Influences of Flow Regime on Development and Desiccation Response of Lotic Diatom Communities

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Abstract
Diatom communities developed in either sheltered or direct—current habitats in the Colorado River below Lake Mead were subjected to short—term desiccation to test the hypothesis that communities inhabiting placid (sheltered) environments are less resistant to unpredictable disturbance than are those from more rigorous (direct—current) habitats. Direct—current communities exhibited lower cell accumulation rates and supported lower biomass and diatom densities than sheltered communities, suggesting that current shear and flow variability limited accumulation in direct current. Diatom communities that developed in sheltered habitats were less resistant to desiccation stress than communities from more rigorous flow conditions. Both biomass and cell densities were reduced by desiccation on sheltered substrates, whereas the same desiccation frequency in direct current had either no effect or caused increases in abundance parameters, depending on community age. Desiccation caused greater taxonomic change in sheltered than in direct—current environments. Low resistance to desiccation in sheltered environments was probably due to lack of nutrient renewal by current and to competition for resources in high—density communities. Temporal variability in ash content was greater in undisturbed than in desiccated communities in both flow regimes, indicating greater binding capacity of desiccated communities and suggesting that mucilage production increased with desiccation. In direct—current communities, where diatom accrual was suppressed by shear forces associated with fast flow, mucilage production after desiccation may have enhanced diatom immigration success, resulting in both higher densities and diversity in desiccated communities.