The variation in the hydraulic conditions showed an
important role in algal accumulation. The slower flume with
lower drag forces recorded higher percent coverage of algae
independently of roughness level. Similar to the highest drag forces slowed down the deposition
and attachment of algal cells in the higher flow regime,
preventing higher accumulations in all surfaces. Neverthe-
less, the roughness stimulatory threshold is visible in the
fast flow flume, since algal SAC increase until intermediate
roughness. Hence, these results corroborate our initial
hypothesis, indicating that hydraulic dynamics can modu-
late the stimulatory threshold mechanisms which regulate
when algae will achieve its maximum growth.
In this context, the response of algal accumulation
showed that stimulatory threshold varied between slower
and faster flumes. The former showed no differences in algal
accumulation among roughness levels, which reveal no
stimulatory effect of increasing roughness. Actually, the
roughness differences among surfaces were not relevant for
algal growth in slower flow. The reason is probably related
to lower drag forces in this flume, which can favor algal
colonization by benefiting cell
migration and development. It is well known that cell
migration is the dominant mechanism of algal accumulation
during early stages of community development, when algal
densities are very low. Furthermore,
a higher sedimentation of detritus within crevices in slower
flumes may have contributed to our results due to the higher
amounts of nutritional resources for algal development in
any roughness level in these hydraulic conditions.
In turn, the higher values of drag force in the faster
flume allow the action of the stimulatory threshold effects
of roughness on algal accumulation. As we predicted, when
fast water flow conditions constraints algal growth, cell attachment was influenced by substrate roughness.
Indeed, our results show that in the faster flume algal
growth was stimulated up to the third level of roughness
following by a reduction after that. In this scenario, there is
a maximum roughness threshold, beyond which there will
be lower algal accumulation. Briefly, the reasons for a
rougher surface exhibit higher algal accumulation are
increased sedimentation efficiency and cell adhesion,
protection from disturbances, such as scour or grazing,
and alteration of flow around the substrate. However, the authors. showed that by increasing the size of crevices, they
no longer act as refuges. In our study, in the faster flow
flume, the higher drag forces showed this relation, indeed.
In addition, larger surface areas found in rougher surfaces
create lower quality attachment sites