Studies of epiphytic periphyton and grazing epifauna were carried out at up to 9 sites in the seagrass beds around Santiago Island, Bolinao, Luzon, Philippines between February and May 1991. Sites were chosen to represent a range of seagrass habitats within different sediment regimes and with differing exposure to wind, currents and wave action.
Dry weight (DW) and ash-free dry weight (AFDW) was determined for periphyton removed from randomly selected shoots (n=20) of Cymodocea serrulata and Enhalus acoroides collected from Pislatan and Malilnep (shore). Biomass of periphyton per unit area of seagrass bed was calculated from periphyton biomass per plant and mean plant density based on unpublished data. Additional periphyton samples (n=5), collected from Lucero (reef flat), Malilnep (shore) and Pislatan were analysed for total organic carbon and total nitrogen (CHN Analyser). Samples were also examined under a compound microscope and the percentage cover of constituents was estimated by the point intersect method.
Primary productivity estimates using respirometry (light-dark bottle) were obtained. Four artificial seagrass units (ASU), comprised of thin plastic strips (ten, 25 mm × 500 mm) suspended from a frame were placed among seagrasses at Pislatan and Malilnep (shore) for 4 weeks to form a cover of periphyton. Seven bottles containing ambient seawater were arranged on the seagrass bed and into each of five bottles was placed a periphyton-coated strip (25 mm x 80 mm). Two bottles without periphyton were controls. Bottles were sealed and incubated for 20 min, either under ambient sunlight, or in complete darkness. Incubations were repeated three times for light exposure and twice for dark, and made under cloudless conditions at both Malilnep and Pislatan in early May between 10:00 and 14:00 h. After each incubation, the bottles were transferred to a boat where the ASU strips were removed. Oxygen concentration in each bottle was measured with a Yellowsprings Scientific: Instruments (YSI) polagraphic O2 probe. Photosynthetic pigments were extracted from the periphyton-coated strips. Absorbance of extracts was read at 666 and 730 nm (Varian spectrophotometer), and the concentration of chlorophyll a was calculated.
Four samples of surface (1 cm) sediment were collected from the Malilnep (shore), Lucero (reef flat) and Pislatan seagrass beds. These were dried to constant weight and subsampled (n=3) for determination of AFDW to DW ratio, silt, clay, sand and gravel contents, and N and total organic C.
The number and type of grazing invertebrates occurring on randomly selected specimens of Enhalus acoroides, Cymodocea serrulata and Thalassia hemprichii (n = 50) during daylight in April 1991 was estimated for all nine sites. At Malilnep (shore) and Pislatan, the number of grazers present the upper and lower half of each plant was recorded. The variation in abundance and distribution of different grazing invertebrates occurring on Enhalus acoroides and Cymodocea serrulata over one 24-hour cycle was assessed at Pislatan. Dominant grazers at Malilnep (shore) and Pislatan were collected randomly (n = 60-70 per species) from plants during daylight for measurement of shell length. The occurrence of grazers on sediments within each of the nine sites was determined by collecting and counting animals within randomly placed 300 mm × 300 mm (in 100 mm x 100 mm partitions) quadrats (n = 10). Sediment was examined to a depth of about 5 mm.
A preliminary estimate was made of the abundance of smaller epifauna on Thallasia hemprichii, Enhalus acoroides and Cymodocea serrulata from four entire plants of each species, collected during daylight at Malilnep (shore) and preserved in separate bags. After l hour, the contents were washed through a 0.5 mm sieve and the epifauna preserved for later examination.
The feeding behaviour of the gastropods Strombus mutabilis and Cerithium tenellum was monitored at the Bolinao Marine Laboratory. Ingestion rate (I) of individuals of these two species was estimated by two independent techniques. In the faeces quantification technique, five groups of freshly collected snails (ten per group) were placed in separate containers with aerated seawater and equal quantities of periphyton-covered fronds of Cymodocea serrulata and Enhalus acoroides collected from the snails' habitat. Over 3.5 days, the faeces produced in each container after successive day and night periods were collected for separate DW and AFDW determinations. Subsamples of periphyton scraped from seagrass fronds were similarly analysed for AFDW and organic carbon. In the grazer manipulation technique, a series of experiments was conducted at Malilnep (shore) using short-term caging of undisturbed plants of Cymodocea serrulata as the feeding substratum. In each experiment, ten plants were enclosed within cages made of Perspex tubing, which was sealed to macrograzers, but provided with mesh covered 3 cm holes to permit water exchange. Half of the plant-chamber replicates were free of snails and other large macrograzers, and two snails (of one species) were introduced to the remaining replicates. The chambers were sealed at the base by a split rubber bung, which enclosed the stem of the plant and then attached to a pole driven into the sediment. After 24 hours grazed and ungrazed plants were harvested for estimation of biomass. The experiment was repeated once for each snail species. Periphyton biomass on each plant was determined as described above.
To investigate the degree of selection of food types by snails, a separate enclosure experiment was conducted at Malilnep (shore) using ten separate plants caged as above, three with Strombus mutabilis, three with Cerithium tenellum and four as controls. Plants were harvested after 24 h and periphyton on each plant preserved for microscopic analysis of composition. Stomach contents of ten specimens of the dominant grazers, Strombus mutabilis and Cerithium tenellum, were examined, and the major components of the diet quantified using the method for microscopic analysis of periphyton composition.
