Investigating the Influence of Tides and Weather Salinity Throughout the Ala Wai Canal
Investigating the Influence of Tides and Weather Salinity throughout the Ala Wai Canal
Andrew Ito, Ashley Salazar, Cameron Dye, David Pham
AP Biology Per 1
1/14/11
Abstract:
For our research on the Ala Wai estuary, we used a YSI 6920 V2 multi-parameter Sonde water sampling device to take measurements on the salinity at .5 meters, 1 meter, and 1.5 meters in five different zones. Weather and tides were two factors that we examined for effect of salinity on the estuary. Due to insufficient data, we have concluded that there exist bands of differing but high concentrations of salt water in the estuary. According to the data, low tides and high tides affect larger depths and affect surface water less. Low tides decrease salinity as the zones become farther away from the mouth. On December 11th, we measured data after increased rainfall and noticed the freshwater layer had increased in volume to occupy 30% of depth.
Introduction: The Ala Wai Canal is a man-made estuary made between 1921 and 1928. It combined the drainage of the Manoa and Palolo Valley Streams into one drainage canal. The Hawaiian Dredging Company was contracted for the making of the canal. In the process, wetlands fed by the Manoa and Palolo Valley Streams were filled in and the streams themselves were diverted to the Ala Wai Canal.
The canal is considered to be a salt wedge estuary; brackish surface water flows out of the canal. Sills in the canal also affect the distribution of salinity and other variables. However, the canal is also a highly eutrophic, polluted body of water; health records indicate that the waters in the canal exceed bacterial counts.
The Ala Wai Canal is an excellent area to study for several reasons. It is particularly convenient for its close location to the University of Hawaii at Manoa campus. The canal also supports high productivity levels due to elevated substances. Organisms are also greatly affected by their geographic position in the canal; tides and salinity levels have a big impact on where organisms congregate (Glenn, 2010).
We looked at salinity throughout the Ala Wai for this project. Salinity is the measurement of the amount of salt dissolved in water. It can be expressed in parts per thousand, parts per million, or as a percentage. Usually, fresh water has a salinity of less than .5 parts per thousand, brackish water (such as that of estuaries like the Ala Wai) have salinity levels of up to 3 parts per thousand. Saline water has salinity levels above 30 parts per thousand.
Salinity plays a huge role in the Ala Wai. As an estuary is a place where freshwater and saltwater mix, the salinity varies widely throughout the area. This leads to huge diversification in the organisms that live there, such as freshwater sturgeon living in the same area as saltwater fish. To understand the salinity levels in different areas of the estuary is a step towards comprehending the organisms that live in it (Office of Naval Research, 2010).
This pattern in salinity is never constant. Environmental effects such as tides, weather, depth, and even topography of the stream bed can affect salinity. Estuaries serve as a connection between the ocean and streams from the mountains. Salinity demonstrates the combination of freshwater from the mountains and salt water from the oceans, constantly changing as the ocean and mountains are affected. High tide brings in ocean water, causing a rise in salinity. During low tide, ocean water recedes, causing a drop in salinity.
Weather can affect the salinity directly at the estuary or through streams that flow from the mountains. High amounts of rain at the estuary can lower salinity as ocean water is diluted from rain. Rain in the mountains flows down to the estuary diluting the ocean water and lowering salinity. If the mountains do not have rain, water will not flow down and salinity will be higher.
A study of San Francisco Bay found that the topography of the bay caused stratification of salinity. This along with gravitational circulation caused Estuarine turbidity maxima, the circulation of currents that affect sediment deposit and agitation (Schoellhamer, 2001).
Analyzing salinity is interesting because an estuary is a mixing of freshwater runoff from streams and salt water that flows and ebbs based on tides. This gradient of fresh to salt water is what we hope to capture. Understanding how the fresh and salt water circulates is important in interpreting our data. Since salt water is denser than fresh, water closer to the surface has lower salinity levels and a current flowing out. Lower, higher salinity values are expected. These circulation patterns fall into four categories.
A salt wedge forms when there is a heavy flow of fresh water from a river with low tidal range effects. The fresh water flowing out makes a majority of the estuary however, it draws a small wedge of salt water that shifts seaward during falling tides and toward the river in high tide. This creates a high concentration of seawater on the bottom of the estuary.
The well-mixed category includes low fresh water with higher tidal effects creates this. Here, salt water and fresh mix thoroughly creating bands that have no change of salinity by depth but increase in salinity as you move seaward.
The partially-mixed category is a cross between the Salt wedge and Well mixed. With heavier inflow and moderately strong tidal currents, a circulating pattern emerges as salt water flows along the bottom, mixes with fresh, and flows out along the top.
The final circulation pattern is the reverse pattern, this occurs where there is a lot of evaporation of salt water along the upper reaches of the estuary. This creates a gradient where salinity concentration is actually greater inside than the seawater. As a study shows, the Alawai Estuary is a Salt wedge. We will use this circulation model to help identify a pattern in the data (Grove, 2010).
Materials and Method: We took data using the YSI 6920 V2 multi-parameter Sonde water sampling device in each of the five zones. The YSI recorded data at each of the fives zone at depths of .5, 1, and 1.5 meters as water depth allowed for one minute intervals. The data was collected over 3 days: October 30th, November 13th and December 11th in 2010. On the data collection days, samples were tested different tidal levels in order to procure a variety of data. For the purpose of more complete data, the portal provided data from other groups for the 1 meter depth.
