Climate change-induced effects on food web structure in an alpine lake: Terrestrial DOM subsidies alter consumer:producer biomass ratios in unexpected ways

Background/Question/Methods

Two primary mechanisms through which climate change influences aquatic ecosystems include direct temperature-driven effects and indirect precipitation-driven effects. One of the primary precipitation-driven effects involves increasing inputs of terrestrially-derived dissolved and particulate resource subsidies including dissolved organic matter (DOM). These two mechanisms of climate change may alter the balance of consumer: producer biomass ratios of an ecosystem in fundamentally different ways. A 21 d microcosm experiment examined the effects of climate-driven changes in terrestrially-derived DOM on consumer: producer biomass ratios in the warmer surface mixed stratum and the cooler, deeper stratum of a low nutrient alpine lake. Based on previous evidence suggesting stimulation of consumers through the microbial loop, and reduced reliance of consumers on terrestrial-based DOM in the cooler deeper waters of lakes, we hypothesized that 1. DOM additions would enhance consumer biomass and increase grazing pressure on phytoplankton producers, increasing the consumer: producer biomass ratio and creating a more strongly inverted trophic biomass pyramid and 2. The variable responses of consumers to terrestrial-based DOM additions at different depths in the water column will result in differing consumer: producer biomass ratio responses between the surface and deeper strata. 

Results/Conclusions

In contrast to predictions, DOM additions resulted in relatively stable consumer: producer biomass ratios, while in the absence of DOM additions, consumer: producer biomass ratios showed strong increases with substantial increases in consumer control, and more strongly inverted trophic biomass pyramids. Consumers responded similarly to DOM additions in the two strata and thus consumer: producer biomass ratio response to DOM did not differ among depth strata. Consumer biomass was greater in the warmer surface stratum than in the deeper stratum in the presence and absence of DOM, while producer biomass did not differ between strata. Consumers were more susceptible to temperature limitation than producers, independent of DOM inputs, while producers were more responsive to DOM inputs. Additionally the effects of DOM on consumer: producer biomass ratios appear to be mediated more by nutrients than by fixed carbon and consumer stimulation through the microbial loop. Climate-related increases in DOM in alpine lakes will likely increase resource rather than consumer control of food webs. By their differential effects on consumers vs. producers, indirect DOM-driven resource subsidies and direct temperature-driven mechanisms of climate change may have strongly contrasting effects on the structure and function of alpine lake food webs.