Diana I. Walker

Mechanistic interpretation of carbon isotope discrimination by marine macroalgae and seagrasses

The literature, and previously unpublished data from the authors laboratories, shows that the δ13C of organic matter in marine macroalgae and seagrasses collected from the natural environment ranges from -3 to -35‰. While some marine macroalgae have δ13C values ranging over more than 10‰ within the thallus of an individual (some brown macroalgae), in other cases the range within a species collected over a very wide geographical range is only 5‰ (e.g. the red alga Plocamium cartilagineum which has values between -30 and -35‰). The organisms with very negative δ13C (lower than -30‰) are mainly subtidal red algae, with some intertidal red algae and a few green algae; those with very positive δ13C values (higher than -10‰) are mainly green macroalgae and seagrasses, with some red and brown macroalgae. The δ13C value correlates primarily with taxonomy and secondarily with ecology. None of the organisms with δ13C values lower than -30‰ have pyrenoids. Previous work showed a good correlation between δ13C values lower than -30‰ and the lack of CO2 concentrating mechanisms for several species of marine red algae. The extent to which the low δ13C values are confined to organisms with diffusive CO2 entry is discussed. Diffusive CO2 entry could also occur in organisms with higher δ13C values if diffusive conductance was relatively low. The photosynthesis of organisms with δ13C values more positive than -10‰ (i.e. more positive than the δ13C of CO2 in seawater) must involve HCO3- use.

Seagrass degradation in Australian coastal waters

Australia has large areas of seagrass, rich in diversity, which flourish in clear, relatively low-nutrient coastal waters. Seagrass losses in recent years have been extensive with over 45 000 ha lost. The major wide-spread human-induced declines of seagrass, from 11 sets of locations around Australia, are summarized. The reasons for these losses are discussed, most being attributable to reduced light intensity, but in many cases, other factors interact to make the process of loss more complex. These declines result in loss of habitat and productivity, and increased sediment mobility. Recovery and recolonization from such losses are rare; thus, the destruction of seagrass has long-term consequences. Increasing awareness of the risks and better understanding of seagrass systems is leading to better management practices.

Effect of boat moorings on seagrass beds near Perth, Western Australia

Boat moorings have been found to produce circular scours in seagrass meadows, ranging from 3 to 300 m2. “Cyclone” moorings (which have three anchors and a swivel) are much less damaging to seagrass meadows than “swing” moorings (with a single anchor and chain). n nThe total area of seagrass meadow lost due to moorings totals some 5.4 ha in the Rottnest Island, Warnbro Sound and Cockburn Sound regions of Western Australia, with most loss (3.14 ha) in the Rottnest region. While the relative area of seagrass meadow lost is small (<2%), there is considerable visual impact in some areas. n nThe scours created by moorings in the seagrass canopy interfere with the physical integrity of the meadow. Though relatively small areas of seagrass are damaged by moorings, the effect is much greater than if an equivalent area was lost from the edge of a meadow.

Radial oxygen loss from intact roots of Halophila ovalis as a function of distance behind the root tip and shoot illumination

Abstract Radial oxygen loss (ROL) was measured as a function of distance behind the tip for roots of the seagrass Halophila ovalis (R.Br.) Hook f. The effects of shoot illumination and leaf area on ROL were also examined as were the porosity and anatomy of the roots, rhizomes and petioles of H. ovalis . For plants taken from the Swan Canning Estuary, Western Australia, the porosities of roots, rhizomes and petioles were 15%, 27% and 17%, respectively. ROL from roots in an O 2 -free and saline medium was measured using root-sleeving cylindrical platinum O 2 electrodes. The shoots were submerged in aerated seawater during the measurements. ROL was substantially higher when the shoots were exposed to saturating light and it decreased markedly in the dark. These findings, and experiments in which the leaves were excised, show that O 2 lost radially from the roots was photosynthetically derived. Moreover, ROL showed a marked gradient along the root; ROL decreased from an average maximum value of 72xa0ngxa0cm −2 xa0min −1 at 0.5xa0cm behind the root tip to only 4xa0ngxa0cm −2 xa0min −1 at 3xa0cm, the most basal position tested. These data show that roots of H. ovalis contain `a barrier to ROL in the more basal regions, an adaptation shown by other workers to enhance the growth of roots of wetland macrophytes into anaerobic sediments.