Data: In addition to our own measurements, we used class data at varying tides and weather conditions. Data is on the Intellesense Portal.
Discussion:
Tide patterns also gave a variance in conditions. In general, the surface of the estuary did not vary in all zones maintaining a distribution of 80% - 90% seawater. In lower depths however, we noticed a fall in salinity at the 1 meter depth of up to 20%. At the 1.5 meter depth, there was no change. We hypothesize that the change was caused by the outflow of Ala Wai. The pull of water current out creates a completely positive outflow of water at a different layer. We do not understand why there is a layer of higher concentration on top of a layer of lower salinity.
For our first two data sets along zones 1-5, we had clear weather conditions with partially cloudy weather. Our results also fell into a clear pattern of 70%-100% saturation of seawater in the estuary. However, we collected data a third time after an extended rainfall from 12/9/19 23:30 to 12/10/10 13:00. The downpour affected the two runs on 12/11/10 where we noticed that the Ala Wai was flushed with fresh water. At the .5m depth, fresh water flowed all the way out to zone 5 creating a distinct layer of pure fresh water.
One factor we did not consider is that this estuary is not open to the ocean. It opens far up on the Ala Wai Canal where water is fairly stagnant and may hold greater concentrations of salt than normal ocean conditions. Having the large volume of water that is also flowing out to sea may have subdued tidal effects to a degree. The Ala Wai too may have specific layers of water concentrations that when forced up the estuary, may have been interpreted as bands exiting the estuary not entering it.
The data we have collected demonstrates that the location tested is an estuary that falls under the Salt wedge characteristic for water circulation. Data taken does vary with weather, tides, and is a mixture of fresh and salt water and are characteristics found in estuaries. In the Ala Wai estuary, data visualizes a layer of fresh water at the top, with denser salt water sinking to the bottom. We classify this separation as a salt wedge found in estuaries that are void of massive water circulation. In the Ala Wai estuary, circulation is minimal, forming a salt wedge. Low water circulation is a result of lower marine activity as tides, waves, and changes downstream do not cause massive water circulation. Continued streams of water from springs or rain in the mountains provide the fresh water floating on the surface, while changes in tides cause in influx of sea water that sinks creating a salt wedge estuary environment.
Our report holds interesting implications may be similar to other estuaries and their conditions. However, we have had many issues that have skewed our data and results. The biggest issue is that we simply do not have enough data to see the full picture. Two of our tests only showed a high mix of fresh and salt water but never pure freshwater. Another factor that we wish we could have controlled was the depth increments. We took samples every .5m until reaching the bottom. Unfortunately, nature does not conform to our standards. Not all the zones were deep enough to take data at 1.5 meters. We also missed a lot of interesting information on specifically where the salt water and fresh water flow meet and saw just pieces of the gradient in between. Again, we largely did not capture the layer of freshwater because our lowest depth was at .5 meters. Our larger increments also prevent us from seeing different patterns such as turbulence between fresh and salt layers which were translated to odd spikes and variations of information. Finally, to interpret our data, we used controls of salt water from the deeper readings in Zone 5 and used online Hawaii Health Organization values of local freshwater for freshwater controls. Though they may be close to actual values, they are not the values specific to the upstream runoffs and natural springs nor, to the Ala Wai.
Conclusion:
· The salinity layers and mixture found are consistent with mixtures of salt and fresh water found in estuaries with a salt wedge.
· The relatively stagnant water of the Ala Wai, creates the environment for the salt wedge, as high levels of salinity and low levels of water movement are found.
· An extension of this lab would simply be to have a more exhaustive data gathering process. We would take more readings in a bigger variety of locations, depths, weather conditions, and tide levels. In doing this we could have a full range of data to analyze. A different lab related to this would be to observe the effect of salinity on wildlife and organisms living in the Ala Wai Canal, such as effects on reproductive cycles and habitats.
· This project is important because salinity affects the environment and habitats surrounding the Ala Wai Estuary. The percentage of salinity in the water affects the growth of plants and animals; differing salinity levels changes the types of predators and prey in each habitat. Salinity also affects the chemical composition of the water in the estuary. If we can understand the chemical composition of the Ala Wai, we might also be able to understand why the estuary is polluted and how to improve the estuary.
Works Cited:
Glenn, C., McMurtry, G. Scientific Studies and History of the Ala Wai Canal, an Artificial Tropical Estuary in Honolulu. Retrieved November 23, 2010, from ScholarSpace Web site: http://hdl.handle.net/10125/2327
Grove, K. "PART I: What is an estuary?." Department of Geosciences Course Home Pages. San Francisco State University, n.d. Web. 24 Nov. 2010. http://geosci.sfsu.edu/courses/geol103/labs/estuaries/parti.html.
Office of Naval Research. (2010). Ocean Water: Salinity. Retrieved January 12, 2011, from the WiseGeek web site: http://www.onr.navy.mil/focus/ocean/water/salinity1.htm
Schoellhamer, D. (2001). Influence of salinity, bottom topography, and tides on locations of estuarine turbidity in northern San Francisco Bay. Sacramento: U.S. Geological Survey.