Since the mid 1960s massive populations of green macroalgae have been reported from Peel Inlet and from the northern end of Harvey Estuary. The algae foul what were once clean, sandy beaches, onto which they are driven by wind-induced water movement, and may smother marsh plants fringing the estuary. They also accumulate through water movement as dense, offshore banks, in which all but the surface layer becomes: a decomposing mass.24 The first dominant species was Cladophora montagneanna, which had not been recorded previously from Australia.25 This occurred as beds of unattached spheres of radiating filaments. diameter 2-3 cm) in the deeper waters of Peel Inlet, beds from which spheres buoyed up by bubbles of oxygen floated to the water surface and were driven to the shore in long windrows; alternatively, spheres brought into resuspension by vertical mixing would be transported in surface currents. Arrival on the beaches was not seasonal, but laboratory and field studies confirmed that growth occurs mainly in the summer months, and that in winter light and temperature become limiting.18,25 Salinity has little effect over the range encountered in the estuary. Of the nutrients, experiments using laboratory culture. together with a knowledge off field-measured concentrations, showed that phosphorus was often at growth-Iimiting concentrations when light was available. 25,27,28 This was confirmed by analysis of tissue concentrations in the field, in relation to critical tissue nutrient concentrations measured in the laboratory.25
Over the years Cladophora has been replaced as a dominant alga by other green algae in the genera Chaetomorpha, Enteromorpha, and Ulva.17 This has occurred as light conditions have generally deteriorated in Peel Inlet: species of these genera have a different growth form than Cladophora, and are higher in the water column. The final demise of Cladophora dominance corresponded with a massive storm in 1979, which disrupted the deep algal beds, and which have never re-established.17Cheatomorpha and Enteromorpha have lower requirements for phosphorus than does Cladophora, a conclusion based on laboratory uptake determinations and the concentrations of nutrients in the tissues of plants collected from the field.17,22,23 Neverthless, light is of critical importance. and there is a close relationship between the amount of algal biomass found at the end of the growth period in Peel Inlet and the light attenuation coefficient during the growth period. 14,17
In Harvey Estuary there is a different symptom of eutrophication: the development of massive blooms of the blue-green cyanobacterium Nodularia spumigena. These first became obvious in 1978, though earlier reports suggest that large blooms had occurred in some previous years. Since 1978, the blooms have occurred in many, but not all summers and as explained above, blooms only occur after years of substantial river flow. The Nodularia blooms are initiated when temperatures rise sufficiently to allow the germination, of akinetes in the sediments, in late spring or early, summer; the bloom then increases to a level dictated by the amount of sediment phosphorus available, the organisms fixing massive amounts of dinitrogen dissolved in the water. 29,30 The blooms finally collapse as salinity rises to 30 ppt in midsummer.10
There is considerable variation in the peak chlorophyll a concentrations reached by Nodularia blooms in different years, and the size of the bloom is related to the minimum salinity reached in the estuary in the previous winter, which is a reflection of river volume and hence, nutrient loading.9,10
A deterioration in the condition of the estuarine system has therefore occurred
since the mid-1960s in Peel Inlet, and since at least the late 1970s in Harvey
Estuary. The total biomass of macroalgae has differed between years but has
in general drifted downwards in recent years, with a deterioration in light
climate.17 In Harvey Estuary
there have been few years without blooms recently, and the presence of post
Nodularia diatom blooms has become more common in summer, suggesting
an increase in nutrient cycling. 21
In some years there have been benthic accumulations of Oscillatoria and
relatively high populations of the coccoid blue-green Synechococcus.
Nevertheless, the available evidence suggests that a reduction of nutrient loading
will be met with a corresponding reduction in Nodularia, rather than
a continuation of large blooms sustained by long-term accumulations in the sediments.1,10,14