A. R. O. Chapman

Disturbance and organization of macroalgal assemblages in the Northwest Atlantic

Large seaweeds are often structurally dominant in subtidal and intertidal rocky shore benthic communities of the N.W. Atlantic. The mechanisms by which these algal assemblages are maintained are surprisingly different in the two habitats. In the subtidal community, kelps are dominant space competitors in the absence of strong grazing interactions. In contrast, the large perennial seaweeds of intertidal zones (fucoids and Chondrus crispus) are competitively inferior to both sessile filter feeders and ephemeral, pioneer algal species. Intertidal seaweed beds are maintained by carnivory of whelks, which reduces filter feeder populations, and by herbivorous periwinkles which reduce ephemeral algal populations. Through most of the intertidal zone, disturbance, both biological and physical, dictates which species shall compete and equilibrium conditions obtain subsequently.The roles of subtidal consumers are quite different. Sea urchins are the major algal herbivores and these voracious animals maintain an equilibrium state in which large tracts of subtidal coralline pavement are kept free of kelp forests. Urchins do not seem to play a successional facilitative role for kelps in the way that periwinkles do for fucoids in the intertidal. Control of herbivore populations is thus a key to the maintenance of subtidal foliose algal beds. It is clear that parasitic amoebas can decimate sea urchin populations so that kelp forest dominance is assured. However, the importance of carnivory in limiting urchins in the subtidal community is unclear in the absence of appropriate manipulation experiments. It is possible that carnivorous decapods and fin fish control sea urchin populations and hence foliose algal abundance, but this must remain speculative. The seaweed-dominated state of the subtidal system is an alternative equilibrium condition to the urchin/coralline alga configuration. The structure of the kelp beds is relatively uniform in responding to frequent small-scale, infrequent large-scale, or no, disturbance.

Gammarid amphipods and littorinid snails have significant but different effects on algal succession in littoral fringe tidepools

Tests on the effects of two herbivore groups (littorinid snails and gammarid amphipods), on algal succession in high intertidal tidepools dominated by Fucus distichus showed that the grazer groups exert different grazing pressures on the algae. The differences can be related to the feeding morphologies of the two grazer guilds. The scraping, microphagous littorinid snails prevent the establishment of both micro- and macroalgae at the microscopic level. The biting, macrophagous gammarid amphipods are ineffective at grazing microalgae and prostrate macroalgae from the substratum, but exert a considerable influence on the erect macroalgae which escape littorinid grazing. Despite differences in feeding mechanisms, the magnitude of the effect of the two grazer groups on the canopy cover of macroalgae is similar.

Seaweed invasions: introduction and scope

At no time in human history has the need to understand invasions of alien species – the process of invasion, impacts of invasion and meaningful options to respond to invasion – been so urgent. The rate of anthropogenically- mediated translocation of species to regions outside native ranges has never been greater.

Herbivory and harvesting: Effects on sexual recruitment and vegetative modules of Ascophyllum nodosum

Abstract Populations of Ascophyllum nodosum (Phaeophyta) have the potential to grow from either sexual recruits or by vegetative propagation of modules (i.e., fronds). In stands that are harvested in southwestern Nova Scotia there are two major potential regulators of these sources of natality, herbivores and commercial harvesting activities. It was shown here that herbivory has a major effect on sexual recruitment. Nearly all of the zygotes (99.9 %) which settled naturally in 1989 were consumed within one year. Slow growth of the survivors and continued herbivore pressure indicated that remaining germlings would not survive in subsequent years. Our results also suggested that harvesting may enhance the production of zygotes but this effect may not result in increased recruitment as most germlings do not survive grazing. Grazing did not affect module survivorship or breakage to any major extent. Hence our study suggested that increased size represents an escape from herbivory. Because sexual reproduction ...

Competitive interactions between Fucus spiralis L. and F. vesiculosus L. (Fucales, Phaeophyta)

Fucus spiralis forms a conspicuous belt in the upper intertidal on rocky shores of the NW Atlantic. The objective of this study was to determine whether competition among congeners plays a role in restricting the distribution of F. spiralis to the upper shore. A replacement series design was used to test the growth performance of F. spiralis and F. vesiculosus in monocultures and in mixtures set up in the mid shore level on an exposed rocky coast. For F. spiralis and F. vesiculosus, all individual and relative crowding coefficients were < 1 and > 1, respectively. Hence, F. vesiculosus succeeded relative to F. spiralis in all mixtures tested. This finding was confirmed by graphical analysis. Ratio diagrams showed that the outcome of long term interaction among the species is extinction of F. spiralis. The results lead to the conclusion that interaction among congeners is a major determinant of the restricted distribution of F. spiralis.

Separating the grazing effects of periwinkles and amphipods on a seaweed community dominated by Fucus 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.

Patch structure in a tropical rocky shore community in Brazil: a mosaic of successional states?

Abstract The community of benthic organisms in the lower zone of a rocky shore at Praia Rasa, Brazil is distinctly patchy in structure. We examined the hypothesis that two types of patch (a. red algal turf and b. colonial coelenterate) represent different temporal stages in the same successional trajectory. We made observations on a) the stability of patch boundaries, b) successional sequence in experimental clearings within both patch types and c) species composition through time in undisturbed patches. In most cases, the coelenterate overgrew the algae at the patch boundaries, thus showing a competitive asymmetry. In the successional sequence, diatoms were followed by ulvoid species. Thereafter, within clearings in the algal patches, a red algal turf was once more established over one year. In contrast, the mid successional period in clearings within the coelenterate patch was characterized by very low cover of sessile organisms. Colonization by red algal turfs and by the coelenterate colony is largely ...

Structure of this issue

This double issue of Botanica Marina has two sets of articles. The first (pp. 265–318) comprises regular submissions to the journal. The second set (pp. 319–464) contains a series of nine invited reviews prepared under the guest editorship of Professor Craig R. Johnson, University of Tasmania, Australia. The reviews and the accompanying introduction and conclusion chapters have been assembled as a Special Issue on Seaweed Invasions. The subject index following the conclusions chapter covers only the 11 components of the Special Issue. As well as appearing as part of this double issue, the Special Issue on Seaweed Invasions will be reprinted as a ‘‘stand-alone publication’’. This reprint will be available for sale in mid-January 2008 (ISBN 978-3-11-019534-7). Orders can be placed with your local bookseller or submitted online to Walter de Gruyter Publishers: http:// www.degruyter.com/. Following the article sections (pp. 465–486), this double issue contains the annual indices for the complete Botanica Marina volume 50 (2007).

First letters to the editor in Botanica Marina's “Forum” section

Abstract I would like to direct readers and contributors to the “Forum” section of this issue of Botanica Marina. It contains the first letters to the editor on a controversial issue. I hope that more of you will use this vehicle to express your opinions on difficult/interesting scientific matters in the field of marine botany. There are no special instructions for formatting letters to the editor, except that they must be prepared in MS Word for online submission at http://botmar.edmgr.com. When there is more than one author, each must sign accepting responsibility for the entire content of the letter. Letters may or may not be externally reviewed.