Mean Percentage Cover Research Articles

Depth distribution and abundance of benthic organisms and fishes at the subtropical Kermadec Islands

Abstract Subtidal areas of the Kermadec Islands have not previously been described; samples from a bay at Raoul Island are discussed here. Abundances and percentage cover of benthic organisms were assessed from the high intertidal to 20 m depth using quadrat sampling. Fish abundances were assessed subtidally by random transects in shallow (3–6 m deep) and deep (14–20 m) areas of the reef. Hard corals occurred at 2–18 m depths at densities of 5 colonies per m2, but colonies were generally small. Soft coral colonies occurred at similar densities, but were confined to depths of 5–13 m. The mean percentage cover of corals did not exceed 20% at any one depth, and coral reefs did not occur. Fucalean and laminarian algae were not seen. The most abundant algae were foliose and filamentous species, and encrusting red algae. These covered over 80% of the reef at some depths. Grazing invertebrates were not abundant. Echinoids were found primarily in shallow water, reaching a density of 5 per m2 at 5 m depth. The mos...

The standing stock, growth and shoot production of Zostera capricorni aschers. in Botany Bay, New South Wales, Australia

Zostera capricorni Aschers. occurs at many shallow sites in Botany Bay, with the exception of one 5-km section of exposed sandy beach; however, bed distribution is very patchy. Details are given for shoot and underground biomass (g fresh weight), shoot density, leaf length and leaf width at 20 representative sites around the Bay. Z. capricorni was found to grow best between −0.2 and −1.0 m, with a lower limit between −2.0 m (northern side) and −3.2 m (southern side). The mean percentage cover was determined for six large areas in the bay. Shoot biomass (g dry weight) was found to be logarithmically related to percentage cover, whereas shoot density (numbers m −2) was linearly related to percentage cover. Large seasonal changes occurred, with a winter die-back characterised by a 4-fold reduction in shoot biomass and a 2-fold reduction in shoot numbers. Flowering occurred from September to April. An equation is presented for determining the total above-ground stock for an area. The total summer above-ground stock of the bay was estimated at 18 ± 8.1 tonnes for a total area of beds of 309 ha. Mean annual leaf production was estimated to be 5.22 ± 0.52 gDW m −2 day −1 for a representative healthy bed at 0.3 m depth, and leaf plus flower production was 5.86 ± 0.59 gDW m −2 day −1. The total above-ground production for all the beds of the bay was estimated to be 512.7 ± 51.3 tonnes year −1, i.e. 1.66 ± 0.17 tonnes ha −1 year −1.

Monitoring of coral reefs with linear transects: A study of storm damage

Monitoring of coral reefs in the U.S. Virgin Islands through repeated sampling of linear transects revealed that Hurricane David (August 1979) caused significant changes in the amounts of live and dead hard coral cover on these reefs, i.e., cover by scleractinians and the hydrozoan Millepora. Mean percent cover of the most abundant coral species, spatial indices (a measure of bottom topographical complexity), the number of species within transects, the diversity index ( H'), and the evenness ( J'), did not, however, change significantly as a result of this storm. Mortality in corals did not appear to be species specific. Monitoring of established transects proved to be an effective way of quantifying storm damage. With the increasing interest in management of coral reefs, this technique could also be useful for assessing other types of reef destruction.

Occupation of patches in the epifaunal communities on pier pilings and the bivalve Pinna bicolor at Edithburgh, South Australia.

The reoccupation of artificially cleared patches in a subtidal epifaunal community was investigated in two field experiments on the pilings of Edithburgh pier, South Austrlia. In most cases, the greatest proportion of the patch was reoccupied by the vegetative extension of established sponge and tunicate colonies adjacent to it. Larval recruitment by sponges, bryozoans, tunicates and serpulids contributed to the reoccupation but resulted in only a small proportion of the mean percentage cover. The relative abundances of individual species established in any patch were shown to be a function of the (1) position in space, (2) age, (3) time of creation, (4) initial size of the patch.There was a large amount of between-patch variation in all cases. Overgrowth interactions occurred frequently within patches, and for many pairs of species, neither species consistently overgrew the other. Overgrowth interactions were tested statistically, and a large number of pairs of species were found to be competitively equivalent. This represents a possible situation additional to the alternatives recognized in the literature, namely competitive hierarchies or networks. Interactions between species should be regarded as stochastic, with a wide range of possible outcomes. The situation at Edithburgh is likely to produce greater between-patch variability than either a network or a hierarchy.Despite this large variation, super-specific taxa differ fairly consistently in capacity for overgrowth. Tunicates overgrow sponges, which overgrow bryozoans, which overgrow serpulids. The occupation of most patches was directional in the sense that bryozoans and serpulids invaded first, but tunicates and sponges excluded them and came to dominate the patch. These realtionships are used to predict patterns of abundance for substrata which are small and isolated, and these predictions are compared with the epifauna of the bivalve Pinna bicolor, which provides such substrata adjacent to the pier.