Resource Enhancement by Indirect Effects of Grazers: Armored Catfish, Algae, and Sediment

Armored catfish (Loricariidae) are the major grazers of attached algae in pools of the Rio Frijoles, Panama (9°9' N, 79°44' W). In the dry season, sunny pools were inhabited by °6 individuals loricariids per square metre of grazeable substratum. At these densities, armored catfish depleted algae and cleared sediment from bedrock substrata, leaving sparse standing crops of small, adnate diatoms (primarily Achnanthes spp.). To study the effects of armored catfish at 1/6 their natural density, I stocked four 6—7 cm (SL), 10—g Ancistrus (the most common size class of the most common species in stream pools) in each of five stream pens. Pens enclosed 4 m 2 of bedrock substratum, and were alternately stocked or left empty during three consecutive periods of 29, 11, and 11 d. At the end of each period, standing crops of sediment and attached algae, and rates of photosynthesis by attached algae, were measured. The attached algae that developed with sparse Ancistrus had higher standing corps with larger cells or colonies, and higher primary productivity, than did periphyton subjected to heavy grazing by unconfined armored catfish. Even heavy grazing, however, was less deleterious to attached algae than prolonged sedimentation on substrata in enclosures left empty for 11 or 29 d. The net effect of Ancistrus on their algal food changed from depletion at high grazer densities to enhancement at low grazer densities, as sedimentation became more limiting to algae than grazing.