Nutrient cycling in lakes

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My research program concerns microbial food webs and nutrient cycles, and in this grant cycle my lab will target four areas. The first area concerns dissolved organic phosphorus (DOP) in lake water. This is the largest dissolved fraction of this limiting nutrient, but our recent work demonstrates that it is mostly derived during the filtration step of sample preparation, probably by cell breakage. Our hypothesis is that truly dissolved organic P is also derived from cell breakage, but due to natural mechanisms such as viral lysis and grazing. To complete this line of research we will test the hypotheses that the high molecular weight fraction of DOP is mostly nucleic acids, and that these become PO4 via low molecular weight DOP that is mostly nucleotides and other substrates available to plankton via cell-membrane associated phosphatases. The second line of research follows from our development of a radiobioassay that can estimate the concentration of PO4, the most important nutrient limiting the growth of algae in lake water. We will use this bioassay in both empirical and experimental approaches to test hypotheses concerning the role of light and food web interactions in determining the concentration of PO4 in P-limited lakes. Our third focus will be the whole-lake P cycle, where we will test a hypothesis arising from our recent review of the literature on TP and sedimentation in stratified lakes. The hypothesis is that P sedimentation increases allometrically with TP such that within an intermediate (mesotrophic) range, lakes resist increases in TP concentration, and during stratification sedimentation moves TP towards a value in the oligotrophic range. Finally, we will shift some of our effort to looking at NH4 in P-limited lakes. While experimental evidence suggests that only P-loading can increase the biomass of such lakes, and N-cycling has received relatively little attention, NH4 is also in low concentration, difficult to measure, and turning over rapidly. Its concentration, along with that of PO4, is also an important determinant of the nutrient environment of phytoplankton and needs to be considered to understand species composition of plankton.