Mud, seeds, seagrass and more mud

Sampling

Sampling for the seagrass seed distribution took place in the intertidal seagrass meadow of Bushland Beach. The sampling area was 180 m by 200 m and was divided into three longitudinal transects, each transect being 100 m apart. A quadrat of 50 cm by 50 cm was sampled every 20 m along the transects. The species composition and percent coverage was recorded in each quadrat, then a seed core sample was obtained, measuring approximately 10 cm and the sample was bagged and taken to the laboratory for freezing.

Laboratory work

The frozen samples were defrosted and measured, separate measurements were recorded for the oxic and anoxic layers before the cores were sliced vertically every centimetre. We anticipated 10 sediment slices per seed core, however not all samples measured 10 cm. The average seed core length between both years was 9.4 cm. Each sediment slice was then sieved in a standard kitchen sieve (mesh size ~ 1.2 mm). Halodule uninervis seeds were counted and recorded. Seeds of Halophila ovalis were excluded from the study as they were too small (< 0.5 mm) to detect.

Results

Graph 1: Depth of the oxic layer

• 
  The oxic layer within a seagrass meadow is the depth to which the rhizomes of the plant are able to effectively oxygenate the sediments. In 2005 this depth was 4.4cm which was slightly deeper than that for 2006, (4.3cm). The ecological significance of this difference in depth would be minimal.

Graph 2: Reproductive success of HU between the years, in relation to the anoxic and oxic layers

• Reproductive success as measured by half (germinated) seeds. More seeds germinated in 2005 than 2006, whilst the proportion of germinated seeds was equal between the oxic and anoxic layer for 2005, a greater proportion of germinated seeds were found in the anoxic layer in 2006.

Graph 3: Number of seeds found each year

• 
  The total number of seeds found in 2005 was greater than the total number of seeds found in 2006. This was also evident in the number of whole and half seeds found during the two year study.

Graph 4: Relationship between percent cover of HU and the number of seeds present

• 
  The data appears to present a trend of growing seed numbers with increases in Halodule uninervis percent cover. However, this increase in percent cover was likely to have been attributed to vegetative propogation, rather than through a means of sexual reproduction, due to the dwindling numbers of half (germinated) seeds (Refer to Graph 3).

Graph 5: Distribution of the average number of seeds within the rhizosphere

• 
  The majority of seeds in 2005 were found in the 5-6 cm sediment slice, whilst in 2006 the seeds were scattered, concentrating in the 5-6 cm slice and 9-10 cm slice. Because seeds of the Halodule uninervis form at the rhizome, which is located in the oxic layer to allow for sufficient root aeration, therefore it was not unusual to find the seeds at this point, approximately half way into the core (Refer to Graph 1).

Graph 6: Distribution of the average number of whole seeds within the rhizosphere

• 
  The pattern of distribution of whole seeds follows very closely to that of the total number of seeds, not surprising as the bulk of the seed bank was comprised of whole (ungerminated) seeds.

Graph 7: Distribution of the average number of half seeds

• 
  In 2005 the largest average number of half seeds was in the 5-6cm slice and the 3 to 4 cm slice in contrast to the the largest number of half seeds being found in 8-9 cm slice in 2006. As previously noted few species of seagrass undergo repeated seedling recruitment and only use sexual reproduction for initial seedling recruitment, hence there were few half (germinated) seeds.