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

Abstract

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.