Distribution and abundance of benthic microalgae in a shallow southwestern Australian estuarine system

Measurements were made of benthic microalgal biomass (chlorophyll mg m-2) and concentration (chlorophyll, µg g-1 dry weight of sediment) from the Peel-Harvey estuarine system, Western Australia. Most chlorophyll was in the top 1 cm of sediment, and less than 10 % of chlorophyll a was non-functional as determined by hexane extraction. Highest biomass occurred at shallow sites and on coarse sandy sediments. Biomass was higher in Harvey (202 mg m-2) than Peel (107 mg m-2) in summer when there was a large population of microalgae in Peel; biomass was similar in winter (Harvey 163 mg m-2, Peel 151 mg m-2). Biomass increased with the onset of riverine nutrient input and decreased when blooms occurred in the water column. Except during Nodularia blooms, the biomass (chlorophyll) of benthic microalgae greatly exceeded (e g 40 times) the biomass of chlorophyll in the water column above. In summer much of the chlorophyll of the water column was due to wind stirring of benthic microalgae. The possible importance of benthic microalgae to the productivity of shallow systems is emphasized

The nutrient status of Wilson Inlet 1984-1985

In response to the concern of local residents about the condition of Wilson Inlet, a study of the inlet and its catchment was undertaken during 1982-83. While the results for that study were clear, it occurred during an atypical year; the winter had a particularly low rainfall and the sand bar between the estuary and the ocean was breached for the shortest period of any winter. To obtain some understanding of the variability likely to be encountered in the estuary between years, another, smaller study was undertaken in 1984-85. The main conclusions about nutrient concentrations in plant tissues, the importance of phosphorus, total plant biomass and general ecosystem behaviour were si111ilar to those reached in the earlier study, despite very different winters. Catchment behaviour, in relation to nutrient losses to streamflow was also generally similar, with somewhat higher concentrations of nutrients associated with a winter of higher runoff. The main contrast was the large amount of nitrogen, especially in the form of nitrate, which came from the larger subcatchments during the present study. Rainfall was much closer to the long-term mean in 1984 and there was a 2.8 times increase in streamflow for the whole Wilson Inlet catchment compared to 1982. The phosphorus load of 19 tonnes was lower than the 30 tonnes predicted for an average year on the basis of the 1982 results. The nitrogen load of 340 tonnes was slightly higher than predicted for an average year from the 1982 study. Salinities were lower and nutrient concentrations and water column loads higher in winter in the present study due to the higher streamflows. Chlorophyll 'a' concentrations and water column load were also higher, presumably in response to the higher nutrient loads. Estimates of total plant biomass in winter were similar, and the results of both studies show that Ruppia biomass is one of the major nutrient banks in the Inlet. Sediment nutrient loads were similar in both studies. A nutrient budget was calculated for one cycle of bar opening and closing. There was a net retention of phosphorus of 9 tonnes (49% of total input), and net retention of nitrogen of 217 tonnes (63%). The percentage of total riverine phosphorus load retained by the inlet was similar in both studies. In contrast, it was estimated that there was net export of nitrogen in the previous study

Distribution and biomass of seagrasses and algae, and nutrient pools in water, sediments and plants in Princess RoyalHarbour and Oyster Harbour

Sediment nutrient concentrations were generally highest in the deepest regions of both harbours due to sediment focusing. The sediments in the two harbours were the largest nutrient pool. Macroalgae were the largest plant nutrient pool, accounting for over 80% of plant biomass in Princess Royal Harbour and over 50% in Oyster Harbour. Macroalgae contained about 90% of the nitrogen and 70% of the phosphorus pool associated with aquatic plants in Princess Royal Harbour, and about 60% of nitrogen and 34% of the aquatic plant phosphorus pool in Oyster Harbour. The high mean areal macroalgal biomass (406 g m•2) in Princess Royal Harbour was indicative of a highly eutrophic system. This was also supported by the characteristics of biomass, cover and carbonate content of algal epiphytes on seagrasses. The results of this study indicated that Princess Royal Harbour was more eutrophic than Oyster Harbour. Dense macroalgal beds were found in the south-eastern comer of both harbours. Prevailing wind directions are such that these areas are the most sheltered and therefore are suitable for macroalgal growth and accumulation. Circulation patterns are also compatible with the accumulation of drift algae in this area of the harbours (Mills, 1987). The rate of loss of seagrass area in the two harbours since 1984 was lower than that estimated between 1981 and 1984 by Bastyan (1986). Considerable losses of biomass are, however, continuing to occur, particularly in Princess Royal Harbour where 60% of above-ground seagrass biomass present in 1984 was lost by 1988. At present, major losses are occurring in the shallow dense seagrass beds, presumably due to macroalgal smothering. Present epiphyte loads were not considered to be high enough to cause significant losses of seagrass. An increase in light supply is probably responsible for the seagrass regeneration recorded in some deeper areas of Oyster Harbour. This trend is expected to reverse if water quality deteriorates again. Macroalgal biomass in both harbours must be reduced to halt the loss of seagrass beds in shallow waters. The role of algal epiphytes in the decline of seagrass in both harbours may be more significant than implied from the results of this study due to the large interannual variation in nutrient loading to the harbours

Water quality and seagrass biomass, productivity and epiphyte load in Princess Royal Harbour, Oyster Harbour and King George Sound

