Phytoplankton communities and stoichiometry are interactively affected by light, nutrients, and fish

We examined the separate and interactive effects of light, nutrients, and zooplanktivorous fish on phytoplankton nutrient stoichiometry, taxonomic richness, community composition, and the associations among these variables. We manipulated light supply, nutrient supply, and zooplanktivorous fish (presence or absence), in a factorial design using mesocosms containing regional phytoplankton and zooplankton assemblages. C : N and C : P were highest under high nutrients and high light. We observed interactive effects on the relative abundance of major phytoplankton taxa. Chlorophytes were most abundant in the majority of the treatments, although cyanobacteria dominated under high light, low nutrients, and with fish present. Cryptomonads were rare in all but the low-light, high-nutrient treatment. Relative abundance of diatoms was negatively affected by light, but only in the presence of fish. Chlorophyte relative abundance (% chlorophytes) was positively correlated with seston C : P, while relative abundances of diatoms and cryptomonads were negatively correlated with C : P. Phytoplankton taxon richness was positively associated with both % chlorophytes and seston C : P, and the number of chlorophyte taxa increased disproportionately with total taxon richness. Assemblage-level stoichiometric responses may be driven not only by direct responses to resource supply ratios, but also by algal community response. In particular, factors that increase taxonomic richness and both the richness and biomass of chlorophytes also result in increased seston C : P. Our results suggest strong feedbacks between phytoplankton community composition and stoichiometry, induced by interactive effects of light, nutrients, and predators. Light, nutrients, and grazers can strongly influence phytoplankton biomass, stoichiometry, and community composition (Reynolds et al. 1993; Sterner et al. 1997; Sterner and Elser 2002). However, because phytoplankton taxa are differentially adapted to particular resource supply and grazing rates, and individual taxa exhibit tradeoffs in their abilities to use resources and deter grazers (Reynolds et al. 1993; Litchman et al. 2007; Litchman and Klausmeier 2008), community-level response to the combined effects of light, nutrients, and grazers may be complex. Phytoplankton life-history adaptations to differential resource and grazing regimes include morphological, chemical, and behavioral (e.g., motility or buoyancy regulation) attributes, and may mediate community-level responses to these factors (Litchman and Klausmeier 2008). For example, some algae deter grazers using spines or sheaths (e.g., some diatoms and chlorophytes); (Sommer 1988), or colonial growth form (Happey-Wood 1988; Paerl 1988; Sandgren 1988). Other taxa have adaptations that help them compete for resources. Fast-growing species, such as many small flagellates (e.g., some chlorophytes and cryptomonads) can quickly utilize pulses of nutrients, but are at a disadvantage under low-nutrient conditions because they require high resource levels for rapid growth (Happey-Wood 1988; Klaveness 1988). Even within a taxonomic group, algae vary in their competitive abilities. For example, Lange et al. (2011) showed a tradeoff between the response of benthic diatom species to grazing and nutrient enrichment. Species that were well-adapted to withstand grazing pressure were not able to respond as strongly to high resource levels, and vice versa. Thus, grazing and resource supply (light and nutrients) may jointly regulate phytoplankton community composition in complex ways. The stoichiometric composition of phytoplankton assemblages also may respond in a complex manner to interactive effects of light, nutrients, and grazing, and may be related to phytoplankton diversity and species composition (Dickman et al. 2006, Hall et al. 2007, Striebel et al. 2008). The light : nutrient hypothesis predicts that an increased light : phosphorus supply ratio leads to increased algal cell carbon : phosphorus (C : P) ratio (Sterner et al. 1997), and studies ranging in scale from microcosms to whole lakes support this hypothesis (Sterner and Elser 2002; Dickman et al. 2006; Striebel et al. 2008). However, because phytoplankton taxa vary in specific requirements for, and responses to, light and nutrient resources (Litchman and Klausmeier 2008), phytoplankton composition may mediate the response of seston stoichiometry to the light : nutrient ratio. Some of the variability in stoichoimetric responses of phytoplankton communities can be explained by differences in resource utilization. For example, N-fixers may exhibit higher cell N : P than non N-fixers, given their ability to fix atmospheric N2 (Healey and Hendzel 1980). Likewise, phytoplankton with higher growth rates and, therefore, high P requirements for ribosomal ribonucleic acid (RNA) may exhibit lower N : P (Sterner and Elser 2002; Klausmeier et al. 2004). Conversely, with low light supply phytoplankton require more N for light-harvesting and, therefore, would exhibit high cell N : P (Rhee and Gotham 1981). Lab monoculture studies clearly show that different species have contrasting * Corresponding author: [email protected] { nee Elizabeth M. Dickman Limnol. Oceanogr., 56(6), 2011, 1959–1975 E 2011, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2011.56.6.1959