Written by: Anna Noreuil, PhD student, Fritz Lab and Maggie Yuan, MS student, College of Edu
Shaded headwater streams are often overlooked by the average hiker, who may only fleetingly consider how they might leap across them to avoid wet feet. However, these streams are critical to aquatic and terrestrial ecological food webs and shouldn’t be simply ‘jumped’ over in terms of research. These food webs illustrate the connections and interactions that can exist between many different species in a natural community. In these illustrations, we can describe the transfer of ‘food energy’ in a system from one group of organisms to another. For example, plants and algae use energy from the sun to grow, which is eaten by herbivores, who will, in turn, be consumed by carnivores. When the sun provides the initial source of energy for the food web, it is said to be a green food web. In contrast, if the system receives little energy input from the sun, and is instead sustained by dead organic debris (utilized by decomposers), it is a brown food web. One common example of a brown food web here in Maryland is a shaded stream in a forest ecosystem.
Think about the limited production of green plants on a forest floor in comparison to an open field. As opposed to streams sitting beneath sparse foliage cover, shaded streams were previously considered to have no algal production because the shade prevents high algal production and so the impact of algae was largely ignored. Although there is not a large amount of algal production in these shaded streams, the algae that is there seems to play a disproportionate role in contrast to its biomass. A little bit of algae is doing a lot in the system! Recent studies of these aquatic systems have demonstrated algae present on leaves in shaded streams may play a significant role in brown food web dynamics and support ecosystem complexity. These algae also influence the decomposition of plant material in stream ecosystems and serve as a high quality food source for an array of aquatic insects (Danger et al. 2013; Guo et al. 2016).
Rebecca Eckert, a Ph.D. student in the Lamp Lab at the University of Maryland, sought to better understand the dynamics of brown food webs in Maryland stream systems (Figure 1). She was interested in assessing how algae growing on leaves in these streams impacts arthropod communities that live in/feed on them. Specifically, she examined (1) patterns between algae distribution and arthropod colonization (2) if certain aquatic insects grow better when leaves have more algal biomass and finally (3) if these insects display a preference for feeding on leaves with algae.
Eckert first examined how varying degrees of leaf-associated algal growth influence insect community composition and abundance. To understand this relationship, Eckert manipulated light in streams with low and high nutrient concentrations. Mesh bags of abscissed red maple leaves were placed throughout the stream study sites in the winter and spring. After a month leaves were brought to the lab where algal biomass and insect abundance were quantified. Insects collected from the streams were analyzed based on how they fed, including: collector-filterers, collector-gatherers, scrapers, shredders, and predators. Eckert found that season and light availability greatly influenced leaf associated algal growth. In turn, the five feeding groups varied in their response to changes in available algal biomass and the response varied by season. In particular, Ephemerellidae mayfly larvae (collector-gatherers) were more abundant with more algae, while the larvae of Tipula crane flies (shredders) decreased in abundance with more algae.
In her next research question, Eckert examined the growth rate of macroinvertebrates when fed leaves with greater amounts of algae. Shredders selectively feed on leaves according to preference. However, in past studies, there are conflicting results regarding shredder growth with algae. In the presence of increased concentrations of algae, shredders have been observed to grow more, grow less, or grow the same than if feeding on leaves with less algae. (Guo et al., 2016; Albariño et al., 2008). With these results, Eckert sought to test another shredder as well as including a collector-gatherer, the mayfly larvae. To determine if algal concentration influences larval mayfly growth, Eckert conditioned leaves in dark and light environments and measured arthropod relative growth rates and leaf consumption. Finally, using carbon and nitrogen stable isotopes, she determined the assimilation of algae into the tissues of both species. With these methods, both the macroinvertebrates were found to have fed on both algae and leaf material. Interestingly, the collector-gatherer species shredded leaves similar to the shredder. Yet, the shredder fed and grew more on light-conditioned leaves, while the mayfly grew the same in both conditions. In regard to algae, Eckhert’s results suggest that they can be important to the diet and growth of isopod shredders in light conditions. However, algal biomass did not influence the collector-gatherer mayfly larvae consumption or growth.
In the last chapter of Eckert’s research, she investigated if macroinvertebrates preferred to feed on leaves with more algae. It’s important to note that growth does not indicate preference just as preference does not indicate food quality (Albariño et al., 2008)! To do this, Eckert included four shredders and one scraper across 5 different groups and measured their preference for leaves with more or less algae present in dispersed or clumped formations. Of all the species tested, most showed no preferences and in those that exhibited a preference, they preferred dark-conditioned leaves.
Eckert’s work demonstrates that the streams in the woods near your house or across from your office building are important and the algae in them play a major role in their ecosystem! Algae on leaves matters for macroinvertebrates in shaded headwater streams--on their assemblages within leaves, on their growth, on their food preferences, and these in turn, affect the broader food webs in which these organisms reside. Green and brown food webs are in fact not separate ecosystems, but intertwined and interconnected through their shared interactions between algae, fungi and macroinvertebrates.
Anna Noreuil is a Master’s student in the Fritz lab studying chemosensory gene expression and host preference in above- and below-ground collected Culex pipiens mosquitoes.
Maggie Yuan is a Master’s student in the College of Education, pursuing a degree in secondary education, biological sciences. She is interested in entomology and has taken several undergraduate-level courses on the topic.