In Lake Tahoe

The famous clear waters of Lake Tahoe. Source

History

The Asian clam made its first appearance in SE Lake Tahoe in 2002 (1). In only a few years the population has boomed, and now densities of over 1500 clams/m² are consistently reported (1). Lake Tahoe and Lake Constance, in Germany, now have the highest concentrations of Asian clams in the world (7). Sadly, the Asian clam is not the first species to be unintentionally introduced to this once-pristine environment.

Currents allow microscopic embryos that clam release to spread to different regions of the lake (7). As Lake Tahoe is a federally protected and delicate ecosystem, the application of pesticides to these clams is strictly forbidden; instead, new mechanisms must be used to control their populations (9). Carp, bluegill, striped bass, blue catfish have been found to eat the clam, however none of those found in Lake Tahoe (5).

A recent history of Lake Tahoe's aquatic invasive species. Source: TERC

Ecological Impacts and Change

Lake Tahoe is considered an Outstanding Natural Resource Water by California and Nevada, which share the lake (10). Tahoe, an oligotrophic lake, owes its crystal-clear water to its depth, small watershed, and low nutrient levels. (11) Over the last 50 years, water clarity has declined up to 30%, while primary productivity has increased annually by 5%, and mean temperatures have also increased (10,12). An increase in urban runoff, which brings nutrients like nitrogen and phosphorous into the lake ecosystem via fine sediment, are the main causes of decreased water clarity (12). This fine sediment scatters light, which combined with the nutrients promotes algal growth (10). Algal growth, in turn, causes hypoxia, which can kill oxygen-demanding organisms. They also decrease subsurface light, disrupting benthic macrophytes and algal growth (13).

All of these changes support a different trophic landscape than used to dominate the Lake Tahoe ecosystem (13). For example, Caires et. Al found that benthic invertebrates were down 75-100% since the 1960s. This disturbance regime has led to the success of many invasive species, such as Corbicula fluminea.

The Asian clam has a devastating effect on Lake Tahoe’s ecology. Their presence alters cycling of organic matter in sediments, shifts benthic biodiversity, outcompetes native bivalves, and leads to less phytoplankton abundance (14). They deposit calcium into the lake, too, which facilitates growth of shellfish such as zebra or quagga mussels, who are risking invasion into Lake Tahoe. These impacts are shown 

in the figure below.

A simplified view of Asian Clam ecosystem dynamics in Lake Tahoe.