Nutrient Limitation of Algae and Heterotrophic Bacteria in Reservoir Ecosystems: Implications for Pelagic Competition along a Tropic Gradient
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In low productivity pelagic ecosystems with low concentrations of inorganic nutrients, bacteria have been shown to play a relatively greater role in C and nutrient cycling. The importance of bacteria is thought to decline as productivity and dissolved inorganic nutrients increases. Plankton ecologists have proposed several mechanisms which lead to this pattern, but it is generally thought that bacteria should exhibit a competitive advantage over algae for inorganic nutrients in unproductive systems with relatively high concentrations of dissolved inorganic C (DOC) and low concentrations of dissolved inorganic nutrients. However, there is a limited amount of data examining if the intensity of competition between algae and bacteria for inorganic nutrients varies with ecosystem productivity. My thesis focused on examining the potential for competition between heterotrophic bacteria and autotrophic algae across a productivity gradient in a group of 19 Texas and Ohio reservoirs. Across reservoirs, DOC:dissolved inorganic nutrient ratios decreased with increasing productivity, signifying a shift in the dominant forms of available nutrients for algae and bacteria along a trophic gradient. The N and P content of algal and bacterial cells (i.e., C:N and C:P) follow a similar pattern of increasing cellular nutrient content with increasing productivity. Concurrent nutrient limitation assays indicated that algae across reservoirs were primarily limited by N or P, whereas bacteria were most frequently primarily limited by P, less frequently by C, and almost never by N. The magnitude of nutrient limitation responses (i.e., calculated response ratios, RRs) were greater overall with P addition over N or C. Both algae and bacteria exhibited heightened response ratios to P than with N or C comparatively due to relatively lower concentrations of P (when compared to N and DOC) found within unproductive systems, thus an important limiting nutrient in these reservoirs studied.