DIATOM EPIPHYTES ON ODONTHALIA FLOCCOSA: THE IMPORTANCE OF EXTENT AND TIMING

The interaction between the epiphytic diatom Isthmia nervosa Kütz and the red alga Odonthalia floccosa (Esp.) Falkenb. was examined in terms of host physiology (photosynthesis and growth), fitness (reproduction and survival), and population dynamics. Most of these characteristics were compared among plants that varied naturally in epiphyte load. Diatom cover was also manipulated in field-deployed containers to study effects on host growth. Increasing levels of epiphytes were correlated with declines in host photosynthesis and growth, and experiments confirmed that reduced growth was directly caused by diatoms. Populations that hosted diatoms throughout the summer contained lower proportions of reproductive plants than did populations colonized in late summer or not epiphytized at all (60% vs. >80%, respectively). Odonthalia with epiphytes experienced greater biomechanical drag when submerged than did unepiphytized individuals of similar size. Despite differences in drag, epiphytized and unepiphytized hosts were equally susceptible to complete mortality (removed with holdfast) relative to partial mortality (shoots broken). In general, higher epiphytism was associated with reduced host performance, and this apparent damage was easier to detect at physiological levels than at population levels. In fact, negative effects of epiphytes were not evident at the population level, and host density and epiphyte load were positively correlated. Based on 4 years of monthly censuses of Odonthalia using marked plants and quadrats, the abundance of Isthmia varies spatially and temporally. Diatoms tend to colonize Odonthalia in late summer, after host growth has stopped and cystocarps and tetraspores have begun to develop. Subsequently, shoots break and plants persist through the winter as shorter perennating basal systems. Life spans were >4 years for some Odonthalia individuals. Many hosts may escape the effects of epiphytes by completing growth and reproduction before diatom colonization.