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SCIE2204 Field Report Jacques Cottesloe

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Cottesloe Invertebrates in Space and Time – A Tale of Resistance

Abstract: One of the major goals of the natural sciences is protecting vulnerable biodiversity. The South-West Region of Western Australia has the second highest endemism of any marine region in the world (Kendrick, 2013). Therefore, species in this area are particularly important to protect for global biodiversity and local ecology alike. Use and non-use values include tourism, education, research, preservation for future generations and preservation for the existence of biodiversity. Invertebrate biodiversity is vital as invertebrates comprise a large proportion of animal diversity and fill a wide variety of ecosystem roles (Ponder et al., 2002). We examined the distribution of invertebrate biodiversity in the Cottesloe reef ecosystem over a three-year period, between 2011 and 2013. Documenting spatial and temporal change in species distribution in the area allows for better understanding and management of this diverse and globally unique ecosystem. Findings of this paper include evidence of spatial variation within the invertebrate assemblage, across different areas of the reef and some indication of stability through time. As a flourishing ecosystem in an area of high population pressure, ongoing high quality monitoring and protection is of utmost importance.

Pyura spinifera – Sea tulip – Photo Credit: The Cottesloe Ecosystem Research Project (http://cottesloeecosystem.wordress.com/photos/)


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Introduction Invertebrates play a significant role in the functioning of marine ecosystems, and are of great importance within the aspects of nutrient recycling and the food chain (Gerlach, 1978). Many invertebrates assist in algae control in the aquatic environment, with Turbo Snails being a prime example of an algae-eating species (Worthington and Fairweather, 1989). Detritus control is considered an important aspect of a healthy ecosystem; detritivores such as Sea Stars and Sea Cucumbers sift through and consume organic matter within sediments – forming an essential part of the biogeochemical cycle and preventing the build-up of harmful chemicals (Gerlach, 1978). Changes in Distribution: This study into the distribution of invertebrates at Cottesloe reef was taken with two main objectives in mind, namely determining spatial and temporal variations within the system over the observed period. Determining a change in spatial variation of a species involves tracking its abundance across an environmental gradient. In the case of marine invertebrates, this can relate to the environment in which a particular species lives, taking into account its requirements (Gaines et al., 1985). Factors influencing spatial variation include; food sources, water temperatures, mobility, oxygen concentrations and hydrologic conditions which can all have a significant effect on the settlement and subsequent fitness of marine organisms (Holmström and Kjelleberg, 1994). These conditioning factors will ultimately determine where a particular species can be found in reference to the reef. Given the basis of the study, our first hypothesis is that there would be a spatial variation in the species assemblage. This variation would be evident between the three areas of the Cottesloe reef system observed, namely the lagoon, flat reef, and broken reef due to the variation in biological and physical factors between these sites. The second focus of the study looks at temporal variation in the Cottesloe reef ecosystem. Data from 2011 and 2012 were combined with the data collected in the current study, in order to test this. Due to this short time-span, annual fluctuations in factors affecting the ecosystem will be examined. The Leeuwin Current and its component currents are the main drivers of annual oceanographic changes in WA (Hanson et al., 2005). The Leeuwin Current brings warm tropical water south past Perth and the Cottesloe Reef FHPA. Not only is the current responsible for change over a short time scale, it is also a major reason for the high endemism in South-West WA (Phillips, 2001). It is an anomalous poleward eastern boundary current, and species have adapted to the unusual factors influenced by this (Ponder et al., 2002). In years when the Leeuwin Current is stronger, south-western WA experiences warmer water, lower salinity and less upwelling, resulting in lower primary productivity (Phillips, 2001). The strength of the Leeuwin Current depends largely upon the El Niño Southern Oscillation (ENSO), with the Leeuwin current being stronger during La Niña than El Niño periods.. Our second hypothesis is that temporal changes in species distribution will be evident between the data from the past three years. These changes are expected to show some connection to the climatic conditions at the time although this study is too brief to pinpoint any major contributing factors. That there is


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some link between Leeuwin Current flux and species distribution is presumed, however, and this connection may be evident.

