ecosystems

Physical, Chemical, and Biological Changes Along the Continuum of an Agricultural Stream: Influence of a Small Terrestrial Preserve

Well, I didn’t quite get this post done the day I meant to, but my day ended up being quite busy. However, the NABS/ASLO joint meeting of 2010 is now officially over! That means this will be my last NABS post until I go to another conference. It’s nice to be back home! Meetings are exhausting and melt your brain after a while. They’re fun, but they’re intense – and I never get enough sleep. No matter how much I enjoy a meeting, I’m always happy to get back home and sleep. And, if I come away from the experience without any new communicable diseases, I’ll consider it a success.

I’m going to skip the Things I Leaned section for today and jump right into the last talk. Today, my focus is a talk by Dr. David Houghton that was given on Day 3 of the conference. Dr. Houghton is an entomologist and Associate Professor at Hillsdale College. Hillsdale is a small liberal arts college (even smaller than the one I went to as an undergrad!) and Dr. Houghton is in the department of biology there. Sadly, there aren’t all that many people from these sorts of schools at most of the meetings I go to. This is doubly sad because Dr. Houghton’s presentation was really interesting and made some excellent points.

In the area in southern Michigan where Dr. Houghton completed his study, the streams used to be surrounded by a wide strip of dense vegetation (the riparian zone). The area is now an agricultural region. This means that, rather than large trees and plants that require a lot of water filling the space adjacent the streams, the native plants have been removed and the agricultural fields go right up to the banks. This has several implications. The lack of trees means the water is warmer than it was before the trees were removed because there isn’t as much shade on the water. A lot of chemicals such as pesticides and fertilizers end up in the water any time water flows over the fields and into the streams (i.e. during rains, heavy irrigation, etc). Those chemicals decrease the water quality, which in turn impacts the plants and animals that live in the water. Overall, the water quality decreases and with it the number of species that can live in the river.

In general, this situation isn’t good for the stream or any of its biota. The river needs the forested areas for everything to work properly. Removing the riparian area means that things in the river change and the “health” of the river goes down. Stream health is a somewhat vague concept that I don’t want to get into here, but it is essentially a measure of how close to naturalistic conditions an ecosystem is. Dr. Houghton’s talk began with this introduction. Then he asked a question: are the small forested areas that are still available along southern Michigan’s streams capable of improving the water downstream so that the area downstream more closely resembles conditions without the influence of agriculture? This has important implications for conservation of aquatic species.

Dr. Houghton’s study was conducted in the St. Joseph River in southern Michigan. Like other rivers in the region, the St. Joseph has agricultural fields along the majority of its length with small forested areas near the headwaters. In particular, Dr. Houghton was interested in one section of the river that had a small terrestrial preserve where the riparian area remained intact. The river running through the preserve looked better than the area upstream, so he thought the water flowing through the area might be improved such that insects downstream of the preserve would fare better than the insects above the preserve.

To study this, Dr. Houghton chose six sites in the St. Joseph River from which he collected water and insect samples. Two sites were above the preserve, two were within the preserve, and two were further downstream. He measured several parameters of the water itself, including the temperature, dissolved oxygen, pH, and conductivity (effectively a measure of the amount of salt compounds in the water). He also measured the insect populations by collecting adult caddisflies at light traps near the river. Measuring the water parameters would tell him whether the water running through the preserve or downstream of the preserve was better than the water upstream of the preserve. Because caddisflies are aquatic as larvae and live in the water for most of their lives, they are strongly impacted by water quality and are excellent indicator species. Counting the number of individual adults and the number of species (also known as species richness) that came to the light traps would tell Dr. Houghton something about how “healthy” the river is.

His results were interesting. There was no difference in any of the measurements of water quality Dr. Houghton collected above and below the preserve. This meant that the river is an agricultural stream for its entire length and the preserve did not improve the water quality downstream. There were two water parameters that improved within the preserve: the temperature (it went down) and the amount of dissolved oxygen (it went up). These two changes can likely both be attributed to the amount of shade the river receives in the preserve versus the areas outside. Shading the water causes the temperature to go down because less sun hits the water. This in turn causes the dissolved oxygen to go up because cooler water holds more oxygen than warmer water. However, once the water flowed back out of the preserve, the temperature and the dissolved oxygen went back to the levels seen above the preserve. The preserve did not appear to be improving the water quality in the river.

Similar results were found using the insect samples. Dr. Houghton found that 7 species of caddisflies made up 90% of all of the specimens coming to the light traps both within and outside the preserve. These 7 species all feed in similar ways (they are collector-gatherers and they eat things that are floating in the water, like leaf particles and floating algae that are of the appropriate size) and have the same level of tolerance to pollution. So, it appears that the majority of the caddisflies in the river were about the same throughout, again suggesting that the preserve didn’t do much to improve the quality of the river.

However, Dr. Houghton did detect one important difference between the caddisflies in the preserve compared to those outside: there were more species of caddisflies inside the preserve, so the species richness improved. 22 species of caddisflies were found only in area of the river where it flowed through the preserve. Most of these caddisflies fed in a similar way (they are shredders, or insects that tear leaves and algaes into pieces small enough to eat – an important component of decomposition in aquatic systems) and the remaining species were ones that required cooler waters than those found outside the preserve. None of these species were very abundant and in fact a few of them were represented by only a single specimen, but the species richness was definitely improved within the preserve compared to outside.

Dr. Houghton ended his talk with a question: is the river “healthier” because of the presence of the preserve? He suggests that the answer to this question depends on what measure of health you are using. The preserve clearly didn’t change the water quality so that the section downstream of the preserve was different from the area above. If your measure of river health is whether the water quality and caddisfly populations downstream of the preserve are better than those above, then the preserve does not have any effect. This could give some policy makers the idea that it’s okay to rip those last few preserves out, making space for more agricultural fields. However, if your measure of river health is species richness, the presence of the preserve had a huge impact. The river above and below the preserve had many fewer species of caddisflies than the area within the preserve. Clearly the preserve is acting as a refuge for species that are unable to live in the more harsh conditions outside of the preserve. Thus, if your goal is to maintain diversity in the stream, the preserve is very important. In fact, building new forested areas along the water might further improve the diversity of the river even further.

I thought this talk was excellent. It was a simple project, but it did everything it needed to accomplish. Dr. Houghton’s talk also highlighted a couple of important points. First, when looking for the biological impacts of a system on a species, you need to identify which measurements of health you want to use. Second, it is good to consider multiple measurements of health within a system. It would be tragic for any study to say that forested areas near a stream aren’t necessary because they don’t improve the water quality downstream. I think what makes Dr. Houghton’s study great is the fact that he identified the changes in the species richness of the forested preserve, which showed that the preserve really did have an impact on the river system, if only in the area within the preserve. It wasn’t exactly the one he might have expected or hoped for, but it does suggest that forested preserves are valuable to river systems and should be protected so that species diversity within the river is maintained.

And that wraps up the Notes from NABS series! I hope you all enjoyed the glimpse into the research that is currently happening in the aquatic sciences and learned some new things. Scientific conferences are an excellent place to gain new insights, think about things in new ways, or learn about things you’ve never even considered. Hopefully I have passed some of these qualities on as they’re just too good to keep to myself.

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Posts in this series:
Day 0 – Introduction to the Series
Day 1 – Invasive Crayfish
Day 2 – Giant Water Bug Dispersal
Day 3 – Dragonfly Captive Rearing
Day 4 – Integrating Service-Learning Programs into College Courses
Day 5 – Impact of a Small Preserve on Stream Health

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