The grazing impact of urchins on seagrass and algal resources, and the
relative importance of this to the lower-level trophic flux of a trop
ical seagrass community were investigated. Thalassia hemprichii (Ehren
b.) Aschers. accounted for 80-93% of seagrass frond biomass at Bolinao
in the Philippines. Growth rate of seagrass was 6.6 mm per shoot day-
1, or 2.3 mg AFDW per shoot day-1. Production of seagrass fronds per u
nit area of seagrass bed varied with location from 870 to 1850 mg AFDW
m-2 day-1. Urchin density ranged from 0.9 to 4.2 m-2, with Tripneuste
s gratilla (L.) and Salmacis sphaeroides (L.) being the most common sp
ecies. Tripneustes gratilla fed mostly on attached seagrass fronds (77
-89% of diet), especially Thalassia hemprichii, whereas S. sphaeroides
was a generalist, consuming Thalassia hemprichii fronds (13-65%), det
ached seagrass debris (5-39%), the red alga Amphiroa fragilissima (L.)
Lamour. (0-30%), algal-coated sediment and rubble (0-51%) in proporti
ons that varied with the availability of preferred food types. Live Th
alassia hemprichii fronds were clearly preferred over macroalgae or de
ad seagrass fronds by Tripneustes gratilla, but S. sphaeroides consume
d all three food types without preference. Both urchins avoided the co
mmon brown alga, Sargassum crassifolium J. Agardh. Urchins absorbed 73
-76% of organic matter in seagrass fronds with epiphytes (75% of DW),
and 55% of that in epiphyte-free fronds. Seagrass debris and the macro
algae A. fragilissima were of lower food quality as they were lower in
organic matter, and this matter was absorbed less efficiently by urch
ins. Rates of ingestion (IR in g WW per urchin day-1) were proportiona
l to body weight (W in g WW) according to the functions: IR=0.56W0.34
(T. gratilla) and IR=0.17W0.53 (Salmacis sphaeroides). Predicted grazi
ng impact of urchins on seagrass resources varied spatially and tempor
ally. Estimated annual grazing rate at the main study site was 158 g A
FDW m-2, equivalent to 24% of annual seagrass production, but owing to
large changes in urchin population structure and density, grazing imp
act is expected to vary from < 5% to > 100% at different times of year
. A synthesis of knowledge on the lower-level trophic pathways in this
system indicates that seagrass-urchin and periphyton-epifauna grazing
interactions are both important in their contribution to overall trop
hic flux.