° 57′ south of the Equator, is about 145 km west of the mid-Atlantic ridge. The position and coastal configuration of the island results in a high level of wave-action throughout. Six categories of marine habitat are distinguished: (1) open littoral rocks (igneous formations; beach rocks); (2) blow-hole areas; (3) pools (tide pools; inland saline pools); (4) turtle ponds; (5) outfall channels from power stations; (6) sublittoral substrata (rocky areas; calcareous cobbles; conch shells). Except in the absence of barnacles, the characterising organisms on wave-exposed igneous rocks are distributed much as reported elsewhere along tropical Atlantic shores, i.e.: small snails define the littoral fringe; in the eulittoral are two subzones, the upper lacking characterising organisms, the lower with lithothamnia; on the sublittoral fringe exists the sea urchin Echinometra lucunter L. Luxuriant and diversified filamentous and fleshy algae develop only where the Ascension blackfish ( Melichthys niger Bloch), which are extremely abundant and widespread, can either not penetrate or not feed readily due to water movement (e.g. in inland pools; tide pools with rocky barriers; beach; rock slabs; the sublittoral in protective niches, crevices and holes). The contents of the blackfish digestive system show that algae, particularly articulated coralline algae, are widely taken by choice, necessity, or accident. Areas which are intensely grazed by fish carry prostrate or short growth forms of normally large or erect algae as a basal turf; only blue-green algae appear as unrestricted growths. Where sea urchins also are present (e.g. in the sublittoral fringe and certain tide pools) only the crustose lithothamnia persist.

Preliminary observations on the structure and ornamentation of the oosporangial wall in chara charales chlorophyta

The fertilization of the oosphere in the order Charales results in the formation of an oosporangium whose highly resistant outer organic wall is frequently decorated or ornamented. Examination by the scanning electron microscope allows for the unambiguous interpretation of such surface features and reveals details of wall structure and ornamentation not described hitherto. In this study of the oosporangia of 28 species of Chara the interpretations of the earlier light microscope investigations are reassessed and the following basic types of wall structure are recognized: a more or less continuous surface although pits, pores, and depressions are evident at c. × 15,000; three-dimensional network of irregularly fused strands; perforate, with the larger pores often more than 0·1 μm in diam. Surface projections include low mounds, papillae, and perforated or unperforated pustules and granules. Often observed on acid-cleaned oosporangia are ribbon-like structures lying over and partly obscuring the region of w...

An Ecological Study on Laminaria Ochroleuca

The distribution of Laminaria ochroleuca along the south-west coast of England and the Isles of Stilly is brought up to date. The pattern of distribution is discussed with reference to certain factors controlling the ‘ performance’ and success of the species on the northernmost edge of its range. The performance of individuals of the species (measured as net stipe production) sampled from populations growing on the coast of south-west England is compared with populations from north-west Spain, where it is growing in the centre of its geographical range. The degree of exposure to direct onshore wave action appears to govern the upper limit of its bathymetric range as well as its distribution along the south-west coast of England. It was found possible to age this species using the method and precautions described by Kain for Laminaria hyperborea . The stipe material sampled from the English populations had a higher ash content per gram dry weight and a higher calorific content per gram of organic material than that from the Spanish populations. Individuals from the English population had a lower weight per unit length of stipe in all age-classes than those from Spain. It is suggested that incident radiation may be a more important factor than temperature in controlling the northward limit of the species.

Factors influencing algal species assemblages on reef and cobble substrata off Ghana

Abstract Subtidal algal assemblages were studied on two substrata, rocky reefs and calcareous cobbles. Although the two occur together at comparable depths, their vegetation differs in species composition and richness, and in patterns of plant size, life form, and longevity. The reef bears a species-rich, patchy cover of small filamentous and crustose forms, with occasional clumps of more robust species. The cobbles support a sparse cover of large leafy and dendritic species in addition to many of the smaller species found on the reef. The floristic separation arises from differential establishment and survival of species under conditions of (1) grazing by fish and urchins (on the reef only), and (2) seasonal physical disturbance during storms leading to the removal of most algae (on the cobbles only). Both substrata show a seasonal floristic cycle, but the trend is more pronounced on cobbles. Species do not depart from randomness in their patterns of co-occurrence on individual cobbles or reef fragments. Interspecific competition appears comparatively unimportant in determining species composition on either substratum.

