Interactions between Herbivorous Fishes and Limiting Nutrients in a Tropical Stream Ecosystem

Abstract

Ecologists have long been interested in understanding the strengths of consumer and resource limitation in influencing communities. Here we ask three questions concerning the relative importance of nutrients and grazing fishes to primary producers of a tropical Andean stream: (1) Are stream algae nutrient limited? (2) Are top-down and bottom-up forces of dual importance in limiting primary producers? (3) Do grazing fishes modulate the degree of resource limitation?We obtained several lines of evidence suggesting that Andean stream algae are nitrogen limited. Addition of nitrogen in flow-through channels resulted in major increases in algal standing crop, whereas there were no measurable effects of phosphorus enrichment. Interestingly, the N 2 -fixing cyanobacteria Anabaena was one of the taxa that responded most dramatically to the addition of nitrogen. Moreover, nutrient uptake rates were significantly higher for inorganic nitrogen (NO 3 -N and NH 4 -N) compared to phosphorus (PO 4 -P).Nutrients and the presence of grazing fishes were manipulated simultaneously in a series of experiments by using nutrient-diffusing substrates in fish exclusions vs. open cages accessible to the natural fish assemblage. We observed strong effects of both nitrogen addition and consumers on algal standing crop, although consumer limitation was found to be of considerably greater magnitude than resource limitation in influencing algal biomass and composition. Finally, the degree of resource limitation varied as a consequence of grazing fishes. Experiments examining nutrient limitation in the presence and absence of fishes showed that the response to nitrogen enrichment was significantly greater on substrates accessible to natural fish assemblages compared to substrates where grazing fishes were excluded. These experiments demonstrate simultaneous and interactive effects of topdown and bottom-up factors in limiting primary producers of tropical Andean streams. Whereas other studies have shown that consumers affect nutrient supply in ecosystems, our findings suggest that consumers can play an important role in influencing nutrient demand.

Effects of behavioral and morphological plasticity on risk of predation in a Neotropical tadpole

“…Marginally higher ammonium concentrations in the predator treatment support the subsidy hypothesis. The positive relationship between algal biomass and ammonium concentration was expected in this nitrogen-limited system, and algal biomass on cobbles in the river (11.8 mg/m 2 ; Flecker et al 2002) is higher than observed in the control treatment but comparable to the predator treatment.…”

Nutrient recycling by two phosphorus-rich grazing catfish: the potential for phosphorus-limitation of fish growth

“…Fish, including Ancistrus and Chaetostoma, are the primary algal grazers in Rio Las Marı´as (Flecker 1992;Flecker et al 2002). Armored catfish are abundant in Rio Las Marı´as, as is typical for many Neotropical streams.…”

“…Armored catfish are abundant in Rio Las Marı´as, as is typical for many Neotropical streams. In addition to potentially affecting nutrient cycling, these and other epibenthic fishes strongly affect algal community structure and the degree of algal nutrient limitation via direct consumption of periphyton and sediment (Flecker 1992;Flecker 1996Flecker , 1997Flecker et al 2002;Flecker and Taylor 2004). We assessed nutrient limitation and nutrient excretion by quantifying mass balance (MB) and by directly measuring excretion rates.…”

Effects of the herbivorous minnow, southern redbelly dace (Phoxinus erythrogaster), on stream productivity and ecosystem structure

“…Effects of grazers on ecosystem structure and function in streams have been reported for a variety of organisms including insects (e.g., Wallace and Webster 1996), snails (e.g., Hill et al 1992;Sarnelle et al 1993;Vaughn et al 1993;Turner 1997), crayfishes (e.g., Gelwick 2000;Evans-White and Dodds 2003), tadpoles (e.g., Nystrom and Abjornsson 2000), and fishes (e.g., Cooper 1973;Power 1990;Matthews 1998;Flecker et al 2002). In prairie streams, which are typically more net autotrophic (i.e., production/respiration % 1) relative to densely canopied streams (Webster et al 2003), we might expect grazing organisms to have a stronger influence on ecosystem structure and function than in more heterotrophic systems because they interact directly with autotrophs (e.g., ingesting them) but only indirectly with heterotrophs.…”