A nutrient is defined as a chemical element necessary for life. In streams, phosphorus is typically one of the most important nutrients and often limits microbial (algae, bacteria, and fungi) growth. As a result, retention of phosphorus within streams largely determines productivity. Factors that influence retention include temperature (Elwood et al. 1981b), velocity (Bencala 1983), and organic matter (Mulholland et al. 1984). Watershed input-output budgets have been commonly used to evaluate nutrient retention characteristics (Borman et al. 1974). These studies provide information about nutrient flux through ecosystems but offer little information about mechanisms governing nutrient dynamics. In contrast, nutrient spiralling, as described by Webster and Patten (1979), provides a method to evaluate retention and the mechanisms governing it. A nutrient spiral is defined as the distance traveled by a nutrient ion as it completes one cycle from dissolved form to particulate form and back to dissolved form. The distance a nutrient ion travels in dissolved form is called the uptake length and typically accounts for > 90% of spiralling length (Newbold et al. 1983). Uptake length is commonly used instead of spiralling length, because unlike spiralling length, uptake length can be measured without the use of radiotracers. Nutrient spiralling, developed in the late 70's and early 80's, is a relatively new concept. Work on spiralling length (or uptake length) has just begun to allude to possible mechanisms of solute retention and the relative importance of these mechanisms (see Solute Working Group 1990 for a review of concepts and methodology). Recent nutrient retention studies have shown phosphorus retention to be affected by both physical (e.g. temperature, velocity) and biological (e.g. microbial activity, organic matter biomass) factors. However, these studies have yielded conflicting information as to the relative importance of these factors. For example, Gregory (1978) and Elwood et al. (1981) demonstrated that uptake was mostly biotic, while Meyer (1979) found that uptake was determined by physical factors in the streams she studied. This contradiction suggests that streams may range from those driven primarily by biological mechanisms to streams driven almost entirely by physical factors with most streams falling somewhere between these extremes. The relative importance of physical and biological factors may vary spatially and temporally within a stream. This study was designed to systematically identify and examine factors that influence nutrient retention. More specifically, the objectives of this study were: 1) Examine microbial colonization and breakdown characteristics of leaves with different amounts of structural rigidity, under different constrainment techniques, to gain insight into how these characteristics may affect nutrient retention. 2) Use artificial streams to separate and identify factors governing nutrient retention by controlling flow and using different amounts and types of leaf material. 3) Evaluate how land-use practices may alter phosphorus retention mechanisms by comparing results of nutrient releases in natural streams draining undisturbed mixed hardwood watersheds with releases in streams draining disturbed watersheds (i.e. watersheds that had been logged and planted in white pine). / Ph. D.