Contribution of epiphyte load to light attenuation on seagrass leaves is small but critical in turbid waters

Quantifying contributors to light attenuation is useful for the management of seagrass meadows. Epiphytic growth on seagrasses can lead to diminished light for the host plant, impairing photosynthesis and growth. Here, we quantify the contributions of the water column and epiphytic load to light attenuation in a Cymodocea rotundata meadow at Chek Jawa, Singapore. Using a modified spectrometer and seagrass mimics (clear polyethene strips) colonised by epiphytes, we determined the relationship between light transmission (400–700nm) and epiphyte load. Subsequently, we derived the percentage of surface light that reaches the leaf surface (PLL) over a range of epiphyte biomass and water-column light-attenuation coefficients (Kd). Results indicated that the relative contribution to light attenuation by epiphytic biomass was greater in clearer waters (Kd<0.5) than in turbid waters. As Kd increases, the amount of epiphytic material required to reduce PLL to minimum light requirement (11%) decreases exponentially. At Chek Jawa, the average epiphytic load was 32mg DW cm−2, which was close to the estimated amount (33mg DW cm−2) required to reduce PLL to 11% at prevailing turbidity levels. Our findings suggest that high epiphyte load is benign in clear waters, but becomes critical in turbid waters.

The use of pasture trees by birds in a tropical montane landscape in Monteverde, Costa Rica

Conversion of forests to agricultural land may require many bird species to use resources in fragmented landscapes in order to persist. Pasture trees can make agricultural landscapes more hospitable for birds, but we do not know what factors promote bird visitation to pasture trees. Bird use of 26 focal trees of a common pasture species, Sapium glandulosum (Euphorbiaceae), was examined in three pastures in a montane landscape in Costa Rica to understand factors influencing bird visitation. Bird visits were analysed in relation to pasture tree size, distance from forest edge, degree of isolation and epiphyte load. Foraging resources (epiphyte or tree substrate) were also measured. From May–July 2012, 52 bird species from 20 families were recorded from 926 unique visits. Bird visitation was best explained by tree size, degree of isolation and epiphyte load such that larger, more isolated trees with higher epiphyte loads attracted more birds. Birds preferred food resources from focal trees (51% of visits) rather than their epiphytes (5% of visits). The results corroborate previous findings that mature pasture trees, even when isolated, may contribute more to species persistence than smaller trees.