Warmer water in rivers and streams may have big consequences for those waterways, as well as the ecosystems around them. That’s among the findings of new research, published in the journal Ecosphere.
Michael Zampini is a postdoctoral researcher at University of California, Berkeley and did his Ph.D. at Northern Arizona University. This study was part of that work done in Flagstaff.
He spoke with The Show about it and started with a thumbnail version of what he was trying to figure out with this research.
Full conversation
MICHAEL ZAMPINI: ... At its core, this study is about how rivers process leaves. And so, in a lot of streams and rivers, especially forested ones, leaves that fall in from trees are actually the main source of energy. So, we wanted to understand how warming changes the way that energy moves through this system, and particularly, how does that energy move up to higher trophic levels, like fish and things like that.
MARK BRODIE: And what would finding that out tell you? Why is that important to know?
MICHAEL ZAMPINI: Yeah, so it might not be super exciting at first, but it’s really fundamentals. So leaves that are broken down by microbes are then eaten by aquatic insects, and then those insects are a major food source for fish.
So, when you change how leaf litter’s processed in in rivers, you’re really changing the entire food web of that whole system. So, that’s sort of a "why should we care" kind of a importance thing.
MARK BRODIE: It kind of moves up the food chain, as it were.
MICHAEL ZAMPINI: Right, yeah. So, thing, most of the things people care about in rivers are, you know, the fish inside them. And actually, people probably don’t think about this very often, but aquatic insects spend some portion of their lives in the river. But then they emerge out and turn into kind of their mature stage, which is like flies and and other other flying insects that end up becoming food for for birds and spiders and kind of like the terrestrial environment, or basically the the forest that surrounds the river.
So, rivers kind of like pump out a ton of important prey for the biology surrounding them.
MARK BRODIE: Well, and you found that as the temperature of the water in these streams rises, it is having a change, right? It is causing a change in how all this works.
MICHAEL ZAMPINI: Yeah, exactly. So, warming definitely speeds things up. So, decomposition of the leaves and twigs and sticks, that all happens faster, but it also changes how efficiently the system works. So, what we found is that more of the carbon gets lost as CO2 and less of it gets turned into biomass that ends up supporting that river food web.
MARK BRODIE: So, what kind of implications might that have?
MICHAEL ZAMPINI: Really, for implications, it’s like if temperatures keep rising, we might push our insects out of their kind of thermal optimal range. So, that means that they’ll still probably be active, but they’re not really using that energy as efficiently, and less of it makes its way up the food web. So, you might have smaller aquatic insect kind of population sizes, and that sort of diminishes the the food base of the river for higher trophic levels, like fish.
MARK BRODIE: And as you were saying, that could also have effects sort of downstream — again, pardon the pun — in terms of things that happen on land.
MICHAEL ZAMPINI: Yeah, so I think it’s sort of especially relevant for rivers in the Southwest, where our rivers are already relatively warm, and that means that they’re kind of already close to this optimal range that we found in this study. So, even small increases in river temperatures could kind of push them beyond that threshold, and we might start to see realized effects of how the systems might function differently.
MARK BRODIE: What kind of of temperature change are we seeing? Are streams and rivers actually getting noticeably warmer?
MICHAEL ZAMPINI: Yeah, so that’s a really good question. So stream temperature and air temperature, it’s not like they increase together, exactly. So, air temperatures are increasing faster than just like the baseline river temperatures are.
But river temperatures are actually a lot more variable than air temperature. So, in a year like this year, we just had where we didn’t get a lot of snowfall in the in the mountains of the West, then we’re going to have really a lot slower moving rivers as the summer progresses this year. And it’s really that change in flow that’s going to change river temperatures probably more. So, the more variability we have in things like precipitation, or when we have droughts, that’s when you have really big changes in river temperature, and mostly for the warmer, in that case.
MARK BRODIE: Is there something to be said also in terms of whether or not there’s a lot of precipitation, that that can impact how much foliage there is, like how many leaves and twigs fall into a river stream in the first place?
MICHAEL ZAMPINI: Yeah, absolutely. So in droughts, it can be kind of a double effect, right? Where you have warmer river temperatures because the air temperature’s warmer and we’re also not getting a lot of flow through the river. And then in those same years, you might have a decrease in sort of tree growth or productivity by the forest surrounding the river, and that’ll result in less food going into the river. And food in this case is that leaf litter that drives the energy base of rivers.
MARK BRODIE: Do you know why this is happening, in terms of why rising water temperatures sort of speeds up some of this decomposition?
MICHAEL ZAMPINI: Well, yeah, it works sort of sort of similarly to like a gas-guzzling car. So, you can have the system kind of moving faster, but you end up having, you know, less efficiency at that higher temperature. So, what tends to happen is organisms have this sort of like baseline need of energy that, you call it their like kind of like maintenance requirements.
And the higher the temperatures go, the higher ... that maintenance cost gets, so then they’re not able to pack on as much biomass from what they’re eating. So, basically, it’s like the price of the house goes up, so they’re not able to buy the the boat in the front yard or go on a vacation or something like that.
MARK BRODIE: Well, can you foresee a time where if, for example, the the temperature of these rivers and streams gets too high, that it could have ... a real noticeable change in in sort of the ecosystem, in terms of what is able to live in it and live around it?
MICHAEL ZAMPINI: Well, yeah, definitely. Ecosystems operate in these kind of alternative states. So, a lot of times, they are resilient up to a point or up to a threshold. And then once that threshold is met, then they can flip to a totally different state. And there’s a ton of studies that have kind of highlighted this interest in thresholds. And, yeah, what my study shows is that our aquatic insects are kind of right up at that threshold. So, with kind of modest increases in river temperatures beyond what we’re already seeing, we might flip ... that threshold.
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