The MCM-LTER project documented how even small variations in climate can drive major changes in polar ecosystems, showing that seemingly slight changes in temperature in the McMurdo Dry Valleys can set off a cascade of responses that affect stream flow, nutrient cycling, and biodiversity. Thus, this research has contributed to the recognition that ecosystem responses to climate change are not necessarily gradual, especially in low diversity ecosystems where harsh conditions dominate. Our studies have shown the influences of a decadal cooling trend and intense seasonal warming events. Based on these observations we predict that the ecological impacts of sustained warming will be mediated by changes in stream flow and increased wind-driven transport of sediment. The dry valley soil ecosystems present one example of an amplified response to these climate changes.  In soils there are only a few soil nematode species, compared to the hundreds in a typical soil sample. As a result, episodic increases or decreases in soil moisture cause a large change in ecosystem activity because of the sensitivity of the dominant nematode species, rather than causing other nematode species to become more active as in typical soils. Overall, the anticipated climate transitions in the Dry Valleys provide an excellent opportunity to understand and predict how “extreme” ecosystems may change in the future.

Another major project outcome was discovering how the dry valley lakes function during the winter, when there is no light for photosynthesis.  These permanently ice-covered lakes can be considered as oases for life in this cold desert because they are some of the few habitats on the Antarctic continent that contain year-round liquid water. All the microbial interactions in these lakes rely to some extent on phytoplankton photosynthesis during the short summer. Because logistical constraints do not allow sampling of these lakes routinely during the long dark winter, little was known about the fate of the microbes in winter. The first studies on the summer-winter transition showed that phytoplankton primary production ceases in mid-April as sub-ice irradiance reaches a low threshold value. Despite the cessation of photosynthesis at this time, chlorophyll-a remained high and bacterioplankton productivity continued at high rates. Further, many of the phytoplankton possessed photochemical systems that resemble those of evergreen trees, in that they maintain their chlorophyll-a content through the winter despite the lack of photosynthesis. During the winter most of the new organic material resulted from carbon dioxide fixation by chemolithoautotrophic bacteria that obtain their energy from chemical energy rather than sunlight. Overall, we found that algae in Antarctic lakes remain active and retain their photosynthetic pigments in spite of the absence of light. This winter activity allows them to begin to photosynthesize as soon as the sunlight returns in the
spring.

Because of the absence of plants, the McMurdo Dry Valleys allowed us to learn about the fate of organic material produced only from microbes, e.g. algae and bacteria, representing another important project outcome.  These microbes not only grow in the water column of the lakes, but also form mats that cover the streambeds. We found that substantial amounts of dissolved organic material in lakes and streams originate from these algae and bacteria, demonstrating their role in the cycling of this important form of carbon in aquatic ecosystems worldwide. Furthermore, this microbial dissolved organic material has chemical characteristics that are distinct from those of dissolved organic material derived from leaf litter on the forest floor or from wetland plants which can give water a yellow-color. Based on these chemical differences, a s...

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