Water and plant samples were collected from Princess Royal Harbour, Oyster Harbour and King George Sound between December 1987 and February 1989. Mean nutrient and chlorophyll a concentrations in the waters of Oyster Harbour were higher than either Princess Royal Harbour or King George Sound. In contrast, water clarity was lower. The water quality of Princess Royal Harbour in 1988/89 had improved significantly since the survey conducted in 1978n9, and was similar to the water quality in King George Sound. A high proportion of phosphorus entering Oyster Harbour is in a dissolved inorganic form. During floods a buoyant, nutrient-rich layer of freshwater flows over the top of the denser marine water of Oyster Harbour and out into King George Sound. Seagrass leaf biomass reached a seasonal maximum in spring/summer at all sites. Seagrass biomass and shoot density was lower in the two harbours than in King George Sound. Stands of P. sinuosa in Princess Royal Harbour were particularly sparse. Nutrient concentrations in seagrasses, epiphytes and periphyton indicated that Oyster Harbour was more nutrient enriched than Princess Royal Harbour and King George Sound. Light was found to be the dominant factor affecting seagrass leaf growth in all three waterbodies. Maximum leaf production rates per shoot were highest in spring, but maximum rates per unit area of meadow occurred in summer. Production rates were less consistent in the harbours than in King George Sound indicating a reduced capacity to lay down below-ground storage reserves, creating an increased vulnerability to unfavourable conditions such as prolonged periods of low light levels. Macroalgal smothering appears to be the major cause of seagrass decline in Princess Royal Harbour. In contrast, epiphytes are implicated as the main cause of seagrass decline in Oyster Harbour, apart from the south-east comer of the harbour where dense accumulations of macroalgae occur. This difference may be due to the better water clarity in Princess Royal Harbour favouring the proliferation of macroalgae, while the higher nutrient loading and relatively poor light conditions in Oyster Harbour may favour the growth of epiphytes

Seasonal changes in macrophyte abundance and composition in the Peel-Harvey estuarine system : report to the Waterways Commission, Perth, Western Australia

This report to the Waterways Commission discusses changes in macrophyte abundance and composition in the Peel-Harvey Estuarine system. It covers the period August 1984 to November 1987 inclusive

Nutrient levels and the development of diatom and blue-green algal blooms in a shallow Australian estuary

In the Peel-Harvey estuary system, Western Australia, some 90% of riverflow and nutrient loading occurs in three winter months. Diatom blooms follow riverflow, but are replaced by blooms of the blue-green Nodularia spumigena Mert., especially in Harvey Estuary. By analysis of time series data from 1977-1983, it is shown that the magnitude of the Nodularia bloom in summer is related to the minimum salinity of the estuary (and hence total river flow), maximum phosphate concentration and total riverine phosphorus loading, in the previous winter. The relationships have a predictive capacity. It is argued that diatom blooms trap phosphorus, which is sedimented largely as faecal pellets; the phosphorus is recycled and supports Nodularia growth under warmer conditions, and the amount available determines Nodularia biomass. Nodularia blooms collapse when summer salinities reach 30

The Peel-Harvey estuarine system, Western Australia

Peel Inlet and Harvey Estuary are two adjoining estuarine basins in southwestern Australia, which in recent years have become highly eutrophic. One of the basins, Peel Inlet, supports a large biomass of green macroalgae; while the other, Harvey Estuary, has dense summer blooms of the blue-green alga (Cyanobacterium) Nodularia spumigena Mert. This estuarine system is fed by three rivers, and communicates with the Indian Ocean through a common inlet channel (Figure 1 ). Beside the inlet channel lies the city of Mandurah (population 30,000), which is within 70 km of the capital city of the state, Perth, and its port of Fremantle. Once a quiet weekend retreat and a place for retirement, the Mandurah area has become rapidly urbanized because of the increasing population of the state and the construction of good access roads to Perth. Canal estates have been established at several points along the shores, especially near the inlet channel and along the Murray River. Depending on the seasons the shallow basins are used for sailing, water-skiing, windsurfing, fishing, crabbing and prawning; for example during a 5-day survey in January 1978, 1314 boats were found to be engaged in crabbing, 427 in fishing, 344 in sightseeing, 76 in water skiing, and 71 in 'miscellaneous activities' . 1 There is a commercial fishery (usually some $A2M per annum) which, although small in relation to marine fisheries of the western coast, is nevertheless one of the largest, estuarine-based fisheries in Australia. The estuaries lie on a sandy coastal plain of low relief, drained for agriculture since the 1920s. The coastal plain is separated from uplands in the catchment by a major fault line, the Darling Escarpment, which runs north-south for approximately 300 km, and is some 300 m tall (Figure 2). The escarpment marks an abrupt change in land use from agriculture on the plain to forested catchment on the immediate uplands; much of the water supply for urban areas is from reservoirs located along the escarpment. Further inland rainfall is reduced, and forested country gives way to land which has been cleared for grazing and wheat farming

Inter-relations between biological and physicochemical factors in a database for a shallow estuarine system

This paper examines data obtained since 1976 in Peel Inlet and the Harvey Estuary, a shallow estuarine system in Western Australia, which has nuisance growths of macroalgae and seasonal blooms of the cyanobacterium (blue-green alga) Nodularia spumigena. Data collected at the same sites at weekly or fortnightly intervals include phytoplankton (chlorophyll a), water nutrients (nitrogen and phosphorus), salinity, temperature and light penetration. Seasonally, the biomass of macroalgae has been measured at a number of sites and used to estimate total biomass. The data are characterised by large season-to-season differences, attributable to the seasonality and volume of river flow. The information has been used to relate the magnitude of summer blue-green algal blooms to the winter loading of phosphorus from the surrounding catchment, and the magnitude of macroalgal biomass to light penetration through the water column