project VIII
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project VIII

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project VIII: the effect of varying DOC concentration on zooplankton community dynamics in freshwater lakes

Dissolved organic carbon is an important integrator in aquatic ecosystem because it is affected by a multitude of stressors and in turn influences almost all chemical and physical aspects of these systems. As the climate warms, and drought becomes more common, DOC may decline in response to reduced DOC export from catchments and increased in-lake photolysis given longer hydraulic retention times (Schindler et al. 1996, Schindler 1998). Furthermore, decreasing pH accelerates photodegradation of DOC because the photodegradation mechanism is highly pH-dependent (Gennings et al. 2001). The combined effect of these stressors is a reduction in DOC within temperate freshwater lakes.

While much is known about DOC-mediated chemical and physical effects, there is less information about how the biological community and food web interactions, especially for higher trophic levels, will respond to changes in DOC. Physically, DOC can indirectly affect the biological community by altering the light and thermal regimes of lakes. Water clarity is an important factor affecting the vertical distribution of zooplankton, and hence predator-prey interactions (Dodson 1990, Wissel et al. 2003a, b). As well, the penetration of UVR can influence trophic dynamics in low DOC transparent lakes by eliminating sensitive species (Bothwell et al. 1994, Persaud and Yan 2003). Photochemical degradation of DOC by UVR can also fuel bacterial activity and recycling via the microbial loop (de Lange et al. 2003). Differences in thermal regimes among lakes due to variation in DOC can influence the abundance and distribution of zooplankton (Wissel and Ramcharan 2003). Although warmer surface temperatures offer a distinct demographic advantage to zooplankton species because egg development and population growth rate is dependent on water temperature (Orcutt and Porter 1983), the effect may be variable among species (Persaud and Williamson 2005).

Chemically, DOC can affect the biological community indirectly through its influence on lake pH, oxygen concentration and complexation with nutrients and trace metals. Since DOC is largely a composite of weak organic acids it can depress the pH of freshwater lakes. However, despite these acid imparting properties DOC, can also act as a buffer against strong acids that can enter lakes via precipitation (Wetzel 2001). Dissolved organic carbon can indirectly alter the availability of trace metals (such as mercury, aluminium, iron and lead) to aquatic biota. At the same time other DOC-mediated physical and chemical characteristics such as temperature, oxygen and UVR can alter the effect of toxic metals on biota (Boening 2000, Peuranen et al. 2003, Preston et al. 1999). Mercury molecules readily form complexes with DOC; consequently lakes with a high influx rate of DOC will receive a greater quantity of mercury (Mierle and Ingram 1991, Watras et al. 1998). Several studies have shown that DOC is positively correlated with mercury concentrations in zooplankton species (Watras et al. 1998).

Since the importance of DOC as a carbon source in freshwater food webs became evident, research has been primarily focussed on the effects of DOC concentration and source (allochthonous vs. autochthonous) on lower trophic levels (phytoplankton and bacteria). Through a comparison between clear and coloured lakes, Nürnberg and Shaw (1999) reported that coloured lakes have higher overall production with higher bacteria biomass and productivity. Jansson et al. (2000) showed that across a DOC gradient, bacteria production increases with DOC, exceeding autotrophic productivity in lakes with DOC concentrations greater than 10 mgL-1.

Stable isotope studies have also explored how zooplankton carbon changes along a DOC gradient and attempted to determine point sources. France (1997) reported that zooplankton ?13C signatures were lower in humic and dystrophic lakes and attributed this to heterotrophic activity. Contrary to France (1997), Karlsson et al. (2003) reported that zooplankton ?13C signatures reflected a conservative use of DOC mobilized by bacterioplankton and phytoplankton. Differences in zooplankton feeding can lead to distinct differences in zooplankton ?13C and ?15N signatures (Matthews and Mazumder 2003), it is therefore imperative that further investigations are done to examine differences among genera and taxonomic groups along a DOC gradient.

The intent of the project is to firstly investigate how direct DOC effects on the algal and microbial communities affect the meso-zooplankton community. Carbon and nitrogen stable isotope analyses of DOM, POM and zooplankton, and feeding experiments will be used to test specific hypotheses. Secondly, it will be determined if and how predation by macro-zooplankton varies along a DOC gradient from high DOC lakes to low DOC lakes. A total of 15 lakes with a wide range of DOC levels in the Dorset area, Central Ontario will be used for this study. All of the lakes are located on the Precambrian Shield.