PARENT SESSION
Symposium 21: Resource pulses in space and time: Linking species, communities and ecosystems
Organized by: LH Yang, MJ Vanni, WH Nowlin , and KA Schmidt
Thursday, August 11, 1:30 PM - 5:00 PM, Meeting Room 517 C, Level 5, Palais des congrès de Montréal
Watersheds and omnivorous fish as sources of pulsed and non-pulsed nutrients for lake phytoplankton communities.
Vanni, Michael*,1, Hale, Scott2, Renwick, William1, 1 Miami University, Oxford, OH, USA2 Ohio Division of Wildlife, Hebron, OH, USA
ABSTRACT- Reservoirs are highly subsidized ecosystems that receive nutrients via both pulsed and relatively constant vectors. Reservoirs receive large, episodic, storm-driven pulses of dissolved and particulate nutrients from surrounding watersheds. In addition, pelagic phytoplankton utilize nutrients released by detritivorous fish (gizzard shad), which feed on sediment detritus and excrete sediment-bound nutrients into the water. Temporal variation in nutrient supply via fish is much less than that of watershed inputs, and is driven by variation in fish biomass and temperature, which change slowly compared to streamflow. We compared the magnitude, temporal variation, and ecosystem consequences of these two nutrient fluxes in a eutrophic reservoir at different temporal scales over a multi-year period. On an annual basis, the watershed always provided more phosphorus (P, the limiting nutrient) than did fish. Over a growing season (April-October), fish provided more P than the watershed in dry years, while in wet years the watershed provided more P. At scales more relevant to phytoplankton generation times (e.g. days), fish provided more P than the watershed during most time periods. P supply via fish greatly reduced the temporal variation of total P supply, and supported a large proportion of summer phytoplankton primary production. Nutrient pulses from watersheds are accompanied by high flows (which flush nutrients and phytoplankton from the lake) and high concentrations of suspended sediment (which reduce light intensity and hence photosynthetic rates). As a consequence, a high proportion of soluble P delivered from the watershed exits the lake without being utilized, whereas most P released by fish is rapidly utilized. Thus, nutrient use efficiency (primary production per unit P supplied) is much greater when P is supplied by fish than when P is delivered from watersheds. At long time scales (years), the two nutrient supply vectors may be synergistically related. Particulate nutrients delivered by watersheds accumulate in lake sediments, possibly providing a food source for detritivorous fish, which slowly convert these nutrients to forms available to phytoplankton. Thus, the relative importance of pulsed vs. non-pulsed nutrient supplies in reservoir ecosystems is greatly dependent on temporal scale.
Key words: detritivory, reservoir, phosphorus, gizzard shad