The Cottesloe reef ecosystem forms an important ecological community of marine organisms, whose ongoing health is critical to the functioning of the biodiversity hotspot. The Cottesloe reef ecosystem is unique in that it is a Fish Habitat Protection Area (FHPA) in an urbanised area and located next to one of Western Australia’s most popular beaches. This makes it ideal for studying how marine ecosystems respond to urban environments. It also makes study of marine ecosystems more accessible to researchers, educational groups and the public. The oceanography and ecological relationships are critical to the functioning of the system. This reef system serves as a suitable structure for a wide variety of marine life due to variability of its rocky limestone makeup (allowing for a wide range of niches) and temperate water conditions (Howard, 1989).


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Methods

To acquire the measurements of invertebrate numbers at Cottesloe over a three-year study period, the reef was divided into north and south zones. Student groups were assigned to collect data in each region. Each zone was then divided into three parts, Lagoon (0-50m offshore), Flat Reef (50-100m offshore) and Broken Reef (100-150m offshore). We split the zones up according to their geomorphology, water depth and difference in seafloor profile. The lagoon hereby compromises the shallowest water column with sandy bottom and extensive seagrass patches. The Flat reef is at about 3 meters depth and is made up of a mix of sandy and rocky limestone bottom, also including plentiful primary producers. The Broken Reef, as the name suggests is a broken scattered part of reef furthest from the shore in our study. It’s maximum depth is at about 5 meters.

The data was collected over two weekends in late March and mid-April. Thereby, the two consecutive days had a two week period between them. For each area two teams of snorkelers haphazardly deployed quadrats and documented the species, their numbers and habitat in a prepared waterproof data sheet. An ID-Guide that was prepared in the run-up to the data collection, to make identification of species easier. For species that were not easily identifiable in situ a photograph was taken and identification was conducted later on shore with the help of professional Ecologists. The quadrants used measured 1 m2 and a 12 quadrant samples were taken in each zone on each day, totalling at 96 measurements - 48 in each the southern and northern zones.

Results:

Spatial Variation in Invertebrate Assemblage

A significant difference was found between the mean density of Porifera in the Lagoon zone and Broken Reef zone, with significantly more found on the Broken Reef. Ascidians also demonstrated a significant variation the Lagoon zone and Broken Reef zone, with again more found on the Broken Reef. No significant differences were found between the densities of the other phyla with respect to zone. A number of large beds of mussel were located within several of the quadrats on the Flat Reef, which will have skewed the density value for this Phylum. The density of molluscs with the effect of the mussel bed removed is shown below (figure 1).


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Figure 1: Average density of each phylum over each zone with the effect of the mussel beds removed. Where no overlap in error bars occurs it can be concluded that a significant variation between zones exists.

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The change in assemblage in terms of the proportion of total invertebrate assemblage occupied by each phylum is shown below (Figure 2). This figure highlights a number of trends. Anemones appear dominant in the Lagoon, and decrease in their dominance with distance from the shore. Molluscs dominant in the Flat Reef. Figure 2 demonstrates the same system with the effect of the mussel bed removed. Without this effect, a trend in increasing dominance of ascidians with distance from shore is noticeable. It is also apparent from this figure that Porifera form a significant part of the Broken Reef assemblage, relative to the other sites.

Figure 2: The percentage makeup of the invertebrate assemblage at each site with the effect of the mussel beds removed.

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Figure 3: The contributions of each phylum to total density of the invertebrate assemblage over the three zones.

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The Flat Reef zone was found to have the highest overall invertebrate density in terms of average number of individuals counted per quadrat, followed by the Broken Reef, and then the Lagoon (figure 3). Even with the effect of the mussel bed removed, this pattern held.

Temporal Variability

Between 2011 and 2013, no significant variation occurred in the densities of any of the phyla (figure 4). Between 2012 and 2013 a significant change in Anemone, Porifera and Echinoderm density is apparent. Between 2011 and 2012 significance is apparent in Anemone, Porifera, and Ascidian density. It is notable that the mean density per quadrat for the entire invertebrate assemblage was much higher in 2012 than both 2011 and 2013 (figure 5).