ALTERNATION OF GENERATIONS IN ALGAE: ITS COMPLEXITY, MAINTENANCE AND EVOLUTION

Life histories of photosynthetic eukaryotes traditionally‐termed algae exhibit a considerably greater degree of complexity than those of ‘higher cryptogams.’ Some algae have a so‐called ‘obligate’alternation between spore‐producing and gamete‐producing phases, but the majority seem capable of following other pathways depending upon environmental conditions. In only four algal classes do life histories show a change in morphological and/or nuclear phases. The following basic life histories are recognized in the Chlorophyceae, Phaeophyceae and Rhodophyceae:(a) monophasic, a diploid or haploid phase, (b) two or more phases, most commonly an alternation of an isomorphic or heteromorphic haploid gametangial phase and a diploid sporangial phase, and (c) three phases (unique to florideophyte Rhodophyceae), with a diploid spore‐producing phase (carposporophyte) developing on the gametangial phase, a diploid phase (tetrasporophyte if meiosis is sporic) and a haploid gametangial phase. Evidence from recent research indicates that in many algae there is an uncoupling of the morphological and nuclear phases. The dominance of one phase and suppression of another has been suggested to be due to the common occurrence in algae of apogamy, apomeiosis and parthenogenesis. Free‐living morphs in heteromorphic life histories may be morphologically so dissimilar that formerly they were attributed to different genera.

A preliminary study of the variations in yield and properties of phycocolloids from Ghanaian seaweeds

The yield and some of the properties of the phycocolloids extracted from Gracilaria dentata, Hypnea musciformis, H. flagelliformis, Gigartina acicularis and Grateloupia filicina have been studied in relation to the season of harvesting and habitat along the coast of Ghana. All plants gave high yields of phycocolloids over the “dry” season (October – April), and this sometimes extended into the start of the rains. Leaching by heavy rain is suggested as a possible factor responsible for the low yields during much of the “wet” season. A relationship exists between declining yields of phycocolloid and reduction in standing crop of a population of Gracilaria dentata growing on open flat rocks. Bleached plants in such declining populations had almost three times less phycocolloid than unbleached individuals. Young, re-estab-lishing populations of G. dentata gave very high yields, which were often greater than those obtained in more stable populations consisting of plants of mixed ages. Reduced salinity, high turbidity, and sedimentation are possible factors leading to the lowered phycocolloid yields and reduced abundance of plants during the “wet” season at a site near a river mouth.

The distribution and net productivity of sublittoral populations of attached macrophytic algae in an estuary on the Atlantic coast of Spain

A short term study was made on the ecology and the net productivity of sublittoral populations of benthic macrophytes in, and near, the Ria de Aldan on the Atlantic coast of Spain. A preliminary survery was made using SCUBA diving techniques and this revealed that the most extensive sublittoral communities were those dominated by Laminaria ochroleuca and/or Saccorhiza polyschides. The effect of depth and the amount and type of water movement on the distribution and productivity of these two populations was studied at 7 localities. There was found to be a narrow zone of transition between the two populations and the depth of this zone was governed by the amount of water movement. The factors which might control the dynamic equilibrium between these two populations are discussed. The total standing crop and productivity per unit area was found to undergo a decrease with both an increase in depth and wave action whilst the highest values of all were in two localities where there was considerable current surge. The maximum productivity of L. ochroleuca (17 metric tons/hectare/annum) was found in the most sheltered locality studied, whilst the highest value for S. polyschides (39 metric tons/hectare/annum) was found where the current surge was greatest but wave action was only moderate. This maximum value for S. polyschides is the highest yet reported for naturally occurring beds of submerged seaweeds. Figures for the net efficiency of energy fixation are presented and discussed.