DISTURBANCE EFFECTS OF DEPOSIT FEEDING ON MICROALGAL COMMUNITY STRUCTURE AND MECHANISMS OF RECOLONIZATION1

Benthic microalgae (BMA) are important primary producers in intertidal and shallow subtidal sediments, serving as a vital food resource for heterotrophs. BMA also release extracellular polymeric secretions that inhibit resuspension of sediments. Key ecological parameters such as abundance, productivity, and species composition of BMA each contribute to the character of these roles. Our primary objectives were to (i) assess the importance of biotic disturbance to the structure of sedimentary microalgal communities and (ii) identify principal modes of recolonization. We employed field comparative studies to test whether deposit feeding by two invertebrates (Leptosynapta tenuis and Balanoglossus aurantiacus) caused removal of BMA, and manipulative experiments to assess rates and mechanisms of recolonization. Both deposit feeders were determined to significantly reduce BMA biomass via ingestion; however, little change in community composition was observed. Recovery of these disturbed patches was followed over the period of intertidal exposure. We distinguished between potential recolonization methods of migration and regrowth by monitoring fecal coils incubated naturally on underlying sediments (regrowth + migration treatment), hydrogen-peroxide-treated coils incubated on ambient sediment (migration only), and coils that were incubated on 0.2 mu m filters and thereby isolated from underlying sediment (regrowth only). BMA biomass recovery was significant in < 3 h, with migration from underlying sediments the dominant means of recolonization. Surprisingly, recovery appeared somewhat slower when natural egesta were exposed to underlying sediments (migration + regrowth treatments) as compared to migration into peroxide-treated coils (migration only). This counterintuitive result was due to the dynamic, bidirectional vertical migrations of diatoms in surficial sediments.