Length Variation Of Hormosira Banksii - An Abundant Alga Species In Australia

Length variation of Hormosira banksii

Hormosira banksii is an abundant alga species in Australia and a foundation species within many intertidal communities. It features various morphological variations that allow it to survive harsh conditions. One such variation, length, was investigated, to determine if it was affected by water depths. Three sites at Point Gellibrand, Victoria, Australia, were sampled. Shallow and deep-water specimens were measured along two transects at each site. The mean lengths of each group, shallow and deep, were compared.

This showed that H. banksii in shallow waters were, on average, 5.1 ± 2.8 cm shorter than those in deep waters. H. banksii were significantly shorter near the shore (mean= 8.9cm, n=47, SE= 0.82) compared to those further away (mean= 14cm, n= 42, SE= 1.2; t-value= 3.5, df= 74, p= 0.00069). The research conducted provides a general insight into the distribution of H. banksii in regards to length. However, the model behind this distribution was not fully investigated. Further research can be done to delve further into this, as the length of H. banksii branches also has implications for its reproductive success. With a species as abundant and ecologically important as H. banksii, it would be useful to explore its morphological variations and reproductive success to provide accurate recommendations for conservation.

Introduction

Species that live in intertidal regions require a particular balance of abiotic factors to survive, grow and reproduce. It is a region of environmental instability and requires species living within the area to be able to withstand large fluctuations in abiotic factors, including salinity, temperature, water availability and sunlight (Vernberg, 1969). In the southern hemisphere, Hormosira banksii is the most successful intertidal fucoid due to its versatility- its morphological variation allows it to survive in high water temperatures, up to 40°C, and high salinity, 140 psu (Kain, 2015).

Hormosira banksii is an important foundation species (Bishop et al. 2009). Its complex structure supports many and varied communities of invertebrates (Bishop et al. 2013). Therefore, it is important to develop a greater understanding of this species and the factors that affect its morphology and distribution. This is so that the distribution and success of other species, which depend on H. banksii, can be better understood. There is growing consensus that environmental conditions affect the influence that foundation species have (Bishop et al. 2009). Morphological traits have been shown to affect both the establishment and continuation of facilitation cascades (Bishop et al. 2013).

This experiment was designed around how these two factors interplay, that is, how environmental conditions affect morphology. It is clear that H. banksii have an important role to play in the biodiversity of Australian intertidal regions. Sampling along the east coast of Australia showed that patches of H. banksii consistently supported a richer array of mollusc species than other nearby substrates (Bishop et al. 2009). This investigation explored whether varying lengths is a morphological difference that allows H. banksii to be successful in differing environments within the intertidal region: both in shallow and deeper waters.

The morphology of H. banksii has implications on which species it can support and how they interact with each other. Manipulative field experiments have shown that H. banksii have reduced biomass in shaded plots compared to unshaded ones (Bishop et al. 2009). This seems to show that the amount of sunlight could affect H. banksii length, one indication of biomass. It was aimed to investigate whether this research can be extrapolated to show that there is also a difference in H. banksii length in deep and shallow water, which have differing access to sunlight. It was hypothesised that length differs between H. banksii in shallow and deep waters, with those in deep water predicted to be longer than those in shallow water. At greater depths, the longer branches would ensure that the algae can reach the surface and obtain the necessary sunlight for growth.

Method

The experiment was conducted at Point Gellibrand, Victoria, Australia. On the south-eastern side of the park, there is an intertidal region, where H. banksii is found. Three sites were sampled: Site 1 (37°52'15.5"S, 144°54'23.1"E), Site 2 (37°52'15.3"S, 144°54'25.7"E), and Site 3 (37°52'13.6"S, 144°54'29.3"E). Data was collected on September 2, 2018, from noon, with site 1 data being collected first, followed by site 2 and then site 3. It was a mild day with typical winter weather for the area; it was cloudy and there was light drizzling rain at a couple of points during data collection. Replicates were collected every 2m, along two 20m transects, one shallow and one deep, at each site.

Data collection began during low tide. Due to the changing water depth during the experiment, deep and shallow water was not defined by a measurement of water depth. Rather, one transect was 25m from the high tide line, defined as shallow water and the other transect was 35m from the high tide line, defined as deep water. The length of a specimen was defined by the length of its longest branch. At each 2m mark, the specimen nearest to the transect point was selected and its length measured, with the aim of getting sixteen replicates for each transect. However, this was not always possible, as there was not always a specimen nearby to sample. No specimen was measured twice.

