Rock Pool Water Quality Field Report

Abstract

Rock pools occur within the intertidal area of any rocky shore. They allow organisms, normally dependent on the sea for survival, to exist for long periods of time above the low- tidemark. The dynamic nature of the environment leads to many factors influencing water quality. Organisms living there must be able to cope with problems of not one environment, but two. They are pounded by waves, exposed to extremes of temperature and salinity, and flooded by seawater and exposed to drying air twice every 24 hours. The study compared rock pools on the North Coast of NSW, where tides refresh the rock pools on a regular basis. The pools were sampled to determine how the physical aspects and the chemical factors of rock pools affect the water quality. The chemical determination of oxygen concentrations (DO) in the seawater was based on the Winkler method. The study was a cross sectional study conducted at low tide on a windy, sunny, dry day. Wind facilitates movement of air, and the sun facilitates photosynthesis and evaporation. The pools had differing volumes and depths. The results indicate that the surface area of the pool, depth, temperature, salinity and abundance of marine life all impact water quality, particularly DO, which is essential for marine life. Salinity was observed to increase in shallow waters due to rapid evaporation, which would affect DO levels, and osmotic balance. Acidity levels increased with temperature. By comparing depths, we were able to see the extreme impact of the fluctuations, and it’s affect on marine life. Some organisms, such as Scleractinian corals, were not observed but anemones were in abundance. This is due to the temperature and pH fluctuations. The acid-base relations of plant environments are complex. Some habitats (e. g. high intertidal rockpools) have pH variations of up to three units over a diel cycle as a result of photosynthesis and respiration. Desiccation was not a problem during our study, but could have impacted before the next tide. Dissolved oxygen levels were mainly consistent with recommended levels, except when the surface area was disproportionate to the depth. A more longitudinal study would have allowed us to gain more concise information of the cyclical nature of rock pool quality.

Introduction

Flat Rock is in East Ballina in New South Wales. It is a rocky platform that stretches out into the open ocean. It is a popular spot for surfing and fishing. It has a flat contour. The date of the study was August 22nd 2018, when the high tide was 0. 99m and low tide was. 3m. During Spring tides the difference could be considerably higher. As our visit was at low tide, the chemical composition of the pools could have varied significantly by the next tide. The pool levels will naturally cycle through a 24 hour period, as at night marine life uses the dissolved oxygen (DO) for respiration, therefore in the morning we would expect a higher CO2 content. During the day this would reverse as the CO2 is utilised for photosynthesis. The pools were randomly chosen and the distance of the pools from the shore had minimum variance. The pools were sampled to determine how volume, depth, pH, salinity, dissolved oxygen and conductivity affect the water quality (in particular DO), and therefore the marine habitat. Dissolved oxygen (DO) is the percentage of oxygen gas dissolved in the water. It is essential for the respiration of fish, aquatic animals, micro-organisms and plants. To maintain a healthy and diverse aquatic ecosystem, the dissolved oxygen must be maintained at high levels. If DO falls, there will be reductions or losses in the more sensitive species. At low DO levels, only a very few hardy species may be present. Excess DO can also lead to ‘gas bubble disease’ where oxygen bubbles can form in the vascular system of marine life.

A constant level of salinity is also important in order to maintain the osmotic balance of marine life. The windy, sunny conditions should have increased available oxygen, but would also increase evaporation. The wave conditions on the day could have lead to the refreshing of rock pools further away from the shore. The pools studied were closer to the shore and therefore would have been isolated for a longer period of time. Levels of pH were also noted. Acidic pH levels can impact on the viability of the rock pool for marine habitation. Acidity in the ocean has an impact on coral calcification rates, which is important for coral skeleton formation. This decreasing pH is due to the increasing atmospheric CO2. The pH scale is logarithmic and as a result each whole unit decrease in pH is equal to a 10 fold increase in acidity. Levels of pH in rock pools will fluctuate from day to night as photosynthesis and respiration affect CO2 levels.

Method

After identifying pools of different sizes, and measuring the width, length and depth, we identified the features. The pools were randomly selected, but all had an approximately equal distance from the ocean. We measured the content of dissolved oxygen in a sample from the rock pool. The chemical determination of oxygen concentrations in the seawater was based on the method first proposed by Winkler, using the dissolved oxygen field kit for the modified Winkler method. The Winkler method has been widely used in DO measurement in fresh water, but it has limitations in saltwater as it may over estimate the amount of DO.

Next we used a conductivity meter (Eutech Model Ecoscan Con6) to measure the conductivity. The conductivity was converted to salinity levels using conversion tables. PH was measured by using pH meter (Eutech Model Ecoscan pH6), using a probe consisting of two electrodes immersed in a KCL solution, which were contained within a permeable glass bulb. Excess H+ or OH- in a water sample interacts with the KCL within the probe to alter conductivity. The meter measures the change in conductivity and converts this to a pH measurement. The meter was not waterproof so it could not be completely submerged, and had to take the measurement at depth of 5cm. Temperature was also taken. The same process was followed for all pools, taking water samples from near the surface and taking measurements from the same depth. This was in some instances dictated by the limitations of the equipment.

