"The Relationship Between Seagrass Cover and Species- richness of Invertebrates" SCIE 2204: Marine Systems The Cottesloe Marine Ecosystem Research Project 2014 By Baronie Shaw, K., Bortoloso, T., Cargill, L., Ellis, E., Gillis, R., de Sousa, A. and Walkey, L.
TABLE OF CONTENTS TABLE OF FIGURES... 2 ABSTRACT... 3 INTRODUCTION... 3 Objectives... 3 Background... 3 Hypothesis... 5 METHODS... 5 Study Site... 5 Experimental Design and Invertebrate Sampling... 7 Method of Data analysis... 8 Assumptions... 8 RESULTS... 8 DISCUSSION... 11 REFERENCES... 14 TABLE OF FIGURES Figure 1: The Cottesloe Fish Habitat Protection Area (Department of Fisheries 2010) 6 Figure 2: North and South Sectors of the Study Site... 6 Figure 3: Topographic and Habitat Zones of the Study Site... 7 Figure 4: Average Species Richness of Invertebrates per m 2 in 2014 Over Three Different Study Zones... 9 Figure 5: Average Percentage Cover of Seagrass per m 2 in 2014 Over Three Different Study Zones... 9 Figure 6: Average Species Richness of Invertebrates per m2 in 2013 Over Three Different Study Zones... 10 Figure 7: Average Percentage Cover of Seagrass per m2 in 2013 Over Three Different Study Zones... 10 Figure 8: Average Species Richness of Invertebrates per m 2 in 2012 Over Three Different Study Zones... 11 Figure 9: Average Percentage Cover of Seagrass per m 2 in 2012 Over Three Different Study Zones... 11
ABSTRACT The Cottesloe marine ecosystem research project was undertaken to determine the species abundance and diversity for a variety of marine flora and fauna. In particular this report is exploring the relationship between percentage cover of seagrass and species- richness of invertebrates. This research was undertaken by students at the Cottesloe Reef Fish Habit Protection Area, completing a field study of the area using scientific sampling methods such as randomly placed quadrats to enable accurate analysis underwater of the species present. The main finding of the project was that as the percentage cover of seagrass increases, the species richness of invertebrates decreases. These results indicate that invertebrates in the Cottesloe study area compete for local habitat with seagrass species. It is possible that certain species of invertebrates are more suited to a habitat largely vegetated by seagrass than others. INTRODUCTION Objectives The key objectives of this research project were to: Conduct a field study of the Cottesloe Reef ecosystem to assess the species abundance and diversity for a range of marine flora and fauna. Examine the results of the field investigation against a hypothesis developed for this project. Background The Cottesloe Reef stretches over 1.5km along the coast of Perth. Located along a limestone shelf the reef is formed from limestone pinnacles, bombies and shelves that includes areas of exposed rocky reef and sandy lagoons. Within the reef area there are a range of different habitats including seagrass meadows, kelp forests and sponge beds. The inner reef can be divided into three main habitat zones, the inshore sandy lagoon, the flat reef and the outer broken reef. The zones closer to shore are subject to reduced wave action and are shallower providing increased light access. As the reef is located near the city it is heavily used by the
public and vulnerable to human impacts (Department of Fisheries 2001). In order to protect the valuable reef ecosystem the Cottesloe Reef Fish Habit Protection Area (CFHPA) was established in 2001. The invertebrate assemblages of the CFHPA are extremely diverse and unique. The reef contains corals, sea cucumbers, sponges, snails and anemones. These anemones are thought to be a unique remnant of the invertebrate fauna found in Cockburn Sound (Department of Fisheries 2001). In addition, invertebrate fauna play an important role in the ecosystem, driving many vital ecosystem processes including decomposition and nutrient recycling, sediment agitation and are also key members of the ecosystem food chain (Gerlach 1978; Underwood & Kennelly 1990). As such the conservation of the invertebrates of the reef system is an extremely important goal. While it has been shown that sanctuary zones have a positive impact on invertebrate diversity (Ryan 2008) the relationship between invertebrate assemblages and the primary producers of the reef, the seagrasses and algae is multifaceted and much is still unknown (Underwood & Kennelly 1990). A more detailed understanding of the interrelationships between these two ecosystem pillars would be of significant help in reef conservation. As the base of ecosystem food chains the presence of primary producers is clearly vital to support invertebrate assemblages found in the Cottesloe Reef area. As a varied group, invertebrates occupy several trophic levels, many such as sea hares, sea urchins and gastropods are herbivorous and to some extent their abundance is determined by the availability of suitable algae or seagrass food sources (Vanderklift & Kendrick 2004). Other invertebrates rely on the complex habitats formed by various primary producers, habitats dominated by turfing algae or seagrass will have significantly different assemblages than compared to those primarily of encrusting algae (Underwood & Kennelly 1985). However the presence of seagrasses and algae is not always beneficial to invertebrate assemblages. Macroalgae can grow rapidly when conditions suit, reducing the levels of turfing algae and damaging the food source of many micro algal invertebrate grazers, causing a reduction in population. As these invertebrates also act as a control on young macro algae that stimulate positive feedback, the cycle can result in the rapid development of a continuous cover of foliose algae and the elimination of all grazers (Underwood &
Kennelly 1990). Algae is also in competition with sessile invertebrates for space as Miller & Etter (2008) demonstrated when areas are artificially shaded, alleviating the competition with the algae they become dominated by invertebrate assemblages while unshaded areas are dominated by macro algae. The cover of seagrasses and algae varies considerably across a reef system like that found in the CFHPA. Exposure to wave energy is one of the most useful predictive tools in determining the prevalence of these primary producers (Ryan et al. 2007). This would indicate that a higher density of seagrass and algae would be found in the inner portions of the reef. In addition, as depth increases light energy is also reduced which may have an effect in the levels of primary producers. It is predicted that as the percentage cover of primary producers, such as seagrasses and kelp, decreases, the abundance and species richness of the invertebrate assemblages present in the reef zones will increase, as caused by the competition between producers and invertebrate assemblages. Hypothesis Based on the information gathered in researching the background to the project, the hypothesis developed was: "As total percentage cover of seagrass decreases, species- richness of invertebrates increases". METHODS Study Site The study was undertaken in a region on the Cottesloe Reef located within The Cottesloe Fish Habitat Protection Area (32 00.8 S, 115 44.9 E) (Figure 1). This region contains one of many unique ecosystems along the coastline of Perth.
