The Effects of Cover on Marine Species Abundance in Tidal Rock Pools on North Stradbroke Island

Andrei Horga

Karen Jacob

Christa Hoy

Cindy Organ

May 7, 2009

Introduction

Tidal rock pools are a haven for organisms of all types (Joerger et al 2008). They come in all shapes and sizes, some have cover, while some are exposed to the elements. Those that are covered will have some sort of canopy, algae, overhanging rocks, or rock shade in the pool.

Canopy algae has been shown to provide shade and protection for organisms in tidal pools (Figueiredo et al 2000). Canopy also makes rock temperatures 5-10 degrees centigrade lower and decreases water evaporation (Bertness et al 1999). It has been noted that when removing canopy cover, this resulted in a bleaching and death of turf species with a consequent loss of entrapped silt (Jenkins et al., 1999). In other words, removing canopy kills marine plants and increases turbidity, which must also affects marine animals in those habitats.

The aims of our research project was to explore the differences between covered and non- covered inter tidal rock pools. We already know that canopy removal results in a loss of turf species and entrapped silt, but does this affect the animal abundance in those tidal rock pools? This leads to our hypothesis, that there is a significant difference in animal abundance between covered inter-tidal rock pools and non-covered inter-tidal rock pools. Our null hypothesis is that there is no significant difference in animal abundance between covered inter tidal rock pools and non-covered inter-tidal rock pools.

Materials and Methods

This study was conducted on the rocky shores outside of the Morton Bay Research Station, North Stradbroke Island, Australia. It was conducted over a one day period on March 22, 2009 during low tide, between 1pm and 4pm. The moon at the time was between a crescent waning moon and a new moon, resulting in a smaller tidal range than at full moon. It was a sunny day with no precipitation and low winds. A tape measure was used to measure the distance from the upper tidal limit to the rocky tidal pools further out, which were 17.55 meters away from the upper tidal limit. A 100 meter transect parallel to the water was laid out from the first rock pool.

Eight quadrats measuring 0.5 meters by 0.5 meters were randomly distributed in tidal pools along the transect. We then estimated the amount of canopy cover in the quadrat, which included any algae, leaves, rocks in the pool, and overhanging rocks. The quadrat was either listed as high cover, where coverage was over 15% of the quadrat, or low coverage, where coverage was below 15% of the quadrat. Photographs were taken of each quadrat using a Cannon PowerShot A480. A physical count was then made of all the different species in the quadrat, and how many of each there were. This data was then saved as an excel file, listing the quadrant, its distance from the upper tidal limit, the percentage coverage, the species viewed, and how many of each species was viewed.

Results

A total of four low coverage and four high coverage quadrats were observed. The mean abundance of species in both low and high coverage quadrats was calculated, along with the standard error (Fig. 1).

Figure 1: Mean total species vs. Percentage Cover in rocky tidal pools of North Stradbroke Island

A one tailed, two sample unequal variance t-test was conducted on the low coverage vs. high coverage species abundance, which resulted in a p value = 0.013, which is much lower than the standard 0.05 needed for a t-test to be significantly valid.

While in the field, we noticed that members of the Caridea family seemed to be more abundant in high coverage pools (Fig. 2) and that members of the Brachyura family seemed to be more abundant in low coverage pools (Fig. 3).

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Animal Abundance vs. Percentage Cover

Figure 2: Mean Palaemon Serrifer abundance vs. Percentage Cover in tidal pools in the rocky tidal pools of North Stradbroke Island

Figure 3: Mean Brachyura abundance vs. Percentage Cover in tidal pools in the rocky tidal pools of North Stradbroke Island

Applying a one tailed, two sample unequal variance t-test to the Caridea resulted in a p value = 0.189 which does not make this result significantly valid. Applying a one tailed, two sample unequal variance t-test to the Brachyura resulted in a p value = 0.293 which again, does not make this result significantly valid.

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Palaemon Serrifer Abundance vs. Coverage

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Crab Abundance vs. Coverage

Discussion

Our results confirm our hypothesis, that there is indeed a significant difference in animal abundance between covered inter-tidal rock pools and non-covered inter-tidal rock pools, with at 1.3% margin of error. Older research found that canopy removal in rock pools did not significantly affect the diversity of the tidal pools, expressed either as the Shannon-Weaver index or as species richness (Cecchi, 1992). However, more recent research has shown that the loss of habitat structure generally leads to lower abundances (biomasses) and often to declines in species richness(Airoldi et al 2008). Our results agree with the more recent research.

As with all experiments, there were limitations that affected our project. There were only a limited amount of tide pools available along the transect, and not all of them were large enough to lay out our quadrat. Out of those pools, there were even fewer available with surface cover, hence our decision to make high coverage pools those with 15% or more surface coverage. Ideally we would have liked high coverage to mean 50% or more coverage. Also, not all tidal pools had the same water depth, which can affect species abundance. In addition, while counting animals in the pool, our disturbances caused some of the animals to hide or move out of the quadrat, which affected our data.

A future study could look into seeing if the distribution of individual species is correlated with percentage tidal pool cover. Our data relating to the barred estuarine shrimp and hermit crabs would seem to indicate this is a possibility. The study should be performed in the same setting, with data collection during all seasons.

Airoldi, L., Balata D., and Beck, M.W. (2008). The gray zone: Relationships between habitat loss and marine diversity and their applications in conservation. Journal of Experimental Marine Biology and Ecology 366, 815.

Bertness, M. D., Leonard, G. H., Levine, J. M., Schmidt, P. R., and Ingraham, A. O. (1999). Testing the relative contribution of positive and negative interactions in rocky intertidal communities. Ecology 80, 2711-2726.

Cecchi, L. B. and Cinelli, F. (1992). Canopy removal experiments in Cystoseira-dominated rockpools from the Western coast of the Mediterranean(Ligurian Sea) Journal of Experimental Marine Biology and Ecology 155, 69-83.

Figueiredo, M. A. d. O., Kain, J. M. J. and Norton, T. A. (2000). Responses of crustose corallines to epiphyte and canopy cover. Journal of Phycology 36, 17-24.

Jenkins, S. R., Hawkins, S. J. and Norton, T. A. (1999). Direct and indirect effects of a macroalgal canopy and limpet grazing in structuring a sheltered inter-tidal community. Marine Ecology Progress Series 188, 8192.

Joerger, K. M., Meyer, R. And Wehrtmann, I. S. (2008). Species composition and vertical distribution of chitons (Mollusca Polyplacophora) in a rocky intertidal zone of the Pacific coast of Costa Rica. Journal of the Marine Biological Association of the United Kingdom 88, 807-816.