Water willow and mussels and deer, oh my!

Primary Article: Lopez, J.W., Parr, T.B., Allen, D.C., and Vaughn, C. C. Animal aggregations promote emergent aquatic plant production at the aquatic terrestrial interface. Ecology 0 (0), 2020, e03126.

Featured Image: Little River, Oklahoma. Wikipedia

Streams and rivers are a dynamic interface between two worlds. Just like arteries carry oxygen-delivering blood to our tissues and organs, stream networks carry energy and nutrients that can be transferred to terrestrial landscapes and their inhabitants. Many insects grow and mature in streams and emerge on land as adults. They provide energy and food to their terrestrial predators, with effects traveling up the food chain. Floods can also deposit aquatic resources (think fish carcasses or dead plant material) onto the terrestrial landscape. In this way, aquatic ecosystems subsidize terrestrial ecosystems, and vice versa.

When scientists think of subsidies, they usually think about organisms that travel between ecosystems or events that blur boundaries. Many aquatic organisms, however, are homebodies: they stay put in the water for their entire lives. Freshwater mussels, for example, are an important, yet endangered, inhabitant of the aquatic world. Like white blood cells removing invaders from the bloodstream, mussels filter sediments and contaminants from stream water.

We know these organisms offer benefits to aquatic ecosystems, but do they have anything to offer to the terrestrial world? A group of University of Oklahoma scientists aimed to examine whether mussels could provide subsidies to terrestrial systems with plants acting as intermediaries.

So what’s the connection between mussels and plants?

Mussel excrement contains nitrogen and phosphorus. Plants take up these nutrients from the environment, and those with higher levels of nitrogen and phosphorus tend to be higher quality food sources for herbivores. If these herbivores are terrestrial, nutrients provided by aquatic mussels to plants may be transferred through herbivores  throughout terrestrial ecosystems.

The American water willow, J. americana, grows along stream banks. It is understudied but widespread in the U.S., as it is often used in habitat restoration (Lopez, pers. comm). It has a surprisingly tight-knit relationship with mussels. In addition to thriving off of the nutrients provided by these shellfish, the plant stabilizes streambed sediments, creating a habitat where aggregations of mussels can settle and thrive.

The researchers hypothesized that high densities of mussels would result in higher amounts of nitrogen and phosphorus in the environment and in J. americana tissues, as well as increased growth of J. americana.

To test these hypotheses, the team used mesocosms, or experimental setups that mimic the natural conditions of a stream. The researchers collected J. americana plants and individuals representing a couple of mussel species from a local river. J. americana was planted in each mesocosm. A variety of measured densities of mussels were introduced to different mesocosms, and some mesocosms were set up without any mussels at all.

After being given some time to acclimate to their new homes, inhabitants of the mesocosms were allowed 9 weeks to live, eat, excrete, photosynthesize, and grow.

Before and after the experiment, the researchers measured the amount of nitrogen and phosphorus in the water of each mesocosm, and measured the wet biomass of J. americana, in order to detect changes in these variables. After the experiment, the biomass of both above and belowground parts of J. americana plants were measured separately to note whether there were differences within where growth occurred. Additionally, nitrogen and phosphorus concentrations within the plant tissues were measured.

More mussels may result in more nutritious plants

Mesocosms with higher mussel density tended to contain plants with a higher biomass and higher tissues levels of nitrogen and phosphorus. Additionally, higher densities of mussels were associated with a higher mass of plant material. These results suggest that J. americana may take up nutrients provided by the mussels, resulting in higher growth and in more nutrient storage. These characteristics may make the plant more palatable.

Interestingly, the plants growing in mesocosms with higher densities of mussels produced more runners, or lateral roots. These structures benefit mussels by holding the sediment of their habitat in place.

Do these nutrients benefit terrestrial ecosystems?

In addition to the mesocosm experiments, wildlife cameras were used to figure out whether terrestrial herbivores were eating J. americana. According to lead author and PhD candidate Jonathan Lopez, this was both the most challenging and rewarding part of the study! The cameras were placed in trees but were lost on a few occasions due to flooding. However, according to Lopez, each of these events helped him to learn “something about what animals were eating water willow in different places. Each time I placed cameras I had to climb higher in the trees in an attempt to keep them out of the water when rain caused the river to rise. This dynamic aspect of the habitat I study makes my work challenging, but also rewarding and informative. It also means I’ve become quite good at climbing trees.”

The footage Lopez obtained revealed that large mammal herbivores, primarily in the deer family, regularly feed on J. americana. This suggests that the nutrient boost that mussels provide for water willow may be passed along to the terrestrial food web.

What’s next?

In the future, Lopez hopes to examine other pathways through which mussels impact the chemical composition of streams. Additionally, he is interested in the services J. americana provides to pollinators. Mussels and the aquatic conditions they generate may influence these services.

There is a lot of interesting work still to come, but for now, this study provides crucial insights into the interconnectivity between aquatic and terrestrial systems. The declines we are seeing in freshwater mussel populations may have more far-reaching impacts than many of us expect.