The structuring role of free-floating versus submerged plants in a subtropical shallow lake

• Published in: Aquatic ecology 2003-10, Vol.37 (4), p.377-391
• Main Authors: Meerhoff, M, Mazzeo, N, Moss, B, Rodriguez-Gallego, L
• Format: Article
• Language: English
• Subjects: Aquatic plants; Community structure; Distribution patterns; Eichhornia crassipes; Fish; Floating plants; Freshwater; Habitat utilization; Microhabitats; Phytoplankton; Pisces; Plankton; Planting density; Potamogeton pectinatus; Submerged plants; Suspended solids; Tropical lakes; Water chemistry; Zooplankton
• ISSN: 1386-2588; 1573-5125
• DOI: 10.1023/B:AECO.0000007041.57843.0b

In shallow temperate lakes many ecological processes depend on submerged macrophytes. In subtropical and tropical lakes, free-floating macrophytes may be equally or more important. We tested the hypothesis that different macrophyte growth forms would be linked with different bottom-up and top-down mechanisms in out-competing phytoplankton. We compared experimentally the effects of submerged and free-floating plants on water chemistry, phytoplankton biomass, zooplankton and fish community structure in a shallow hypertrophic lake (Lake Rodó, 34°55'S 56°10'W, Uruguay). Except for the retention of suspended solids, we found no other significant bottom-up process connected with either Eichhornia crassipes or Potamogeton pectinatus. Free-floating plants had a lower abundance of medium-sized zooplankton than any other microhabitat and submerged plants were apparently preferred by microcrustaceans. Fish showed a differential habitat use according to species, size-class and feeding habits. Dominant omnivore-planktivores, particularly the smallest size classes, preferred submerged plants. In contrast, omnivore-piscivores were significantly associated with free-floating plants. The density of omnivorous-planktivorous fish, by size class, significantly explained the distribution of medium-sized zooplankton, the high number of size 0 fish being the main factor. The abiotic environment and the structure of the zooplankton community explained little of the fish distribution pattern. Our results suggest that bottom-up effects of free-floating plants are weak when cover is low or intermediate. Top-down effects are complex, as effects on zooplankton and fish communities seem contradictory. The low piscivores:planktivores ratio in all microhabitats suggests, however, that cascading effects on phytoplankton through free-floating plant impacts on piscivorous fish are unlikely to be strong.[PUBLICATION ABSTRACT]