M.L.H. Thomas

A physically derived exposure index for marine shorelines

Abstract A new exposure index is described which can be derived from wind velocity, direction, duration and the effective fetch. Effective fetch is calculated from actual fetch modified by shoreline and offshore extents of shaIlow water. The index is calculated from the sum of these components for each unobstructed 22.5 degree compass sector of the shoreline. Data are presented for tests of the index as a predictor for littoral zonation phenomena in Bermuda and the Bayof Fundy, Canada. For the zones in both locations the index satisfies the majority of variance in the data. The vertical distribution of species is weIl modeIled in many cases, but not for others. In many cases significantly better fits are obtained if the basic index is modified using a component for shore slope. The index is useful as a numerical index of exposure which can be derived from standard weather data and hydrographic charts. It is also useful in pointing out littoral species that either do not respond to exposure, or respond in ...

Sessile invertebrate coelobite communities from reefs of Bermuda: Species composition and distribution

Sessile invertebrate coelobite communities attached to the walls and roof of cavities in coralgal reefs on the annular rim of the Bermuda platform near North Rock (4 sites) and in the algal cup reef tract on the south shore (3 sites) have been studied by belt photo-transects and direct sampling. Irradiance measurements reveal a light gradient which appears to exert a strong influence on the composition, relative coverage and zonation of the attached biotic communities. Two main communities are recognised from cluster analysis and relative areal coverage data. Near the cavity entrances is a community dominated by crustose coralline algae, with subsidiary ascidians, demosponges, bryozoans and Foraminifera. Species richness is high and there is total biotic coverage of walls and roof. This community grades laterally into an exclusively animal community characterised by encrusting sponges and Foraminifera, with subsidiary bryozoans and unidentified branching organisms. Coverage varies from 100% to 30%, the substrate often exhibiting high micro-relief from the branching growth forms of the Foraminifera. Species richness is high at North Rock sites, less so on the south shore. The distribution of coelobite species is compared with that described from previous studies in Bermuda, Grand Cayman and Madagascar.

Depth zonation of epibenthos on sublittoral hard substrates off Deer Island, Bay of Fundy, Canada

Abstract Three locations were selected for detailed study of the epibenthos of sublittoral hard substrates in the Deer Island region of the Bay of Fundy. A total of 10 transects, using photographic and quadrat methods, yielded data on percentage coverage, density and diversity of biota in relation to depth. A cluster analysis, using the Jaccard Coefficient of Association, produced five major clusters, representing three depth zones. The shallow and mid-depth zones lie within the infralittoral, the deep zone within the circalittoral. The shallow zone extends from mean low water (MLW) to a mean depth of 5 m below MLW and consists of two clusters representing minor biological differences. It is characterized by crustose coralline algae and Petrocelis middendorfii which together cover over 70% of the primary substrate. Other macro-algae are rare, as are bryozoans, while sponges are absent. The sea urchin Stronglyocentrotus droebachiensis, the limpet Acmaea testudinalis and chitons belonging to Tonicella are very common and may exert a significant influence on the community structure in terms of grazing pressure. The mid-depth zone has a mean depth of 10 m and consists of two clusters, one representing well-illuminated upward-facing surfaces, the other representing shaded steeply-inclined cliff faces. The zone is characterized by higher species richness (relative to the shallow zone); greater coverage of sponges, bryozoans and hydroids; lower densities of sea urchins and limpets; and less areal coverage by encrusting algae. The cliff-face cluster is characterized by enrichment of bryozoans, anemones, sponges and brachiopods. The deep zone has a mean depth of 18 m, and is animal-dominated, supporting the greatest species richness, with sponges, hydroids, anemones, brachiopods and tunicates common, but algal coverage much reduced. Organisms colonizing the upward-facing surfaces in the shallow and mid-depth zones are here regarded as belonging to the encrusting algae-urchin community, while biota of the shaded cliff faces of the mid-depth zone, together with the biota of the deep zone, are regarded as belonging to the Terebratulina septentrionalis community of previous authors.

Communities of constructional lips and cup reef rims in Bermuda

Constructional lips and cup reef rims are little studied reef features typical of very turbulent conditions. The relatively low diversity community of these structures consists of three components, and algal mat dominated by Herposiphonia secunda, an encrusting calcareous biota characterised by crustose coralline algae and the vermetid gastropod Dendropoma corrodens, which together are the main hermatypic components, and an endolithic component with very abundant boring sponges, polychaetes, crustaceans and sipunculids. In northerly areas of Bermuda, the hydrozoan Millepora alcicornis is a prominent member of the community while to the south the urchin Echinometra lucunter is abundant. On the south side of Bermuda the community traps sediment which is deposited in the void spaces. The environment shows reduced grazing and this may account for the presence of several relict and rare species.

Growth and production of a Littorina littorea (L.) Population in the Bay of Fundy

Abstract The rocky shore intertidal population of the periwinkle, Littorina littorea, at Welchs Cove, Bay of Fundy, is composed of three or four cohorts depending upon the time of sampling as related to the time of recruitment. The size (spire height) range of individuals present was from 2 to 26 mm and the population was numerically dominated by older and larger (> 15 mm spire height) animals, a result of the method of recruitment of smaller individuals to the population. Cohort growth was relatively constant with time as a result of a comparatively short life span (3-4 years), brought about by predation pressure and severe physical conditions experienced during the winter. Daily growth rate of shell deposited at the aperture lip was found to vary significantly with time in the period from May until November 1985. The minimum daily short-term growth rate was 0.0647 mm for the 17-day period from 21 May until 6 June, compared to the maximum rate of 0.0968 mm for the 15-day period from 1 to 15 September 198...

Ascidian depth zonation on sublittoral hard substrates off deer island, New Brunswick, Canada

The upper surfaces of sublittoral hard substrates in the Deer Island region of the Bay of Fundy, New Brunswick, support diverse, depth-zoned epibenthic communities of which ascidians form a minor part. Their population density was quantitatively studied from photo-transects taken between mean low water (MLW) and 30 m depth at four sites off the Deer Island coast and from 30–140 m depth along two photo-transects in Head Harbour Passage. All photo-analyses were aided by collections from transect survey sites, wharf pilings and salmon cage floats, to yield a total of 15 ascidian species encountered. Ascidians were found at all depths at the four shallow sites. Halocynthia pyriformis and Boltenia ovifera are most common at depths of less than 20 m, while Aplidium pallidum, Didemnum albidum and other species exhibit a marked increase in abundance below this depth. Cluster analysis of ascidians shows an association between B. echinata and B. ovifera , which may reflect resource partitioning, and between A. pallidum-D. albidum and Molgula sp.— A. stellatum , the ecological significance of which are as yet unknown. The community in Head Harbour Passage is animal-dominated and in its deeper sections often shows three-dimensional bottom relief from horse mussel shells. D. albidum , the commonest ascidian, shows a close association with Modiolus modiolus , to which it is normally attached, suggesting that mussel beds may minimize the possibility of dislodgement and even confer a feeding advantage on this ascidian.