- Effects of current velocity, irradiance, and grazing on the
biomass, taxonomic structure, physiognomy, and chemical composition
of lotic algal assemblages were investigated. Four experiments,
lasting 32 to 75 days, were conducted in laboratory streams to
determine how these factors affect algal assemblages singly, and in
concert with each other.
Regardless of light level, low current velocities (5 cm.s⁻¹)
enhanced initial rates of colonization, presumably because of high
settling rates. However, in streams exposed to equal irradiances,
greater biomass levels eventually developed in streams with high
current velocities (15 cm.s⁻¹). Few taxonomic differences could be
attributed solely to current velocity regimes.
Algal assemblages exposed to high (400 and 150) as opposed to
low (50 and 15 μE.m⁻²s⁻¹) irradiances were characterized by higher
biomass, a greater amount of filamentous chlorophytes, and lower
concentrations of total lipid and the 18:1 and 20:5 fatty acids. The physiognomy of the assemblage was also more complex at high
Effects of grazing on taxonomic structure were primarily a
function of algal growth form and herbivore feeding behavior.
Large, overstory cells were vulnerable to grazing and decreased in
relative abundance as grazing pressure increased, while small,
adnate cells increased in relative abundance. The snail Juga
silicula had little effect on algal dynamics at low densities
(125/stream = 62/m²), although at higher snail densities (500 and
1000/stream) and at all densities of the caddisfly Dicosmoecus
gilvipes, grazing resulted in low algal biomasses and a dominance
of adnate cells. Fatty acids were more robust indicators of algal
taxonomic structure than amino acids.
A separate experiment showed that at densities of 500
snails/stream (250/m²), algal biomass levels were similar in grazed
and ungrazed streams by day 43 in channels exposed to 100 and 400 μE.m⁻².s⁻¹. At 15 μE.m⁻².s⁻¹, algal biomass levels were similar
(and low) throughout the experiment. Grazing reduced amounts of
taxa with large growth forms, regardless of irradiance level.
A detailed process model of herbivory was developed, which
allows both quantitative (i.e. biomass) and qualitative (i.e.
taxonomic and chemical composition) components to be assessed. The
proposed model can be used to generate hypotheses about how algal
assemblages respond to current velocity, irradiance, and grazing in