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The influence of storm subsidies on nutrient and light limitation of reservoir phytoplankton growth.
Andrews, Jeffrey*,1, Vanni, Michael1, Renwick, William1, 1 Miami University, Oxford, Ohio, USA
ABSTRACT- Phytoplankton in most lakes are nutrient-limited during summer. During storms, reservoirs erratically receive pulses of nutrients and sediment from their watersheds. These pulses may alleviate nutrient-limitation by providing nutrient subsidies and by inducing light limitation. However, the dynamics of nutrient- and light-limitation in relation to storm events have not been rigorously quantified. The goals of this study were to (1) determine the magnitude of nutrient-limitation before, during, and after storm events; (2) evaluate the time scale over which storms alleviate nutrient limitation and over which nutrient-limitation returns; and (3) determine if storms alleviate nutrient limitation by providing excess nutrients, inducing light-limitation, or both. Nutrient and light-limitation were evaluated in spring and summer 2002 with a series of algal growth bioassays that utilized varied nutrient combinations (control, N, P, N+P) and light intensities (200, 100, 50, 25 umol m2/sec) on natural phytoplankton assemblages from Acton Lake (SW Ohio, USA), a reservoir in an agricultural watershed. We found that phytoplankton were P-limited during relatively calm periods, and that storms alleviated P-limitation. During storms, phytoplankton became light-limited. In addition, during large storms phytoplankton were not P-limited even when grown at saturating light intensities, showing that storm events deliver quantities of nutrients sufficient to saturate growth. Phytoplankton became nutrient-limited within a few days after a storm subsided, and the magnitude of nutrient limitation was negatively correlated with discharge into the lake over preceding time periods. Maximal correlations occurred with discharge over the preceding 10-14 days; correlations over shorter and longer time scales were not as strong. These results suggest that phytoplankton are light-limited due to storm-induced turbidity and then shift to nutrient limitation as storm inputs subside, with severity of nutrient limitation being linked to flow over a time period equal to several phytoplankton generations.
Key words: light limitation, storm events, nutrient limitation, phytoplankton