It is useful to comment briefly about the variation encountered in the variables studied. Table 1 summarises information obtained over 24 hr periods, one in the 'winter phase' during a period of river discharge, the other in the 'summer phase', when there was no river flow. The data are for a single site in each basin.
Salinities were higher in summer than winter, and the Harvey Estuary site was more saline than the Peel Inlet site in summer, and less saline in winter, mainly because it is further removed from the channel to the ocean. In winter there was marked salinity stratification at the Peel Inlet site; this was less obvious in the Harvey Estuary, and stratification was lost in the afternoon. In summer the water in both estuaries was hypersaline, with some stratification in Peel Inlet, but not in the Harvey Estuary.
Dissolved oxygen concentrations were higher in winter than in summer, and this was attributed to the higher concentrations of phytoplankton present at the time. The effect of stratification can be seen in reduced oxygen concentrations at depth in winter.
Light was more strongly attenuated in winter than summer, and in summer light penetration was poorer in the Harvey Estuary than in Peel Inlet. Attenuation could not be attributed to phytoplankton, as absorbance curves lacked chlorophyll maxima. Using the equation of Bannister (1974), the chlorophyll afraction accounted for about 7% of the attenuation coefficient. The poor light penetration is due largely to wind-induced suspension of fine particulate detritus from the river and sediments (Gabrielson and Lukatelich, 1985).
Nutrient concentrations were generally higher in winter than in summer. Nitrate concentration was much higher in Peel Inlet in winter than summer, and this is attributed to riverine inflow of nitrate; it is concentrated in the surface freshwater. Ammonia concentrations were also high, especially in the bottom water, probably from the decomposition of organic detritus. In the Harvey Estuary there was a relatively high level of inorganic nitrogen in summer, attributed to the breakdown of a large Nodulariabloom which had occurred in late spring. In both estuaries phosphate concentrations were higher in the winter, and this is attributed to a combination of river loading, recycling from sediments under anaerobic conditions, and uptake by phytoplankton in surface waters in summer.
Chlorophyll concentrations were much higher in winter than in summer, and were
mainly due to planktonic diatoms, of which Skeletonema costatum, Cerataulina(2
spp) and Chaetoceros(3 spp) were the dominant taxa. In winter the chlorophyll
concentrations were higher in Peel Inlet than the Harvey Estuary, but this is
not typical - weekly data show that is more characteristic for winter chlorophyll
concentrations in the Harvey Estuary to exceed those in Peel Inlet (McComb
et al.,1981), as they do in the summer data in the table. In summer
the presence of chlorophyll is largely accounted for by diatoms from benthic
taxa, stirred into the water by wind mixing.
There can be large spatial differences in nutrients (Figure 2) and salinity (Figure 3). However, such large differences are found only during particular inflow events (for example after a severe winter storm), and because of the logistics of sampling such a large system, we chose to restrict routine sampling to three sites in each basin (Figure 1), but to sample those sites at a relatively high frequency (weekly or fortnightly). Regular monitoring has been combined with two other types of investigation. Firstly, intensive, process-oriented studies have been carried out over shorter time scales (days) (e.g. Black et al.,1981; Gabrielson and Lukatelich, 1985).
Secondly, we have sampled on a few occasions at an additional 36 sites ('grid studies'), to check how representative the regular sampling sites are under summer and winter conditions . In four studies it was found that for Peel Inlet the three regular sampling sites provide a good estimate of the status of the Inlet. There was no significant (p<0.05) difference between the means for the three regular sampling sites and the 20 grid study sites (which included the regular sites) for all of the measured variables .
A similar comparison of the three regular sites in the Harvey Estuary with
the 16 grid study sites revealed that for all of the measured variables, except
one, there was no significant (p< 0.05) difference between the means.
The exception was for nitrate plus nitrite during one of the studies (August,
1978) when the mean for all sites was 119 mg L-l
, but the mean for the three regular sampling sites was only 9 mg
L-l. It is not surprising that the three sites are not always representative
of such a large and complex water body, but on most occasions they apparently
did provide a good approximation of the status of the Harvey Estuary as a whole.
SAMPLING MACROALGAE
Sampling of macroalgae poses different problems, in that while biomass changes
slowly compared with phytoplankton; there are extreme spatial differences at
all times. For this reason 'grid' studies are undertaken in all seasons. At
each of 42 sites biomass is recorded from 5 cores, and the standard error of
the 5 is generally about 20% of the mean. Even so the system is grossly undersampled,
and average biomass values for the estuary would be meaningless. Instead we
routinely use the contouring package SYMAP (Dougenik
and Sheehan, 1977) to provide a distribution map (Figure
4), and then sum the area between contour lines to estimate total biomass
for the water body. As sampling and data processing is undertaken in the same
way on each occasion, trends should be revealed, even if absolute values of
total biomass remain somewhat conjectural.