Effects of long-term resource additions in arctic lake, stream, and tundra environments: Beyond NPP

Background/Question/Methods

Global change is altering nutrient inputs and nutrient cycling in many ecosystems. Experimental nutrient addition has been frequently used to better understand the nature of nutrient limitation and to aid in predicting responses to global change. Reported individually and in recent meta-analyses, these experiments often cause dramatic increases in net primary production (NPP) in the short-term, thus affecting resource availability for higher trophic levels and regional biogeochemistry. Less often described are effects on the physical and chemical environment brought about by the NPP increase. Here we review changes to the physiochemical environment caused by primary producers responding to 10+ years of nutrient addition in representative lakes, streams, and terrestrial tundra communities on the North Slope of Alaska. We examine how responses of algae and plants affect light availability, temperature, oxygen levels, and perennial structures available for consumers, and how these may accumulate and feedback to cause further changes in community and ecosystem properties over the long-term.

Results/Conclusions

For some variables, responses were similar across two of the three ecosystems. For example, in both tundra and lakes, light (at the soil surface or through the water column) was significantly reduced by the increased growth of primary producers following fertilization. In tundra, added nutrients favor a dwarf deciduous shrub that grows larger and shades the substrate. In fertilized lakes, pelagic algae bloom following fertilization, restricting light penetration through the water column to the benthos. Primary producers in tundra and streams increased the amount of perennial structure, with more wood biomass produced in the former and benthic mosses covering the substrate in the latter. These changes provided important additional habitat niches for epiphytic species and invertebrates under fertilized conditions in addition to altering food availability. Life history characteristics (particularly longevity, dispersal and detrital production) of the primary producers as well as physical characteristics of the environment affected the responses to and recovery from nutrient amendments differently across these ecosystems. As global change persists and nutrients become more readily available in the arctic landscape, these physiochemical feedbacks must be accounted for to better understand current and future changes and how they may compound each other over time.