Zebra mussels are filter-feeding freshwater bivalves who originally hail from the Caspian Sea region in Eurasia. After being introduced to North America via ballast water exchange in the Great Lakes, they spread rapidly because of their planktonic larvae, rapid reproduction and fast growth rates. Their ability to achieve very high biomass along with their high filtering rates allow them to significantly reduce biomass and productivity of phytoplankton in some lakes and rivers, sometimes with negative consequences for other organisms that depend on this resource. In the Hudson River, for example, algal biomass after the invasion of zebra mussels is on average only 15% of that prior to the invasion. The invasion has also depleted oxygen levels, altered community structure in plankton and in the benthos and altered the paths of energy and material flow through the Hudson.

The persistence of zebra mussels and their ecosystem effects poses a conundrum. Top-down control of algal biomass by predators is typically difficult to sustain over the long-term because predators eventually deplete prey leading to starvation of the predator. The result is reestablishment of high prey abundance or alternations between periods of high prey abundance and high predator abundance. Calculations suggest that since the invasion of the Hudson River algal biomass has been persistently below the threshold needed for zebra mussels to break even energetically for nearly two-decades. The question is therefore, why do zebra mussels persist in high enough abundance to have large ecosystem effects?

One explanation is that zebra mussels are benefitting from a relatively constant external subsidy that stabilizes their population despite low amounts of particulate food. The presence of such a hidden subsidy is indicated by the fact that the amount of carbon respired by zebra mussels in the Hudson River is larger than the amount of carbon fixed by photosynthesis of rooted plants and algae in the river combined. Past work has indicated that zebra mussels, like their marine ancestors, can directly absorb dissolved amino acids from solution. Moreover, exposure to particle-free Hudson River water increased the period over which of zebra mussels could survive without particulate food by 5-fold. Dissolved organic matter in aquatic system is a relatively large and invariant pool of unused energy that is derived from the terrestrial environment. Its use would therefore constitute a terrestrial subsidy.

We have recently been funded to build on this work. These efforts will develop a more thorough accounting of the substances taken up by zebra mussels, with the aim of establishing an index of DOM availability that can be used to predict variations in metabolism of zebra mussels. We will also assess the importance of the DOM subsidy to the population as a whole, taking into account effects of temperature, physiological state and body size on filtration rate and absorption efficiency. Finally, we will use these findings to develop models that show how the terrestrial subsidy influences zebra mussel dynamics and ecosystem properties. This work will lay the foundations for predicting zebra mussel dynamics in other systems that experience different levels of terrestrial subsidies due their position in the landscape or residence times. It should also for a blueprint for studying other organisms which may also benefit from dissolved organic subsidies. This includes native and invasive bivalves and even zooplankton and some filter feeding fish.