The mean lengths of the longest branches were found for deep water and shallow water overall, as well as at the three sites. Welch's unpaired t-tests were also completed on the grouped shallow and grouped deep water data, as well as for shallow and deep data at each individual site.

Results

The lengths of H. banksii were analysed in shallow and deep waters over three sites at Point Gellibrand. The number of replicates, mean, standard error, t-value, degrees of freedom and p-value are shown for the sites individually as well as grouped together. Site Number Shallow Deep T-Test between shallow and deep-water data Number of replicates Mean (cm) SE Number of replicates Mean (cm) SE T-Value Degrees of freedom p-value1 16 8.4 1.2 11 13 1.5 2.1 20 0.0462 15 6.6 0.91 15 8.5 0.91 1.5 27 0.143 16 12 1.7 16 20 1.9 3.3 29 0.0024Overall 47 8.9 0.82 42 14 1.2 3.6 74 0.00069 Figure 1. The mean lengths of H. banksii in shallow and deep water from three sites at Point Gellibrand were plotted with standard error shown.There was no significant difference (p-value= 0.14; Table 1) in mean lengths of H. banksii between shallow (mean= 6.6 ± 2.0 cm) and deep waters (mean= 8.5 ± 2.0 cm) at Site 2. Significant difference in mean lengths between shallow and deep water were obtained for site 1 (t-value= 2.1, df= 20, p-value= 0.046; Table 1) and site 3 (t-value= 3.3, df= 29, p-value= 0.0024; Table 1). Overall, Fig. 1 shows that the means differ quite markedly between the shallow and deep waters. T-tests show that near the shore, H. banksii were significantly shorter (mean= 8.9cm, n=47, SE= 0.82; Table 1) than farther away (mean= 14cm, n= 42, SE= 1.2; t-value= 3.5, df= 74, p= 0.00069; Table 1). H. banksii in deep waters were, on average, 5.1 cm (95% CI [2.2, 7.9] longer than those in shallow water.

Discussion

The results show that the length of H. banksii in water further from the shore is significantly longer than those closer. While the significance of the results differs between the sites (Site 2 showed an insignificant difference in mean length), the overall combined data show a significant difference in the mean lengths of H. banksii in deep water compared to shallow water. This experiment defined deep water as 35m from the high tide line and shallow as being 25m from the high tide line. This was done instead of measuring the depth of water because it changes with the tide.

At each site, the samples taken further out were consistently found to be attached deeper underwater than those closer to the shore. However, there were particularities to each site, such as more large rocks on which H. banksii grow on at Site 2, which meant that the actual depth that the H. banksii were found in varied between sites; the deep water at site 2 was much shallower than the deep water at the other two locations. This coincides with the fact that the difference in means at site 2 was not significant. Previous research has shown that human trampling has a marked effect on the abundance of a species. H. banksii at another location experienced a percentage cover decrease of 25% with as few as ten tramples during a single tide cycle (Schiel and Taylor, 1999).

Trampling would have a greater impact in the more accessible areas closer to shore, meaning that algae is destroyed before having the opportunity to reach greater lengths. Further research into the extent of trampling at Point Gellibrand Park would give a better idea of whether H. banksii grow longer due to the abiotic conditions of the environment that they are in, or if it is due to external factors. While it can survive harsher conditions than other species, H. banksii does not recover well from desiccation (Kain, 2015).

Hence, the higher temperatures and greater salinity that H. banksii in shallower water is exposed to could also be a reason that the species cannot reach the same lengths seen in deep-water H. banksii. This study, with a small sample size collected on a single day, should be taken as a cursory investigation. The hypothesis was correct in that H. banksii in deeper waters are longer than those in shallower waters. However, while it has been shown that different water depths affect the length of H. banksii, time and resource constraints hindered research on the model behind the difference: whether the access to sunlight drives the morphological difference.

Nevertheless, the experiment has introduced interesting points warranting further research. H. banksii can reproduce through detachable fronds, with the number of reproductive structures being proportional to the length of these fronds (McKenzie and Bellgrove, 2008). An interesting area for further research would be to examine whether the differing environments of shallow and deep water affect the species’ reproductive success. This would be important in developing policies for park management, particularly in response to human trampling that occurs in shallow areas.