H. Kirkman

Ecological significance and commercial harvesting of drifting and beach-cast macro-algae and seagrasses in Australia: a review

This review provides an overview of aspects of the ecology of drifting and beach-cast macroalgae and marine angiosperms in respect to present and potential commercial use of that resource in Australia. It sets the scene with sections on industries that utilise macro-algae and seagrasses, the ecology of littoral and nearshore sublittoral ecosystems and the processes of deposition of beach-cast macro-algae and seagrasses on beaches. It then describes the major economic macro-algae and seagrasses that occur as beach-cast wrack, with an emphasis on known information on habitat distribution, geographical range, and harvesting issues. Gaps in scientific knowledge are pointed out. The priority areas of future research were found to be: • The importance of beach accumulations of macro-algae and seagrasses on feeding and nesting shorebirds; • Whether available resource allows for ecologically and economically sustainable harvesting; • A survey of present and potential commercial macro-algae and seagrasses: studying biomass, density and annual production rates, interannual variability of recruitment into living stands, the effect of harvesting on trophodynamics and community structure and the stability of the resource base for economically sustainable harvesting; • An assessment of the importance of wrack in recycling nutrients and detritus to nearshore coastal ecosystems at wider geographical scales than previous work. This research should assess the dependence of offshore production on nutrients and detritus that are broken down in beachwracks.

Standing stock and production of Ecklonia radiata (C.Ag.): J. Agardh

The monthly productivity, standing stock, plant size and density of Ecklonia radiata (C.Ag.) J. Agardh is presented for a 2-yr period. Annual production was 20.7 kg wet wt · m−2 with maximum growth of 0.9% per day in spring (October–December) and minimum growth of 0.2% per day in late summer. (March–April). A close negative correlation was found between spring and summer growth and water temperature. Maximum biomass (18 kg wet wt · m −2) did not coincide with maximum growth but occurred in late summer. Minimum biomass (6 kg wet wt · m −2) occurred in winter. An estimate of erosion of plant material from the kelp bed was made from these data and a hypothesis concerning the ultimate destination of eroded and removed kelp plants was formulated.

Pattern and process in southern Western Australian seagrasses

Abstract Posidonia and other seagrass genera cover vast areas along the coast of temperate Western Australia, where it seems that disturbance and recolonisation are the dynamic forces which govern the pattern of seagrass beds. Some species of Posidonia may take 60–100 years to cover cleared substratum while species of other genera may come and go within a year. Three case studies of spatially widely separated seagrass beds are given. They illustrate that depth, weather and location make great differences to the extent and diversity of seagrasses. Comments on reproductive ability and observations on seedling growth and vegetative propagation are given with emphasis on the “ Posidonia ostenfeldii ” complex.

A Study of the Role of the Seagrass Posidonia australis in the Carbon Budget of an Estuary

Abstract The budget and fate of organic carbon from the leaves of the seagrass Posidonia australis Hooker f. were studied in a small sunken river valley in Port Hacking, New South Wales. Standing stock and leaf growth were measured over 12-month periods. Estimated average relative leaf growth was 2.3 mgC g dry wt. −1 day −1 . Estimated losses totalled 2.6 mgC g dry wt. −1 day −1 of which 48% was in the form of dissolved organic carbon, while grazing by herbivores (3%), leaves floating off (12%) and sinking leaves (37%) accounted for the remainder of the carbonaceous material lost from the seagrass leaves.

The first year in the life history and the survival of the juvenile marine macrophyte, Ecklonia radiata (Turn.) J. Agardh

Abstract Small Ecklonia radiata (Turn.) J. Agardh plants were tagged in March 1979 and observed monthly for 1 yr. Growth appeared related to the amount of light received by the plants. Plants in shaded areas showed little growth. Plants in unshaded positions grew to full size in 8 months, showing an exponential growth curve with time. Unshaded plants went through three morphological stages; a juvenile first stage at settlement, a second stage observed within the 1st month and a mature stage reached 4 months after tagging. When canopy plants were cleared artificially, small repressed plants responded by developing rapidly. A larger (36 m 2 ) artificial clearing was not re-vegetated by E. radiata for at least 9 months after clearing. The survival of the marine E. radiata is compared with survival of seedlings in terrestrial forests. Different types of disturbance and their effects on the community are discussed to explain the single-species dominance of E. radiata on the reef.

The seagrass communities of Moreton Bay, Queensland

In this paper the composition of the seagrass communities of Moreton Bay, Queensland, is described. Six species were found: Zostera capricorni Aschers., Halodule uninervis (Forsk.) Aschers., Halophila ovalis (R.Br.) Hook.f., Halophila spinulosa (R.Br.) Aschers., Cymodocea serrulata (R.Br.) Aschers. and Magnus and Syringodium isoetifolium (Aschers.) Dandy. These were found in five different distinct phanerogamic communities: two monospecific communities, one multispecific community, a community characterised by a single dominant species, and a community in which two species were of equal importance. The zonation of these was determined by depth, salinity, turbidity and substrate characteristics. It is postulated that the absence of Posidonia has enabled various species to co-exist and remain intact instead of being part of a series leading to a Posidonia meadow.

Community structure in seagrasses in southern western Australia

Abstract The southern Western Australian coast is relatively diverse inseagrass species. This paper discusses the effects of storm-caused disturbance and follows the changes in seagrass communities over a 5-year period. The intermediate distrubances caused by winter storms prevent Posidonia sinuosa Cambridge and Kuo from dominating all available space and allow pioneering species to be present at all times. During the study no new spaces were cleared by storms in the dominant community, but changes amongst the pioneers were noted as well as some succession.

Measurement of health, and echinoderm grazing on Posidonia oceanica (L.) Delile

Abstract The decline of Posidonia oceanica (L.) Delile along the Mediterranean coast near Marseille has hitherto been described, but not examined experimentally. The biomass per shoot, shoot density and growth rate of P. oceanica at various depths in polluted and unpolluted areas were measured during July and August 1978. A pilot investigation of the grazing of this seagrass species by the sea urchin Paracentrotus lividus (Link.) was attempted. Biomass per shoot was not significantly different at any of the locations examined. Shoot density was negatively correlated with depth and with pollution levels. Growth was at a maximum in polluted areas (0.003 g g−1 day−1). Experimental field studies, with P. lividus in enclosures revealed that this species will eat seagrass leaves in the wild.

Biomass and growth of Zostera capricorni aschers. in port hacking, N.S.W., Australia

Abstract The biomass and net productivity of the leaves of Zostera capricorni Aschers. were measured in an estuary near Sydney, New South Wales, at various times during the period 1976–1979. The mean standing crop biomass of leaves was 55 g dry wt. m−2, while mean relative growth rate ranged from 0.014 g g−1 day−1 in winter to 0.028 g g−1 day−1 over the summer months. Productivity was estimated by a leaf marking method, and showed a closer relationship to water temperature than to solar radiation. Data on leaf length composition indicate that new leaf production continues throughout the year.

Fruits, seeds and germination in the seagrass Halophila ovalis (Hydrocharitaceae)

Fruits of Halophila ovalis (R. Br.) D. J. Hooker develop on rhizome nodes of the female plants during summer to autumn (February to April) in southern Western Australia. The fruit is protected by a pair of bracts and fruit walls consisting of several enlarged parenchyma cells which contain starch grains. In each fruit there are between one and sixteen seeds with a mean of 7.4 seeds per fruit. The seed has an embryo with a distinct leaf primordium protected by a curved cotyledon, and an enlarged hypocotyl. The hypocotyl consists of numerous uniform parenchymatous cells containing packed starch, protein and lipid as nutrient storage (...)