Causes of Depth Zonation in Emergent and Floating Aquatic Plants

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Emergent, floating, and submersed aquatic plants show a characteristic but unexplained zonation by depth in lakes throughout the world. The PI proposes to test a new hypothesis to account for this pattern in terms of competition, as influenced by differences between growth forms in leaf height, allocation to relatively unproductive support tissue, and ability to maintain a positive whole-plant carbon balance at different depths. This hypothesis accounts for several trends (including some recently documented) in zonation as a function of environmental conditions and plant characteristics. Field manipulations are proposed to determine: Whether emergents competitively exclude floating species from shallow water (based on removal experiments); Whether emergent and floating species are near net annual growth at their maximum depths (based on measurements on cutting transplanted to a variety of depths); Whether the observed pattern of decline in net growth with depth is attributed to increased allocation to support tissue (based on studies of leaf demography, the allometry of support tissue, and tissue construction costs as a function of depth); Whether dense beds of submersed plants limit the maximum depth of water in mesotrophic and eutrophic lakes (based on removal experiments); and Whether large-leaved waterlilies (e.g., Nymphaea) competitively exclude smaller-leaved floating species (e.g., Brasenia, Potamogeton) from shallow water via aggressive competition ( mechanical abrasion) or leaf overriding (based on removal experiments and analysis of leaf overlap and leaf demography where the two groups co-occur). These experiments would be conducted in a representative subset (Helen Lake, Wind Pudding Lake, Wildcat Lake, Beautiful Lake) of a 10-lake fertility gradient that is currently the subject of comparative studies.