Theoretical predictions on shifts in phytoplankton life-strategies along different thermal regimes
Abstract: Bloom phenology is shaped by different responses of phytoplankton functional groups to environmental changes. While freshwater diatoms generally form blooms in cold temperatures in spring and autumn, cyanobacteria are assumed to benefit from increasing temperatures. However, some cyanobacteria also thrive under cold conditions and can even blooms under ice cover. The mechanisms underlying this phenomenon are still speculative, as cyanobacteria reach their maximum growth rate at about 25-30°C. To investigate the mechanisms, I used the theoretical concept of microbe life strategies, e.g. r, K, ruderal, copiotroph, and oligotroph, characterized by maximum growth rate (r), carrying capacity (K) and minimum resource requirement (R*), of a microbe population relative to other populations in the community. Using empirical thermal performance curves, I derived theoretical r, K, and R* values for freshwater and marine phytoplankton functional groups with small/medium/large size classes under a temperature range of 0-35°C, in olig- and eutrophic environments. Then I projected shifts in their life-strategies on the temperature gradient, considering different regimes from cold (below 10°C) to extreme (above 30°C).
The results show that indeed freshwater cyanobacterial "blooms also like it cold", using the K-strategy (stress toleratoring) to hold high biomass under cold temperature regime and form summer blooms by high r and K values (r/K trade-up). Freshwater diatoms are also predicted to perform well under cold temperature regime using an r-strategy, but they have a high risk of extinction when the temperature is above 30°C. On the other hand, marine diatoms are predicted to perform relatively well in all temperatures using r- or r/K trade-up strategy. Small marine phytoplankton, like cyanobacteria and coccolithophores, are better resource competitors (smaller R*) in warm-hot and low-nutrient environments. Marine dinoflagellates keep r and K large (r/K trade-up) rather than reducing R* for nutrient competition in oligotrophic environments, while using K-strategy in eutrophic environments.
The results of this study give mechanistic insight into phytoplankton ecology and specifically on shifts in major quantities related to life-strategies of planktonic primary producers, providing an important basis for bloom predictions.
Speaker: Patch Thongthaisong
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