The distribution, biomass and primary production of the seagrass Halophila ovalis in the Swan/Canning Estuary, Western Australia

The seagrass Halophila ovalis (R.Br.) Hook f. is the dominant benthic plant of the Swan/Canning Estuary, southwestern Australia. This paper describes the biomass, distribution and primary production of this plant in relation to environmental factors. n nHalophila ovalis occupied 550–600 ha in the lower reaches of the estuary, approximately 20% of the area of the main estuarine basin. Over 99% of the seagrass was in water less than 2 m deep (relative to “datum”, an extreme low water reference mark set in 1892). Distribution in the main estuarine basin differed little between 1976 and 1982, although the species was more ephemeral in the Canning Estuary. n nUniform stands of Halophila ovalis reached a biomass of up to 120 g dry weight (DW) m−2 in late summer/early autumn, and maximum productivities of up to 40 g DW m−2 day−1 in summer. At peak biomass, the area of Halophila ovalis in the estuary represented approximately 350 t DW of plant material, 4200 kg of nitrogen and 630 kg of phosphorus. Average productivity was 500 g C m−2 year−1, although uniform stands in shallow waters attained up to 1200 g C m−2 year−1. n nThe biomass, productivity and biometry of Halophila ovalis were strongly influenced by salinity, temperature and light supply. The main growing period was summer, when marine salinities prevailed, and light supply and temperature were highest. Salinity, temperature and light were lowest during winter. Field and laboratory studies indicated that during years of average river discharge (1980, 1982), Halophila ovalis was little affected by the salinity range experienced (15–35‰). However, during 1981, a year of high discharge, conditions of low salinity and poor light supply caused severe declines in biomass, particularly in the Canning Estuary. Light was considered the more important factor controlling growth, since the waters of the estuary are generally turbid, and subject to sudden increases in turbidity. The effects of salinity, temperature and light were investigated by growing sprigs in artificial seawater culture and measuring growth increments. Each factor was investigated separately; salinity values ranged from 5 to 45‰, temperature from 10 to 25°C and light from 0 to 400 μE m−2 s−1. Halophila ovalis grew actively at salinities from approximately 10 to 40‰. Saturating irradiance was approximately 200 μE m−2 s−1 (10% of surface PAR) and compensation point was approximately 40 μE m−2 s−1 (2% of full sunlight PAR). Temperatures lower than 15°C severely limited productivity, and at 10°C no growth occurred, although plants did not die. Productivity increased from 15 to 20°C by a factor of seven, and a further 30% from 20 to 25°C. The highest observed growth rate, approximately 2.1 mg DW per apex day−1, was reached at 25°C. n nThese results were incorporated into a model to determine how much of the variance in productivity could be accounted for by these three factors, assuming independent action. The model was relatively successful at predicting seasonal growth responses, but underestimated spring productivity, probably because the unpredictable light climate in spring in the Swan River was not fully simulated.

Threats to Macroalgal Diversity: Marine Habitat Destruction and Fragmentation, Pollution and Introduced Species

Macroalgae have not been the subject of serious conservation attention. The conservation of biological diversity in terrestrial environments has become recognised as requiring conservation of habitats in order to preserve diversity. This concept is particularly applicable in the marine environment, where macroalgae are limited to the photic zone, and usually grow attached to hard substratum. Three of the major threats to marine macroalgae biodiversity are habitat alienation, pollution and the introduction of exotic (alien) species. The development of the coastline, particularly related to increased population pressure in coastal areas, leads to construction, for example, of marinas, port facilities and canal estates. Developments result in direct destruction of existing communities and indirect changes in hydrodynamics and sedimentation. They may produce extra habitat in the form of new surfaces for colonisation, but there is unlikely to be a net gain in habitat. Reduced water quality in association with development, and from point source and diffuse pollution, also results in macroalgal loss. Generally these losses do not lead to complete loss of a taxon, but the extent of human population growth in the coastal zone will continue to increase the impact on algal populations and certainly lead to zones of diversity depletion resulting from multiple, chronic pollution events. A third major threat to macroalgal diversity is that of introduced species. The examples of Sargassum muticum in Europe, and Undaria pinnatifida in France, Australia and New Zealand, suggest species introduction can cause replacement of dominant macroalgae by introduced species resulting in shifts in communities and their trophic food webs.

Dispersal of propagules of Sargassum spp. (Sargassaceae : Phaeophyta) : observations of local patterns of dispersal and consequences for recruitment and population structure

Abstract Propagule dispersal in Sargassum spp. was studied from reproductive adults as a single point source of reproductive adults and distributed naturally throughout a bed. Dispersal from single sources was investigated by staining 80 reproductive thalli, tying them together at one location, and sampling released propagules by suctioning the substratum with a venturi suction pump at distances up to 2 m from the stained thalli. Dispersal from many reproductive adults within the bed was determined using limestone settlement plates placed within and outside a subtidal Sargassum bed containing reproductive thalli that were releasing propagules. Numbers of settled propagules were subsequently compared with the numbers of macroscopic recruits visible 2–3 months after propagule release and then with mortality of these recruits. From a single source, dispersal of propagules was highly localized with most propagules settling within 1 m (≈98%). Densities of settled propagules declined exponentially with distance from their source. The rate of decline was variable and significantly different (at p = 0.05) between repeated experiments. Propagule settlement was greater within Sargassum beds than beyond them and the source of many of the propagules settling beyond the Sargassum bed appears to be the local bed itself. Mortality of settled propagules was very high with 0.0045% surviving to visible recruits. Further exponential losses occurred such that only 0.0001% survived for 12 months. Propagules of Sargassum disperse locally, most settling within their bed of origin. Highly localized propagule dispersal and settlement could lead to patchy distributions of recruits within and outside a Sargassum bed. The high mortality of recently settled propagules suggests that recruitment into the local adult population could at times be uncoupled from local propagule dispersal. The causes of high mortality of recently settled propagules is unknown and deserves further study.

