Effects of the zebra mussel, an exotic freshwater species, on seston stoichiometry

We examined the effect of the zebra mussel, Dreissena polymorpha, an exotic species, on seston stoichiometry by conducting laboratory experiments in which we varied nutrient composition of seston and mussels over time. Zebra mussels altered the stoichiometry of seston through removal of particulate organic nutrients and changed the stoichiometry of the dissolved nutrient pool through nutrient excretion. Grazers had stronger effects on carbon : phosphorus (C : P) and nitrogen (N) : P ratios than on the C : N ratio of seston. Elemental residence time in tissue and high mass-specific nutrient excretion by small mussels caused small mussels to be more efficient nutrient recyclers than larger mussels. Zebra mussels reduced P availability through enhancing C : P and N : P molar ratios of seston during the period extending from June to August, when P was limited in the lake, and increased the C : N molar ratio of seston in June, when N was at the minimum level in the lake. C : P and N : P molar ratios for zebra mussel tissue were higher in August and somewhat in September than in all other months. N was retained more efficiently than P in Dreissena tissue. Nutrient mass-specific uptake rate was higher than excretion rate by zebra mussels. Ecologists are increasingly interested in understanding the balance of multiple chemical elements in ecological interactions using the ecological stoichiometry approach (Elser et al. 1996, 2000a; Hessen 1997). This is because stoichiometric imbalances between resources and consumers have been found to have implications both for consumer growth and for nutrient recycling in food webs and may constrain or change key ecosystem processes (Sterner and Hessen 1994; Elser and Urabe 1999; Sterner and Elser 2002). Measuring ecological stoichiometry has emerged as a powerful tool for understanding mechanisms and processes important for the structure, biodiversity, and function of aquatic communities. It has been applied to address important questions in ecology, including those related to population dynamics (Andersen et al. 2004), trophic interactions (Elser et al. 2000b), nutrient cycling, and food web dynamics (Elser and Hassett 1994; Elser and Urabe 1999; Sterner and Elser 2002). Aquatic exotic species pose a serious threat to the economy, ecology, and biodiversity of aquatic ecosystems, in which their effects are difficult to invert or mitigate. The zebra mussel, Dreissena polymorpha, is an exotic species capable of altering community structure and ecosystem function of lakes through a large grazing capacity and selective feeding (MacIsaac 1996; Naddafi et al. 2007b). In addition, Dreissena may have large ecosystem-level outcomes as a result of its ability to cycle phosphorus and/or nitrogen (Mellina et al. 1995; Arnott and Vanni 1996; Conroy et al. 2005). Zebra mussels filter a large but variable size range (0.7 mm–1.2 mm) of particles (Naddafi et al. 2007b) from the water column. Phosphorus (P), nitrogen (N), and carbon (C) associated with seston are entered into mussels’ inhalant siphon and ingested. The ingested nutrients are either assimilated or egested. Egested nutrients deposited as feces may become available to primary producers through mixing or microbial decomposition. Assimilated nutrients are either sequestered at relatively constant concentrations in mussels’ tissues and used for growth and reproduction (mussels as a nutrient sink) or excreted in dissolved inorganic form (mussels as a nutrient source), which can provide a substantial proportion of the nutrient demands of primary producers. In addition, the phytoplankton that are rejected in pseudofeces (Naddafi et al. 2007b) have higher access to nutrients because of lower remaining phytoplankton biomass. Consequently, zebra mussels may change resource supply to the prey through nutrient excretion and may enhance resource availability. N and P can limit phytoplankton growth and the abundance of other primary producers. Further, the N and P ratio is an important aspect of resource-related indirect effects of consumers, and, thus, the ratio at which zebra mussels excrete N and P will determine the relative degree of N and P limitation, which in turn could affect algal composition (Smith 1983; Elser and Hassett 1994; Arnott and Vanni 1996). Evidence indicates that zebra mussels may shift the nutrient regimes in lakes. For example, both ammonia and nitrate have increased in the Acknowledgments We thank Helena Enderskog, Olga Matzen, Jan Johansson, and Björn Mattsson for assistance in laboratory and fieldwork. We also greatly appreciate the comments of two anonymous reviewers, which improved this manuscript considerably. Funding for this research was provided by the Malméns and the Olsson–Borgh foundations as well as Erken laboratory (R.N.) and The Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (P.E.). Limnol. Oceanogr., 53(5), 2008, 1973–1987 E 2008, by the American Society of Limnology and Oceanography, Inc.