Figure 1: The Cottesloe Fish Habitat Protection Area (Department of Fisheries 2010) The study site was divided into two sections, North and South (Figure 2). Each of these sections was then divided into three zones due to the region having varying topography and habitat structures (Figure 3). Dividing the North and South components of the study site into three zones enabled the observation of the invertebrates distribution as it varied according to the different habitats. Figure 2: North and South Sectors of the Study Site The three zones (Figure 3) that extend outwards from the shore are: the lagoon, the flat reef and the broken reef. The lagoon (0-50m offshore) is the region connected to the shoreline that has the shallowest water column and consists mainly of a sandy bottom and extensive
seagrass patches. The flat reef (50-100m offshore) is roughly 3 metres in depth, consisting of a mix of sandy and rocky limestone bottom and containing a large array of primary producers. The zone furthest away from the shore was the broken reef (100-150m offshore). This zone consists of scattered areas of reef with its maximum depth of 5m. Figure 3: Topographic and Habitat Zones of the Study Site Experimental Design and Invertebrate Sampling For the abundance of the invertebrate species to be measured throughout the study site, the class was divided into a total of 6 groups consisting of 6 to 8 students each. Before entering the study site groups prepared for analysis by creating identification guides (on waterproof paper) to enable the identification of different species present in the field study. A results table was printed on waterproof paper to enable the measurement of species numbers that were observed within the different zones during the field study. Each group was given a 1m2 quadrat; these were placed in 6 random locations within each of the three zones (Figure 3), within the 1m2 quadrat the number of inverts were counted. A total of 36 samples were taken in each of the three zones. Overall 108 quadrat samples were taken within the study site: 54 in the northern region and 54 in the southern region. These samples were taken by positioning the quadrats on the bottom of the seabed. The quadrats were released over randomly selected areas and divers recorded the species type and number. The depth and visibility at the sample sites were also noted. One diver remained on the surface and recorded this information on the results table.
Method of Data analysis All the invertebrate data that was collected (108 samples) through the sampling was uploaded on an online database together with data from previous years. The computer software Excel was used to produce graphs of invertebrate species richness over the three zones as well as percentage sea grass coverage. We compared the data of both invertebrate species richness and percentage seagrass cover from 2012 to 2014. The data was also analyzed and enabled us to determine whether there were any correlations seen within our data. Assumptions The haphazardly placed quadrats were adequately distributed stochastically to remove bias The quadrats accurately represent the organisms present in the sample area. Quadrats are the most reliable method for estimating species richness and percentage cover of benthic organisms in a marine survey. RESULTS Data collected over the three years of field studies indicated a range of values for both species richness and seagrass cover. Figure 4 shows that in 2014 the lagoon had a species richness of 1.62, the shallow reef had a value of 2.31 and the broken reef had a value of 2.89. As seen in Figure 5, the average seagrass cover for 2104 was 56.7% for the lagoon, 19% for the shallow reef and 0% for the broken reef. This was inversely correlated with the species richness for the 3 study zones.
3.5 3 2.5 No.Species 2 1.5 1 0.5 0 Lagoon Flat Broken Zone Figure 4: Average Species Richness of Invertebrates per m 2 in 2014 Over Three Different Study Zones Figure 5: Average Percentage Cover of Seagrass per m 2 in 2014 Over Three Different Study Zones Species richness is described in Figure 6. Values for 2013 with were 1.23 for the lagoon, 2.56 for the flat reef and 2.73 for the broken reef. Figure 7 shows that in 2013, the average seagrass cover for the lagoon zone was 16.7%, the average cover for the flat reef was 1.08% and the broken reef had an average of 0.46% seagrass cover per quadrat.
