What goes on both underwater and underground?
Have you heard of the stonefly Leuctra tenuis? Likely not; it is pretty underground. No, really. Like some other stoneflies, a type of aquatic insect, this species takes kindly to the cramped and dark environment of the hyporheic zone, the part of a stream between the streambed and the deeper groundwater. Though this environment is quite different from the streambed where most stoneflies live, the hyporheic zone has its benefits. This zone acts as a refuge from predators, droughts, and floods above, allowing it to be quite the happening place even when the surface stream is not.
In addition to being a refuge, the hyporheic zone is the site of high rates of secondary production, or the growth of organisms who consume primary producers like plants or algae. Despite its importance in understanding the food web and more, our knowledge of secondary production in the hyporheic zone is unfortunately quite limited, given the inherent difficulty of sampling an environment both underwater and underground. Nathan Dorff and Debra Finn were some of the first to study the secondary production of hyporheic macroinvertebrates in an Ozark stream. They did so by examining the stonefly Leuctra tenuis to understand how much this zone contributes to the overall production of streams.
The study was conducted at Bull Creek and one of its tributaries, Peckout Hollow, in southern Missouri’s portion of the Ozark Plateau. Wells were installed and pumped for stoneflies which were preserved in ethanol. To compare surface and subsurface stonefly populations, emergence traps were set up on the surface to capture adults before they flew away. Finally, in addition to collecting bugs from the wells, temperature loggers were deployed in a subset of them.
How to make sense of a whole lot of bugs
The collected bugs were sorted in the lab for L. tenuis individuals, and the immature nymphs’ lengths were measured to estimate body mass. Changes in biomass for a given area across sampling dates were used to estimate rates of secondary production. Multiple environmental conditions act to affect organisms and their growth rates, but temperature and the accumulation of heat are often some of the most important conditions. 350°F might be a better temperature to cook food at than 200°F, but one minute at 350 will do less than an hour at 200; so too is it with stoneflies (but please don’t cook them!) This accumulation of heat is measured in degree days, the sum of the temperatures experienced across a number of days in time.
Dorff and Finn found the rate of this accumulation of degree days to be a strong predictor of how quickly the stoneflies developed and when they emerged from the hyporheic zone to the surface as adults. Analyses of these developmental rates and estimates of hyporheic biomass indicated L. tenuis being adapted to developing entirely within the hyporheic zone, annually producing roughly 32 adult stoneflies’ worth of mass in a 1 x 1 x 0.15 meter chunk of that environment, even where there was no permanent surface water! The hyporheic populations accounted for the majority of all Leuctra tenuis production in Bull Creek and all of it in Peckout Hollow. These large ratios are consistent with similar studies.
Updated thinking on energy flow in Ozark streams
Even with data from just one species, this study provided strong evidence that the hyporheic zone of Ozark streams hosts large rates of secondary production and may be even more important to the total production in stream ecosystems than ever before thought. Many Ozark streams go part of the year without surface flow, but if stoneflies and perhaps other macroinvertebrates can survive and still grow during these times, biological conditions within the stream may be more stable than surface conditions may indicate. Furthermore, stream ecosystems are not isolated from their surrounding landscapes: adult stoneflies become food for reptiles and amphibians, birds and mammals, and other invertebrates like spiders and dragonflies. They surely appreciate having a reliable source of food! A more complete picture of which organisms may thrive in the Ozark streams’ hyporheic zone and just how much they contribute to their stream’s total production, however, will require even fewer stones left unturned!
Dorff, N.C., Finn, D.S. Hyporheic secondary production and life history of a common Ozark stonefly. Hydrobiologia 847, 443–456 (2020). https://doi.org/10.1007/s10750-019-04105-z