A.J. McComb

The loss of seagrasses in Cockburn Sound, Western Australia. I. The time course and magnitude of seagrass decline in relation to industrial development

The areas of seagrass meadows in Cockburn Sound, a marine embayment in Western Australia, were estimated from historical aerial photographs supplemented by ground surveys, studies on meadows in adjoining areas, and coring for rhizome remains. Ten species of seagrasses with different habitat tolerances are recorded for the area, with Posidonia sinuosa Cambridge et Kuo forming the most extensive meadows. It is estimated that from 1954 to 1978 the meadow area was reduced from some 4200 to 900 ha. Based on measurements of aboveground productivity at several sites, this represents a reduction of leaf detritus production from 23 000 to 4000 t (dry wt.) y−1. The major loss of seagrass occurred during a period of industrial development on the shore, and the discharge of effluents rich in plant nutrients.

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.

The loss of seagrass in Cockburn Sound, western Australia. II: Possible causes of seagrass decline

This paper examines possible reasons for the extensive loss of seagrass in Cockburn Sound following industrial development. Transplanted seedlings survived poorly in Cockburn Sound compared with an adjoining embayment. Altered temperature, salinity, sedimentation and water movement do not explain the death of seagrass over wide areas, and there is no evidence for a role of pathogens. Oil refinery effluent reduced seagrass growth in aquaria at concentrations similar to those at the point of discharge, but could not account for the widespread deterioration observed in the field. Severe grazing by sea urchins was observed on meadows already under stress and does not appear to be a primary cause of decline; caged, transplanted seedlings also deteriorated. Increased light attenuation by phytoplankton blooms may have affected the ddepth to which seagrasses could survive, but would have had little significant effect in shallow water; marked phytoplankton blooms were recorded only after extensive seagrass decline had taken place. Light reduction by enhanced growth of epiphytes and loose-lying blankets of filamentous algae in nutrient enriched waters is suggested as the most likely cause of decline. Heavy epiphyte fouling was consistently observed on seagrasses in deteriorating meadows, as well as on declining, transplanted seedlings, and is known to significantly impair photosynthesis in other systems. Extensive seagrass decline coincided with the discharge of effluents rich in plant nutrients.

The loss of seagrass in cockburn sound, Western Australia. III. The effect of epiphytes on productivity of Posidonia australis Hook. F.

The hypothesis was examined that increased epiphyte growth was responsible for a reduction in seagrass meadows in Cockburn Sound during the discharge of nutrient-rich effluent. One study site was in a deteriorating meadow near an effluent outfall, the other at similar depth in an unaffected meadow in more oceanic water. Seagrass production at the first site was less than that at the second, with 33% lower growth per shoot and 29% less dense meadow. Water at the former site had higher mean concentrations of chlorophyll and phosphate than the latter, but light reaching the seagrass meadows was not significantly different. Epiphyte loads (as dry weight or chlorophyll per unit leaf area) were 2–8 times higher at the former site. Seasonal changes in epiphyte loads were well correlated with periphyton biomass on glass slides or plastic seagrass. Photosynthesis of leaf segments, with and without epiphytes, was measured using an oxygen meter in the laboratory; epiphyte photosynthetic rates were similar to those of periphyton on plastic, expressed per unit chlorophyll. The percentage reduction in light by known periphyton loads was measured, and used to calculate light reduction by epiphytes in the field, which was estimated to be 63% on average at the first site and 15% at the second. Pooling data for sites and seasons, there was a negative log-linear relationship between leaf production and epiphyte load. The observations provide support for the suggestion that seagrass loss in the Sound may be attributed to enhanced epiphyte loads following nutrient enrichment.

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). The 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. The 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.

