Green Lake 4 is an alpine lake in the Silver Lake Watershed of Boulder Colorado. Many alpine lakes similar in productivity and elevation to Green Lake 4, have shown a recent shift in diatom species. Nitrogen has been shown to be increasing in alpine lake systems of the Rocky Mountain Front Range due to atmospheric deposition. Species associated with oligotrophic systems are being replaced with more common species that are tolerant of higher nutrient concentrations. However, little is known about the role phosphorous. The bedrock of the watershed in which Green Lake 4 sits is composed of apatite. When eroded, dissolved, and introduced into alpine lakes by snowmelt, apatite acts as a phosphorus surrogate. With recent warming in the alpine area of Green Lake 4 and the melting of alpine permafrost, a pulse in stream flow has been observed in the fall months of the last twenty years unrelated to any precipitation events and is likely the result of thawing permafrost. The pulse is especially significant during drought years when nutrients become more concentrated. For example, the frequent but rare diatom Fragilaria cyclopum was observed in significant abundance during the 2002 drought. This example of a short-term diatom shift, due to nutrient concentrations, highlights the role of diatoms as indicators of environmental change.

In order to further understand the effects of the pulse in stream flow and mobilization of available phosphorous, the top 5 cm of the sediment core representing approximately the last 150 years, will be analyzed and compared to a previous Green Lake 4 studies. Taxonomic counts and descriptions of the diatoms will be conducted and made available on an online database supported by the Niwot Ridge LTER. The Rocky Mountain Lake Algae Website will include diatom and soft alga descriptions and data from annual sampling and previous studies. The goal is to better facilitate taxonomic identification and encourage collaboration between the LTERs and research institutions.