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Physicochemical effects on community structure in intertidal pools at the Western Cape Province, South Africa

Diana K. Guzmán Colón1, Erin Wilkus2 and Amanda Frankel31University of Puerto Rico at Bayamón, 2Reed University, 3Ithaca University

Species Richness and Pool Characterization:We found sixty species occupying the tidal pools.Thirty of the surveyed species were algae, four were classified as fast moving, thirteen as medium moving, five as slow moving, and six as sessile. Regression analyses to assess how different physical pool characteristics such as size y=5.9029x + 6.9798 (r2 = 0.2842, P<.05) and distance from ocean (y= -0.0102x+0.8526 (r2= 0.0605, P>0.05) affect the physico-chemical characteristics of the pools showed that those relationships did not have a strong trend.

Species richness and physico-chemical characteristics:As oxygen increased, algae showed a slight decrease in species richness, y = 0.0132x + 4.3366 (r2= 0.0302, P<0.05), while the rest of the functional groups showed as slight increase in species richness, y = 0.0656x + 0.1523 (r2=0.185, P<0.05). Salinity was not significant with both algae, y=5.2465x - 167.83 (r2=0.2112, P>0.05) and the other functional groups, y= 10.923x - 353.3 (r2=0.2499, P>0.05).

Algae was unaffected by changes in temperature, y= 0.3538x+0.0584 (r2= 0.0071, P>0.05) and the other functional groups showed an increase in species richness with increased temperature, y= 2.2315x-29.654 (r2= 0.0766, P>0.05). Algae was also unaffected by changes in pH, y= 0.7029x+0.2514 (r2= 0.0016, P>0.05) and the other functional groups showed increased species richness in more alkaline conditions y=8.1267x-59.628 (r2= 0.0601, P>0.05)

Oxygen and pool characteristics across the temporal scale:Oxygen fluctuated depending on the time of the day, the optimal content was seen during the day with an average of 200% oxygen, whereas at night oxygen content minimum was 45% oxygen. The largest range in oxygen that was experience between night and day was a difference of 155%. Oxygen content was influenced by pool size or distance from the ocean.

Similarity analysis Similarities in community composition considers both species richness and abundance. Pools 4,5, 6, and 7 which are found far away from the ocean have similar species compositions and pools 2 and 3, which are found close to the ocean had similar species compositions. A similarity matrix showed that average similarity between tidal pools was 46.16% similar which suggests that pools were overall, were not extremely different in community composition.

Study SitesTwenty pools of varying sizes and varying distances from the shore were randomly selected on the rocky

intertidal shore at the marine protected area of Koppie Alleen in De Hoop Nature Reserve. This study was conducted at low tide on October 26, 27, and 28 between the hours of 5:30 and 11:00 and on October 26 between 20:00 and 23:00. These times were chosen to correspond with low tide during the spring season, when the rock pools are above the sea surface. Three teams of students collected information on pool characterization, physical and chemical characteristics, and biodiversity. Manipulated experiments were also conducted to gauge how different organisms respond to oxygen level changes within pools.

Marine species distributions are strongly controlled by the physicochemical conditions of their habitat created by percent oxygen in the water, salinity, temperature, and pH. . The South African coastline experiences a semi-diurnal tidal pattern with two low and two high tides each day. The area between the low and high tide line is known as the intertidal zone and consists of water-filled depressions in the rocky shore known as tidal pools. The microhabitats that exist within the individual tidal pools experience extreme physical conditions, because of this they may constitute a specialized habitat for a variety of marine organisms. The island biogeography theory has been used to explain community structure in these pools. This theory postulates that the main predictive variables in determining community structure are the island’s (pool) size and distance from the mainland (ocean). Thus, the larger tidal pool and proximity to the ocean will consist of a greater variety of habitats and therefore support a greater diversity of species. . Nevertheless, the theory overlooks important physicochemical properties that may have a large influence on community structure, such as oxygen content in the water, salinity, temperature ranges, and pH. While island biogeography theory suggests some explanation for species composition and distribution in tidal pools, this study argues that temperature and oxygen content specifically might be playing a more direct role. We predict that how close or far a pool is from the ocean and the pool size will affect the physicochemical conditions of the pool and therefore the species within it. Therefore, this study looks at pool size, distance from ocean, and physiological factors to determine what characteristics contribute to intertidal species distribution.

Background

Introduction

Results

Figure 1. Spatial relationship of pools located on the rocky intertidal shore at Koppie Alleen in De Hoop Nature Reserve. The blue areas represent the ocean and the line separating the white and blue regions represent the edge of the rocky shelf where the waves broke during low tide. The object labeled rock is depicted twice to indicate that pools 10, 11, and 12 follow pools 8 and 9. Pools 1-12 follow the coastline east and 13-20 are located around a corner in a northeast direction.

