NUTRIENT CONTENT OF FRINGING VEGETATION
The nutrient dynamics of the marshes were examined, particularly because of their possible relevance to the apparent trend of eutrophication in the Peel-Harvey system. Marshes may act as nutrient 'buffers', taking up nutrients and storing them in plant material, and conversely, contributing nutrients through decay. In this work a preliminary assessment was carried out, largely to discover if a significant nutrient pool might be present in the fringing marshes. The estimates were made for only two of the fringing plants, chosen to represent prominent vegetation of somewhat extreme growth form, Scirpus maritimus and Sarcocornia blackiana.
Representatives of the two plants were harvested from dense stands, in the case of Sarcocornia from the shore of Roberts Bay, and for Scirpus near the mouth of the Harvey River. The harvested material was dried (80 ºC, 72 hr) and weighed. Nitrogen and phosphorus contents were determined from ground material by standard techniques (Atkins, 1978).
The area of salt marsh likely to contribute to overall productivity was determined using the original large-scale version of Figure 1.1.
(i) Sarcocornia: The above ground crop of S. blackiana was 2.8 kg m-2. This is higher than that obtained by Congdon and McComb (1981) (0.8 kg m-2) for Sarcocornia growing at the Blackwood River estuary, Western Australia, and is also higher than that obtained by Mahall and Park (1976) for the related Salicornia virginica (0.55 - 0.96 kg m-2) in the United States. Sarcocornia growing in the region from which the sample was collected appeared healthy, with thick green stems. The material was collected in February and was possibly at a maximum standing crop for the year.
(ii) Scirpus maritimus: The standing crop of this plant was 3.85 kg m-2. This is higher than for Sarcocornia and is considerably higher than that obtained by Congdon and McComb (1981), 0.8 kg m-2, for the Blackwood River estuary.
(b) Total Biomass: Using the values of Congdon (1977) for the ratio of above ground to below ground standing crop (Sarcocornia 4:1; Scirpus 1:3), the respective values for total biomass of 3.5 and 15.4 kg m-2 were obtained, indicating a considerably greater biomass of Scirpus as compared with Sarcocornia. It might be noted that the areas sampled were relatively dense stands, and commonly values may range between 20% and 50% of these measured.
(c) Nutrient Content of Marsh Vegetation:
(i) Nitrogen: The mean total nitrogen concentrations in above ground material of both species was 6.2 mg N g-1 dry wt. There was no significant difference between the two plants (p < 0.05, 't' test). Replicate analyses showed little deviation from this figure. Using this mean value, and an estimated ratio of Scirpus to Sarcocornia of about 20 to 1, and the calculated area of marshland, a total nitrogen pool for the marsh can be calculated as shown in Table 3.1.
(ii) Phosphorus: The phosphorus concentrations were 1038 µg g-1 dry wt. for Scirpus and 876 µg g-1 dry wt. for Sarcocornia. These means were significantly different (p < 0.05, 't' test). Replicate samples for each plant were in close agreement. An estimate of the total amount of phosphorus in the marshes was obtained in the same way as for nitrogen - Table 3.1.
Table 3.1: Total nitrogen and phosphorus in marsh vegetation1
|
|
Scirpus maritimus |
Sarcocornia blackiana |
Total |
| % of marsh occupied |
5 |
95 |
100 |
| Area occupied (m2) |
64 x 104 |
1216 x 104 |
1280 x 104 |
| Above-ground standing crop (tonnes) |
24.6 x 104 |
341 x 102 |
366 x 102 |
| Total biomass (tonnes) |
99 x 102 |
428 x 102 |
527 x 102 |
| N content (mg g-1) |
6.2 |
6.2 |
- |
| Total N, above-ground (tonnes) |
15.2 |
210 |
255 |
| 2Total amount N (tonnes) |
61 |
264 |
325 |
| P content (mg g-1) |
1.04 |
0.88 |
- |
| Total P, above-ground (tonnes) |
2.56 |
30.0 |
32.6 |
| 2Total amount P (tonnes) |
10.3 |
37.5 |
47.8 |
| 1Data based on relatively dense stands of vegetation 2Assumes concentration is the same in above- and below-ground material |
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