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 pla nt in relation to environmental factors. Halophila oualis 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 wate r less than 2 m deep (relative to ''datum'', an extreme low water refe rence mark set in 1892). Distribution in the main estuarine basin diff ered little between 1976 and 1982, although the species was more ephem eral in the Canning Estuary. Uniform stands of Halophila oualis reache d 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 s ummer. At peak biomass, the area of Halophila oualis in the estuary re presented approximately 350 t DW of plant material, 4200 kg of nitroge n and 630 kg of phosphorus. Average productivity was 500 g C m(-2) yea r(-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 su pply. The main growing period was summer, when marine salinities preva iled, and light supply and temperature were highest. Salinity, tempera ture 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 parts per thousand). However, during 1981, a year of high disc harge, conditions of low salinity and poor light supply caused severe declines in biomass, particularly in the Canning Estuary. Light was co nsidered the more important factor controlling growth, since the water s of the estuary are generally turbid, and subject to sudden increases in turbidity, The effects of salinity, temperature and light were inv estigated by growing sprigs in artificial seawater culture and measuri ng growth increments. Each factor was investigated separately; salinit y values ranged from 5 to 45 parts per thousand, temperature from 10 t o 25 degrees C and light from 0 to 400 mu E m(-2) s(-1). Halophila ova lis grew actively at salinities from approximately 10 to 40 parts per thousand. Saturating irradiance was approximately 200 mu E m(-2) S-1 ( 10% of surface PAR) and compensation point was approximately 40 mu E m (-2) s(-1) (2% of full sunlight PAR). Temperatures lower than 15 degre es C severely limited productivity, and at 10 degrees C no growth occu rred, although plants did not die. Productivity increased from 15 to 2 0 degrees C by a factor of seven, and a further 30% from 20 to 25 degr ees C. The highest observed growth tate, approximately 2.1 mg DW per a pex day(-1), was reached at 25 degrees C. These results were incorpora ted into a model to determine how much of the variance in productivity could be accounted for by these three factors, assuming independent a ction. The model was relatively successful at predicting seasonal grow th responses, but underestimated spring productivity, probably because the unpredictable light climate in spring in the Swan River was not f ully simulated.