Pool CharacterizationCalculated volumes and distances from the ocean for the twenty tidal pools assessed. Volume was calculated using the length, width, and depth of each pool, while distance from the ocean was measured from the pool’s edge that was closest to the ocean. Tidal pools were classified as small, medium, and large depending on their volume The distance between the tidal pools and the ocean was measured from the edge of each pool closest to the ocean to the edge of the rocky shelf where the waves broke during low tide.

Physical/Chemical Oxygen concentration (recorded as percent oxygen in the water), standardized against percent oxygen in the atmosphere, was measured using a YSI oxygen meter DO200. YSI conductivity meter EC300 was used to indicate how salty and nutrient rich a pool was. Temperature and pH were measured using a Cryson pH meter pH25.

Biodiversity SurveysA survey of algae, sessile, slow moving, medium moving, and fast moving organisms was conducted for each tidal pool. Sessile organisms included mussels, barnacles, and sponges. Slow moving organisms included urchins and anemones. Medium moving organisms included limpets, winkles, and sea stars and fast moving organisms included crabs and whelks. Percent cover and the abundance of individuals in each pool were recorded for all organisms except algae, for which we only recorded percent cover. For smaller pools we surveyed the entire pool, while for larger pools students in wet suits, masks, and snorkels tossed 0.5 x 0.5m quadrats into random areas of the pool ten times and evaluated biodiversity within the quadrat.

Experimental Oxygen manipulation:To observe the behavior of three different species, Fissurella mutabalis (Keyhole limpet), Parechinus angulosus (Cape urchin), and Burnupena cincta (Ridge burnupena). Normal sea water taken from edge of the rocky shelf of the intertidal area was used as a control to represent the optimal oxygen conditions that exist in pools closer to the sea. The experimental variable tested was oxygen depleted water to replicate oxygen conditions similar to pools further away from the sea. Sea water collected the day before running the experiment was boiled to deplete the oxygen concentration of the water and stored it in sealed jars to keep the water from oxygenating until use. Three organisms of the same species, one organism placed in each of the three normal salt water jars, were observed for 30 minutes. The same organisms were then placed in the deoxygenated water jars and observed again for 30 minutes.

Materials and Methods

Figure 2. Regression analysis of species richness in algae (A) and clumped functional groups (fast- F, medium- M, slow moving -Sl, and sessile-S organisms) across twenty different pool sizes and distances surveyed in Koppie Alleen, DeHoop Nature reserve, South Africa on October 26, 27, and 28 between the hours

of 5:30 and 11:00 and on October 26 between 20:00 and 23:00

Conclusions

Figure 2. Regression analysis of species richness in algae (A) and clumped functional groups (fast- F, medium- M, slow moving -Sl, and sessile-S organisms) in response to average percent oxygen (a), average salinity (b). Species richness was surveyed and physico-chemical measurements were taken in Koppie Alleen, DeHoop Nature reserve, South Africa on October 26, 27, and 28 between the hours of 5:30 and 11:00 and on October 26 between 20:00 pm and 23:00

Figure 3. Regression analysis of species richness in algae (A) and clumped functional groups (fast- F, medium- M, slow moving -Sl, and sessile-S organisms) in response to average temperature (a) and average pH (b). Species richness was surveyed and physico-chemical measurements were taken in Koppie Alleen,

DeHoop Nature reserve, South Africa on October 26, 27, and 28 between the hours of 5:30 and 11:00 and on October 26 between 20:00 pm and 23:00

Figure 5. Similarity in tidal pool community composition between pools 1-20 located in in Koppie Alleen, DeHoop Nature reserve, South Africa. Similarity is based on sessile, slow moving, medium moving, and fast moving species compositions. Minimum stress value, 0.12

Our results support the postulations suggested by the island biogeography theory which states that larger pools are made up of more niches and can therefore support more species they are also easier to colonize due to their proximity to the ocean. The effects of oxygen, salinity, temperature, and pH on the number of species that colonized pools consistently affected each of the functional groups. All functional groups except algae showed a preference for pools with higher average levels of oxygen, higher salinity, warmer temperatures and more alkaline. Algae were equally distributed across pools of various conditions . . The only algae present in tidal pools was ephemeral algae, which is tough and resilient suggesting that the algae that are established in pools are well adapted for the variety of microhabitats within tidal pools. the fluctuations that were found in oxygen, salinity, temperature, and pH were not large enough to affect the life strategies of the organisms we observed within the pools. Our study shows that at this temporal scale there are fluctuations in oxygen levels between night and day. Therefore, organisms inhabiting the pools are adapted for extreme shifts in oxygen in a short period of time.