Ever wonder where that large, granulated salt dumped all over your sidewalks and roads during winter truly goes? For many of us, we simply don’t notice when it has been washed away as the snow and ice gradually melts. But give it some thought, and you might come to the same conclusion as us: the salt goes wherever the melted snow goes. In many cases, this means being picked up by the nearest body of water. Small, freshwater streams are a common resting place for these concentrated road salts, and becoming combined with these otherwise unsalted ecosystems can cause unprecedented impact on stream organisms and nutrient retention. Join us as we explore what kind of impact increased salt levels have on stream life in freshwater ecosystems!
About the Authors
AJ Speicher
Hello all! My name is AJ Speicher, and I am rising Junior and Biology major. I’m from Seattle, Washington, and enjoy outdoor activities like hiking and running, as well as games and movies. I’m especially interested in genetic ecology and organismal relationships, and hope to apply my knowledge in the field in the future. I chose to work in Dr. Eckert’s lab because I believe the issues we research are extremely important to better understand current environmental shifts.
Justin Sawina
Hello, my name is Justin Sawina. I am an environmental studies major and anthropology minor and rising senior working on Professor Eckert’s project this summer. This is my first research experience here at Gettysburg college, and I have learned a lot during my time working on it.
Experiment
Project Overview
Our project studies the effects that salinity has on freshwater streams, specifically the decomposition rates of oak leaves. Many small streams are supported by “brown” food webs, as they derive their primary flow of nutrients and energy from the decomposition of leaves or other organic matter, opposing “green” food webs which obtain nutrients and energy through photosynthesis. This decomposition is mainly carried out by fungi and bacteria and can be aided by algae. Together, these organisms are able to break down the tough composition of sturdy leaves and allow tiny insects or other invertebrates to consume the otherwise inaccessible nutrients. However, this is a carefully balanced act, and any outside pollutants could disrupt this vital process. To a fragile freshwater stream, road salt runoff can be a significant pollutant to all lifeforms calling the stream home. Research has been conducted to gain insight into the severity of this pollution, yet we do not fully understand the impact of increased salt levels in streams.
Varying salts are often spread on roads and sidewalks in preparation for snowstorms or slippery terrain. This salt can be brought to freshwater systems through snowmelt or runoff from rain, resulting in wildly increased salinity concentrations. Currently, there is evidence that enough of this salt contamination may disrupt the decomposition of leaves which may alter energy flow in the food web of these streams, posing dire consequences to all stream life relying upon those sources of energy. Our laboratory’s goal is to better understand how this increased salinity can harm invertebrate life and alter the relationship with their energy source, decomposing leaves. This involves analyzing not only the growth and composition of the organisms themselves, but how the consumption and decomposition of oak leaves changes at different salt levels. Additionally, understanding leaf decomposition requires information on the microorganisms colonizing the leaves, such as varying species of algae and bacteria.
Methods
To test the effects of salinity on stream systems, we decided on four levels of salt to put into our water ranging from near zero to 230 mg/L (this is the chronic level of salt as defined by the EPA, and the range that most streams in the USA fall under). To begin, we headed out into the field to collect macroinvertebrates which often feed on the microorganisms colonizing the leaves, as well as accurate readings for the stream conditions (e.g., pH, water temperature, amount of dissolved oxygen) to replicate as best as possible in our lab. We settled on focusing on the amphipod species Gammarus minus, a small freshwater crustacean that appears similar to a small shrimp, and collected them in Boiling Springs, PA (big thanks to Professor Fong for showing our group this spot!).
AJ and Justin out in the field at Boiling Springs collecting amphipods and algae samples
We then punched out a whooping 1200 round leaf discs out of pin oak leaves over the course of many days and put them into the four separate treatments of water to naturally colonize with microorganisms. Our experimental setup consisted of individual flasks, 8 replicates per treatment, with 5 including an amphipod and 3 containing only leaves to assess differences in decomposition and algal/bacterial growth due to feeding. Changes in the amphipods, leaf decomposition, and algal and bacterial communities on the leaves were monitored for the next three weeks. Leaf samples were measured and replaced each week.
Captured images of amphipods and leaves to be measured and analyzed
After every week of the experiment, we would remove the leaves from each flask, measure their weight and photograph their area, and preserve them for later analysis. New leaves would be added from the respective salinity treatment.
The flask setup for the experiment, with tubing to oxygenate water and replicate stream conditions
Justin’s work
My work mainly consists of analyzing the effects that salinity has on algae. Algae is not directly responsible for breaking down leaves, but it seems that they play a significant role in the process by providing both bacteria and fungi with an alternative source of carbon than the leaf itself. Currently, our hypothesis is that the increased salinity will decrease the diversity of the types of algae we see. My role in to analyze the algae samples we collected during the experiment under a microscope. Generally, I see three types of algal organisms in these samples: green algae, diatoms (single cellular organisms), and cyanobacteria (small photosynthetic bacteria). I then identify the diatoms down to the genus level which is one classification above the species level. I then classify the algae by their shape, or morphology. I’ve definitely enjoyed this aspect of the work; there is no end to interesting combinations of green algae, cyanobacteria, or diatoms that you can find in the microscopic world.
Fun fact: Cyanobacteria were the first lifeforms to start using photosynthesis and are responsible for adding oxygen to our atmosphere billions of years ago.
An example of what Justin sees under the microscope when analyzing algae samples
AJ’s work
I primarily look into what kind of bacteria are present at varying salinities. Bacteria largely impact many processes in stream ecosystems, such as leaf decomposition and nutrient cycling. The bacteria help break down tough leaves into their core components and allow small organisms to acquire the otherwise inaccessible nutrients. In a similar manner to Justin’s analysis, I view and record what kinds of bacteria are most commonly found at each salinity level, and determine if any one group is growing better than others. I can’t identify quite down to genus, and instead focus on the shape of the bacteria themselves, such as lines, circles, ovals, or linked chains. I do this by staining the DNA of the bacteria with a dye, which reveals their structure under specific microscope conditions. Determining which of these shape groups is most prevalent at each salinity concentration allows our lab to have a better understanding of how the nutrients in our ecosystems flow.
This is what the dyed bacteria look like under a microscope. Small, barely visible clusters try their best to pop out from the colored background.
This is a similar image of bacteria, specifically edited to make the bacteria as visible as possible. Here we can see instances of lines, ovals, curved lines, and even a few chains.
While our ultimate conclusions might show that road salt does negatively impact stream ecosystems, we also understand that safety during the winter months is especially important. Rather than completely stopping road salt use, it would be valuable to assess what alternates can be applied that still keep us safe from slipping, but don’t pollute or harm local streams. So the next time you see the salt trucks out during a winter storm or feel the crunch of salted sidewalks under your feet, be sure to give some though as to where that salt ends up.
Cross-Disciplinary Science Institute at Gettysburg College - Summer Research 