Discussion
This experiment was conducted to test the effects of increased salinity on freshwater organisms collected from the Los Angeles watershed. We hypothesized that the sample jars with the lowest salt concentrations would contain the greatest number of organisms, and the jars with the highest salt concentrations would contain the least amount. Looking at our data regarding the total number of organisms observed by concentration, our hypothesis was supported. The concentration with the most salt (5.0g) showed the least number of observed organisms. Yet, when looking at each individual phyla, it’s more difficult to determine if our hypothesis was supported or refuted. For example, diatoms were found to be most observed in the sample containing 5.0g of salt, which would refute our hypothesis. On the other hand, the smallest number of protozoa were seen in the 5.0g sample, which would support our hypothesis.
Although not measured by number, it is likely that the populations of green algae and cyanobacteria were affected in some way by the experiment. We can assume this due to the significant coloration changes that both underwent, and how relatively no zooplankton were seen feeding on the edges of the algae by Week 4. We assume that the discoloration is most likely occurring because the plant is unable to photosynthesize, likely due to the increasing salt content, lack of sunlight and lack of carbon dioxide (“Plants”). Then, we found that the six samples containing added salt were observed to have significantly less moving organisms, and were even completely absent of some of the same organisms seen in the control at Week 4. In addition, in all four different concentrations, the populations of diatoms and protozoa decreased over time. Contrastingly, euglenoid numbers increased and decreased at different times and did not show a clear trend over time. There are a few major salt related reasons these different observations could have occurred. The first is that the organisms that decreased were stenohaline species which cannot tolerate high salt levels. The second could be that the salt amounts altered abiotic factors in the water such as pH or nutrient content causing changes in ecosystem processes. A third factor, related to food web and species interactions, could be that the consumer organisms didn’t have a sufficient food and energy supply once the primary producers like algae began to die. However, it’s important to note that certain factors such as a limitation to sunlight may have also affected the ecosystem’s processes along with the salt.
There are a few significant errors that may have skewed collection, observation or analysis of data. First, the one large water sample was initially stored in a sealed container for two days before we split it up into the jars. It’s also possible the indoor storage of our jars did not give the organisms in the samples enough sunlight, and the plastic wrap did not allow for adequate flow of oxygen within the sample. This lack of oxygen and light may have had an adverse effect on the sample initially, before salt was even added. Second, definitions and terminology for data collection were not consistent among group members at the start of the experiment. For example, one member might be recording quantitative measurements for a category that another member was recording in coloration and quality. This made it difficult to organize and analyze the data since the week to week data recording was not consistent within a phyla. In addition, when analyzing data, both moving and nonmoving organisms for each phyla were added together, and it’s possible many of the organisms counted were nonliving. Moreover, the difference in skill between group members as far as microscope use and microorganism identification played the biggest role in this experiment as far as being a problem. Group members tended to identify certain organisms better than others, especially as time went on. Therefore, I think this skewed data in that members tended to look for the organisms that they knew more easily and recorded more of them. To further add, members were simply better able to identify organisms with more practice as time went on, so data between the beginning and the end seemed to be inconsistent as far as the numbers of organisms we observed. For example, there were a significant increase in the number of euglenoids observed in the 5.0g sample between Week 3 and 4. This change was likely due to the fact that the observer was better at identifying euglenoids as time went on, not because there were actually more euglenoids in the sample. There are many other confounding variables that may have skewed the data and therefore analysis, but these are the most likely for this given case. In the next experiment, it would be important to have a better basis for data collection as far as making it more consistent between group members.
This experiment’s results are relatively similar to those found in previous studies. As found in the study by Sudhir and Murthy, “Salt stress causes decrease in plant growth and productivity by disrupting physiological processes, especially photosynthesis.” This is similar to the data we found in our study in that we assume much of the decline of our sample’s diversity and species richness was due to photosynthetic changes in the autotrophic plants that resulted in significant changes in the overall food web. Species interactions may be greatly affected by the changes that salinity can causes in the food web. As discussed in the introduction, when a species in the food web become unable to function or provide its ecosystem services, it affects not only that species but all the others it connects to. To further add, energy transfer between trophic levels becomes altered when certain species can no longer interact with others. For example, in this experiment, a decline in primary producers in the aquatic food web such as the green algae resulted in a decline in a food and energy source for many secondary and higher level consumers such as the protozoa. In addition, the increasing salt levels may be directly affecting the organisms, or indirectly altering the abiotic elements of the ecosystems so that certain organisms are unable to survive there due to their specific environmental tolerances. In this case, organisms are forced to move to better locations, or eventually die out from the competition with species who are better equipped for the changing environment.
It’s important to understand how salt is affecting our freshwater organisms because unless a change is made, it’s going to affect human life whether we are aware or not. Human life is regulated and supported by ecosystem services from what we eat and drink, to what diseases we are at risk of, to even what pharmaceutical medications we have available (“Ecosystem Goods and Services for Health”). When the ecosystem becomes compromised, so do its services, and we will be greatly impacted by this. Increasing salt is being observed to have adverse affects on both the organisms in an ecosystem and the services they provide collectively. These increases are due to both natural and human related causes, many of which can be regulated and stopped in order to prevent further increase. In fact, a main part of the reason salt has increased so much in recent decades is due to increased human activity within these ecosystems. Unless we make a change, salt levels are only going to increase, and more adverse effects on the ecosystem are only to come.