3.5 3 2.5 no. species 2 1.5 1 0.5 0 Lagoon Flat Broken Reef system Figure 6: Average Species Richness of Invertebrates per m 2 in 2013 Over Three Different Study Zones % cover 20 18 16 14 12 10 8 6 4 2 0 Lagoon Flat Broken Reef system Figure 7: Average Percentage Cover of Seagrass per m 2 in 2013 Over Three Different Study Zones As seen in Figure 8, the species richness of the lagoon zone for 2012 was 1.1, the species richness of the flat reef was 2.77 and the species richness value for the broken reef was 3.71. Figure 9 shows that the 2012 seagrass cover for the broken reef was 0.1%, with the flat reef and lagoon zones possessing 10.5% and 26.6% respectively.
4.5 4 3.5 No. Species 3 2.5 2 1.5 1 0.5 0 Lagoon Flat Broken Zone Figure 8: Average Species Richness of Invertebrates per m 2 in 2012 Over Three Different Study Zones 30 25 20 % cover 15 10 5 0 Lagoon Flat Broken Zone Figure 9: Average Percentage Cover of Seagrass per m 2 in 2012 Over Three Different Study Zones DISCUSSION On analysis of the data from 2012, 2013 and 2014, it is evident that there is an inversely proportionate relationship between the average species richness and the percentage cover of sea grass. The hypothesis for this experiment is reflected in the results, concluding that the presence of seagrass may have an influence on the species richness of invertebrates in
the Cottesloe habitat. However the experiment contained errors that may have affected the results. By using measurements obtained from other group s data, our experiment was able to encompass more quadrat studies improving the reliability of measurements, looking at multiple years in the study also increased validity of findings. However, this also increases the number of experimental errors that may cause results to be inaccurate. In our group study, movement of quadrats with the currents, lack of visibility, inability to stay under water long enough and human error with estimates as well as species identification were among the biggest problems affecting the accuracy of data collection. Despite this, all three years performed these tests within the same time period to remove changes in seasonal variations as well as utilising similar methods. This resulted in a definite trend across all three years with fluctuating averages, which suggests that although there were problems with the data collection method, the trend may be caused by definite relationships. Our interpretation of this trend is due to the nature of invertebrate habitat. The relationship shows that only a limited amount of marine invertebrates are able to survive within high seagrass density areas and that the majority of marine species found in the study prefer areas with little or no sea grass coverage (Seen in Figures 4, 6 and 8). The most probable reasoning for this may be because the marine invertebrates studied are competing with the seagrass for habitat. As many of the observed invertebrates are filter feeders that remain in a single position, or have limited mobility, there is significant competition with the seagrass for habitat on stable ground such as the limestone outcrops seen at Cottesloe (Kenneth et al. 2000). Another reason may be the feeding mechanisms of the invertebrates. With most feeding on detrital and/or epibiota (Kenneth et al. 2000). However, with seagrass surrounding and covering most invertebrates, food availability may be limited, as it has reduced access and does not reach the invertebrates as effectively as being uncovered. However, due to their feeding strategy, it is unclear whether these organisms need to be directly underneath or nearby benthic plants to receive their food supply. Studies such as Kenneth et al (2000), have found that in areas of comparatively low biomass, microorganisms and epiphytic algae are available for consumption by invertebrates. Further investigation and research is needed to determine whether invertebrates are suited to areas
of low seagrass cover due to competition for space and whether food is in sufficient supply in areas of lower vegetation. Literature suggests that the complexity of a habitat area is a key factor in the species richness of invertebrates (Kenneth et al. 2000). In an area dominated by seagrass, which lacks any other marine vegetation species, the complexity of the habitat is low. This may be a factor that causes the low values for species richness in the lagoon zone rather than the presence of seagrass influencing variations. Differences that occur between literature and the Cottesloe data on the species richness of invertebrates may be due to the data collection occurring at different times of year. The Cottesloe study has kept the collection of its data at a consistent time of year to remove seasonal variations, but this is likely to be different from most literature on the subject. Measuring species richness and assessing the location of organisms, groups species without accounting for the differences in their behaviour throughout the year or in reaction to any disturbances. Invertebrates are likely to behave in different ways to each other and some may prefer highly vegetated habitats to others. The Cottesloe data found that high seagrass coverage correlated with low species richness but it is possible that not all the invertebrate species or genera prefer the low seagrass habitat. These differences may become more evident with a greater sample size. In summary, the data presented indicates that the hypothesis stating that as the species richness of the invertebrate population increases, the percentage cover of seagrass decreases is correct. However, it cannot be concluded that there is a definite relationship between the two variables due to many experimental errors that could be affecting the data. Other factors could also potentially be giving rise to this trend. However there is also reasonable evidence that makes logical sense of invertebrate spatial patterns, so the hypothesis should not be completely rejected.
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