Heather McNair Asian Clams in Lake Tahoe Spring 2010 1Asian Clams Corbicula flumina contribution to algal growth in Lake Tahoe Heather McNair ABSTRACT Over the past 80 years Asian Clams, Corbicula flumina, have extended their reach from Asia; spreading first to Oceania then across the Pacific to the Americas then recently to Lake Tahoe. During the summer of 2008 a filamentous algae was observed growing over many of the clam beds in Lake Tahoe. This study seeks to better understand how clam excrement affects the growth of algae; specifically do Asian Clams facilitate algal growth in Lake Tahoe? Three data sets helped explore this question: field chlorophyll, a lab experiment, and theoretical nutrients levels expelled by clams. Continuous chlorophyll measurements taken in the field showed the affect of a storm on the distribution of chlorophyll. To test the affect of clam excrement on algal growth we used three different concentrations, 0%, 2% and 50%, of clam excrement and a control. Carbon and Nitrogen, measures of biomass, were highest in the 50% treatment, lower in the 2% and 0% and at a constant minimum in the control, until the last day of the experiment. Results of the lab experiment were not significant because there were no replicates. Average clam densities and known clam excrement nutrient outputs suggest high theoretical levels of phosphorus and nitrogen in the water in close proximity to clam beds. The results of this study suggest that Asian Clams do facilitate the growth of algae; more growth in the lab with more clam excrement, and high theoretical levels of P and N that could support algal growth. KEYWORDS nutrients, oligotrophic, chlorophyll, Redfield ratio, clarityHeather McNair Asian Clams in Lake Tahoe Spring 2010 2INTRODUCTION Invasive species disrupt food webs and alter nutrient flows of the native ecosystems they invade (Klerks et al. 1996; MaMahon 2002). In North American freshwater ecosystems, aquatic invasive species are one of the leading causes of high extinction rates of native aquatic fauna (Dextrase and Mandrak 2006). Some examples of this are the wide spread colonization of the Great Lakes and other inland lakes by Rainbow smelt and the quick spread of Zebra mussel in North American waterways. Both of these organisms have been instrumental in the extinction of one or more native species and are considered substantial threats to a number of endangered species (Dextrase and Mandrak 2006). Successful invasive species usually have r-selected life styles. In the case of invasive bivalve species this translates to high filtration and assimilation rates, and high fecundity (McMahon 2002). Asian clams have become a widespread successful invasive bivalve species in North America (McMahon 2002). Aside from competition concerns Asian clams affect ecosystems by altering nutrient cycles. Through the deposition of feces and psedufeces Asian clams release nitrogen and phosphorus to the substrate and waters that they invade (Klerks et al. 1996; Whittmann et al. 2009). It is for this reason that Asian Clams are considered a threat for the oligotrophic (nutrient poor) Lake Tahoe (Whittmann et al. 2009). Lake Tahoe is a high altitude, freshwater lake, nestled in the Sierra Nevada Mountains between California and Nevada (Goldman 1988). Deep blue water and intense clarity are unique trademarks of Lake Tahoe. Over the past 70 years as anthropogenic activity increased on and around the lake, clarity has decreased from 70 m to about 30 m depth (State of the Lake 2009). Increased anthropogenic activity has caused a flux of nutrients and particulates to be released into the Lake. The flux of particulates and nutrients reduces clarity by increasing suspended sediment loads and fertilizing algae (Schuster and Grismer 2004). Many conservation efforts around the lake, such as storm water treatment, center on the idea of ‘Keep Tahoe Blue’. One of the reasons why Lake Tahoe is blue is because algal growth is limited by very low nitrogen and phosphorus concentrations in the water (Goldman 1988). One of the most recent threats to Lake Tahoe’s clarity is the recent invasion of the freshwater bivalve Asian clam, Corbicula fluminea, because of their potential to add limiting nutrients to the water (Whittmann et al. 2009). Over the past 80 years Asian clams have extended their reach from Asia; spreading first to Oceania then across the Pacific to the Americas (Sousa 2008). After a history of invasion in North America, Asian clams successfully established in Lake Tahoe. Large beds of Asian clamsHeather McNair Asian Clams in Lake Tahoe Spring 2010 3were first found in Lake Tahoe in 2008 (Whittmann et al. 2009). The invasion is focused on the southeastern shore of Lake Tahoe (figure 1), where large area, high-density, beds of clams are found in 3-5m of water (Whittmann et al. 2009). The potential impacts that Asian clam could have on Lake Tahoe include, outcompeting other benthic organisms, decline of plankton communities through high filtration rates, declining the aesthetics of recreational beaches when dead clams and algae wash ashore, and degradation of water clarity (Whittmann et al. 2009). Some of these impacts have occurred; clam matter has been washed up on some of the southeastern beaches of Lake Tahoe, and water clarity has been impacted with an increase in algal growth near the clam beds. Figure 1. Occurrence, distribution and densities of Asian Clams in Lake Tahoe. During the summer of 2008 a filamentous algae, primarily Zygnema sp., was observed growing over many of the clam beds in Lake Tahoe (Whittmann et al. 2009). The unattached Zygnema sp. floats above the clam beds during warmer summer months and dies as the watersHeather McNair Asian Clams in Lake Tahoe Spring 2010 4cool during the fall (Whittmann et al. 2009). Algal growth in many lakes shows temporal growth with highest production rates in the warmer summer months and lower production in the winter months (Nozaki et al. 2003; State of the Lake 2009). The occurrence of algae in close proximity to Asian clam beds in Lake Tahoe shows a seasonal pattern, however because the algal growth is localized perhaps there is a localized source of nutrients (Whittmann et al. 2009). This study seeks to answer the question; do Asian Clams facilitate algal growth in Lake Tahoe? In order to answer this question I first looked at a record of chlorophyll a in the field to