Changes in chlorophyll a, salinity and phosphate concentrations during a particular year are shown in Figure 5 for the Harvey Estuary. The pattern is essentially the same in different years, but the magnitude of the events differs. Chlorophyll concentrations are very low in summer, and chlorophyll ais dominated by contributions from diatoms of essentially benthic taxa stirred into the plankton by wind mixing (Gabrielson and Lukatelich, 1985; Lukatelich and McComb, 1986b). Chlorophyll concentrations rise during winter as blooms occur of planktonic diatoms. Then in late spring there is a marked rise in chlorophyll aconcentration with the onset of a Nodulariabloom; this collapses in mid summer.
Salinity is depressed in mid winter because of river flow; it later increases because river flow ebbs, marine water penetrates, and evaporation rates are high. Nodulariablooms are initiated when temperatures are sufficiently high to allow germination of resting achinetes on the sediment, and rapid growth of filaments; the blooms collapse when salinity reaches about 30 (Huber, 1984; Lukatelich and McComb, 1986a).
Phosphate reaches high concentrations in winter because of river flow, in which about 80% of the total phosphorus is present as free phosphate (Lukatelich and McComb, 1986a). The winter loading of nutrients supports the growth of planktonic diatoms. The diatoms are extensively grazed by the copepods Gladioferansand Sulcanus(Lukatelich and McComb, 1986a; McComb and Lukatelich, 1986), so diatom biomass reaches high levels only transiently, if at all. These blooms provide the main mechanism for the trapping of nutrients from the water column, and their sedimentation in particulate form to the estuary floor occurs as faecal pellets and other detritus.
In contrast, during a Nodulariabloom there is very little available phosphate in the water column; Nodulariarelies on the sediments for its phosphorus supply. During the peak of a bloom the amount of phosphorus accounted for by Nodulariabiomass is equivalent to about 18-70% of the amount calculated to be discharged from the river in winter. Conditions of darkness and salinity stratification lead to anaerobic conditions at depth, which favour phosphorus release from sediments. Field release rates up to 70 mg Pm-2 d-l have been recorded, while in the laboratory the potential for release has been measured at 190 mg Pm-2 d-l (Lukatelich and McComb, 1985). These phosphorus release rates readily account for the amount of phosphorus required to support the growth of Nodulariathat is observed in the field.
Evidence that phosphorus is indeed responsible for controlling the amount of plant biomass in the estuary comes from a variety of studies, including interpretation of the time-course data, study of N:P ratios, use of algal assays, and the occurrence of phosphatase in the system (Huber, 1984; McComb et al.,1981;1984). When rivers flow in winter phosphate is readily available and nitrogen may be limiting for a time (especially in Harvey Estuary), but Nodulariaactively fixes nitrogen and so the biomass reached is controlled by the amount of phosphorus available.
For the macroalgae, laboratory studies on growth under controlled conditions (Gordon et al.,1981) and the interpretation of field data using modelling methods based on those data (Hornberger and Spear, 1980; Spear and Hornberger, 1980; Humphries et al.,1984; Gordon and McComb, 1988) also suggest that phosphorus is in relatively short supply compared with nitrogen, a conclusion born out by analysis of tissue nutrient concentrations (Birch et al.,1981; Lukatelich and McComb, 1985).