PARENT SESSION
Poster Session #12: Marine Ecology.
Monday, August 5. Presentation from 5:00 PM to 6:30 PM. Exhibit Hall B & C, TCC
125
Assessment of a trophic cascade model in a temperate seagrass system under a nitrogen gradient.
JORGENSEN, PABLO*,1, IBARRA-OBANDO, SILVIA1, 1 Centro de Investigacion Cientifica y Educacion Superior de Ensenada (CICESE}, Ensenada, 22830, Baja California, Mexico
ABSTRACT- In order to test a trophic cascade model which postulates that an increase of small fish predators control the abundance of mesoherbivores leading to algal overgrowth and seagrass loss, we assessed the effect of a naturally abundant fish, Syngnathus leptorhynchus on the growth of Zostera marina. The experiment took place in San Quintin Bay, a "Y" shape coastal lagoon, in July 2001 during upwelling conditions. We selected 3 seagrass beds representing a fertilization gradient: base Y, BY (maximum); west arm, BF (intermediate), and east arm, SQ (least). After 30 days, the growth of Z. marina leaves inside cages with 40 S. leptorhynchus.m-2 (inclusion treatment, I) was compared with the one in cages without the fish (exclusion treatment, E). Seagrass growth differed between both beds and treatments (p<0.05, n=4). At sites with high dissolved inorganic nitrogen levels, BY and BF, inclusion of the fish resulted in reduced seagrass growth (p<0.05): 1.4 (BY) and 2.9 cm.day-1.shoot-1 (BF), compared with 2.9 and 3.8 cm.day-1.shoot-1 respectively in E treatment. At SQ, dissolved nitrogen levels were significantly lower and the inclusion of the fish did not have an effect on seagrass growth (p>0.05; I: 2.8, E: 2.3 cm.day-1.shoot-1). These results suggest that top-down control of Z. marina growth varies as a function of nutrient status. To evaluate this hypothesis we are, at present, repeating the experiment under low nutrient conditions, winter time.
KEY WORDS: plant-animal interactions, food web, top-down control, bottom-up