Warming up food webs: Implications for trophic energy transfer
Our present understanding of climate warming on marine organisms is often restricted to the temperature dependence of individual species, while consequences for food web interactions and energy transfer efficiency are still unknown. The variety of diverse species responses might prevent extrapolation to the community and ecosystem level, because biotic interactions might lead to a dampening or amplification of single species effects. Here we investigate effects of increased temperature on energy (carbon) and nutritional transfer efficiency from primary producers to terminal zooplankton consumers using a natural microbial community from Espegrend Fjord, Bergen and two focal zooplankton groups: copepods and filter-feeding appendicularians. The cycling of organic matter was investigated using 13C labeled bicarbonate added to all mesocosms to follow the transfer of carbon from dissolved inorganic carbon (DIC) into phytoplankton, heterotrophic bacteria, and zooplankton. In addition to carbon flow, the biochemical (fatty acid, FA) composition as an index of seston food quality was investigated, as there is an increasing awareness of food quality as a limiting factor for consumers. The conversion of phytoplankton into mesozooplankton consumer biomass for carbon and production rates measured using combined FA-specific 13C isotope analysis (CS-SI).
Enrichment of mesocosm with 13C indicated both exchange and drawdown over time, which was strongest at increased temperature. Phytoplankton FAs were enriched after one day of label addition, whereas the signal appeared later in bacteria and zooplankton consumers and differed between temperature treatments. Taxon-specific FA markers showed that phytoplankton groups differed in their response to warming. Along with shifts in phytoplankton community composition, the FA profile of seston and zooplankton changed, suggesting that in synchrony with altering species composition food quality changes for primary consumers can be expected under climate change. CS-SI analyses indicate that carbon flow to copepods and appendicularians varied with increased temperature. While overall bulk biomass did not change with temperature, turnover rates increased at higher temperature, suggesting that climate warming largely affects carbon transfer rates. This study suggests that CO2-induced shifts alter carbon flow through the planktonic food web, which will have significant effects on trophic transfer, particle sedimentation and nutrient cycling.