Figure 4: Average density of each phyla in each year. Where no overlap in error bars occurs, then it can be concluded that a significant variation between zones exists.

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Figure 5: Total assemblage density for each year, with the contributions of each phylum included.

Figure 6: The percentage makeup of the invertebrate assemblage in each year.

Figure 6 demonstrates the proportional change in assemblage in terms of density per quadrat. The decrease in relative anemone between 2012 and 2013 is clearly evident in this figure, supporting the findings presented in figure 4. A large increase in the proportion of Molluscs and Annelids is also evident in this figure.

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Discussion

Spatial Change

A variation in species distributions across different sections of the reef is prevalent in this sample area. Distinct areas of the reef show substantial differences in the abundances of the phyla (Fig 1-3) found. This supports our first hypothesis, that an altering diversity exists between the sections of the reef.

These differences in abundance can be attributed to abiotic factors, such as distinct wave and erosion intensities across the sections. Since most marine invertebrates are either immobile or have limited mobility, any disturbance in their surroundings would affect them in a very direct fashion – an example being waves moving boulders with molluscs living on them (Eleftheriou & McIntyre, 2008 2-3). This would logically lead to areas more exposed to destructive and violent waves having fewer organisms present there that would find those conditions adverse. A study done around Portugal supports this assumption, finding that filter feeding organisms were the dominant species amongst shallower sub-tidal rocky habitats (Boaventura et al. 1999). This is in turn supported by our findings, in that our main source of filter feeding organisms was the flat reef – a sub-tidal area both relatively rocky and shallow.

Biotic factors could also explain this very patchy distribution, an example being that most benthic invertebrate’s reproductive processes don’t allow for much expansion past their immediate area of fertilization, since most are immobile and rely on broadcast spawning they tend to form dense clusters of organisms, mussels being an prime example (Quinn & Ackerman, 2011). This observation is supported by a previous study, also testing for spatial variation but between several different reefs over many kilometres of Australian coastline. The study area for which is inclusive of the Fremantle area, which is situated close to, if not exactly where we tested (Vanderklift & Kendrick 2004). Although this study was looking at a far broader scale between reefs, spatial variation within the reefs themselves was also found – namely between gastropods from the Turbo and Australium genus and sea urchins. While we did not observe many sea urchins, we did find that our highest observed abundances of molluscs were in the flat reef section (when the mussel beds are taken into account) – consistent with the findings of (Vanderklift & Kenrick 2004). Overall, it can be concluded that variability of species across the sections of the reef add a layer to the diversity of species in the ecosystem. Specific areas with their differentiating recreational uses (such as surfing, scuba diving, boating etc.) are equally important to the health of the system. Therefore, they need to be managed and protected according to their recreational impact variables to ensures the best protection from human impacts.


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Temporal Change

Examining the data collected over a three-year period it is a tough feat to draw solid conclusions for long-term temporal change. However, by looking at the results presented, it can be said that species assemblages appear relatively stable over all (Figure 6). This is quite surprising as we hypothesised that the distribution would change according to climatic variations in the system. A strong La Niña period is evident during March 2011 but neutral periods are seen for March 2012 and 2013. La Niña Periods are visualised in the Southern Oscillation Index as all figures above + 8 and El Niño as those below – 8 (Figure 7). Furthermore, the effects of the unprecedented high temperatures during the Marine Heat Wave on the Western Australian Coast in the summer of 2010-2011 would suggest that changes would have been most evident between 2011 and 2012 (Pearce et al., 2011). On the contrary the population at Cottesloe Reef seems very stable between those years. One strand of explanation could be the high metabolic rate in intertidal invertebrates that make them overall capable to survive very well in a system in which rapid fluctuations in temperature occur (Nwewll & Northcroft 1967). Furthermore, the highly clustered endemism of this areas species as “restricted-range species” in a temperate biodiversity hotspot is a possible explanation for a high level of heat resistance of invertebrates (Roberts et al. 2002, Wernberg et al. 2011). This resistance is dependent on temperature conditions of “formation and existence of a species rather than on its taxonomy” (Andronikov 1975). Therefore, it seems that the species in Cottesloe reef ecosystem have over the years of varying oscillation influxes and regular annual temperature changes, effected by the Leeuwin current, formed a resistance to temperature variations.