Discussion

The results indicate that the physical and chemical characteristics combined with the environmental conditions and marine life, all impact water quality. Pools 1, 2 and 4 had DO levels consistent with the range recommended by the Australian and New Zealand Environment and Conservation Council (2000) Australian Water Quality Guidelines for Fresh and Marine Waters, which recommends levels should be between 90% and 110% saturation. Pool 1 had average salinity, and pH. Do levels correlated with the large surface area. Pool 2 had an acceptable salinity level and low DO levels. This could be because of the time of day (utilization of DO overnight), and abundance of marine life. It was more alkaline than the other pools, which suggests it still had less CO2. Pool 3 had very low levels of DO. Higher salinity and a proportionally small surface area for interacting with available oxygen, would affect this. Due to the level of marine life, it would be expected that the DO levels would increase during the day. Pool 4 was not ideal to support marine life, and it had very few organisms. It had high DO levels due to its surface area and low depth, but the pH was acidic. The average pH of the oceans near the surface is around 8. 1. Acidity is a problem for marine life, especially corals, which rely on a stable pH for their skeletons.

The results show that the amount of dissolved oxygen is inversely proportional to temperature and salinity, and that the availability of oxygen is influenced by abundance of marine life, surface area and environmental conditions. This is confirmed by research. The solubility of oxygen decreases as temperature increases. This means that warmer surface water requires less dissolved oxygen to reach 100% air saturation than does deeper, cooler water. In addition dissolved oxygen decreases exponentially as salt levels increase. That is why, at the same pressure and temperature, saltwater holds about 20% less dissolved oxygen than freshwater. On a cool wet day we would expect a lower temperature. Daniel, M. J. and C. R. Boyden (1975) in their field study found that rockpool temperature was related to physical features such as volume, depth, surface area and aspect, but primarily dictated by the amount of direct sunlight. They also concluded that “pool oxygen, carbon dioxide and pH were closely linked with respiration and photosynthesis, the rates of which are primarily determined by temperature and illumination”, but the differing levels were due to the proportions of animals and plants combined with the depth and surface area. Rain has an impact on increasing oxygen levels as the rain interacts with oxygen in the air as it falls.

Salinity would be lower on a cold wet day due to dilution and lower evaporation rates. But “Pools of large volume are less prone to large fluctuations in salinity and temperature”. The smaller rock pools would have lower salinity proportionally than the large rock pool on a cold rainy day and greater on a hot sunny day. If we consider that “the physicochemical factors present within rock pools are highly dependent on the amount of time for which they are isolated from the sea during low tide”. The study needs to be repeated over a period of time with different weather and tidal conditions. Also due to diffusion of oxygen, CO2 concentration may have been more useful for determining rates of pool metabolism because exchange with the atmosphere is low. The range of pools and the precision of measurement were limited by the strict time constraint and the equipment used (pH and Conductivity meters could only reach a shallow depth). We observed that the fluctuation of water quality in rock pools, has an impact on the species found. Anemones were in abundance because they have the ability to adapt to the changing environment. They have extremely pliable bodies that can undergo drastic, shape changes, thanks to the thick mesoglea (gel) within their body wall. These properties also allow sea anemones to resist sudden forces, like a strong ocean current, but to adapt their shape deliberately if needed to open for feeding or to become compact for protection (or to prevent desiccation at low tide).

The anemone has a symbiotic relationship with algae, which utilise sunlight for photosynthesis. The sea anemone benefits from the products of the algae's photosynthesis, namely oxygen and food and the algae in turn are assured a reliable exposure to sunlight and protection. Sea anemones, are also carnivorous animals that consume animal matter. They move if the conditions are not suitable and have stinging tentacles that protect it from predators. Some species cannot adapt, like the Scleractinian corals. Temperature variation and the decline in pH affect the calcium carbonate requirements of the coral for their skeleton. This skeleton protects the coral animals from predators. The effect of acidity results in more hydrogen ions, which bond with carbonate (CO3-2), a key component of calcium carbonate (CaCO3) shells. To make calcium carbonate, shell-building marine animals combine a calcium ion (Ca+2) with carbonate (CO3-2) from surrounding seawater, releasing carbon dioxide and water in the process. Like calcium ions, hydrogen ions tend to bond with carbonate — but they have a greater attraction to carbonate than calcium. When two hydrogen ions bond with carbonate, a bicarbonate ion (2HCO3-) is formed. Shell-building organisms can't extract the carbonate ion they need from bicarbonate, preventing them from using that carbonate to grow new shell. Even if corals are able to build skeletons in more acidic water, they may have to spend more energy to do so, taking away resources from other activities like reproduction. If there are too many hydrogen ions around and not enough molecules for them to bond with, they can even begin breaking existing calcium carbonate molecules apart — dissolving shells that already exist. The corals also have to combat the effects of coral bleaching, which occurs when temperatures rise. When this happens the corals will expel the algae (zooxanthellae, that it lives symbiotically with) causing the coral to turn completely white. “Coral bleaching occurs when the sea level temperature increases by one degree. In a rock pool the temperature can vary by up to 10 degrees, ".

In conclusion, even with limitations, we could see the combined effects of the external environment, physical attributes, and internal rock pool environment, on the water quality. The fluctuating chemical composition of the rock pools mean that the marine life must be constantly adapting, and this will limit what can survive there.