The Vegetation of the Blackwood River Estuary, South-West Australia

Abstract

The standing crop and productivity of the main primary producers (the sources of autochthonous detritus) were measured in a West Australian estuary which has a shallow estuarine basin, 2 small lagoons and a tidal river and marked seasonal changes in salinity. Standing crops of phytoplankton (mainly diatoms) are small in the open water; no blooms were observed during 3 yr. In sporadic sampling the highest concentration of chlorophyll a was 1.98 .mu.g l-1. Sediments contain more chlorophyll (as microscopic algae) per unit area than does the water column. Small phytoplankton stocks are attributed to low nutrient concentration when light flux and temperature are high and water-residence time low. Macroscopic algae (20 spp.) are most prominent in the summer marine phase. Their distribution is very patchy and they have a small total biomass. There are 4 benthic angiosperms in the estuary. The greatest contributor to production in open water is Ruppia, which is the most widely distributed of these, and has a peak dry wt standing crop of 503 g m-2. The fringing marshes are mainly dominated by Juncus kraussii, which reaches a dry wt standing crop of about 5 kg m-2. Based on general distribution, standing crops and published seasonal data, it is estimated that Ruppia produces .apprx. 700-1900 tonnes (dry wt) yr-1, Juncus about 1000-4500, and the paperbark trees (Melaleuca spp.) of the marsh about 800 tonnes yr-1 in leaf litter. Most of this organic material must become available as detritus, as only Ruppia is grazed appreciably.