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Abstract
The elemental composition of phytoplankton (C:N:P stoichiometry) is a critical factor regulating nutrient cycling, primary production and energy transfer through planktonic food webs. Our understanding of the multiple direct and indirect mechanisms through which temperature controls phytoplankton stoichiometry is however incomplete, increasing uncertainty in the impacts of global warming on the biogeochemical functioning of aquatic ecosystems. Here, we use a decade-long warming experiment in outdoor freshwater ponds to investigate how temperature-driven turnover in species composition and shifts in stoichiometric traits within species through local thermal adaptation contribute to the effects of warming on seston stoichiometry. We found that experimental warming increased seston C:P and N:P ratios, while the C:N ratio was unaffected by warming. Temperature was also the dominant driver of seasonal variation in seston stoichiometry, correlating positively with both C:P and N:P ratios. The taxonomic composition of the phytoplankton community differed substantially between the warmed and ambient treatments indicating that warming resulted in differential sorting of species from the regional pool. Furthermore, taxonomic composition also changed markedly over the year within each of the warmed and ambient treatments, highlighting substantial temporal turnover in species. To investigate whether local adaptation also played an important role in shaping the effects of warming on seston stoichiometry, we isolated multiple strains of the cosmopolitan alga, Chlamydomonas reinhardtii from across the warmed and ambient mesocosms. We found that warmed isolates had higher C:P and N:P ratios, shifts that were comparable in direction and magnitude to the effects of warming on seston stoichiometry. Our results suggest that both species sorting and local adaptation are likely to play important roles in shaping the effects of warming on bulk phytoplankton stoichiometry and indicate that major shifts in aquatic biogeochemistry should be expected in a warmer world.
Highlights
The stoichiometry of carbon (C), nitrogen (N), and phosphorous (P) in phytoplankton biomass set important constraints on the biogeochemistry of aquatic ecosystems, shaping patterns of nutrient limitation (Elser et al, 2009; Bonachela et al, 2013; Alexander et al, 2015), recycling (Sterner and Elser, 2002), material transfer, and C sequestration in planktonic ecosystems (Galbraith and Martiny, 2015)
Central to this issue is a grasp of the relative importance of rapid evolution and species sorting in shaping how ecosystem-level properties respond to environmental change (Lomas et al, 2014)
We tackled this issue by assessing the extent to which the effects of long-term experimental warming and seasonal changes in temperature on the C:N:P stoichiometry of phytoplankton in pond mesocosms reflected temperature-dependent changes in the composition of the communities vs. evolutionary shifts in stoichiometric traits within component species
Summary
The stoichiometry of carbon (C), nitrogen (N), and phosphorous (P) in phytoplankton biomass set important constraints on the biogeochemistry of aquatic ecosystems, shaping patterns of nutrient limitation (Elser et al, 2009; Bonachela et al, 2013; Alexander et al, 2015), recycling (Sterner and Elser, 2002), material transfer, and C sequestration in planktonic ecosystems (Galbraith and Martiny, 2015). It is widely recognized that the C:N:P stoichiometry of phytoplankton is highly variable across multiple spatial, temporal and organizational scales (Geider and La Roche, 2002) Such variation has been linked to directional changes in abiotic factors (e.g., light, CO2 and nutrients), with temperature often cited as a key determinant of phytoplankton stoichiometry (Woods et al, 2003; Hessen, 2005; Martiny et al, 2013; Toseland et al, 2013; YvonDurocher et al, 2015b). The “temperature-dependent physiology” hypothesis predicts that organisms growing at higher temperatures should have higher N:P ratios because they require fewer P-rich ribosomes, relative to N-rich proteins, to sustain growth and maintenance (Woods et al, 2003; Toseland et al, 2013; Yvon-Durocher et al, 2015b). The current weight of evidence indicates that physiological plasticity in response to rapid changes in temperature can cause substantial shifts in algal stoichiometry within species that are at least as large as those observed across species, irrespective of the underlying molecular and biochemical mechanisms
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