MEPS 333:81-93 (2007)  -  doi:10.3354/meps333081

ABSTRACT: Trophic cascades usually regulate structure and dynamics of valuable benthic vegetation communities (e.g. kelp forests, salt marshes), resulting in alternating high and low abundances from predators at the highest level, down through herbivores, to basal producers at the lowest trophic level. While herbivores may have direct strong negative effects on kelp and salt marsh vegetation, grazing invertebrates (mesograzers) within seagrass meadows consume and thus control the biomass of opportunistic epiphyte algae that can outcompete seagrasses for light. Small fish predators, however, may control mesograzer abundances, thus releasing epiphytes from grazing pressure, with resultant widespread losses of seagrasses under eutrophic conditions. In order to assess whether trophic cascades operate in seagrass systems, we compared patterns of abundance and composition of the benthic vegetation community, the associated epifaunal invertebrates, and their predators in eelgrass meadows distributed across a natural gradient of algal productivity. Within this gradient we selected and sampled in summer and winter 3 eelgrass meadows differentially affected by strong coastal upwelling. High mesograzer abundance repeatedly coincided with low epiphyte biomass, emphasizing top-down control of epiphytes, despite high nutrient availability. Consumption of live eelgrass tissue by mesograzers was potentially important at the lower end of the productivity gradient. Alternating patterns of abundances between successive trophic levels under intermediate and high productivity conditions suggest that small predator abundances may ultimately govern the success of eelgrass in eutrophic meadows. When pipefish were abundant and eelgrass complexity was low, patterns were consistent with a destabilizing trophic cascade. Evidence supporting a trophic cascade dynamic from small fish through to epiphytes may indicate susceptibility of eelgrass systems to higher order interactions.

KEY WORDS: Seagrass · Zostera marina · Trophic cascade · Epifauna · Herbivory · Coastal upwelling · Baja California