THE DISTRIBUTION OF SEAGRASS SPECIES IN SHARK BAY, WESTERN AUSTRALIA, WITH NOTES ON THEIR ECOLOGY

Twelve species of seagrass were found in Shark Bay (26°S, 114°E), forming some of the largest seagrass meadows reported. The distribution of the species within the bay and descriptions of typical habitat types are given. The area is dominated by Amphibolis antarctica (Labill.) Sonder ex Aschers. which covers 3700 km2, approximately 85% of the area covered by seagrasses, with smaller areas of Posidonia australis Hook. f. (200 km2). Smaller seagrasses occupy an additional 500 km2, which includes patches of high species richness, with up to 9 species within a few m2. Factors influencing the distribution of seagrasses within Shark Bay are discussed.

Landscape-scale changes in seagrass distribution over time: a case study from Success Bank, Western Australia.

Abstract Seagrasses in temperate Australia persist on sand habitats in shallow coastal environments by recruitment from seedlings and lateral spread of rhizomes from existing meadows. These colonizing processes, combined with seagrass loss from physical disturbance, result in a mosaic of sand and seagrass habitats. Here we describe these changing seagrass landscapes on Success Bank, Western Australia over a 20-year period, using aerial photographs. The 4xa0ha landscape units (LUs), selected from areas of current Posidonia coriacea Cambridge and Kuo and Amphibolis griffithii (Black) Den Hartog meadows, were analyzed for seagrass cover from aerial photographs from 1972, 1982 and 1993. Two LUs for each species were chosen from three regions (west, central and east) across Success Bank. Changes in landscape features of LUs were then summarized into total area and length of edge to area ratios of seagrass patches and meadows. Seagrass cover in LUs increased by 20,000 to 30,000xa0m −2 between 1972 and 1993. Such a large increase in seagrasses has not been documented elsewhere in Australia for these seagrass genera. Seagrass expansion was observed as an increase in the number and size of seagrass patches ( 2 ). A simple model of seagrass colonization, based only on radial extension via rhizome growth, was constructed to test whether such large increases in seagrass cover could be accounted for solely by rhizome elongation. The model fitted observed increases in seagrass cover in some, but not all landscape units. The greatest divergence was in the western region where observed cover was higher than modeled rates. In central and eastern regions the modeled and observed increase in seagrass cover were similar. From aerial photographs, seagrasses have been actively colonizing Success Bank over the last 20 years. These observed changes can be accounted for by published rates of horizontal rhizome elongation for some, but not all, landscape units, suggesting that rhizome elongation is only part of the active seagrass colonization process observed on Success Bank. Future studies should target more accurate assessment of rhizome elongation rates, and colonization by seedlings of P. coriacea and A. griffithii , which were observed in great numbers. Whether the observed increase in seagrass cover is a phenomenon unique to Success Bank and to the seagrass species studied, or more generally applicable to other locations and seagrasses, also requires further study.

Nitrogen uptake and allocation in the seagrass Amphibolis antarctica

Abstract Ammonium acquisition and internal allocation of nitrogen in the seagrass Amphibolis antarctica ((Labill.) Sonder ex. Aschers.) were studied on freshly collected plants in laboratory experiments. The uptake kinetics were studied from the depletion of ammonium in split chamber experiments, while N uptake by entire plants and internal allocation patterns were studied using 15 N techniques on culture plants. The uptake of ammonium was concentration dependent and followed Michaelis-Menten kinetics. Maximum uptake rates for leaves were 5–38-fold higher than for the root-rhizome complex and the ammonium uptake by leaves was transiently enhanced when plants were suddenly exposed to ammonium. Transiently elevated uptake rates were relatively short-lived and only significant at very high substrate concentrations. Amphibolis rarely, or never, experience nitrogen concentrations that high, and so, surge uptake has only little ecological relevance. The uptake of ammonium at low and ecologically relevant substrate concentrations could only supply about 70% of the nitrogen demand of rapid Amphibolis growth during summer and the remaining 30% had to be met from internal sources. The 15 N experiments showed that both young and old leaves took up nitrogen but most of the nitrogen taken up by old leaves was immediately exported to young actively growing plant parts. Also, nitrogen was re-mobilized and subsequently exported from old to young plant parts. This export of re-mobilized nitrogen could supply about 36% of the nitrogen incorporated into young actively growing tissues, thus lowering the demand for external nitrogen by an equivalent amount. Re-mobilization and subsequent allocation of nitrogen from old plant tissues seem to be an important way to reduce the demand for external nitrogen in Amphibolis and, therefore, this seagrass seems well adapted to sustain rapid growth in nutrient-poor environments.