Document: ALE-3-73-19

Interactions between grazing fish and nutrients in a tropical stream ecosystem.

FLECKER, A.S.* ¹, B.W.TAYLOR ¹, J.P.HOOD ², E.BERNHARDT ¹, W.K.CORNWELL ¹, S.R.CASSATT ¹ and M.J.VANNI ²

Cornell University, Ithaca, NY 14853 USA ¹
Miami University, Oxford, OH 45056 USA ²

Abstract:
Ecologists have long been interested in understanding the strengths of consumer and resource controls in regulating community structure and function. Here we ask three questions concerning the relative importance of nutrients and grazers in influencing algal assemblages of a tropical Andean stream: 1) Are stream algae nutrient limited?, 2) Do top-down and bottom-up controls dually influence Andean stream periphyton?, and 3) Do grazing fish modulate the degree of resource limitation? Addition of nutrients using flow-through channels and nutrient diffusing substrates revealed that algal accrual was strongly nitrogen limited, whereas phosphorus enrichment resulted in no measurable effect on periphyton. Likewise, nutrient uptake rates were significantly higher for inorganic nitrogen compared to phosphorus. Although nitrogen enrichment stimulated algal accrual, both nutrients and herbivores acted simultaneously on periphyton, and consumer control was found to be of considerably greater importance than resource control in influencing algal standing crop and composition. Finally, the relative degree of resource limitation varied as a consequence of grazing fish. Experiments examining nutrient limitation in both the presence and absence of fish showed that the response to nitrogen enrichment was significantly greater on substrates accessible to natural fish assemblages compared to substrates where grazing fish were excluded. These findings suggest simultaneous and interactive effects of top-down and bottom up factors in regulating periphyton of tropical Andean streams.

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This abstract is being presented at: 9:15 AM in session:
Oral Session #70: Aquatic Ecology.