Periphyton responses to a hydraulic gradient in a regulated river in New Zealand

TitlePeriphyton responses to a hydraulic gradient in a regulated river in New Zealand
Publication TypeJournal Article
Year of Publication1994
AuthorsBiggs BJF, Hickey CW
JournalFreshwater Biology

1. Periphyton species composition, chlorophyll a concentration, organic matter biomass, and metabolic activity were analyzed at a site in a regulated river with low nutrient concentrations to investigate population and community level responses to a spatial gradient in hydraulic conditions. The communities were dominated by diatoms over the full hydraulic range [0.1-0.5m depth, 0.1-1.5 ms-1 velocity, and 0.01-1.5 Froude number (Fr)] with Cymbella kappii, Synedra ulna, and Gomphoneis herculeana having the highest relative biovolumes. 2. Unexpectedly, Cymbella kappii and Synedra ulna were abundant or co-dominant at all levels of velocity and Fr. Gomphoneis herculeana was most abundant within the mid-velocity range (velocity = 0.8-1.2 ms-1). 3. The physiognomy of the communities changed with a change in hydraulic conditions. There was progressively more diatom mucilage as velocity and Fr increased which resulted in a macroscopic change from relatively thin films (1.2 mm) at low velocities and Fr (in pool habitats), to thick (approximately 10 mm) mats at higher velocities and Fr (in riffle habitats). Associated with this, ash-free (AFDM) increased strongly, but chlorophyll a concentration did not, resulting in a decrease in %chlorophyll a over the gradient. 4. The results of experiments concluded in an in situ benthic chamber showed no significant differences in gross primary production between two communities of different biomass, but indicated a slightly decreasing trend with increasing velocity (0.14 to 0.38 ms-1). In contrast, community respiration increased greatly with mat biomass and also as a function of increased water velocity. The combined AFDM, chlorophyll a and metabolic results indicated that the zone of photosynthesis was maintained at the surface of the mats, with variable amounts of mucilage being secreted below, depending on hydraulic conditions. 5. The diatom community had considerable physiognomic plasticity through the accumulation of mucilage. It is suggested that this high mucilage secretion may be an active feedback mechanism to help ameliorate environmental stress. If so, present theory on velocity control of periphytic algal development in streams, which is essentially based on passive responses, needs expanding. It is concluded that the response of periphyton to spatial differences in habitat hydraulics in streams is highly complex and it may be difficult to define clear hydraulic habitat preference curves for periphyton communities in nutrient-poor streams.