Macroalgal-sediment nutrient interactions and their importance to macroalgal nutrition in a eutrophic estuary

The potential for algal banks to influence water quality and sediment nutrient flux was examined through laboratory experiments and in situ monitoring of algal banks. Loose macroalgal banks displayed seasonal changes in tissue nutrient concentrations suggesting a strong dependence on water column nutrients. These banks fail to generate conditions suitable to sediment nutrient release. Dense banks generated low oxygen conditions in the inter-algal water (0–1 mg l−1), corresponding to zones of high, and relatively stable, phosphate and ammonium concentrations (up to 96 μg l−1 PO4P and 166 μg l−1 NH4N). Laboratory experiments confirmed that macroalgal banks can generate reducing conditions at the sediment surface, regardless of the aeration regime, through the decomposition of macroalgal tissue. Platinum electrode potentials as low as −200 mV were recorded in the inter-algal water. In such banks, redox-dependent sediment nutrient release and anaerobic accumulation of nitrogen accounted for inter-algal nutrient concentrations of over 60 μg l−1 phosphate and 800 μg l−1 ammonium. The generation of reducing conditions in inter-algal water required 7 days of still conditions and so this mechanism of nutrient generation is unlikely to be important in winter, when strong winds frequently shift the algal banks. It is suggested that in summer this mechanism may provide a source of nutrients to dense algal banks, supplementing reserves stored in winter.

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. Halophila 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. Uniform 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. The 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. These 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.

Changes in the biomass and species composition of macroalgae in a eutrophic estuary

More than 20 years of data are presented on the macroalgal biomass, species composition and water quality of Peel-Harvey estuary in south-western Australia. The occurrence of macroalgal blooms was a sudden event in the late 1960s, and appears to have resulted from nutrient availability surpassing a threshold of some kind. Cladophora dominated the system until 1979 and appears to have had a competitive advantage in deep-water areas because of its morphology. A catastrophic event compounded by a series of unfavourable conditions resulted in the loss of Cladophora from the deep areas and its estuary-wide replacement by Chaetomorpha, which was more competitive in the shallows. Since 1979, changes in water quality have been reflected in changes in biomass and species composition in the system. Average annual biomass is linearly related to average light attenuation over the summer growth period. Periods of high nutrient concentrations favour Ulva and Enteromorpha, while Chaetomorpha resumes dominance during periods of lower mean nutrient concentrations. Nutrient concentrations appear to be more influential on an inter-annual than seasonal scale, except in the case of Ulva which, on the basis of tissue N and P concentrations, is seasonally nitrogen-limited. Light attenuation appears to have seasonal and long-term effects. The data support the hypothesis of other workers that inter-annual differences in hydrographic events and phytoplankton dynamics influence macroalgal dynamics. The concept is examined further in light of this extensive database.

Plant resins—their formation, secretion and possible functions

Plant resins pose interesting ecological, taxonomic, physiological, and biochemical problems. This chapter briefly describes the resins in chemical terms and presents their contrast with certain other plant products. Resins are nonvolatile products of plants, from which they exude naturally (surface resins) or can be obtained by incision or infection (internal resins). They are insoluble in water but soluble in organic solvents. Stable, inert, and amorphous, they become sticky when heated and are fusible with no sharp melting points. They are mixtures of compounds, including flavonoids, terpenoids, and fatty substances. Resins are usually produced in specialized surface glands (glandular hairs) or internal ducts. Such ducts are widespread in certain families and occur in both woody and nonwoody plants. They are more common in gymnosperms and dicotyledons than in monocotyledons. The chapter focuses on the external resins that are secreted onto leaf surfaces, but it also provides information on resins, which remain within the plant, and other related plant products.

The nutritional eco-physiology of Chaetomorpha linum and Ulva rigida in Peel Inlet, Western Australia

The uptake rates and critical tissue concentrations of nitrogen and phosphorus were determined for Chaetomorpha linum and Ulva rigida , the dominant algae in Peel Inlet, Western Australia. Both species had rate-saturating mechanisms of phosphate uptake described by Michaelis-menten type functions; C. linum had the faster uptake rate (667 c.f. 272 mu g P g/dwt/h) although U. rigida had a lower half-saturation value. Both species displayed linear relationships between ammonium uptake rates and substrate concentrations with C. linum having the greater slope (4.4 c.f. 1.7). Chaetomorpha linum also had a linear increase in uptake rate with increasing concentration of nitrate, but U. rigida showed rate-saturating kinetics; below 750 mu g/L, U. rigida had the higher rate of uptake. Ulva rigida had critical tissue nitrogen and phosphorus concentrations of 20 and 0.25 mg g/dwt respectively. Corresponding concentrations for C. linum were 12 and 0.5 mg g/dwt. Ulva is frequently nitrogen limited during spring in Peel Inlet, reflecting the high nitrogen requirements of this plant compared to Chaetomorpha as well as the reduced ability of Ulva to store nutrients over winter.