Figure 7: The Southern Oscillation Index showing El Niño (at below – 8) and La Niña (at above + 8) fluctuations. (Source: Australian Bureau of Meteorology http://www.bom.gov.au/climate/currents/soi2.shtml)


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Adversely to general invertebrate numbers on a smaller scale a change in numbers of species between the years of 2012 and 2013 is evident. The change shows mainly in form of decreasing relative percentage of Anemones and an increase thereof for Molluscs and Annelids. This change may be a late effect of increased temperatures in 2010 and 2011, but may also be due to natural fluctuations in those assemblages or changing food availability for each group. The changes in chlorophyll concentrations during the periods that the data was collected support the postulate that higher primary productivity occurs during La Niña periods. This is evident in 2011 data showing higher chlorophyll concentrations (Figure 8). Primary productivity is the input of energy into an ecosystem and therefore directly and indirectly influences herbivorous, filter-feeding and predatory species. A possible decrease in predatory anemone may be directly correlating to the decrease in chlorophyll levels. However these species-specific changes are not conclusively evaluable, due to limited data availability rather then the likelihood of no changes occurring (Wernberg et al. 2011). Therefore, it seems that species at Cottesloe Reef Ecosystem, have over the years of varying oscillation influxes and permanent annual temperature changes, affected by the Leeuwin current formed a resistance. Therefore we express a dire need for ongoing data collection at the Cottesloe Reef Ecosystem.

Figure 8. Chlorophyll concentrations are shown in mg/m3 in southwestern WA for March 2011, 2012 and 2013. Source: http://imos.aodn.org.au/imos/


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The importance of long-term studies for the field of ecology is not a new concept. It has been discussed, tested and established thoroughly over the years (e.g. Strayer et al., 1986). Drawing on Strayer (1986) and his likes, it is of utmost importance to engage in ongoing research and data collection in crucial ecosystem such at the Cottesloe reef with extreme care. A critical element for the success of such studies is a simple and accommodating design, such as the methods used for this project. Methods need to be “unambiguously repeatable even by staff lacking sophisticated training” (Strayer et al., 1986, p. 11). The most important factor in ensuring the straightforward experimental treatment to be consistent is attributed to sophisticated leadership. This leadership can ensure for the example of the Cottesloe Project, that quadrats will always be placed haphazardly and not based on participant’s preferences or particularly biota rich segments of the reef. This will ensure higher quality datasets and unbiased results, because the fact that benthic animals are extremely patchy in distribution is already included in the consideration of the experimental design for this project (Eleftheriou & McIntyre, 2008).

By engaging in long-term studies more and more temporal conclusions can be drawn with time. Examples for such could be more reliably measurable changes due to extreme weather events, such as the Marine Heat Wave on the Western Australian coast in 2010-2011 (Pearce et al., 2011), for which in this case no baseline is available to make definite conclusions feasible. Except for extreme weather events also periodical oscillation changes and their effects on chlorophyll levels and biodiversity on the reef can be measured better in the future. However, the effectiveness of the experimental design is limited given the small time scale of a several weeks in late summer. By adhering to the study methods closely and collecting rich data sets though, phyla specific research will be possible for research such as distribution change of specific genera due to climate change impacts. Fields for study could include range contractions and range extensions for certain species, as well as a movement of populations towards the pole or deeper into the water column to escape high temperatures in shallow water (Wernberg et al. 2011). Our finds of natural spatial distribution and relative stability of invertebrate species, however, paint a positive picture about the health of the ecosystem as a whole and it’s ability to withstand natural temperature and food availability variations. With ongoing monitoring, research and protection the Cottesloe Reef habitat stands good chances to continue thriving at our doorsteps.


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